Alpha, beta and gamma radiation (QCE Physics Unit 1)

What this dot point is asking

QCAA expects you to know the three classical types of ionising radiation (alpha, beta and gamma), the conservation laws governing decay equations, and the inverse relationship between ionising power and penetrating power.

The three types

Beta-plus ($\beta^+$, the positron) also exists; it is the antiparticle of the electron and behaves with the same magnitudes but opposite charge.

Why ionising power and penetration are inverse

Alpha particles interact strongly because of their charge and slow speed; they deposit their energy quickly and stop in a short distance. Gamma photons have no charge and interact weakly; most pass through matter, so penetration is high but the dose deposited per metre is low.

Balanced nuclear equations

Two quantities are conserved in any nuclear decay or reaction:

• mass number $A$ (total number of nucleons),
• atomic number $Z$ (total charge).

Alpha decay: $A$ drops by $4$, $Z$ drops by $2$.

Beta-minus decay: A neutron becomes a proton plus an electron plus an antineutrino. $A$ unchanged, $Z$ increases by $1$.

Gamma emission: Often follows alpha or beta decay. The daughter nucleus is in an excited state and releases a gamma photon to drop to its ground state. $A$ and $Z$ unchanged.

Biological effects

Ionising radiation knocks electrons off atoms and breaks chemical bonds. The risk to biological tissue depends on the radiation type and where it lands.

• Alpha is dangerous if ingested or inhaled (a radon decay product in lungs is the classic case) because all energy deposits in a small volume.
• Beta penetrates skin and damages tissue over centimetres.
• Gamma travels through the body; whole-body dose matters.

Examples in context

Example 1. Smoke detectors in Brisbane apartments use americium-$241$ alpha emitters. The $5.5 \text{ MeV}$ alphas ionise air in a small chamber, sustaining a microcurrent that drops when smoke enters. Alphas (high ionising power, very low penetration) cannot escape the metal cover, so the residential safety case is robust; a sheet of paper or the chamber wall stops them. The same dot-point property table that QCAA Unit 1 students learn is the regulatory basis for ARPANSA's exemption of these devices.

Example 2. Gladstone industrial radiography uses iridium-$192$ gamma sources to weld-test pressure vessels. Gammas have low ionising power but high penetration (several centimetres of lead required for half-value shielding). The team enforces a $30 \text{ m}$ exclusion zone, scaled by the inverse-square law, when activity is $300 \text{ GBq}$. Beta-particle emitters such as strontium-$90$ are not used here because they will not penetrate the steel weld. The QCAA EA Unit 1 thematic table maps directly to this industrial choice of isotope.

Try this

Q1. State the charge and approximate mass of an alpha particle and a beta-minus particle. [2 marks]

• Cue. $\alpha$: $+2e$, $4.00 \text{ u}$; $\beta^-$: $-e$, $5.5 \times 10^{-4} \text{ u}$.

Q2. Balance the decay $^{14}_{6}\text{C} \rightarrow ? + \beta^- + \bar{\nu}_e$. Identify the daughter and explain why a neutrino is emitted. [3 marks]

• Cue. Daughter $^{14}_{7}\text{N}$; antineutrino conserves lepton number and energy.

Q3. A radiographer in Gladstone uses an iridium-$192$ source. (a) Compare the ionising and penetrating power of gamma to alpha and beta. (b) Justify why gamma is selected. (c) Calculate the activity at $7 \text{ m}$ from a $300 \text{ GBq}$ source treated as a point, using $I \propto 1/r^2$ relative to $1 \text{ m}$. [3+2+3 marks; ISMG: Knowledge and conceptual understanding, Analysis and interpretation]

• Cue. (a) Gamma: low ionising, high penetration; (b) penetrates steel weld; (c) factor $1/49$, $6.1 \text{ GBq}$ equivalent.