Energy and particles · GCSE Physics
Radioactivity
Isotopes, alpha, beta and gamma, half-life, irradiation versus contamination, and uses and risks of nuclear radiation.
Alpha is a helium nucleus — highly ionising, stopped by paper. Beta is an electron from the nucleus — through paper, stopped by aluminium. Gamma is an electromagnetic wave — the most penetrating, the least ionising.
The important bits
What you need to know
- 1
Some nuclei are unstable and decay, emitting radiation and becoming more stable. This is random for a single nucleus but predictable for a large sample using half-life.
- 2
Alpha (α): two protons and two neutrons (helium nucleus), charge +2. Highly ionising, short range in air, stopped by paper or skin. Deflected towards a negative plate.
- 3
Beta (β): a high-speed electron emitted when a neutron turns into a proton. Moderately ionising, stopped by a few millimetres of aluminium. Deflected towards a positive plate.
- 4
Gamma (γ): electromagnetic radiation from the nucleus. Weakly ionising, very penetrating, reduced by thick lead or concrete. Not deflected by electric or magnetic fields.
- 5
Half-life is the time for the number of unstable nuclei (or the count rate, after subtracting background) to fall to half. It is independent of the starting amount.
- 6
Irradiation is exposure to radiation from an outside source; the object does not become radioactive. Contamination is unwanted radioactive material on or in an object, which then keeps emitting.
- 7
Uses include smoke alarms (alpha), thickness control (beta), sterilising equipment and treating cancer (gamma), and tracers. Risk is ionisation damaging DNA; reduce it with time, distance and shielding.
Go deeper
Ionising versus penetrating is a trade-off
Alpha dumps a lot of energy in a short track, so it is dangerous if swallowed or breathed in, but a dead layer of skin stops it from outside. Gamma goes through the body, so it is useful as a tracer or for treating a tumour from outside, but it is harder to shield. Beta sits in the middle, which is why it is used to monitor paper thickness: some is absorbed, some gets through to a detector, and the count rate changes if the sheet is too thick. Learn a source, a range, a shield and a use for each type. “Radiation is dangerous” without the type and the route (contamination or irradiation) will not score.
Go deeper
Half-life is a graph skill
Subtract background count first if it is given. Plot count rate against time, or use a table. Find the time from a value to half of that value; repeat for a second pair and average if the data wobble. After one half-life, half remains; after two, a quarter; after three, an eighth. A medical tracer needs a half-life long enough to take the measurement but short enough that the patient is not radioactive for weeks. Carbon-14 dating uses a half-life of about 5700 years — useless for a two-week tracer, ideal for archaeology. Never say a nucleus “uses up half its radiation”. The decay is random; half-life describes the sample.
See the idea in action
A sample has a count rate of 880 counts per minute, including a background of 80. Net count = 800. The net count falls to 400 in 3 hours and to 200 in 6 hours. Half-life = 3 hours. After 9 hours the net count would be 100, so the measured count including background would be 180. If this were a tracer in the body, three half-lives would leave one eighth of the original activity, plus whatever background the hospital measures.
Exam technique
Turn knowledge into marks
Always subtract background before finding half-life. For comparisons, mention ionisation, penetration and a shield. Distinguish contamination (radioactive material on you) from irradiation (dose from a source that is then taken away). Write nuclear equations with mass and atomic numbers balanced if they are asked.
Common mistakes
Do not give these marks away
- 01
Forgetting to subtract background count, or treating half-life as half the time the source exists.
- 02
Saying gamma is the most ionising because it is the most penetrating — it is the least ionising.
- 03
Confusing irradiation with contamination, or claiming an irradiated object keeps emitting.
Which statement about alpha radiation is correct?
AIt is the most penetrating and the least ionising
BIt is a helium nucleus, highly ionising, and is stopped by paper
CIt is an electron from the nucleus
DIt is not deflected by an electric field
Show the answer
It is a helium nucleus, highly ionising, and is stopped by paper. Alpha is ⁴₂He, charge +2. It ionises strongly, has a short range, and is stopped by paper or skin. Gamma is the penetrating, weakly ionising one.
Quick questions
If this is the bit you searched
What is half-life?
The time taken for the number of unstable nuclei in a sample, or the net count rate, to fall by half. It is constant for a given isotope.
What is the difference between contamination and irradiation?
Contamination is radioactive material on or in an object, which continues to emit. Irradiation is exposure to a source outside; once the source is removed, the object is not radioactive.