Nuclear Reactions vs. Normal Chemical

ChangesNuclear reactions involve the

nucleus

The nucleus opens, and protons and neutrons are rearranged

The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy

“Normal” Chemical Reactions involve electrons, not protons and neutrons

The Nucleus

Remember that the nucleus is comprised of the two nucleons, protons and neutrons.

The number of protons is the atomic number.

The number of protons and neutrons together is effectively the mass of the atom.

Isotopes

Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

There are three naturally occurring isotopes of uranium:Uranium-234Uranium-235*Uranium-238

Nuclide Symbols

Nuclide Symbols

IsotopesTwo Categories

Unstable – isotopes that continuously and spontaneously break down/decay in other lower atomic weight isotopes

Stable – isotopes that do not naturally decay but can exist in natural materials in differing proportions

RadioactivityIt is not uncommon for some

isotopes of an element to be unstable, or radioactive.

We refer to these as radioisotopes.

There are several ways radioisotopes can decay and give off energy known as radiation.

Types of Radioactive Decay Alpha Decay

Loss of an -particle (a helium nucleus)

He42

U23892

Th23490 He4

2+

Types of Radioactive Decay Beta Decay

Loss of a -particle (a high energy electron)

0−1 e0

−1or

I13153 Xe131

54 + e0

−1

Types of Radioactive Decay Gamma Emission

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

00

Types of Radiation

e01

He42

• Alpha (ά) – a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons

•Beta (β) – an electron

•Gamma (γ) – pure energy; called a ray rather than a particle

Penetrating Ability

Geologic Time Radioactive Isotopes used in Geologic Dating

Parent Daughter half-life (y)

U-238 Lead-206 4.5 billion

U-235 Lead-207 713 million

Thorium 232 Lead 208 14.1 Billion

K-40 Argon-40 1.3 billion

R-87 Sr-87 47 billion

C-14 N-14 5730

Half-life = time it takes for 1/2 of the parent mass to decay into the daughter mass

Geologic Time

14Carbon Dating

Dating is accomplished by

determining the ratio of 14C to non-

radioactive 12C which is constant in

living organisms but changes after the

organism dies

When the organism dies it stops taking in 14C which disappears as it decays to 14N

Forensic 14Carbon Cases

Dead Sea Scrolls – 5-150 AD

Stonehenge – 3100 BC

Hezekiah’s Tunnel - 700 BC

Forensic 14Carbon Cases

● King Arthur’s Table in Winchester Castle, England 14C dated to 13th

century AD

● Cave painting at Lascaux, France

14C dated to 14,000 BC

● Rhind Papyrus on Egyptian math 14C dated to 1850 BC

Forensic 14Carbon Cases

●The Shroud of Turin was

14C dated 1260-1390 AD

which suggests that it is a fake

● However, recent evaluation

shows that the sample measured

was from a medieval patch and/or

that it was seriously contaminated

with molds, waxes, etc

●New estimates date the shroud from

1300-3000 ybp bases on vanillin

retention

Forensic 14Carbon CasesNuclear testing during 1955-63 put large amounts of 14C into the atmosphere which was incorporated into the enamel of human teeth. Because such testing stopped the 14C input ended and the 14C in the teeth decayed at a fixed rate allowing dating of the teeth

Nuclear ReactionsAlpha emission

Note that mass number goes down by 4 and atomic number goes down by 2.

Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

Nuclear ReactionsBeta emission

Note that mass number is unchanged and atomic number goes up by 1.

Write Nuclear Equations!Write the nuclear equation for the

alpha decay of radon-222

222Rn 218Po + 4He

Write the nuclear equation for the beta emitter Co-60.

60Co 60Ni + 0e

8486 2

27 28 -1

BellringerFill in Chart

Radioactive Particle Nuclear Symbol Pertinent Information

alpha

beta

gamma

positron

4

He2

0

e-1

0

γ0

0

e+1

helium atom without any electrons

high energy electron, no mass with a negative charge

high energy ray with no mass and no atomic number

mass of an electron but a positive charge

Neutron-Proton RatiosAny element with more than

one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.

A strong nuclear force helps keep the nucleus from flying apart.

Neutrons play a key role stabilizing the nucleus.

Therefore, the ratio of neutrons to protons is an important factor.

Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Neutron-Proton Ratios

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1

Neutron-Proton Ratios

As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei

Nuclei above this belt have too many neutrons.

They tend to decay by emitting beta particles.

Stable Nuclei

Nuclei below the belt have too many protons.

They tend to become more stable by positron emission or electron capture.

Positron = 0e+1

Stable Nuclei

There are no stable nuclei with an atomic number greater than 83.

These nuclei tend to decay by alpha emission.

Radioactive Series

Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Nuclear TransformationsNuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

These particle accelerators are enormous, having circular tracks with radii that are miles long.

Measuring Radioactivity

One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.

The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Energy in Nuclear ReactionsThere is a tremendous amount of energy stored in

nuclei.

Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

In chemical reactions the amount of mass converted to energy is minimal.

However, these energies are many thousands of times greater in nuclear reactions.

Nuclear FissionHow does one tap all that energy?

Nuclear fission is the type of reaction carried out in nuclear reactors.

Nuclear Fission

Bombardment of the radioactive nuclide with a neutron starts the process.

Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

This process continues in what we call a nuclear chain reaction.

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Trafficking Nuclear Materials

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Man-made Radioactive Isotopes

Smuggled Plutonium – can identify the reactor type in which the fuel was originally radiated and the type of plant where the material was subsequently reprocessed

In 1997, two pieces of stainless steel contaminated with alpha-emitters were found in a scrap metal yard in Germany.

Source was identified as a fast-breeder reactor in Obninsk, Russia

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Weapons-grade Plutonium

The isotopic composition of plutonium can indicate INTENT

In 1994, a small lead cylinder discovered in a garage in Tengen on the Swiss-German border was found to contain plutonium metal, isotopically enriched to 99.7%

Weapons-grade Pu-239

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Radioactive Fingerprints

Preserving the conventional chain of evidence whilst dealing with radioactive samples can be problematic

For example – lifting fingerprints and swiping for radioactive contamination cannot both be carried out

The first ever radioactive fingerprint has recently been identified on an object contaminated with alpha-emitting isotopes

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear ReactorsThe reaction is kept in

check by the use of control rods.

These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Nuclear Fusionnuclear fusion is a nuclear reaction in

which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus.

During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy which is released.

Fusion is the process that powers active stars.

Fusion would be a superior method of generating power.The good news is that the

products of the reaction are

not radioactive.The bad news is that in order to achieve fusion, the material

must be in the plasma state at several million kelvins.

Weapons whose explosive output is exclusively from fission reactions are commonly referred to as atomic bombs or atom bombs

misnomer because the energy actually comes from nucleus

Many fission products are either highly radioactive (but short-lived) or moderately radioactive (but long-lived), and as such are a serious form of radioactive contamination if not fully contained. Fission products are the principal radioactive component of nuclear fallout.

Thermonuclear bombs work by using the energy of a fission bomb to compress and heat fusion fuel.

When the fission bomb is detonated, gamma rays and X-rays emitted first compress the fusion fuel, then heat it to thermonuclear temperatures

Tsar Bomb