6.10–6.11, 6.15–6.16Radioactive decay 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.
Worked example
A smoke detector uses a source whose radiation must ionise the air in a small gap but must not escape the plastic case. Which type should be used, and why?
- The radiation must ionise the air strongly, so that a current flows across the gap.
- Alpha is strongly ionising; beta and gamma are much less so.
- The radiation must not escape the case, for safety.
- Alpha has a range of only a few centimetres in air and is stopped by paper, so it cannot get out.
- Both requirements point the same way.
Answer: Alpha — it is strongly ionising, so it ionises the air in the gap, and its very short range in air means it cannot escape the detector.
- Radioactive decay
- the random process in which an unstable nucleus gives out radiation as it changes to become more stable
- Activity
- the rate at which a source of unstable nuclei decays, measured in becquerel (Bq)
- Count-rate
- the number of decays recorded each second by a detector such as a Geiger-Muller tube
6.12–6.14Background radiation
Physics only — not on the Combined Science specification
Background radiation is around us all of the time. It comes from:
• natural sources such as rocks and cosmic rays from space • man-made sources such as the fallout from nuclear weapons testing and nuclear accidents.
The level of background radiation and radiation dose may be affected by occupation and/or location.
Radiation dose is measured in sieverts (Sv), and 1000 millisieverts (mSv) = 1 sievert (Sv). You will not need to recall the unit of radiation dose, so it will be given if a question needs it.
Background radiation matters practically as well: when measuring the count-rate from a source, the background count must be measured first and subtracted, or the source will appear more active than it is.
Common mistake
Most background radiation is natural, not man-made. Rocks and cosmic rays contribute far more than weapons testing and accidents for almost everyone.
- Background radiation
- radiation around us all the time, from natural sources such as rocks and cosmic rays and from man-made sources such as fallout from weapons testing and nuclear accidents
- Radiation dose
- a measure of the risk of harm from the body being exposed to radiation, measured in sieverts
6.18–6.22Nuclear equations
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.
Worked example
Complete the alpha decay equation: ²²⁶₈₈Ra → ?? + ⁴₂He
- Balance the mass numbers, the top numbers: 226 = ? + 4, so the daughter has 222.
- Balance the atomic numbers, the bottom numbers: 88 = ? + 2, so the daughter has 86.
- Write the daughter with those numbers.
- You are not required to name the element, only to balance the numbers.
Answer: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He — the mass number falls by 4 and the atomic number by 2, as alpha decay always does.
Worked example
Complete the beta decay equation: ¹⁴₆C → ?? + ⁰₋₁e
- Balance the mass numbers: 14 = ? + 0, so the daughter still has 14.
- Balance the atomic numbers: 6 = ? + (−1), so the daughter must have 7.
- The beta particle's atomic number is −1, which is why the daughter's goes up rather than down.
- Write the daughter with mass number 14 and atomic number 7.
Answer: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e — mass unchanged, charge increased by one.
Common mistake
The beta particle carries an atomic number of −1. Subtracting a negative means the daughter's atomic number increases — which is why beta decay moves an element up the periodic table while alpha moves it down.
Before you practise
- Alpha is a helium nucleus, beta-minus an electron from the nucleus, beta-plus a positron, and gamma an electromagnetic wave.
- Alpha is stopped by paper and ionises most; beta is stopped by a few mm of aluminium; gamma needs thick lead and ionises least.
- Penetration and ionising power run in opposite directions — say both.
- Background radiation comes from rocks (especially radon), cosmic rays, food, and medical and industrial sources.
- Detect radioactivity with photographic film or a Geiger-Müller tube.
- In beta-minus decay a neutron becomes a proton plus an electron: mass number unchanged, atomic number up by 1.
- In alpha decay the mass number falls by 4 and the atomic number by 2; gamma emission changes neither.
- Balance a nuclear equation by making the mass numbers and the charges match on both sides.