The Universal Nuclide ChartAn Interactive JAVA-based Chart on CD-ROM

J. Magill, Y. Galla, J. GalyEuropean Commission,

Institute for Transuranium Elements,Postfach 2340, 76139 Karlsruhe

Germany

Contents

CONTENTS.......................................................................................................I

1. INTRODUCTION.......................................................................................31.1. Discovery of Radioactivity.........................................................................31.2. Structure of the Atom and Nucleus............................................................4

1.2.1. Thomson’s “Plum Pudding” Atom..................................................................41.2.2. Rutherford’s Nuclear Atom..............................................................................41.2.3. Bohr’s Model...................................................................................................51.2.4. The Nucleus.....................................................................................................6

1.3. The Atomic Mass Unit...............................................................................71.4. Binding Energy..........................................................................................81.5. The Elements............................................................................................9

1.5.1. Synthesis of the Elements................................................................................91.5.2. Classification of the Elements........................................................................10

1.6. Nuclide Stability – The Nuclide Chart......................................................11

2. NUCLIDE CHARTS................................................................................132.1. Introduction..............................................................................................132.2. Decay Processes and Nuclear Reactions...............................................132.3. Paper-based Nuclide Charts...................................................................14

2.3.1. Karlsruhe........................................................................................................152.3.2. Strasbourg......................................................................................................152.3.3. General Electric..............................................................................................152.3.4. JAERI.............................................................................................................17

3. RADIOACTIVE DECAY..........................................................................183.1. Alpha () decay.......................................................................................183.2. Beta-minus (ß-) decay.............................................................................203.3. Gamma emission and Internal Conversion.............................................213.4. Isomeric transitions (IT)...........................................................................213.5. Beta-plus (ß+) decay (positron emission).................................................223.6. Electron Capture ().................................................................................233.7. Spontaneous Fission (SF).......................................................................243.8. Proton Decay...........................................................................................253.9. Special beta-decay processes.................................................................263.10. Heavy-ion or Cluster Radioactivity........................................................263.11. Magic Radioactivity................................................................................28

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4. NUCLEAR REACTIONS.........................................................................294.1. Nuclear Interactions.................................................................................294.2. Cross-sections.........................................................................................294.3. Particles Interactions...............................................................................30

4.3.1. Neutron Reactions..........................................................................................314.3.2. Decay and Transmutation..............................................................................32

4.4. Fission and Fission Yields.......................................................................33

5. THE UNIVERSAL NUCLIDE CHART.....................................................355.1. Introduction..............................................................................................355.2. Installation and system requirements......................................................355.3. Features..................................................................................................365.4. How to use the program..........................................................................39

5.4.1. Decay chain simulator....................................................................................39Displaying the decay chain:.................................................................................41

5.4.2. Nuclide chart viewer......................................................................................425.4.3. Reaction path simulator..................................................................................43

5.5. Data-sources...........................................................................................44

6. SOME APPLICATIONS..........................................................................456.1. Basic Nuclear Physics: Halo Nuclides.....................................................456.2. The Super Heavy Elements.....................................................................466.3. Geology: the Age of the Earth.................................................................476.4. Radionuclides in Medicine.......................................................................486.5. Nuclear Waste Disposal and Natural Analogues.....................................49

GLOSSARY...................................................................................................53

Index...............................................................................................................54

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1. Introduction

In the fifth century B. C., Democritus proposed that the “elements” – earth, water, fire, and air were composed of extremely small particles called atoms (from the Greek word atomos meaning indivisible). This was based on the reasoning that if matter is continually subdivided, one would eventually obtain a particle so small that it could not be further subdivided.

1.1. Discovery of Radioactivity

In November 1895 Wilhelm Conrad Röntgen discovered X-rays. In a meeting of the French Academy of Science, the following January in Paris, Becquerel heard Poincaré report the recent discovery. The X-rays discovered by Röntgen were the result of fluorescence produced by cathode rays in a cathode ray tube. Becquerel wondered if luminescence was a precondition for the observation of X-rays – he had already studied phosphorescence of uranium compounds.In one of his early attempts, Becquerel exposed uranium-containing minerals to sunlight to cause the material to glow (phosphorescence). The sample was placed on top of a photographic plate wrapped in black paper. Following development of the plate he could observe if radiation had penetrated the black paper. This was indeed the case. By placing various objects (coins etc.) between the mineral and the photographic plate, he could reproduce the shapes of the objects.From his detailed records, it is known that Becquerel decided to develop plates which had been in his drawer together with the uranium mineral. These had not been exposed to sunlight. Remarkably, the plates had been “fogged” by the uranium without activation by sunlight. The uranium was emitting rays by itself. Becquerel had discovered radioactivity.Shortly after this event, Pierre and Marie Curie showed that thorium also acted like uranium. In an effort to try to isolate the source of the rays, they discovered the elements polonium and radium. Rutherford started working with these newly discovered uranium rays believing that they were similar to the X-rays discovered by Röntgen. In 1899, he discovered that these “rays” could be bent by a magnetic field and that there were two types of rays: alpha and beta. Today we know that the alpha particle is a nucleus of helium and the beta particle is an electron. In later experiments, Rutherford would use these alpha particles to probe the structure of atoms.

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1.2. Structure of the Atom and Nucleus

The discovery of radioactivity, and J.J. Thomson’s discovery of the electron, were the starting points for theories of atomic structure.

1.2.1. Thomson’s “Plum Pudding” Atom

In the early 1900s, J.J. Thomson proposed one of the first models of the atom. Based on the fact that atoms are electrically neutral, he proposed a configuration in which the negative charges of the electrons and the positive charges cancelled out. He considered the atom to consist of a sphere in which the mass and charge are distributed uniformly with electrons embedded like plums in a pudding. In this model, electromagnetic radiation would be emitted if external forces caused the electrons in the atom to vibrate.

Figure 1.1 Thomson’s “plum pudding” atom

1.2.2. Rutherford’s Nuclear Atom

In 1911, Rutherford postulated that, in contrast to the "plum pudding" model, the positive charge of the atom was concentrated in the central part of the atom called the nucleus, with the negative electrons situated in remote points i.e. that atoms consisted of open space.

Figure 1.2. Rutherford’s nuclear atom (schematic)

These ideas were put to test by two of Rutherford's students Geiger and Marsden in 1913. In a classic experiment, they bombarded thin gold foils with alpha particles emitted from radioactive polonium. If Rutherford's ideas on the structure of the atom were correct, then most of the alpha particles would pass through the thin foils with no interaction while a few would be scattered strongly due to close contact between the projectile and the nucleus.

= electrons

uniform distribution of positive charge

Orbiting electron

Postive nucleus

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The experiments confirmed Rutherford's ideas by showing that a collection of atoms consisted of positively charged nuclei with diameter of 5x10-15 m and that these nuclei were separated by distances of approximately 10-10 m. It was also found that the number of unit charges in the nucleus was approximately equal to the atomic numberof the atom and to about one half of the atomic weight.

Although these ideas were revolutionary at the time, there were two unsatisfactory aspects of Rutherford's proposed atomic structure. The first concerned how electrons are held in place outside the nucleus and the second how the protons can be held together in view of the strong repulsive forces of the positively charges particles. The idea of a small solar system with the electrons orbiting the positively charged nucleus seemed attractive but this was unacceptable to classical theory. According to accepted theory, such orbiting electrons experience a radial acceleration and should emit radiation. In doing so they would lose their kinetic energy and spiral into the nucleus.

Rutherford's ideas were indeed irreconcilable with the classical theory of electromagnetism. Around this time new ideas were emerging on the structure of the atom. In 1913 Bohr developed a model based on the new "quantum" theory proposed by Planck for radiation. Bohr proposed that the electrons orbit around the nucleus at certain fixed radii. In these orbits, the electrons do not emit radiation. Radiation is emitted only when the electron moves from one orbit to a lower energy orbit. The great success of Bohr's model lay in the fact that it could explain the sharp line atomic spectra which results from, for example, the electrical excitation of gases.

1.2.3. Bohr’s Model

To explain the fine structure of atomic lines, Bohr's ideas had to be developed further into a full quantum theory of the atom. Circular orbits, with the main or principal quantum number n, had to be extended to elliptical orbits with the nucleus at one of the foci. These elliptical orbits, including the circular orbit, for one quantum number n have the same length of the major axes and with this the same energy levels, but different lengths of the minor axes. Different proportions between the major axes and minor axes (i.e. the eccentricity of the elliptical orbit) show different azimuthal quantum numbers l. The number of different quantum numbers l depends on the main quantum number m through the relation lmax = n-1. The different l numbers characterise the different orbitals:

l 0 1 2 3orbital s p d f

This model is outlined in the figure 1.3.

In general, the number of orbitals with the same azimuthal quantum number l is given through the expression 2l + 1. Each of these orbits is characterised by the orbital magnetic moment ml, where ml has the values l, l-1, l-2, …, -l+1, -l. From this, we get obtain:

l 0 1 2 3number of orbitals 1 3 5 7

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ml 0 1, 0, -1 2, 1, 0, -1, -2 3, 2, 1, 0, -1, -2, -3

To explain the very fine structure of atomic lines, it was necessary to postulate that orbital electrons can spin in one of two directions about its own axis in the same way that the earth spins about its axis as it orbits the sun. The associated spin magnetic moment ms can take one of two values, +½ and -½, with the condition, that the two electrons in one orbital have different spins.

Figure 1.3. Bohr’s atomic model

The total magnetic moment, the quantum number m, is the sum of the orbital magnetic moment ml and spin magnetic moment ms. The magnetic moment may be positive or negative depending on the direction of the orbital motion. When the atoms of a substance are placed in a magnetic field, the electrons will arrange themselves in definite directions with respect to the applied field.

With Bohr’s atomic model, all elements of the periodic table can be categorised with these four quantum numbers n, l, ml and ms.

