INTRODUCTION DEFINITION:- An unstable nuclide spontaneously emits a particle, without the stimulus of any outside agency, transforming itself into a different nuclide. Such a nuclide is said to be radioactive and the process of transformation is termed as the RADIOACTIVE DECAY.

The generic name of this process is RADIOACTIVITY.

DISCOVERER

Radioactivity was discovered in 1896 by A.H. BECQUEREL while studying the fluorescence and phosphorescence of compounds eradiated with visible light.

AN INTERESTING PHENOMENON WHICH HE OBSERVED

After illuminating some pieces of uranium-potassium sulphate with visible light, he wrapped them in black paper and separated the package from a photographic plate by piece of silver. After several hours exposure, the photographic plate was developed and showed blackening due to something that must have been emitted from the compound and was able to penetrate both black paper and silver.

STABILITY OF DIFFERENT ISOTOPES

The nuclear atoms of all isotopes of an element have the same number of electrons and hence the same chemical properties.

Some isotopes of an element may be stable while the others may be unstable.

For example:- hydrogen,the simplest element has three isotopes,hydrogen,deuterium,and tritium. Of these, the first two are stable while tritium is unstable.

For a sample of tritium gas in a closed vessel, the transmutation into 3He occurs smoothly, and the concentration of 3He gradually builds up as tritium disappears. After about 12 years, half of the sample of tritium is converted into 3He .

A plot of known nuclides. The dark shading identifies the band of stable nuclides, the grey shading the radionuclides. Low mass, stable nuclei have essentially equal number of protons and neutrons but more massive nuclei have increasing number of neutrons. The figure shows that there are more stable nuclei for Z>83.

NUCLIDIC CHART

The activity of radioactive material is the result of three different kinds of emanations termed as alpha, B and Y radiations(rays). The properties of these radiations are-

Alpha- rays are 4He nuclei, emitted from radioactive nuclei are completely stopped by a sheet of paper or by a few centimetres of air. Emission of alpha particle reduces the mass number of the radionuclide by 4 and its atomic number by 2.

B- rays are electrons or particles called positrons. After emission of b particles the mass of the radioactive nucleus is unchanged but its atomic number is increased or decreased by one.

Y-rays are energetic photons, which can penetrate through considerable thickness of lead. Since photons carry no charge or mass, emission of Y- rays does not change the isotope.

LAW OF RADIOACTIVE DECAY dN / dt = - λN(t)

Or N(t) = N0e-λt

Where λ is the probability per unit time for a nucleus to decay and N(t) is the number of radioactive nuclei present at time t.Half- life, T ½

is the time in which one-half of the number of nuclei decay.

T ½ = In2/λ = 0.693/λ

The decay rate also called the activity of the sample,

R(t) =λN (t)SI unit of activity is becquerel, and is equal to

one disintegration per second.

SI UNIT OF ACTIVITY The total decay rate R of a sample of one or more radionuclides is called the activity of that sample. The SI unit for activity is the becquerel,

named after the discoverer of radioactivity, Henry Becquerel.

1 becquerel = 1 Bq = 1 decay per second. An older unit, the curie, is still in common use:

1 curie = 1 Ci = 3.7 * 1010 Bq Some other units of activity in common use are :

1mCi(milli curie) = 3.7*107 Bq 1µci ( microcurie) = 3.7*104 Bq.

ALPHA DECAY When a nucleus undergoes alpha decay, it transforms to different nucleus by emitting an alpha particle (a helium

nucleus, 42He ). For Example, when 238

92U undergoes alpha decay, it transforms to 234

90Th: 23892U ->

23490Th + 4

2He . In this process, it is observed that since 4

2He contains two protons and two neutrons, the mass number and the atomic number of the daughter

nucleus decrease by four and two respectively. Thus the transformation of a nucleus A

ZX into a nucleus A-4 Z-2 Y can be expressed as: A

ZX-> A-4 Z-2 Y + 42He . where

AZX is the parent nucleus and A-4 Z-2 Y is the daughter

nucleus.

The alpha decay of 23892U can occur

spontaneously (without an external source of energy) because the total mass of the decay

products 23490Th and 4

2He is less than the mass of the original 238

92U. Thus, the total mass energy of the decay products is less than the mass

energy of the original nuclide. The difference between the initial mass energy and the total

mass energy of the decay products is called the Q of the process or the disintegration energy. Thus, the Q of an ALPHA decay can be expressed as :

Q = (mx - my -mHe ) C2 .

This energy is shared by the daughter nucleus

A-4 Z-2

Y, and the alpha

particle, 42

He. As the parent nucleus

AZ

X is at rest before it undergoes alpha

decay, the alpha- particles are emitted with

fixed energy, which can be calculated by

applying the principle of conservation of

energy and momentum.

The potential energy function for the emission of an alpha - particle

by 23892U . The horizontal black line marked at 4.25 MeV shows the

disintegration energy for the process. Thick grey portion of this line

represents separation R that are classically forbidden to the alpha -

particle. The alpha - particle is represented by a dot both inside and

outside the potential barrier after the particle has tunnelled through.

BETA DECAY

A nucleus that decays spontaneously by emitting an

electron or a positron is said to undergo beta decay. This is a spontaneous

process, with a definite disintegration energy and half- life. It is also a

statistical process governed by certain equations, that are -

32

15P -> 3216S + e- + ν- ( In beta - minus)

2211Na -> 22

10Ne + e+ + ν ( In beta plus)

The distribution of the kinetic energies of positron emitted in the

decay of 6429Cu.

SIMILARITY BETWEEN ALPHA AND BETA DECAY

In both alpha- decay and beta- decay, the

disintegration energy Q is characteristic of the radionuclide. In the alpha decay of the particle

radionuclide , every emitted alpha particle has the same sharply defined kinetic energy. However, in beta decay the disintegration energy, Q, is shared

between the three decay products, the daughter nucleus, electron or positron and the antineutrino

or neutrino, As a consequence, the kinetic energy of an electron or a positron in beta

decay process is not unique. It may range from zero to certain maximum kinetic energy (kmax).

This Kmax of an electron or a positron must equal the disintegration energy Q.

GAMA DECAY

Like an exited atom, an exited nucleus can make transitions to state of lower energy by

emitting a photon. As the energy of the nuclear states of the order of million

electrons volts (MeV), the photons emitted in transitions between nuclear states can have energy of the order of several MeV.

The wavelenght of photons of such energy is a fraction of an angstrom. The short wavelenght electromagnetic waves

emitted by nuclei are called gamma rays.

Most radionuclides after an alpha decay or a beta decay leave the daughter nucleus in an exited state. The daughter nucleus by single transition or sometimes by successive transitions reaches the ground state by emitting one or more gamma rays. A well- known example of such a process is that of

6027Co. By beta emission, the 60

27Co nucleus transforms into 60

28Ni nucleus in its exited state. The exited 60

28Ni nucleus so formed than de- excites to its ground state by successive emissions of 1.17 MeV and 1.33 MeV gamma rays

A radioactive nuclide spontaneously emits a particle, transforming itself in the process into a different nuclide.

In radioactive decay ,there is absolutely no way to predict whether any given nucleus in a radioactive sample will be among the small number of nuclei that decay during the next second. Each nucleus has the same chance of decay.

MADE BY:- SAPNA GARG

AND

NATASHA

GOEL

( XII - D)