A-level Physics (Advancing Physics)/Radioactive Emissions/Worked Solutions

You will need a periodic table.

1. Americium-241 is an α emitter. What element, and what isotope, is produced by this decay?

$_{95}^{241}Am\to \;_{a}^{b}?+\;_{2}^{4}\alpha$

b = 241 - 4 = 237

a = 95 - 2 = 93

So, $_{93}^{237}Np$ (Neptunium-237) is produced.

2. Iodine-129 is a β^- emitter. What element, and what isotope, is produced by this decay?

$_{53}^{129}I\to \;_{a}^{b}?+\;_{-1}^{0}\beta ^{-}+\;_{0}^{0}{\bar {\nu }}$

b = 129

a = 53 + 1 = 54

So, $_{54}^{129}Xe$ (Xenon-129) is produced.

3. Gamma rays are used to kill microbes in food. Why doesn't the food become radioactive?

Gamma radiation only interacts with the electrons of the atom, not with the nucleus. Radioactivity is due to nuclear properties, not chemical properties.

4. Plutonium-244 decays by emitting an α particle. It does this twice, emits a β^- particle, and then emits a further two α particles. The nucleus becomes a different element each time. What element is produced at the end?

$_{94}^{244}Pu\to \;_{a}^{b}?+\;4\;_{2}^{4}\alpha +\;_{-1}^{0}\beta ^{-}$

b = 244 - 16 = 228

a = 94 - 8 + 1 = 87

So, at the end of this decay chain, $_{87}^{228}Fr$ (Francium-228) is produced.

5. Carbon-11 changes into Boron-11 by a radioactive emission. What was emitted?

$_{6}^{11}C\to \;_{5}^{11}B+\;_{a}^{b}?$

b = 11 - 11 = 0

a = 6 - 5 = 1

So, a positron ($_{1}^{0}\beta ^{+}$) was emitted. However, in order for the lepton numbers to balance, a neutrino ($_{0}^{0}\nu$) must also be emitted.

6. Uranium-236 decays, following the equation:

$_{92}^{236}U\to \;_{90}^{232}Th+\;X$

Identify the particle X in this equation.

Solve these type of problems by applying the conservation laws: e.g. the decay must balance electric charge, nucleon number, proton number, lepton number etc. Nucleon number doesn't balance (236 = 232 + ?), so X must have a nucleon number of 4.

The Proton number does not balance either (92 = 90 + ?) therefore X must have 2 Protons.

Therefore it can be concluded that X is an alpha particle.