1.2.4. The Nucleus

The second difficulty of Rutherford's model of the atom was to explain how the protons can be held together in view of the strong repulsive forces of the positively charged particles. Hydrogen, the simplest atom, consists of a single proton in the nucleus. If heavier nuclei contained multiple protons, then the mass number and the atomic numbers should be the same. This was not the case. The mass numbers were found to be approximately twice the atomic number for light nuclei (for nuclei up to h mass number 20). Similarly, the Coulombic repulsion of these multiple proton nuclei would be enormous - what can hold the nuclei together? These difficulties were

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resolved by the discovery of the neutron by Chadwick in 1932. Its presence in the nucleus explains the difference between the atomic and mass numbers. More importantly, the neutron is responsible for the cohesive force that holds the nucleus together. This nuclear force is attractive and extremely short range - about 2-3x10-15

m.In addition, because of the very short range of the nuclear force, neutrons can only interact with their nearest neighbour nucleons, in contrast to the longer-range repulsive electrical forces of the protons. For this reason, in a stable nucleus, the number of neutrons must increase more rapidly than the number of protons.In addition to the proton, neutron, and electron, there are considerably more than 100 other fundamental particles which have been discovered or hypothesized. The majority of these fall into one of two classes, leptons (particles without any structure e.g. electrons, muons, and neutrinos) or hadrons (particles with structure e.g. protons and neutrons). These elementary particles can be grouped in a similar manner to chemical elements in the periodic table. From these groupings, it has been proposed that hadrons are composed of simpler components called quarks. This has led to the speculation that hadrons are composed of three (or possibly more) simpler components called quarks, and that these quarks are “glued” together by gluons which carry the strong nuclear force.

1.3. The Atomic Mass Unit

Atomic masses can be expressed in grams or as dimensionless atomic mass units (amu). The mass of a single atom can be computed from the fact that one mole of any substance contain 6.02214199(47)x1023 atoms (Avogadro's number) and has a mass equal to the molecular weight in grams. For 12C, 1 mole has a mass of 12.0000 g, hence the mass of one atom is given by:

Mass of one atom of 12C = 12.0000/(6.02214199x1023)

= 1.9926465x10-23 g

Since the mass of 1 mole of 12C was found to be 12.0000g, it was chosen as the reference standard for the system of (relative) atomic weight ratios (AWR). The atomic mass unit, amu (also known as a Dalton) is

1 amu = 1.9926465x10-23 g / 12 = 1.66053873(13)x10-24 g

The energy equivalent of the amu is:

E = mc2 = 1.66053873x10-27 kg * (2.99792458x108 m/s)2 = 1.49243971x10-10 J

Fig. 1.4 Variation of the binding energy per nucleon with atomic number. Data from Nuclides.net database.

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= 931.494013 MeV (where 1 eV =1.602176462x10-19 J)

1.4. Binding Energy

The binding energy, BE, of a nucleus is the energy which must be supplied to separate the nucleus into its constituent component nucleons. The sum of the masses Mc of the constituents of an isotope is given by

Mc =ZmH + (A-Z)mn

where Z is the atomic number, A is the atomic mass number (number of nucleons in the nucleus), mH is the mass of a hydrogen atom and mn is the mass of the neutron. Consider the case of 17O, hence

Mc (17O) = 8(1.007825) + (17-8)(1.008665) 17.140585 amu

However, the actual atomic mass of 17O is M 16.999132. This difference between the atomic weight and the weight of the constituent masses is the mass defect i.e. = Mc - M and is the mass equivalent of the energy required to separate the components. In energy units, the binding energy is

BE (Mc - M) (931 MeV/amu)

This binding energy is a measure of the cohesive energy of the nucleus. The average binding energy per nucleon E is given by

<BE> 931(Mc - M)/A (MeV/nucleon)

In the case of 17O, <BE>=7.75 MeV per nucleon.A plot of the binding energy per nucleon, <BE>, versus the atomic mass number A, in fig. 1.4, shows a broad maximum in excess on 8 MeV per nucleon between mass numbers 50-100. At lower and higher mass numbers, the binding energy per nucleon is less. It is for this reason that energy can be released by splitting heavy elements of by fusing light elements. Notable departures from the smooth behaviour of the binding energy per nucleon vs. the mass number are provided by 4He, 8Be, 12C, and 16O. The binding energies per nucleon of these isotopes are higher than their immediate neighbours indicating that they are very strongly bound. These nuclei contain respectively one, two, three, and four sub-units of 4He. This tends to suggest that nucleons form stable sub-groups of two protons and two neutrons within the nucleus.

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1.5. The Elements

1.5.1. Synthesis of the Elements

The distribution of elements in nature is by no means uniform. Studies of stars reveal that hydrogen is by far the most abundant element, accounting for approximately 75% of the mass of the universe. The remaining mass is mostly in the form of helium gas with other elements contributing only to a small extent.

The chemical composition of the universe is undergoing continuous change. Hydrogen is being converted into helium, and helium is being changed into heavier elements. As time goes on, the ratio of heavier elements increases relative to hydrogen. Presumably, the process is not reversible.The observed abundances of elements in the universe require the postulation of at least eight different processes for their formation. The main processes are:

hydrogen burning (formation of helium-4 by the fusion of hydrogen nuclei), helium burning (formation of carbon-12 from helium-4-nuclei), s process (slow neutron capture processes, starting from iron group nuclei,

close to the stability line), r process (rapid neutron capture processes, starting from iron group nuclei,

produce nuclei far from the line of stability), rp process (similar to the r-process, except it goes by successive proton

absorptions), X process (spallation of heavier elements into lighter elements by cosmic

rays).

Light elements were probably produced in the early history of the galaxy whereas heavy elements were formed in supernovae.

According to current theory, the universe is about 1010 years old and started as a “soup” of quarks at enormous temperatures and pressures. As the universe expanded, the temperature and pressure decreased rapidly to the point where neutrons and protons condensed out. Shortly after, nuclear fusion took place and led to the formation of deuterons followed by tritons and onto helium-3 nuclei and alpha particles (hydrogen burning), thereby giving rise to the present hydrogen to helium ratio of 3:1. In addition, some lithium-6 and lithium-7 were produced at this time. As the universe further expanded and cooled, the fusion reactions stopped and the gases condensed and eventually led to the formation of stars consisting of the primordial elements.

As a cloud of interstellar gas contracts and condenses to form a star, the temperature will rise as the gravitational potential energy is converted to kinetic energy. When the temperature increases to 107 K and beyond, fusion reactions will begin to take place and further nucleo-synthesis leads to the conversion of elements hydrogen and helium to carbon, silicon, oxygen, sulphur and heavier elements up to iron. The energy

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released in these fusion reactions provided the driving force for these reactions to take place. Fusion can proceed no further than iron since this element has the highest binding energy. Fusion of heavier elements will consume energy rather than release it.

The synthesis of elements heavier than iron require a supernova. As stated above, the sequence of stellar burning more or less stops when the core of the star is composed of nuclei with mass numbers close to 56 (i.e. iron). At this point the central region of the star has consumed all the nuclear fuel and cannot support the stars’ interior against gravitational collapse into a neutron star. In this process all protons are converted into neutrons with the release a large number of neutrinos within a short time. These neutrinos interact with the outer layers of the star resulting in a large fraction of the mass being ejected into space. In addition, large numbers of particles are produced, in particular neutrons. It is then these neutrons, which lead to the formation of heavier elements by transmutation through r(rapid)-process neutron capture events.

At least 24 elements are believed essential to living matter. The most abundant of these in the human body are, in order of importance: hydrogen, oxygen, carbon, nitrogen, calcium, phosphorus, chlorine, potassium, sulphur, sodium, magnesium, iron, copper, zinc, silicon, iodine, cobalt, manganese, molybdenum, fluorine, tin, chromium, selenium, and vanadium.Ninety-one elements occur naturally on earth supporting the theory that heavy elements were produced during creation of the solar system. Exceptions are technetium and promethium although these are present in stars.

1.5.2. Classification of the Elements

Until recently, there were 112 known chemical elements. In the last few years, evidence for the existence of elements 114, 116 and 118 has been reported. Each element is characterized by atoms containing a fixed number of protons, denoted by the atomic number Z, in the nucleus and an equal number of orbital electrons to ensure electrical neutrality. In addition to protons, the nucleus contains a variable number N of electrical neutral neutrons. Atoms of an element with different numbers of neutrons but fixed number of protons are known as isotopes of that element. More than 3000 nuclides are known, but only about 10% of these are stable. The total number of protons plus neutrons is known as the mass number A of a nuclide. Nuclides with the same N and different Z are called isotones, and nuclides with the same mass number A are known as isobars. In general, an atom with atomic number Z, and neutron number N is known as a nuclide. A nuclide can be specified by the notation:

Z is the atomic (proton) numberN is the neutron numberA is the mass number (N+Z)

X is the chemical element symbol

Because of the relationships between Z, A, N (A=Z+N) and X, a nuclide can be uniquely specified by fewer parameters. A particular chemical element is uniquely specified by its symbol X or the proton number Z. A nuclide is uniquely specified by

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the element name X (or proton number Z) together with the mass number A. An example is 60Co which refers to the element cobalt (chemical symbol Co) with mass number 60 (number of protons plus neutrons). A variety of ways of referring to this nuclide are in current use i.e. Co60, Co-60, Co 60.

1.6. Nuclide Stability – The Nuclide Chart

A plot of the number of protons versus the number of neutrons for stable isotopes (represented by the black squares), is shown in fig.1.5. This particular way of arranging nuclides was first proposed by Segrè.

From fig.1.5. it can be seen that stable isotopes lie within a relatively narrow range indicating that the neutron to proton ratio must have a certain value or range of values to be stable. Radioactive nuclei (white squares in fig. 1.5) mostly lie outside this range. The plot also shows that for low atomic numbers, the neutron to proton ratio is unity. At higher atomic numbers, this value increases indicating a higher ratio of neutrons to protons in heavy atoms.

The extremities of the white regions above and below the region of stability are known as the proton and neutron “driplines” beyond which nuclei are extremely unstable (i.e. if a nucleon is added it will “drip” out again). As nucleons are successively added to a nucleus on the stability line, the binding energy of the last

Fig. 1.5. Nuclide stability diagram. Stable nuclides (black) fall in a narrow range of neutron to protons ratio. Unstable nuclides (white) have neutron to proton ratios outside this range. Also shown are the proton and neutron magic numbers represented by the horizontal and vertical lines.

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neutron decreases steadily until it is no longer bound and the nucleus decays by either neutron emission (at the neutron dripline).

Nuclei with even numbers of protons and neutrons are more stable than nuclei with other combinations of neutrons and protons. For uneven numbers of protons and neutrons, there are only very few stable nuclides.

