Reference: Nelson Physics VCE Units 1&2 Chapters 10, 11 Pages 213 - 256

Before we can discuss the processes involved in radioactivity we must look briefly at the structure of the atom and how it is involved radiation.

As we know all things are made up of "particles" and the smallest of these "particles" are called Atoms.

In around 1897 it was discovered by J. J. Thomson (Nobel 1906, Knighted 1908) that electrons were a part of the atom. Thompson suggested that positive and negative charges in the atom were mixed together as in a ' plum pudding', the pudding having the same volume as the atom.

Between 1909 and 1911 Sir Ernest Rutherford (Nobel 1908, Knighted 1914) together with his assistants Hans Geiger (of Geiger Counter fame) an Ernest Marsden, performed a series of experiments that cast doubts on the Thompson ' plum pudding' model. They performed an experiment in which they bombarded a thin gold foil with a -particles. To their surprise most of the a -particles passed straight through , but some were deflected through large angles. This could not be explained by the Thompson model. They performed may more experiments and from their observations Rutherford concluded:

• most of the atoms mass is concentrated at the positively charged nucleus.
• surrounding the nucleus are a number of electrons.
• most of the atom is empty space.
• the total negative charge of the electrons balances the positive charge of the nucleus, thus the atom remains neutral.

The deflections observed by Geiger and Marsden can easily be explained as follows:

Direct hit bounces back

Near direct hit, greatly deflected

Miss, slight or no deflection. Since most of the atom is empty space, most of the a -particles behave like this.

This is the model that is accepted today, it has a nucleus which consists of particles which are of two kinds: Protons which have a positive charge and neutrons which have no charge. Orbiting this nucleus there are other particles called electrons which have negative charge. A neutral atom has the same number of electrons as it does protons.

Improvements on the model for the atom were made by each of these scientists and experiments to improve the model of the atom are still being made today, but the basic structure has remained unchanged. It looks like the following:

Chemical changes involve electrons either being shared between atoms or transferring between atoms. Nuclear reactions/changes involve changes in the nucleus. So isotopes are important when we are looking at nuclear reactions and changes, but what are isotopes?

It is known that atoms of the same element can have differing masses. This can happen if there is a differing number of neutrons in the nucleus of the atom.

Thus isotopes of an element are defined as having the same numbers of protons and electrons but differing numbers of neutrons.

Eg. Hydrogen can have 3 different isotopes.

It is this variation in neutrons that gives radioactive substances their properties.

Isotopes are also known as nuclides.

In 1920 Rutherford suggested that in a nucleus a proton and an electron may join to form another particle called a neutron.

In 1932 James Chadwick (Nobel 1935, Knighted 1945) suggested that penetrating radiation consisted of neutral particles of the same mass as a proton. He proposed these particles to be an electron and a proton in some combination.

The neutron is now considered a fundamental particle of the atom. Its charge is neutral and mass is that of a proton a (close). A neutron by itself is unstable and will decay.

Atoms are often symbolised as follows:-

Mass Number (A)

X (Symbol)

Atomic or Proton Number (z)

So the two Examples in section 2.1.1 would be written as and respectively.

The protons and neutrons are known as nucleons. The number of neutrons in the nucleus of an atom is the difference between the mass and atomic numbers.

Atoms were once thought of as stable and unchangeable. But experiments performed by scientists such as Henri Bacquerel, Rutherford, Marie and Pierre Curie (Both Nobel 1903, Marie Nobel 1911) showed that changes in radioactive decay are different to chemical changes.

Radioactivity can be defined as the spontaneous and uncontrollable decay of an atomic nucleus resulting in the emission of particles and rays. Some elements may have isotopes that are stable and others that are radioactive.

For example Carbon-14 and Carbon-12

Carbon-12 has 6 protons and 6 neutrons it is also very stable.

Carbon-14 has 6 protons and 8 neutrons it is unstable and decays radioactively.

An isotope such as carbon-14 is called a radioisotope since it decays radioactively.

There are three types of radiation Alpha (a ), Beta (b ) and Gamma (g ).

a -particles have the following properties:

They are a Helium nucleus He^2+.

The charge is two elementary charges, positive.

The mass is four atomic mass units, i.e. 4 X mass of a proton.

The penetration is a few cm in air and absorbed by paper.

Easily able to ionize atoms.

Very small deflection due to electric and magnetic fields.

b -particles have the following properties:

They are an electron moving quickly from the nucleus.

The charge is that of an electron.

It has the mass of an electron, of an a -particle.

Penetration of a few metres in air, ~ 3.5 cm in lead.

Weak ionization ability.

Large deflection in electric and magnetic fields.

g -rays have the following properties:

They are high frequency (short wavelength) electromagnetic radiation.

They have no charge.

They have no mass.

Penetration of ~ 30 cm in steel, no maximum in air, never really completely absorbed.

Very weak ionization ability.

No deflection in electric or magnetic fields.

Lethal effect on living tissues (used in medicine for cancer treatment).

Originating from the nucleus of the atom.

When a nucleus decays it becomes a nucleus of a different atom. The original nucleus is called the parent nucleus and the remaining nucleus is called the daughter nucleus. The emitted particles and the daughter nucleus are called decay products.

