Describe the properties of alpha, beta and gamma radiation, including charge, mass, ionising and penetrating power, and represent decay reactions using balanced nuclear equations

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.

Worked example

Carbon-14 undergoes beta-minus decay. Write the balanced equation.

$^{14}_{6}\text{C} \to ^{14}_{7}\text{N} + ^0_{-1} e + \bar{\nu}_e$

A neutron in carbon-14 becomes a proton, so mass number $14$ is unchanged and atomic number rises from $6$ to $7$ (nitrogen). The emitted beta particle and antineutrino carry away energy and momentum.

Common traps

Forgetting the antineutrino in beta-minus decay. QCAA marking accepts the equation without it for shorter responses, but extended responses should include it.

Writing $Z$ change in the wrong direction. Beta-minus increases $Z$ (the daughter has one more proton). Beta-plus decreases $Z$.

Calling a gamma emission a "decay". Gamma emission is not a decay in the sense of changing the element. It is a transition between energy states of the same nuclide.

Forgetting that alpha is two protons plus two neutrons. Some students write the alpha particle as $^4_4$He or $^4_1$He. Always $^4_2$He.

In one sentence

Alpha radiation is a $^4_2$He nucleus (charge $+2$, very strongly ionising, stopped by paper); beta-minus is a high-speed electron (charge $-1$, moderately ionising, stopped by aluminium); gamma is a high-energy photon (uncharged, weakly ionising, stopped by thick lead); and every decay equation must conserve both mass number $A$ and atomic number $Z$.