Figure 1.6. Number of stable nuclides

The stability of nuclei is extremely significant for special numbers of protons and neutrons. These numbers are 2, 8, 20, 28, 50, 82 and 126 and correspond to full shells in the shell model of the nucleus. The element tin with the proton number Z = 50, for example, has 10 stable isotopes, more than all other elements. When the proton and neutron numbers have both magic values, the nucleus is said to be “doubly magic”. Double magic, stable nuclides are for example 4He, the alpha particle, as well as the nuclide 208Pb, which is reached in several decay processes, for example in the decay chain of 232Th.

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2. Nuclide Charts

2.1. Introduction

A representation of all known nuclides is commonly made under the so called the Segré Chart (it was first proposed by Segré) or more commonly the Chart of Nuclides. It is essentially a plot of the number of protons, Z, vs. the number of neutrons, N in the nucleus, which follows the logical pattern of atom building. In addition of displaying basic information on each isotope (such as half-life, thermal cross section, etc...), this particular way of arranging nuclide is a useful way to describe radioactive decays and nuclear interactions.

Such a chart is useful to find out information about the different elements, showing how many isotopes it has (along the Z axis) and which are stable or unstable.Various kind of information for each particular nuclide can be found on chart, and, even it the basic structure of the chart is common, the amount of information contained in each depends on the publisher. In the past national chart have been developed for in house used and then some of them became standards commonly used.

The most common paper based charts are described in a following section: the Karlsruhe chart from Germany, the Strasbourg chart from France, the General Electric chart from US and the JAERI chart from Japan.

2.2. Decay Processes and Nuclear Reactions

The Nuclide or Segrè Chart is essentially a plot of the number of protons, Z, vs. the number of neutrons, N in the nucleus. This particular way of arranging nuclide was first proposed by Segrè. As will be seen, this plot is a particularly useful in revealing nuclear reaction processes and in particular nuclear decay.

In fig. 2.1, a nuclide with co-ordinates Z, N “decays” to the position shown depending on the decay process. A nuclide located in the centre (dark grey) will, following , -, +/EC, n, or p decay processes, resulting in a daughter nucleus at the

Fig. 2.1 Nuclide or Segrè chart ( a plot of Z vs. N) showing radioactive decay processes. A nuclide with co-ordinated Z, N “decays” to the position shown depending on the decay process.13

positions shown by the various arrows. Hence, if one knows the mode of decay, one can trace out the decay path or chain.

The Nuclide Chart is also very useful for investigating activation and nuclear reactions as shown in fig. 2.2. A target nuclide of element Y (with Z protons and N neutrons, (A=Z+N) will transform by reaction with a particle a to nuclide Y (with Z’ protons and N’ neutrons, A’ = Z’+N’) with the emission of particle b. This reaction can be written

AX + a A’Y + b

Or more compactly in the form

AX(a,b)A’Y

The result of interaction of a variety of particles a (neutrons, alpha particles, deuterons, gamma radiation, protons etc.) with a target nuclide with co-ordinates Z, N is shown in fig. 2.2

2.3. Paper-based Nuclide Charts

As explained previously, all the charts plot all known nuclides in a number of protons vs. number of neutrons representation. They show basic information for all nuclides: symbol, mass number, half-life, modes of decay with main energies, abundance for the stable or radioactive available nuclides, isomeric states. Other information and color schemes are particular to each edition as described as follows.

Fig. 2.2 Activation processes and nuclear reactions on the Segrè chart. A target nuclide with co-ordinates Z, N transforms to the nuclide Z´, N´ through the processes shown.

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2.3.1. Karlsruhe

The Karlsruhe chart has been developed in at the Institute for Radiochemistry in the Forschungszentrum Karsruhe in Germany. The decay scheme is based on the decay mode. The possible decay modes are:

: ß-: : ß+: sf:

p:C1:

alpha decaybeta decayelectron capturepositron decayspontaneous fissionproton decaycluster emission

ß- ß+ sf p C12 Stable

There is also a color for stable nuclides (black), so we have 7 different colors in this color scheme. For each isobar, the thermal fission chain yield of the 235U is given.

2.3.2. Strasbourg

The Strasbourg chart is similar to the Karlsruhe one since the colour scheme of both are based on the main decay modes of the nuclide. The colours used to describe to the decay processes, however, are different:

: ß-: : ß+: sf:

p:IT:

alpha decaybeta decayelectron capturepositron decayspontaneous fissionproton decayIsomeric Transition

ß- ß+ sf p IT Stable

2.3.3. General Electric

The color scheme used in this chart is based on half-lives and cross-sections. The lower half of the cell, defining a single nuclide, is used to identify the larger of thermal neutron absorption cross-section. The following identification of range of absorption properties is used:

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Absorption cross section range Colour

10 to 100 barns blue

100 to 500 barns green

500 to 1000 barns yellow

Greater than 1000 barns red

The upper half of the nuclide cell identifies half-life, within the following ranges:

Half-life Colour

less than 1 day white

between 1 and 10 days red

between 10 and 100 days yellow

100 days and 10 years green

between 10 and 500 million years blue

more than 500 million years or

Stablegray

Other information such as spin and parity, thermal neutron capture cross section followed by the resonance integral are displayed. For each isobar, the thermal chain yield is given for the 233U, 235U and 239Pu fissioning systems.

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2.3.4. JAERI

This Japanese Chart, developed at the nuclear data center JAERI tokai, use a four color code to highlight the half life of the nuclide with the corresponding scheme:

Half life Colour

less than 10 min yellow

between 10 min and 30 days red

between 30 days and 5.108

yearsgreen

greater than 5.108 years blue

If the nuclide has not been synthesized (the chart goes up to the mass 300 and the element 121!) the block nuclide has no color. On the other hand, if the nuclide has been synthesized but that its half-life has not been measured, the block nuclide upper part is colored in yellow and its lower part is left without color.

The JAERI chart uses block hash method to point out the main decay modes of the nuclide and their range of intensity (per decay).

The paper based chart gives only the half-life, the mass number and the element symbol as basic information.

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3. Radioactive Decay

Radioactive decay is a spontaneous nuclear transformation which results in the formation of new elements. In this process, an unstable “parent” nuclide is transformed into a more stable “daughter” nuclide through various processes. The process is usually accompanied by the emission of gamma radiation. If the daughter nuclides is also unstable, the radioactive decay process continues further in a decay chain until a stable nuclide is reached. Radioactive nuclides decay spontaneously by the following processes.

Alpha () decay Beta-minus (ß-) decay Gamma emission Isomeric transitions (IT) Beta-plus (ß+) decay Electron capture () Spontaneous fission (SF) Proton decay (p) Special beta-decay processes (ß-n, ß+, ß+p) Heavy-ion radioactivity (14C, 24Ne etc.)

Radioactive decay is a nuclear process and is largely independent of the chemical and physical states on the nuclide. The actual process of radioactive decay depends on the neutron to proton ratio and on the mass-energy relationship of the parent, daughter, and emitted particle.

3.1. Alpha () decay

The highly energetic alpha particle emitted in this type of radioactivity is a doubly charged ion of helium, i.e 4He(2+) or simply 4He. Energies of the alpha particles lie in the MeV range. An example is the decay of uranium-238:

23892U 234

90Th + 42He ()

In this reaction, the neutron to proton ration increases from 1.59 for 23892U to 1.6 for

23490Th. For this reason, alpha decay is mainly found for nuclides above the regions of

stability, where the neutron to proton ratio is too low for stability. The process of alpha decay is found mainly in high atomic number nuclides due to the fact that electrostatic repulsive forces increase more rapidly in heavy nuclides than the cohesive nuclear force. In addition, the emitted particle must have sufficient energy to overcome the potential barrier in the nucleus. The height of the potential barrier in fig.2.1 is about 25 MeV. Nevertheless, alpha particles can escape this barrier by the process of quantum tunnelling.

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The known alpha emitters are shown in yellow in fig.3.1. There are in total 815 alpha emitting nuclides listed in the Nuclides.net database.

Fig. 3.1 Alpha emitters (yellow) in Nuclides.net

For alpha emission to occur, the conservation of mass and energy requires that

MP= MD + M + 2Me + Q

Where Mi denotes the mass of species I (I = P, D, , e, corresponding to the parent, daughter, alpha particle and two orbital electrons lost in the transition). Q is the total energy released in the transition. Re-arranging this expression, the energy released Q can be obtained from the known masses. For the decay of 238U, this gives:

Q = MP – MD - M - 2Me = 236.005810 - 232.033047 - 4.00277 –2 * 0.00055 = 0.0048 atomic mass units or 4.47 MeV

If no gamma energy is released, this energy Q appears as kinetic energy of the recoil nucleus, the daughter and the alpha particle. If E is the energy of the alpha particle, it can be shown by conservation of energy and momentum that

E = Q/(1+m/M)

where M is the energy of the recoil nucleus and m the mass of the alpha particle. For the decay of 238U, E = 4.47/(1+4/234) = 4.39 MeV. All alpha particles emitted by a nuclide either have the same energy or have at most a few different discrete energy values dependant on the excited state of the daughter.

In their interaction with matter, the alpha particles give up their energy and become neutral helium atoms. Their range in solids and liquids is very short - of the order of µm. In air the range is typically a few cm. Because of this short range, they do not normally constitute a hazard to humans. They are absorbed in the outer layers of the skin before they cause injury. If the alpha particles are taken internally, for example by ingestion or inhalation, they are very toxic because of the large amount of energy

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released in a short distance within living tissue. Indeed, this property can be used for killing cancer cells in the process of alpha-immunotherapy.

3.2. Beta-minus (ß-) decay

The process of ß- radioactivity occurs when a nucleus has an excess of neutrons and emits a negative electron from its unstable radioactive nucleus. Theoretical considerations, however, do not allow the existence of a negative electron in the nucleus. For this reason the beta particle is postulated to arise from the nuclear transformation of a neutron into a proton through the reaction

n p +ß- +

Beta emission differs from alpha emission in that beta particles have a continuous spectrum of energies between zero and some maximum value, the endpoint energy, characteristic of that nuclide. The fact that the beta particles are not monoernegetic but have a continuous energy distribution up to a definite maximum energy, implies that there is another particle taking part i.e. the neutrino .

Fig. 3.2 ß- emitters (blue) in Nuclides.net

This endpoint energy corresponds to the mass difference between the parent nucleus and the daughter as required by conservation of energy. The average energy of the beta particle is approximately 1/3 of the maximum energy.