In radioactive decay the mass number and the atomic number must balance.

The a -particle decay process emits a Helium nucleus He²⁺.

Examples.

We notice that the mass number of the atom decreases by four and the atomic number of the atom decreases by two.

In general

Overall the number of nucleons remains the same

What is the nuclide Y in the following equation?

The b -particle decay process emits an electron. For the purpose of writing equations the electron is written as (no mass, -ve charge).

Example

Note: 1. the electron comes from the nucleus.

2. this particular decay includes g -ray emission

When a nucleus undergoes b -decay the mass number stays the same, but the atomic number in creases by one.

In general

Question: What is the nuclide Y when undergoes b -decay according to the following equation?

Since g -ray emission is the emission of a photon of energy rather than a particle it causes no change to the atomic or mass numbers. g -rays are not emitted by themselves, but along with an a -or b -particle.

Evidence suggests that the nuclear force that holds atoms together weakens as the proportion of neutrons to protons becomes too large. When the number of neutrons exceeds the number of protons the nucleus tends to become unstable and decays radioactively. This is especially true of the heavier elements, particular those with an atomic number greater than lead (82).

Different nuclei decay at different rates. The decay rate is not affected by physical or chemical conditions e.g. temperature, pressure.

The time required for the decay of ¹/₂ of the original sample is called the half life of the material.

Put another way:

The half-life of a radioactive isotope is the time taken for half the nuclei (atoms) present to decay into another element.

eg. iodine-131 has a half-life of 8 days. If we start with a sample of 40 million iodine atoms then over time the following will happen.

There are two formulae that are used in half-life calculations

N = N_o (¹/₂) ⁿ

where n = no. of half-lives

N_o = original no. of atoms

N = final no. of atoms

and N = N_o e^{-l t}

where l = decay rate

t = time in days

Note: N and N_o can be substituted by

Activity A and A₀ (in Bq) (Becquerel)

1 Bq = 1 decay per second

Example:

A radioactive element decays according to the equation m = 20 e^-0.06t

How much is initially present?

How much remains in 5 days?

What is the half life of the substance?

How long will it take until 2.5 gm remains?

Scientists noticed that many of the products produced by radioactive decay are themselves radioactive. They were able to identify a series of radioactive nuclides that were formed before a stable end product was reached. The end product of many of these series is .

The effects of radiation on the human body depends on the amount of radiation that the body is exposed to and the type of radiation. The energy absorbed is a way of measuring the effect of the radiation. This is known as absorbed dose. Absorbed dose is the amount of energy absorbed per kilogram of body tissue. It is measured in a quantity called the gray (Gy). A dose of one gray means that 1 kilogram of tissue absorbs 1 joule of energy. 1Gy = 1 J/Kg.

To take into account the amount of damage caused by the various forms of radiation, the dose equivalent measure was developed. The units for dose equivalent are sieverts (Sv).

The quality factor is determined by the type of radiation that delivered the energy.

One sievert of radiation causes the same amount of biological damage, no matter what type of radiation to which you may be exposed.

The effects of nuclear weapons are well known. Nuclear weapons cause destruction in two ways. First there is the shock wave caused by the initial explosion. The second is the fallout, the radioactive dust is spread for many kilometers by the explosion and by the wind. There are many different radioactive materials contained in nuclear fallout. For example Strontium 90 is absorbed into human bone marrow, which makes blood cells and destroys it. The energy from radiation damages living cells causing genetic mutations that are passed on to the children of the survivors of the blast.

There are also dangers associated with the use of nuclear fission to produce energy for peaceful purposes. There is the possibility of radioactive leaks or meltdown at power stations. There is also the problem of what to do with the radioactive wastes. Some of these are buried at sea, in underground containers or in very stable geological formations. But how safe are these methods? The containers may leak. There is also the possibility of the waste being stolen by terrorists and converted into nuclear weapons. Even the mining of Uranium has it's problems, it exposes the workers to radiation as does the refining of the ore. Nuclear power stations produce large amounts of hot water from there cooling systems that flows into surrounding lakes and streams upsetting the ecological balance.

This last problem is not confined to nuclear power plants. Fossil fuel power plants also produce hot water. There are problems with conventional power stations that do not apply to nuclear power stations. Burning coal produces noxious fumes containing sulphurous and carboniferous wastes. The mining and drilling operations for coal, gas and oil have cost many live in accidents and mining-related lung diseases.

Radioactive isotopes are used for industrial, medical and research purposes.

Carbon-14 is used by archeologists in determining the age of ancient remains.

Radioactive dating using Uranium-238 has enabled geologists to estimate the age of the Earth's rocks.

Biochemists use Carbon-14 to study photosynthesis in green plants. Agriculturists, by using radioactive tracers in fertilizers, can find the best time to fertilize plants.

In industry radioactive isotopes can be used to study both the density and thickness of materials in production without damaging the materials.

Doctors use radioactive isotopes in a number of ways.

Some uses are as follows:

1. to study and treat diseases such as cancer
2. to sterilize instruments and dressings
3. to measure the rate of flow of blood through the heart
4. to follow the path of salt through nerve cells and other parts of the body.