Atoms and nuclear radiation

Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.

Activity is the rate at which a source of unstable nuclei decays. Activity is measured in becquerel (Bq).

Count-rate is the number of decays recorded each second by a detector, such as a Geiger-Muller tube.

The two are not the same. Activity is what the source is actually doing; count-rate is what a detector happens to pick up, which is always less because the detector only catches radiation heading its way.

The nuclear radiation emitted may be:

• an alpha particle (α) — this consists of two neutrons and two protons; it is the same as a helium nucleus • a beta particle (β) — a high speed electron ejected from the nucleus as a neutron turns into a proton • a gamma ray (γ) — electromagnetic radiation from the nucleus • a neutron (n).

Required knowledge of the properties of alpha particles, beta particles and gamma rays is limited to their penetration through materials, their range in air, and ionising power — so nothing else about them is examinable.

You should be able to apply your knowledge to the uses of radiation and evaluate the best sources of radiation to use in a given situation.

That evaluation always comes back to the table above. A source used inside the body should not be strongly ionising and must be able to get out to a detector, so gamma is chosen. A source used to check the thickness of aluminium foil must be partly stopped by it, so beta is chosen — alpha would all be absorbed and gamma would all pass through, and in either case the reading would not change with thickness.

Nuclear equations are used to represent radioactive decay. In a nuclear equation an alpha particle is represented by the symbol ⁴₂He, and a beta particle by ⁰₋₁e.

The emission of the different types of nuclear radiation may cause a change in the mass and/or the charge of the nucleus.

• Alpha decay causes both the mass and the charge of the nucleus to decrease — the mass number falls by 4 and the atomic number by 2. • Beta decay does not cause the mass of the nucleus to change, but does cause the charge of the nucleus to increase — the mass number is unchanged and the atomic number rises by 1. • The emission of a gamma ray does not cause the mass or the charge of the nucleus to change.

Beta decay looks strange until you see where the electron comes from: a neutron in the nucleus turns into a proton, and the electron is ejected. The total number of particles in the nucleus is unchanged, which is why the mass number stays the same, but there is now one more proton, so the atomic number rises by one.

You should be able to use the names and symbols of common nuclei and particles to write balanced equations that show single alpha and beta decay. This is limited to balancing the atomic numbers and mass numbers — the identification of daughter elements from such decays is not required, so you will not have to name the new element. You are also not required to recall the two worked examples printed in the specification.

Radioactive decay is random.

The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate — or activity — from a sample containing the isotope to fall to half its initial level.

You should be able to explain the concept of half-life and how it is related to the random nature of radioactive decay.

The link between the two is the point of this section. It is impossible to say when any individual nucleus will decay — that is what random means. But a sample contains an enormous number of nuclei, and although each one is unpredictable, the proportion decaying in a given time is highly predictable. Half-life is a statement about that reliable average behaviour of a huge number, not a prediction about any one nucleus.

You should be able to determine the half-life of a radioactive isotope from given information — usually a table or a graph of count-rate against time. Find the initial value, halve it, and read off the time at which the curve reaches that level.

Higher tier students should be able to calculate the net decline, expressed as a ratio, in a radioactive emission after a given number of half-lives.

After n half-lives the fraction remaining is 1 ÷ 2ⁿ. The net decline expressed as a ratio compares what is left with what there was at the start.

Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard.

Irradiation is the process of exposing an object to nuclear radiation. The irradiated object does not become radioactive.

That last sentence is the whole distinction, and it is examined constantly. Contamination means the radioactive material itself is on or in the object, so the hazard continues until it is removed. Irradiation means only the radiation reached the object; move the object away, or remove the source, and the exposure stops immediately.

You should be able to compare the hazards associated with contamination and irradiation. Alpha is the greatest hazard inside the body because it is strongly ionising and all its energy is deposited in a very short distance in nearby tissue. From outside the body it is the least hazardous, because it cannot even penetrate skin. Gamma is the reverse on both counts.

Suitable precautions must be taken to protect against any hazard that the radioactive source used in the process of irradiation may present — for example handling sources with tongs, storing them in lead-lined containers, keeping exposure times short, and standing well back.

You should also understand that it is important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists, so that the findings can be checked by peer review. Peer review lets other scientists examine the method and the data before conclusions are accepted, which catches errors and bias — and on a subject as consequential as radiation exposure, that matters.