More precisely, the "neutrino" emitted in beta-minus decay is the anti-neutrino (with neutrino being emitted in beta plus decay). The neutrino has zero charge and almost zero mass. The maximum energies of the beta particles range from 10 keV to 4 MeV. Although beta minus particles have a greater range than alpha particles, thin layers of water, glass, metal, etc can stop them. An example is the decay of Th-234:

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+ - +

There are 1281 ß- emitters in the Nuclide.net database.Beta radiation can be an external radiation hazard. Beta particles with less than about 200 keV have limited penetration range in tissue. However, beta particles can also give rise to Bremsstrahlung radiation which are highly penetrating.

3.3. Gamma emission and Internal Conversion

Gamma emission is not a primary decay process but usually accompanies alpha and beta decay. Typically this type of radiation arises when the daughter product resulting from alpha or beta decay is formed in an excited state. This excited state returns to the ground state through the emission of a gamma photon. Instead of having a well-defined range like alpha and beta particles, gamma rays loose characteristically a certain fraction of their energy per unit distance through matter. Gamma rays are highly penetrating and can result in considerable organic damage. Gamma emitting sources require heavy shielding and remote handling.

Alternatively, the excited nucleus may return to the ground state by ejecting an orbital electron. This is known as internal conversion and results in an energetic electron and X-rays due to electrons cascading to lower energy levels. The ratio of internal conversion electrons to gamma emission photons is known as the internal conversion coefficient. Conversion electrons are monoenergetic. Consider the case of the internal conversion of 137Cs. This nuclide is transformed by ß- emission to the excited state 137mBa. This daughter nuclide then emits a 0.661 MeV photon which undergoes internal conversion in 11% of the transitions. These conversion electrons are seen in the beta spectrum of 137Cs. Following the internal conversion, outer orbital electrons fill the deeper energy levels and result in characteristic X-ray emission. These X-rays can in turn lead to the ejection of outer electrons through an internal photoelectric effect. These low energy ejected electrons are known as Auger electrons.

3.4. Isomeric transitions (IT)

In contrast to normal gamma emission that occurs by dipole radiation, isomeric transitions must occur by higher order multipole transitions that occur on a longer time-scale. If the lifetime for gamma emission exceeds about one nanosecond, the excited nucleus is defined to be in a metastable or isomeric state. The decay process from this excited state is known as an isomeric transition (IT). An example is the decay of the isomer or metastable state of protactinium i.e.

234m91Pa 234

91Pa +

with a half-life 1/2 = 1.17 min. and a branching ratio of 0.0016. The letter m after the mass number denotes the metastable state.

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Fig. 3.3. Nuclides which decay by isomeric transition (blue) in the Nuclides.net database

There are 475 nuclides which decay by isomeric transition in the Nuclides.net database.

3.5. Beta-plus (ß+) decay (positron emission)

In nuclides where there are too many protons, i.e. the neutron to proton ratio is too low, and alpha emission is not energetically possible, the nucleus may become more stable by the emission of a positron (a positively charged electron). Within the nucleus a proton is converted into a neutron, a positron, and a neutrino i.e.

n p + ß+ +

Similarly to the ß-, the positron ß+ is continuously distributed in energy up to a characteristic maximum energy. The positron, after being emitted from the nucleus, undergoes strong electrostatic attraction with the atomic electrons. The positron and negative electrons annihilate each other and result in two photons (gamma rays) each with energy of 0.511 MeV moving in opposite directions. An example is the decay of C-11:

116C 11

5B + ß++

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Fig. 3.4. ß+ emitters (red) in the Nuclides.net database

There are 1497 ß+ emitters in the Nuclides.net database

The radiation hazard from positrons is similar to that from ß- particles. In addition, the gamma radiation resulting from the positron-electron annihilation presents an external radiation hazard.

3.6. Electron Capture ()

Neutron deficient nuclides can also attain stability by capturing an electron from the inner K or L shells of the atomic orbits. As a result, a proton in the nucleus transforms to a neutron i.e.

p + e- n +

The process is similar to ß+ decay in that the charge of the nucleus decreases by 1. The resulting nucleus is unstable and decays by the ejection of an unobservable neutrino () and the emission of an x-ray when the electron vacancy in the K or L shell is filled by outer orbit electrons. An example is the electron capture process in beryllium i.e.

74Be +e- 7

3Li +

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Fig. 3.5. Nuclides which undergo electron capture (red) in the Nuclides.net database

The nuclides.net database lists 162 nuclides which undergo electron capture.

3.7. Spontaneous Fission (SF)Spontaneous fission was discovered in 1940 by Petrzhak and Flerov in 238U. Although the alpha emitting properties of 238U were well known by that time, the much less common spontaneous fission had been “masked” due to its very small branching ratio of about one SF in 2x106 alpha emissions. With the exception of 8Be (which decays into two alpha particles), SF has not been detected in any elements lighter than uranium. In the 1960s, sources of 252Cf became available and detailed measurement of the fissioning of this system contributed much to our understanding of the process.

Actinides (U, Pu, Am, Np ,Cm, etc.) and trans-actinides can undergo radioactive decay by spontaneous fission. In this process the nucleus splits into two fragment nuclei, with mass and charge roughly half that of the parent, and several neutrons.  For fermium, one such reaction is as follows:

256100Fm 140

54Xe + 11246Pd +4n

where only one of many fission product combinations is shown. The kinetic energy release in this process, due mainly to large electrostatic repulsion of the fragments, is approximately 190 MeV.The nuclides.net database currently lists 123 spontaneously fissioning nuclides.

Fig. 3.6. Spontaneously fissioning nuclides (green) in the nuclide chart

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3.8. Proton Decay

A review of the early theoretical speculations on the subject of radioactive proton emission has been given by Goldansky in 1966. This decay process, discovered in 1970 by Jackson et al., is exhibited by the metastable state of cobalt-53.

This decay process, discovered in 1970, is exhibited by the metastable state of cobalt-53, i.e.

with branching ratios of 1.5% (p mode) and 98.5% (ß+ mode). Proton radioactivity from a ground state, i.e. 151Lu, was first reported by Hofmann et al in 1981. In the following three years, three more proton emitters were observed in the region of nuclei near 151Lu with 147Tm, 147mTm, 150Lu, and two in the region near 100Sn i.e. 113Cs and 109I. This work is summarised in the review article by Hofmann (1995). More recently, due to various experimental iumprovements, proton transitions have been found in nuclei near to N=82 (146Tm, 146mTm, 156Ta, 160Re) and also in 165,166,167Ir, 171Au, 185Bi and 105Sb. As of 1995, 21 radioactive proton transitions have been established definitively. Since then more proton emitters have been either claimed or identified. The nuclides.net database contains 95 proton emitters.

The phenomena of two-proton radioactivity has been discussed by Brown (1991). In total there are 13 two-proton emitters in the Nuclide.net database

Fig. 3.7. Proton emitters (brown) in the Nuclides.net database

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3.9. Special beta-decay processes

Neutron emission immediately following ß- emission (beta delayed neutron emission ß-n) is observed in many neutron-rich nuclides. The phenomena of delayed neutron emission is very important in the control of nuclear reactors since neutron emission occurs on a timescale much longer than that associated with fission – this allows a response time long enough to move control rods and thereby control the fission reactor. An example of this type of emission is given by 17N i.e.

177N 17

8O* + ß- +

178O* 16

8O + n

where the asterisk denotes the short-lived intermediate excited states of oxygen-17. The effective half-life for this process is 4.17s. There are 279 ß-n emitters in the Nuclides.net database.

Some positron emitters beta-decay partly to excited states that are unstable with respect to emission of an alpha particle (β+α). Examples for this sort of decay are boron-8 as well as 20Na, i.e.

2011Na 20

10Ne* + ß+ +

2010Ne* 16

8O +

There are 39 ß+ emitters listed in the Nuclides.net database. Likewise, some negative electron emitters beta-decay partly to excited states that are unstable with respect to emission of an alpha particle β-α. Examples for this sort of decay are lithium-8 as well as 18N, i.e.

187N 18

8O* + β- + ν18

8O* 146C + α

There are 11 β-α emitters in the Nuclides.net database. In a few cases, positron decay leads to an excited nuclear state not able to bind a proton. In these cases, proton radiation appears with the half-life of the beta transition. One example of this is the decay of 111Te i.e.

11152Te 111

51Sb* + ß+ +

11151Sb * 110

50Sn + p

There are 176 ß+p emitters in the Nuclides.net database

3.10. Heavy-ion or Cluster Radioactivity.

In 1984, Rose and Jones at Oxford University announced the discovery of a new rare type of radioactive decay in the nuclide 223Ra. Their article entitled “A new kind of natural radioactivity” was published in Nature. The possibility that such a decay process, intermediate between alpha decay and spontaneous fission, may exist was

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postulated by A. Sandulescu, D.N. Poenaru, and W. Greiner a few years earler in 1980.Rose and Jones showed that part of the 223Ra parent nuclide decays directly to 209Pb by the emission of a 30 MeV 14C ion i.e.

223Ra 14C + 209Pb + 31.87 MeV

The transition in the nuclide chart is shown in fig.3.8. Observations also have been made of carbon-14 from radium-222, radium-224, and radium-226, as well as neon-24 from thorium-230, protactinium-231, and uranium-232. Such heavy-ion radioactivity, like alpha decay and spontaneous fission, involves quantum-mechanical tunnelling through the potential-energy barrier. Shell effects play a major role in this phenomenon and in all cases observed to date the heavy partner of carbon-14 or neon-24 is close to doubly magic lead-208.

The ratio of carbon-14 decay to alpha decay is about 5.510-10. This low value explains why the spontaneous decay mode had not been observed earlier. Since the probability of cluster emission is expected to be greatest when the daughter nuclide configuration is close to that of a full shell, attempts have been made to observe the phenomena with parent nuclides near Z = 82 (Z=82 corresponds to a magic proton line). Hence the search has concentrated on the elements francium and actinium with potential daughter of bismuth and thallium e.g.

Fig. 3.8 Emission of 14C cluster from 223Ra was discovered by Roseand Jones in 1984.

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221Fr 207Tl, and 225Ac 211Bi

Oxygen cluster emission was discovered by Hussonois et al. (1991) in the decay of thorium i.e.

