Types of radiation and nuclear equationsEdexcel A-Level Physics: Revision notes

Background radiation is the ionising radiation that is always present around us. Sources include radon gas from rocks (the largest contribution in many places), cosmic rays from space, gamma radiation from rocks and buildings, radioactive isotopes such as potassium-40 in food and in our bodies, and man-made sources such as medical X-rays and nuclear fallout.

It varies with location and time. A GM tube always detects it, so it must be corrected for: measure the background count rate with no source present, over a long time, and subtract it from the count rate with the source. The result is the corrected count rate.

Worked example: with the source, 1440 counts in 2.0 minutes is 720 per minute; background of 96 counts in 5.0 minutes is 19.2 per minute. Corrected count rate $= 720 - 19.2 = 701$ per minute.

Alpha (α): a helium nucleus (2 protons, 2 neutrons), charge +2. Strongly ionising, so it loses its energy quickly. Range of a few centimetres in air, stopped by paper or skin.

Beta-minus (β⁻): a fast electron emitted from the nucleus, charge −1. Moderately ionising. Range of about a metre or more in air, stopped by a few millimetres of aluminium.

Gamma (γ): a high-energy photon of electromagnetic radiation, no charge. Weakly ionising and highly penetrating. It has no definite range in air and is only reduced, never fully stopped, by thick lead or concrete.

The more strongly a radiation ionises, the shorter its range, because each ionisation takes energy from it. Alpha is the most dangerous if the source is inside the body, and gamma is the most dangerous from outside.

In a nuclear equation, the nucleon number $A$ (top) and the proton number $Z$ (bottom) are both conserved. The emitted particles are written with their own $A$ and $Z$.

Alpha decay: $A$ falls by 4 and $Z$ by 2. ${}^{241}_{95}\text{Am} \rightarrow {}^{237}_{93}\text{Np} + {}^{4}_{2}\text{He}$

Beta-minus decay: $A$ is unchanged and $Z$ rises by 1, as a neutron changes into a proton, with an electron and an electron antineutrino emitted. ${}^{60}_{27}\text{Co} \rightarrow {}^{60}_{28}\text{Ni} + {}^{0}_{-1}\text{e} + \bar{\nu}_e$

Gamma emission: a nucleus in an excited state loses energy by emitting a photon. $A$ and $Z$ do not change, so it often follows alpha or beta decay.

Method. Fix a gamma source and a GM tube a set distance apart in clamps, and place lead sheets of measured thickness between them. First measure the background count rate with the source removed. Then, for each thickness, count for a long enough time to reduce random fluctuations, repeat and average, and subtract the background to get the corrected count rate.

Analysis. Gamma absorption is exponential: $I = I_0 e^{-\mu x}$, where $\mu$ is the absorption coefficient. Taking logs, $\ln I = \ln I_0 - \mu x$. A graph of $\ln I$ against $x$ is a straight line with gradient $-\mu$. The half-value thickness is $x_{1/2} = \ln 2/\mu$.

Safety. Handle the source with tongs, store it in a lead-lined box, keep it away from the body and minimise the time it is out.