226Th 18O + 208Pb

According to the “golden rule”, 34Si cluster should result from the decay of 214Am and 240Pu.In total there are 26 cluster emitters in the Nuclides.net database.

3.11. Magic Radioactivity

The discovery of trans-tin cluster emitters may confirm the idea of “magic radioactivity” proposed by Sandulescu in 1989. Magic numbers in the trans-tin region are at N = 50 and 82 and Z = 50. The doubly magic closed shell nuclides 132Sn and 100Sn lie far from the line of stability as can be seen in fig 2.4. In the proton rich region around Ba-Sm cluster emission would lead to nuclides close to the doubly magic 100Sn. Expected cluster emission reactions could be

114Ba 12C + 102Sn, 128Sm 100Cd + 28Si

Fig. 3.9. Magic numbers in the trans-tin region areat N = 50 and 82 and Z = 50. The location of doubly

magic tin nuclides 132Sn and 100Sn are indicated.

Sandulescu has also proposed the idea of cold fission as a special case of cluster radioactivity where the fission fragments lie in the Z=50 region. An example is the decay of fermium i.e.

264Fm 132Sn + 132Sn

in which the neutron rich fermium splits into two identical doubly magic tin fragments with a probability comparable to that of alpha decay.

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4. Nuclear reactions

4.1. Nuclear Interactions

Nuclear interactions involve absorption of a bombarding particle by the nucleus of a target material. The absorption of the projectiles produces first a compound nucleus in an excited state, which then breaks up to yield the final products. The idea of a compound nucleus has actually been put forth by Bohr in 1936, describing the idea that an atomic projectile entering a nucleus may be thought to remain in the nucleus, putting the nucleus in an exciting state, after which emission of a particle or gamma ray, an ensuing nucleus is created.

Three main different kinds of interactions can occur:

Elastic Scattering. In this process, the incident particle strikes the nucleus, which is almost always in its ground state. After while, the particle reappears, and the nucleus is left in its ground state. The particle is said to have been elastically scattered by the nucleus. For an incident neutron particle, this interaction is abbreviated, in nuclear notation, by (n,n).

Inelastic Scattering. This process is identical to elastic scattering except that the nucleus is left in an excited state. In the case of a neutron incident particle, the inelastic scattering is denoted by (n,n'). The excited nucleus decays by the emission of -rays.

Absorption. Absorption reaction occurs when the incident particle is fully absorbed into the target atom to form a new compound nucleus in an excited state and then breaks up in a very short time. The compound nucleus may decay on various possibilities, which are in competition with one another, and also depend on the energy brought to the nucleus by the incident particle.

4.2. Cross-sections

The likelihood of an interaction between a bombarding particle and a nucleus is described by the concept of cross section denoted by the Greek letter As there is no guarantee that a particular bombarding projectile will interact with the target nucleus to bring about a given reaction, provides only a measure of the probability that it will occur, i.e. the cross section is use in describing the efficiency of a nuclear process.

The cross section is often described as being equal to the number of collision per second with one nucleus per unit intensity of the beam, but it also appears to be the

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effective cross sectional area of the nucleus. From the previous definition one can see that the cross section has the unit of an area (cm2) per target nucleus, although it is not a physical area. Cross sections are usually within the range of 10 -24 cm2. Nuclear physicists use the unit of barn, abbreviated b, where:

1 b = 10-24 cm2

The story says that one of the early researchers in the field was so surprised of the large values of the calculated and measured cross sections (a barn is on the order of the square of a nuclear radius) that he called out that it was ``as big as a barn'' and the name remained.

Cross sections are defined for all type of reaction resulting from the interaction of an incident particle and a nucleus. In the case of an absorption reaction, the de-excitation of the compound nucleus may involve the emission of an alpha particle, electrons, positrons, protons, -rays or, in the case of fission, fission fragments. All the different decay “paths” can be refereed to a specific cross section as for the emission, for the emission, f for the fission cross section, and etc.

For a given type of incident particle, the sum of the cross-sections for all possible interactions is called the total cross section t. This total cross section represents the probability that an interaction, whichever type it can be, may occur from the particle bombarding. The same way, one can define a total absorption cross section which is equal to the sum of all absorption reaction cross sections (fission cross section, emission cross section, etc.).

4.3. Particles Interactions

The idea of creating a nucleus by artificial means was first achieved in 1919 by Rutherford when he bombarded ordinary nitrogen with high-speed alpha particle emitted by Bi-214. From his investigations, he concluded that some less massive particle must have been ejected from the nitrogen nucleus and by conservation laws he concluded that the particle would be a proton. This conclusion was beautifully confirmed later and opened a new field of the physics often called transmutation1. However, in order to get detectable effects with alpha particle when heavy nuclei with high barrier are bombarded, the incident particles needed a much higher energy. This has led to a search for means of accelerated particles to higher energy.

The first reactions to be produced by artificially accelerated particles were actually the proton-induced reaction in 1930 by Cockcroft and Walton. They used accelerated protons first to 100,000 volts and later to 700,000 volts to bombard a target of lithium 7 and leading to the release of two alpha particles:

From then, and with the developments of accelerators and cyclotrons, many reactions have been experimented using as incident particle not only alpha and proton particles but also deuterons, neutrons, high-energy gamma rays.

1 “Don't call it transmutation... they'll have our heads off as alchemists.”Ernest Rutherford, 1902.

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To represent a particular reaction, the abbreviated symbol of the incident and emitted particles within parenthesis is used.For example:

Alpha-proton ( ,p)Alpha-neutron (,n)Neutron-proton (n,p)Gamma-proton (,p)Proton-gamma (p,)Neutron-fission (n,f)

etc.

The corresponding cross section are abbreviated the same way e.g. the neutron-fission cross section is noted (n,f).

4.3.1. Neutron Reactions

A particle with a mass about the same as the proton and with no charge has been initially suggested by Rutherford. But it is in 1932 that Chadwick actually measured it and the zero charge property of it led to the name of neutron.

Due to the low power of the accelerators in the 30s, it was not possible to produce particles of sufficient energy to disrupt the nuclei of the heavier atoms. In 1934, Fermi irradiated with neutrons some sample of Uranium, the last element known at this time, in order to extend the table of Mendeleev, after an electron emission, and has created what he presumed to be the first transuranic element. The observation of a score of radioactive periods has shown a surprising process and more complex even than the formation of the element 93. In December 1938, the presence of barium in the reaction process has revealed the presence of a new nuclear process: the fission process was thus discovered. It was then after World War II that the discovery of fission, and Fermi's belief that he had produced transuranic elements, was amply confirmed.

With Fermi's experiments and since them, more neutrons reaction experiments have been produced than by any other means. They still are quite common nowadays since neutrons are used in the nuclear reactors for both researches and energy production. The following table shows the likely neutron reactions which have been measured:

Reaction Nametotal

elasticnonelastic

n,2nn,3nn,4nn,nn,n2n,2n

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n,npn,ndn,ntn,

n,fissionn,pn,2pn,dn,t

n,3Hen,

n,t2

4.3.2. Decay and Transmutation

The half-life of a radioactive nuclide, T½, is defined as the time it takes for half of the atoms of a radioactive source to undergo transformation. If one takes the standard decay equation:

where N(t) is the number of atoms at the time t, and is the decay constant, the solution is N(t) = N(0) exp(-t). It follows that ½ = exp(-T½) and hence T½ = ln2/.

In a reactor, the fuel is burnt by the neutron flux following the equation:

where N(t) is the number of atoms at the time t, the absorption cross section and the neutron flux of the reactor.

As we discussed in a previous section, the word “transmutation” is used to indicate the transformation of a nucleus to another by the process of absorption of an incident particle (proton, neutron or other) and emission of particles or -rays. This word is commonly used nowadays when speaking about nuclear waste reduction by means of nuclear transformations.The same way, one has defined the half-life, a burnout time is defined to be the amount of time it takes for half the atoms of a fuel to burn (if one supposes the flux time independent), i.e. Tb = ln2/.

If the burn-out and the decay equations are combined we obtain:

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In this equation, when T1/2 >> Tb (long half-life, short burnout time), one can neglect the decay process as the absorption of a neutron occurs much faster than the decay, and when Tb >> T1/2 (long burnout time and short half-life), one can neglect the nuclear reaction as the nuclides has decayed long before a neutron is absorbed.In the case of the uranium cycle, one can suppose that, by neutron absorption, 238U gives 239U, as the burnout time for 238U is much shorter than its half-life of 4.47x109

years. This then decays to 239Np as the half-life of 239U is, with only 23.4 minutes, shorter than its burnout time. The 239Np than decays to 239Pu as its half-life of 2.35 days is much shorter than its burnout time (for a standard neutron flux of 5x1013) of 1.18 years. The 239Pu than absorbs neutrons and goes on to 240Pu and 241Pu which all have a half-life of several years, so that the burnout time is much shorter and the neutron reaction takes place.

Figure 4.1: First 6 steps of reaction path of U 238

You have to be careful with the neglect of the nuclear reaction or the decay process. If the half-life and the burnout time for the standard neutron flux of 5x1013 aren’t very different, only a little change in the neutron flux will change the behaviour of the nuclide. In the Thorium cycle, for example, 233Pa has a half-life of 26.9 days and a burnout time of 1.09 years. In this case, the 233Pa decays to 233U. But if you increase the neutron flux only up to 8x1014, the burnout time sinks to 25 days, so that the probability for the neutron reaction is higher than for the decay, this means that 233Pa absorbs a neutron and gives 234Pa more often than decaying to 233U.As result, you see that you can only neglect nuclear reaction if the burnout-time is much bigger than the half-life (half-life of less than about 10 days) and the neutron flux is near to the standard flux. The decay process can be neglected if the half-life is more than 100 years and the neutron flux is near the standard flux.

In more realistic calculations, one should take into account that the flux can vary within the irradiation and, on the other hand, that the effective sections are related to the neutron spectrum (in hardness and energy), which can vary with the concentration of the different nucleus, and which depends on the irradiation rate.

4.4. Fission and Fission Yields

The fission interaction is the basis of all nuclear chain reactions, without it there would be no nuclear reaction. In fission, the neutron bombards an atom of fissionable material splitting it apart and producing additional free neutrons to perpetuate the reaction, as well as heat energy and radiation energy. A fissionable element will have a heavy nucleus from which an average of two or three neutrons are emitted when it is struck by a neutron.

Among the processes of nuclear decay modes, fission is certainly the most complicated one. Indeed fission reactions differ according to the mass and nuclear

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charge of the fissioning nuclides, according to the excitation energy of the compound nucleus, and furthermore, more than 900 primary fission products are produced in the fission process.

In the fission process, the probability of measuring the production of an isotope or nuclide is generally expressed as yield (given in units of production per unit fission or in percent). Thus, the mass and charge distributions reflect the yields of fission products. Furthermore, in fission, like in other complicated nuclear reactions, the spectrum of the products changes with time. Indeed, primary products, due to their radioactivity, transform into other products that can also be formed directly in the fission process. Facing the complexity of the nuclear fission, different types of yields are defined and distinguished.

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5. The Universal Nuclide Chart

5.1. Introduction

This new electronic Chart of Nuclide is aimed to simulating the existing paper-based charts and to display information on the nuclide decay processes with a user-friendly interface.This book/application is addressed to both students and professionals who will take advantage of its powerful features. Indeed, one can use the Universal Nuclide Chart and its features for educational purposes and for displaying basic nuclear data commonly proposed in a nuclide chart, but with the fabulous potential of an interactive interface and powerful electronic data implementation.

5.2. Installation and system requirements

The program is platform independent and it can then be run on any operating system. On the other hand, we recommend a processor frequency greater than 500MHz and 64 Mo of RAM

The application is launched from an HTML page which can be read directly from the CD-ROM with an appropriate browse. Any browser which supports JAVA can be used. Internet Explorer 5.x or greater, Netscape 5.x or greater or even Opera is supported.

The starting HTML Page is shown in Fig. 5.1.

The whole directory “Universal_Chart” can be copied to the hard disk for a quicker access/run of the application (recommended).

The application is written in JAVA 1.2.2 but, except the latest version of Netscape 6.1, actual browsers only support Java 1.1, therefore a special plugin is needed to run the applet. This plugin exists for several operating systems and machines (Windows, MacOS, LINUX, UNIX, SOLARIS, IBM-AIX and others) and it is available from the sun website (http://java.sun.com).If you have not already installed the Java-Plugin 1.2.2 or higher, you will be prompted to download and install the appropriate plugin for your machine before running the applet.

A simple click on the “Start” button will run the application.

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Fig. 5.1. The Universal Nuclide Chart starting HTML page launched by the Internet Explorer browser.

5.3. Features

The Universal Nuclide Chart presents the following features:

1. An electronic nuclide chart programmed in the JAVA language for platform independence.The Java Applet can be viewed with the newest versions of Internet Explorer or Netscape Browser. With older browser version, an additional, licence-free plugin is needed.

2. Simulate radioactive decay and nuclear reactions.During the simulation of radioactive decay, all decay chains of the selected nuclide are evaluated and shown on the screen in the format Z vs. N or A-2Z vs. Z. For the nuclear reactions, the burnout-time and the half-life are compared. If the burnout-time is shorter than the half-life, a neutron is absorbed; otherwise the main decay chain is followed.

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Fig. 5.2. Decay chain of the 238U simulated in the Universal Nuclide Chart Applet.

3. Display nuclear decay and cross-section data for more than 3000 nuclides.The following data is given as far as available: half-life, decay modes, daughter products, branching ratios, abundances, two most important gamma lines, thermal neutron cross sections, burnout-time.

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4. Simulate existing paper-based nuclide charts (Karlsruhe, Strasbourg, General Electric, JAERI).Radioactive decay and nuclear reactions can be simulated with the most important color schemes for nuclide charts, as well as a representation of the nuclide chart in sizes 5x5 up to 15x15 is available for these color schemes.

Fig. 5.3 a) and b). a) Display of the half-life, the burn out time and the available thermal cross section for 235U. b) Display of the Half-life, modes of decay, daughters and branchings, and the two main gamma lines and the corresponding emission probability for the selected 235U nuclide.

a)

b)

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Fig. 5. 4. a), b), c) and d). Simulation of the existing paper based chart by the Universal Nuclide Chart displaying the decay of 238U. The figures a), b), c), and d) represent the Karlsruhe chart, the Strasbourg chart, the General Electric chart and the JAERI chart, respectively.

a)

b)

c)

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5.4. How to use the program

You can start the application by pressing the “start” button on the starting HTML page.

The applet is optimized for a desktop area of minimum 800*600 pixels, better 1024*768 pixels. With a resolution of 1024*768 pixels, the whole applet can be shown in the browser at once, as it has a size of 770*575 pixels.

The applet simulates radioactive decay schemes for over 3000 nuclides in two different arrangements and with five different standard color schemes. Also shown are nuclides with a high gamma radiation (blinking).

5.4.1. Decay chain simulatorAfter having started the program, a window with a size of 770 * 575 pixels is opened. In this window, there is an area on the top with different text-fields, buttons and combo boxes. Under this area, there is an empty area. In this area, decay chains are shown as soon as the user has started the simulation with the Start-button.

Fig. 5.5. The Decay Chain Simulator toolbar.

For this simulation, the user can set the following:

Select Nuclide:With the two combo boxes, the user can select an element and the isotope of the element, characterised by the atomic mass. The m or n after the mass shows that this is the metastable state of the isotope. The default value for the selected nuclide is U238.

d)

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Time Step:Here the user can change the time interval in which the daughters are painted onto the screen. The value is given in milliseconds with a default value of 200 ms.

Start / Stop:With the Start-Button, the user starts the simulation with the chosen selections. During the simulation, the applet can be stopped by pressing the Stop-Button. With the simulation halted, it is only possible to change the time step, and then to restart. As soon as the simulation finishes, the Stop-Button is automatically changed to a Start-Button and all input and selection fields are enabled again.

Plot:There are two different ways of showing the simulation. The first one is the Normal Plot where the number of protons Z in the nuclide is shown as a function of the number of neutrons N.The second plot is the Compact Plot. In this case the quantity A-2Z is shown as a function of the number of protons Z, where A is the atomic number (N+Z). This leads to a more compact representation of the decay scheme.

Colour Scheme:The applet can be run with five different standard colour schemes. The first is the Karlsruhe Colour Scheme, where the colours depend on the main mode of decay (alpha, ß-, ß+, IT, n, SF, p, ec, stable).The second is the Strasbourg Colour Scheme in which different colours are used for the decay processes.The third colour scheme is the General Electric Colour Scheme. Here the colours depend on the half-lives of the nuclides (<1d, 1-10d, 10-100d, 100d-10y, 10y-5x108y, Stable), not on the decay mode. The fourth colour scheme is the JAERI Colour Scheme, where different colours are used for half-lives under 10 minutes, between 10 minutes and 30 days, 30 days and 5x108 years and stable nuclides. The fifth colour scheme is the Nuclides 2000 Colour Scheme, which shows the main mode of decay as the Karlsruhe Colour Scheme.The default colour scheme is "Karlsruhe".

Th232 / U235 / U238:With these buttons the user can simulate the three natural decay families (Th232, U235 and U238) directly. These nuclides can of course also be selected through the use of the combo boxes.

MinBR(%):This field shows the threshold branching ratio for a daughter, that this is shown in the decay chain. With this feature, it is possible to follow the historical way of discovering the decay chain.

E(MeV):This field shows the threshold gamma energy per disintegration in mega electron volt (MeV) to initiate "blinking". This can be changed also while the simulation is

41

running. As decimal separator, a point (.) must be used. The default value is 2.0 MeV, and this is also introduced when the user deletes the contents of the field.

Displaying the decay chain:

All nuclides, that are reached on the decay chain, are shown as part of the decay chain. The colour shows, depending on the selected colour scheme, the main decay mode of this nuclide or the half-life of this nuclide. Metastable states of elements reached through decay are shown in the top right corner of the element box. They either have another colour than the ground state of the nuclide or they are made visible with a black border around the metastable state. If only the metastable state is part of the decay chain, this is show in the correct colour and the rest of the box is transparent. If there is no information available on a daughter product, this is shown transparent and the decay scheme ends there. Also if all branching ratios of a nuclide are less then the threshold value for the branching ratio or the branching ratios are not known, the nuclide is shown transparent with no further daughters.

Fig.5.5. Display of the decay of the 239Pu with the ITU color scheme. Basic decay information of the nuclide 239Pu can be obtained by mouse over.

On dragging the mouse over the nuclide boxes shown in decay chain, the basic decay data are shown. For the metastable states, one must be careful to position the mouse accurately. This data is for radioactive nuclides: Element symbol, mass, half-life, all decay-modes with the corresponding daughter products and branching ratios and, if available, the two most important gamma lines. For stable nuclides, this data is: Element symbol, mass, information that the nuclide is stable, abundance.

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By clicking into a nuclide box shown in the decay chain, another window with a size of 620 * 612 pixel is open. In this window, a part of a nuclide chart with the size 9x9 nuclides is shown with the clicked nuclide in the middle, highlighted with a red border.

5.4.2. Nuclide chart viewer

If one clicks on the 238U nuclide on the decay chain window, one obtains the following chart window:

Fig. 5.6 Nuclide Chart window showing the surrounding nuclides around 238U. Thermal cross section, half-life and burnout time are displayed on the left.

The chart size can be zoomed in and out from 5x5 nuclide representation to a 15x15 nuclide representation. Half-life and burnout time for the shown nuclides are displayed on mouse over. When available, thermal neutron cross section are as well displayed for the neutron reactions available in the database.For likely fission products, the chain yield of the isobar, which the selected nuclide belongs to, is given for the thermal fission of 233U, 235U and 239Pu.

The user can navigate over the chart by double-clicking on a specific nuclide, which will have for effect to place the chosen nuclide in the center of the display.

5.4.3. Reaction path simulator

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Pressing the “Reaction Path” button the user reach the reaction path simulator. This feature shows the likely neutron reactions occurring in a reactor with a given neutron thermal flux (exception is made of the fission reaction) based on the cross sections, the burnout time and the half-life of the selected nuclide.

The user can input the neutron thermal flux (the default value is equal to 5.1013 n.cm-

2.s-1, which correspond to the average thermal neutron flux in a PWR reactor type). The reaction index which correspond to the number of nuclides to display from the total reaction path.

As explained in section 4.3.2, depending of the input thermal neutron flux, there is a competition between the decay term and the burn-out term of the equation. This can be illustrated by the following example.

If one takes the example of the 238U: with a high thermal neutron flux, the burnout term will predominate in the total decay-burnout equation.By absorption, 239U excited nucleus will be formed and it will decay to 239Np by ß- emission, which will decay to 239Pu and so on. The reaction will be shown in the limit of the given reaction index number as shown in Fig.5.7.

On the other hand, if one sets up a low thermal neutron flux, then the decay process will predominate over the burnout process. Then we will recognize on the reaction path the starting path of the decay chain (which may be stopped by a nuclide with a long half-life where the neutron absorption may predominate, e.g. in our example by 230Th with a half life of 7.54104 years).

The Fig.5.8 shows the decay process of 238U starting (the 238U decays to 234Th by -decay, and etc). The chain ends with 230Th due to its large half-life.

Fig. 5.7. Simulation of the reaction path of the 238U irradiated

with a 5.1013 n.cm-2.s-1 thermal neutron flux. The reaction number

index is set to 10 nuclides.

Fig. 5.8. Simulation of the reaction path of the 238U irradiated with a 5.101

n.cm-2.s-1 thermal neutron flux. The reaction number index is set to 10

nuclides.

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5.5. Data-sources

The basic radioactive decay data used in the nuclides.net database is from the NUBASE evaluation (see the "ATOMIC MASS DATA CENTER" http://www-csnsm.in2p3.fr/AMDC/web/nubase_en.html). The first version of the NUBASE evaluation of nuclear and decay properties appeared in Nuclear Physics A, 1997, vol. 624, page 1-124. The evaluation contains experimentally known nuclear properties, and some that have been estimated from extrapolation, for 3010 nuclides: mass, isomeric excitation energy, half-life, spin, parity, decay modes and intensities. Spectral information such as the gamma energies and emission probabilities is not contained within NUBASE.

Spectral data is from the Joint Evaluated File (JEF) version 2.2, with a few corrections for known inaccuracies in the file. The JEF project is managed by the OECD/NEA Data Bank in Paris, France, and the original JEF-2.2 evaluated nuclear data library is available, free of charge, from the NEA Data Bank (http://www.nea.fr). The OECD/NEA makes every effort to ensure the accuracy of the radioactive decay data, but assumes no responsibility for errors and omissions in the data nor any liabilities with respect to the use of, or for damages resulting from the use of, these data.

Averaged thermal cross section data is based on the JENDL-3.2 data file reported in the JEF Report 13. Fissions yields are also from the JENDL data files (JENDL-3.2: Nakagawa T., Shibata S., Chiba S., Fukahori T., Nakajima Y., Kikuchi Y., Kawano T., Kanda Y., Ohsawa T., Matsunobu H., Kawai M., Zukeran A., Watanabe T., Igarasi S., Kosako K. and Asami T.; J. Nucl. Sci. Technol., 32(12), 1259-1271 (1995)).

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6. Some Applications

6.1. Basic Nuclear Physics: Halo Nuclides

For most nuclides, the density of nucleons in the nucleus is more or less uniform. However near the neutron drip line, some neutrons are only weakly bound to the inner core nucleons. In this recently discovered phenomena, the nuclei consist of a normal nucleus surrounded by a halo of extra neutrons with a diameter very much larger than the core nucleus. The halo neutrons are mostly outside of the strong nuclear force between the nucleons. Typically half-lives of these nuclei are around 100ms. One such example of a halo nuclide is lithium-11. It consists of a lithium-9 core surrounded by a halo of two loosely bound neutrons requiring only 0.3 MeV to remove them. The lithium-11 nucleus is similar in size to that of calcium-48 which has a neutron binding energy of about 8 MeV. Other examples of halo nuclei are helium-6 (He-4 + 2n), helium-8 He-4 + 4n), beryllium-11 (Be-10 + 1n), beryllium-14, boron-17 and carbon-19.

Borromean Nuclei

In Borromean nuclei, three separate parts of the nucleus are bound together in such a way that if any one is removed, the remaining two become unbound. The expression originates from the Borromean Ring which consists of interlocking rings.Carbon-12, in its excited state, is one example of a Borromean nucleus. It consists of three sub-units of helium-4. If one is removed, the result is beryllium-8 which is not bound. Other examples are helium-6, lithium-11, beryllium-14, and carbon-22.

Fig. 1.2 The neutron halo in 11Li extends to fill the volume equivalent to 208Pb, with very dilute, pure neutron matter. Courtesy Wilton Catford, from the SIRIUS Science Booklet.

Fig. 1.3. The Borromean rings provide an analogy for the structure of halo nuclei in which the removal of any one of the three major components breaks the whole system. Courtesy Wilton Catford, from the SIRIUS Science Booklet.

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6.2. The Super Heavy ElementsThe idea of the existence of a group of stable elements out-with the main nuclear “island” dates back to the thirties and received considerable attention in the sixties. The location of a smaller island of stability at Z=114, N = 184 was suggested in 1966 and, at this time, prompted an intensive search for super heavy elements in nature. The isotope Z = 114, N = 184 has a doubly magic configuration with both the protons and neutrons being in complete shells. In addition, the idea of creating super heavy elements experimentally through heavy ion collisions led to a number of laboratories being set up to investigate this. The wave of enthusiasm, at the time depicted in Professor Flerov’s Nuclide Chart shown in the diagram, continues to this day.

In the years 1981-84, the GSI group at Darmstadt, Germany, reported the synthesis of elements 107 (Bohrium), 108 (Hassium), and 109 (Meitnerium). In 1994 elements 110 and 111, as yet unnamed, were reported by the same group.

In 1998, a Russian team in Dubna, produced for the first time element 114. This was achieved by bombarding (fusing) nuclei of plutonium-242 with calcium-48. The scientist had to bombard the plutonium with calcium nuclei for a period of six weeks to produce a single nucleus of element 114. The reaction gives rise to a compound nucleus, 289114 shown below, which decays by alpha emission. The compound nucleus of element 114 had the remarkably “long” half-life of 30 s before undergoing a series of alpha decays to element 108 over a time period of approximately 30 minutes.

Fig. 6.2. The nuclide chart as depicted by Prof. Flerov in 1974. The ships, which are afloat, represent the different laboratory strategies and facilities dedicated to this endeavour (picture by kind permission from Prof. G. N. Flerov, Dubna, USSR).

Fusion: 2048Ca + 94

242Pu = 289114 + n

Decay: 289114 285112 281110 277108 ......47

In 1999, researchers from the University of California at Berkeley and Oregon State University claimed to have detected three atoms of element 118 in collisions between high-energy krypton ions and a lead target. The results, however, were later retracted.

6.3. Geology: the Age of the EarthTowards the end of the 19th century, the age of the Earth was of great scientific controversy. Lord Kelvin had estimated the age to be around 20 million years based on the rate of cooling assuming that the Earth was originally in a molten state. Already by 1840, from the work of Hutton and Lyell, it was known that rocks were laid down on top of one another in an orderly manner. This gave rise to age estimates based on sedimentation geology. Darwin had estimated the age of the Earth at 300 million years from cliff erosion assuming an erosion rate of 1 inch per century.

In 1896, Becquerel discovered radioactivity in uranium. Shortly thereafter, the Curies discovered polonium and radium in pitchblende in addition to uranium. Unknown at that time, these discoveries would eventually lead to the development of techniques for accurate age determinations.

Rutherford showed that through radioactive processes, one element could be transformed into another - for example radium (a metal) became radon (a gas). Rutherford and Soddy suggested also that the observed presence of helium in rocks might be due to radioactive "decay". The same authors first established the "decay chain" of the unstable "parent" uranium. They identified 14 stages in the decay of uranium producing 8 atoms of helium. In addition, a new stable element was found.

Figure 6.2. Decay chain of uranium with the 14 stages established by Rutherford.

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Based on these observations, Rutherford and Soddy concluded that if the amount of helium in a rock could be measured, one could estimate how long this process took i.e. the age of the rock. In particular, by measuring the amount of radium and helium, Rutherford was the first person to date the age of a rock at 500 million years. The problem with this method was, however, that helium gas could escape from the rock and that an age determination would result in a minimum age. Evidence was however gathering that Kelvin's age determination was much too low. In 1907, Boltwood had observed that along with helium, large amounts of lead were found in rocks containing radioactivity. He postulated that lead was the stable product in the decay of uranium. Based on this, Arthur Holmes proposed that the age could be determined by measuring the amount of lead, rather than helium i.e. on a uranium/lead technique. The basic assumption here was that the "ordinary" lead in a rock was present in much smaller quantities than the amount produced by the decay of uranium.

Around this time, Soddy discovered "isotopes" – nuclides of one element which had different atomic masses. The existence of these isotopes would complicate matters considerably in the development of age techniques - they would, however, ultimately lead to an accurate dating technique. Not only did lead have four such isotopes Pb208 (parent Th232), Pb207 (parent U235), Pb206 (parent U238), Pb204 ("ordinary" lead) but natural uranium had also three isotopes (U238, U235, U234 with relative abundancies of 99.28%, 0.72% and 0.0055% respectively). Following the discovery of the new isotope of uranium U235, which decayed much faster than U238, Rutherford in 1929 determined the age of the Earth by assuming that, at formation, equal amounts of U238 and U235 were present. The result obtained, 3400 million years, was the first age determination based on isotope ratios.

It also followed from this new isotope of uranium, that the uranium lead pathways contained two geological clocks that could be used to check one another i.e. the decay of U238 to Pb206 and the decay of U235 to Pb207. In addition, a third "clock" based on the ratios of the isotopes Pb206, Pb207 relative to the constant value of Pb204 was proposed.. This new lead-lead method is still use today in age dating techniques. The first estimates of the age of the Earth based on these lead ratios (the isochron method) in meteorites are due to Paterson in the 1950s. The current value of the age of the Earth is 4550 70 million years.

6.4. Radionuclides in Medicine

Radionuclides were first used for therapeutic purposes almost 100 years following the observation by Pierre Curie that radium sources brought into contact with the skin produced burns. Already by 1915, sealed sources of radium-226 and radon-222 were in use. By the 1950s radiotherapy had become much more widespread due to the development of remote source handling techniques and the availability of reactor produced radionuclides such as cobalt-60.

Ionising radiation from radionuclides kills cells by damaging the DNA thereby inhibiting cellular reproduction. The energy of the radiation (in the form of photons, electrons, heavy particle, etc) required to damage DNA should be greater than a few electron volts (eV) corresponding to the binding energy of the outer electrons. In external beam radiotherapy, where radiation is delivered from outside the body,

49

photon energies of millions of electron volts are required to penetrate the tissue and reach the tumours inside the body. Today, however, hundreds of thousand of patients are treated each year based on brachytherapy (Brachys is the Greek word for near) in which sealed sources of radiation are brought into the body and placed in or near the tumour to be treated leaving the surrounding healthy tissue undamaged. Using these brachytherapy implants, radionuclides with photons energies as low as 20 keV can be used. The radionuclide palladium-103, for example, used for prostate implantation has an average energy of only 21 keV. Some gamma ray emitters commonly used are 192Ir, 137Cs, 125I, 103Pd and have an effective range of a few centimetres. Beta emitters include 90Y, 188Re, and 32P with effective ranges of a few millimetres in tissue.

In the last ten years, the technique known as radio-immunotherapy has been under investigation. In this technique, a radionuclide is chemically attached to an antibody which is then injected into the bloodstream. The antibodies go to the source of the tumour and the attached radionuclides, for example 131I, emit charged particles to kill the tumour cells. The three main applications of radionuclides for therapeutic purposes, however, remain a) sealed sources for prostrate therapy, b) sources for intravascular therapy and c) radiopharmaceutical therapy. It is expected that these therapies will see rapid growth in the near future.

6.5. Nuclear Waste Disposal and Natural Analogues

Nuclear waste disposal is a problem of radionuclide "packaging" in the extreme. One of the challenges facing the nuclear industry is to demonstrate that an underground repository can contain nuclear waste for very long periods of times and that any releases that might take place in the future will pose no significant health or environment risk. It must be taken into account that the engineered barriers which initially contain the wastes will degrade, and that some residual radionuclides may return to the surface in low concentrations at some time in the future due to groundwater movement and environmental change.One way of building confidence in engineered barriers is by studying the processes which operate in natural and archaeological systems and by making appropriate parallels with a repository. These studies are called "natural analogues". The natural analogues are particularly relevant in the event that nuclear waste transmutation is introduced*.

Natural Analogues:

There are many radioactive materials which occur naturally and can be found in rocks, sediments etc. In particular, uranium which is the main component in nuclear

* Recently proposals have been put forward to introduce transmutation of nuclear waste reduce the burden of underground repositories. Through transmutation, the mass and the radiotoxicity of the waste are significantly reduced as is also the time needed to reach the radioactivity level in natural ores. The proposed schemes lead to the radiotoxicity of the waste reaching reference levels in about 500-700 years rather than hundreds of thousands of years in the natural decay process.

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fuel, occurs in nature. By studying the distribution in nature, information can be obtained on the move of uranium in rocks and ground waters.Natural analogues provide a way of informing the wider public on the principles on which repositories are built, without using complex mathematical demonstrations of "safety" and "risk". One of the concepts which can be presented using analogues is the very slow degradation of materials over thousands of years. Some notable natural analogues are:

The Inchtuthil Roman nails

The most northerly fortress in the Roman Empire at Inchtuthill in Perthshire, Scotland had to be abandoned hastily in 87 AD. In an attempt to hide metal objects which could be used for weapons, the Roman buried over one million nails in a 5 m deep pit and covered them with 3 m of compacted earth. These nails were discovered in the 1950s. It was found that the outermost nails were badly corroded and had formed a solid iron oxide crust. The innermost nails, however, showed only very limited corrosion. This was attributed to the fact that the outer nails removed the oxygen from the infiltrating groundwater such that by the time they came into contact with the innerlying nails the waters were less corrosive. In the same way, the large volumes of iron in waste canisters are expected to maintain chemically reducing conditions in an environment which is oxygen rich due to the radiolytic decomposition of water.

The Kronan cannon

The Kronan was a Swedish warship built in 1668 and which sank in 1676 during the Battle of Öland. One of the bronze cannon on board the Kronan had remained partly buried in a vertical position, muzzle down in clay sediments since the ship sank.. This cannon is a good analogue for canisters to be used in the Swedish and Finnish spent fuel repositories which have a copper outer shell since the cannon had a very high content of copper (96.3%. From an analysis of the cannon surface, a corrosion rate of 0.15µm/y was established. At this rate of corrosion, it would take some 70,000 years to corrode 1cm thickness of copper. This provides evidence for the very long life of copper spent fuel canisters in the repository.

Hadrian's Wall

In AD 122 Emperor Hadrian ordered construction of a wall to separate the lands of the Britons from that of the Picts to the north. Hadrian's Wall was over 100 km long and 5 m high and was built from stone blocks cemented together. The Wall is of interest as an analogue due to the longevity of Roman cement used to bind the stones together. Modern Portland cement is

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very similar chemically and mineralogically. From these studies, conclusions can be drawn with regard to the stability and longevity of modern cements in repositories.

The Dunarobba forest

In the Dunarobba forest near Todi in Italy, dead tree trunks approximately 2 million years old have been found in their original upright position. Remarkably, in contrast to typical fossilised trees, the Dunarobba trees are still composed of wood.. The wood has been preserved due to the surrounding clay. This aly stooped oxygenated waters reaching the wood thereby limiting the aerobic decomposition. The Dunarooba trees are of relevance in repository concepts since the wood is considered to be analogous to the organic/cellulosic materials which comprise a large part of the waste.

The Needle's Eye

This site in south-west Scotland, close to a natural rock arch known as the Needle's eye, comprises a sea-cliff in which the mineralised veins of uranium and other metals are partly exposed. Uranium is present as pitchblende (UO2) associated with

secondary minerals.

The pitchblende has undergone dissolution by two processes. In the first, slow leaching results in a preferential loss of 234U relative to 238U. The second is dissolution by oxidising waters. The mobilised uranium is redeposited in close proximity to the vein as stable oxidised uranium minerals. In

contrast to uranium, the dissolution and transport of thorium is negligible.The Needle's Eye is ideal for investigating radionuclide migration behaviour and for testing geochemical codes in simulation exercises.

The Oklo natural fission reactors

In 1972 scientists in France found that the 235U content of ore being processed to make nuclear fuel pellets had been depleted from the normal 0.72% to 0.62%. The ore had been obtained from Oklo in the south-east part of the Republic of Gabon in West Africa. Further investigations revealed that nuclear fission had taken place. The uranium ore bodies at Oklo are the only known examples of natural fission reactors. The criticality took place approximately 2 billion years ago as a result of dissolution, mobilisation and accumulation of uranium in sufficient mass to achieve criticality. The chemistry of the uranium is such that it is insoluble in water

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under oxygen-free conditions, but readily soluble in water in the presence of oxygen. The fission reactions operated intermittently for between 105 and 106 years.The natural fission reactors at Oklo can be considered as analogues for very old radioactive waste repositories and can be used to study the transport behaviour of transuranic nuclides and stability of uranium minerals which have undergone criticality.

References:

Geological Disposal of Wastes and Natural Analogues: Lessons from Nature and Archaeology, William Miller et al., Pergamon Press, 2000.http://www.uoguelph.ca/~mbrookfi/hutton3.htm (needles eye)http://www.bbc.co.uk/scotland/education/seeyouseeme/fort/nails.html (nails)

A European Roadmap for Developing Accelerator Driven Systems (ADS) for Nuclear Waste Incineration, The European Technical Working Group on ADS, April 2001, ENEA, Rome.

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Glossary

54

Index

112C 7, 8, 2816O 817O 8

44He 8, 18

660Co 11

A

Actinides 24Alpha 18Alpha emitters 19alpha particle 18alpha-immunotherapy 20anti-neutrino 20Atom 4ATOMIC MASS DATA CENTER 44Atomic Mass Unit 7atomic number 5, 6, 8, 10, 18atomic orbits 23atomic weight 5, 7, 8Auger electrons 21Avogadro's number 7

B

Beta emission 20binding energy 8Bohr 5branching ratio 21branching ratios 25

C

cancer 20Chadwick 7cluster emission 27, 28cohesive force 7cold fission 28

control rods 26

D

Dalton 7Decay Processes 13doubly magic 27, 28Dunarobba forest 51

E

electron 20, 21, 22, 23, 24Electron 18, 23Electron capture 18electrons cascading 21electrostatic repulsion 24endpoint energy 20

F

fission 24

G

Gamma emission 18, 21Geiger 4gold foils 4golden rule 28

H

Hadrian's Wall 50Heavy-ion radioactivity 18, 26helium 18, 19hydrogen 8

I

Inchtuthil 50Inchtuthil Roman nails 50ingestion 20inhalation 20internal conversion 21internal conversion coefficient 21isobars 10isomeric transitions 21

55

Isomeric transitions 18, 21isotones 10isotope 8isotopes 10

J

JEF 44Joint Evaluated File 44

K

Kronan cannon 50

M

Magic numbers 28magic radioactivity 28Marsden 4Mass 7mass defect 8mass equivalent 8mass number 10, 21multipole transitions 21

N

natural analogues 49, 50NEA Data Bank 44Needle's Eye 51neutrino 20, 22, 23neutron 10, 11, 22, 23, 26, 28Neutron 23, 26neutron-rich 26NUBASE 44nuclear force 7, 18Nuclear Reactions 13nuclear reactors 26nuclear waste transmutation 49nucleus 4, 5, 6, 7, 8, 10, 13, 18, 19, 20,

21, 22, 23, 24Nuclide Chart 14Nuclides.net database 19, 22, 23, 24,

25, 26

O

OECD/NEA Data Bank 44Oklo natural fission reactors 51

P

Planck 5polonium 4

positron 22, 26principal quantum number 5Proton 18, 25Proton decay 18, 25proton emission 25proton rich 28protons 5, 6, 7, 8, 10, 13, 14, 22

Q

quantum 5, 18, 27

R

Radioactive Decay 18Radioactivity 28radionuclide migration 51repository 49, 50, 51response time 26Rutherford 4, 5, 6

S

Segrè 11, 13Segrè Chart 13SF See Spontaneous Fission See

Spontaneous Fissionsolar system 5Special beta-decay processes 18, 26Spectral information 44spin 44spontaneous fission 24, 27Spontaneous fission 18Spontaneous Fission 24

ß

ß- emitters 20, 21ß-n 18, 26

T

tissue 20trans-actinides 24Types of Radioactivity 18

U

uranium-238 18

W

waste disposal 49

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