Radioactive decay and types of radiationIB MYP Physics: Revision notes

Section 1

Unstable nuclei and radioactive decay

Some nuclei are unstable. They become more stable by giving out radiation, which is called radioactive decay. Decay is:

Spontaneous: it happens without any outside trigger, and it is not changed by temperature, pressure or chemical reactions.
Random: you cannot predict when a particular nucleus will decay. With a very large number of nuclei, the overall rate of decay is predictable.

The radiation is ionising: it removes electrons from atoms to make ions.

Key termsunstable nucleusradioactive decayspontaneousrandomionising

Section 2

Alpha, beta and gamma radiation

Alpha (α): a helium nucleus (2 protons and 2 neutrons), charge +2. Strongly ionising, weakly penetrating: stopped by paper or a few centimetres of air.
Beta (β): a fast-moving electron from the nucleus, charge -1. Moderately ionising: stopped by a few millimetres of aluminium.
Gamma (γ): an electromagnetic wave, no charge. Weakly ionising, highly penetrating: reduced by thick lead or concrete.

The more strongly ionising a radiation is, the less far it penetrates, because it loses its energy more quickly.

Key termsalpha particlebeta particlegamma raypenetrationshielding

Common mistake

A beta particle is an electron from the nucleus, not one from the electron shells.

Section 3

Deflection in electric and magnetic fields

Charged particles are deflected by electric and magnetic fields, but uncharged radiation is not.

Alpha (positive) and beta (negative) are deflected in opposite directions.
Beta is deflected more than alpha because it has a much smaller mass.
Gamma has no charge, so it is not deflected.

Key termsdeflection

Exam tip

Remember: positive and negative bend opposite ways, light particles bend more, gamma goes straight.

Section 4

Background radiation and the Geiger-Muller tube

Background radiation is always present. Natural sources include radon gas from rocks, cosmic rays from space, rocks and soil, and food and drink. Artificial sources include medical X-rays and nuclear fallout.

A Geiger-Muller (GM) tube detects ionising radiation. Each particle or ray that enters produces a pulse, and a counter records the count rate (counts per second or per minute).

To find the count rate from a source, measure the background count rate with the source removed and subtract it.

Key termsbackground radiationGeiger-Muller tubecount rate

Section 5

Decay equations

In a decay equation the mass numbers (A) and atomic numbers (Z) must balance on both sides.

Alpha decay: A decreases by 4 and Z decreases by 2.
Beta decay: A stays the same and Z increases by 1 (a neutron changes into a proton and an electron).
Gamma emission: A and Z do not change.

Worked example: radium-226 (Z = 88) emits an alpha particle: $^{226}_{88}\mathrm{Ra} \rightarrow {}^{222}_{86}\mathrm{Rn} + {}^{4}_{2}\mathrm{He}$ .

Beta decay of carbon-14: $^{14}_{6}\mathrm{C} \rightarrow {}^{14}_{7}\mathrm{N} + {}^{0}_{-1}\mathrm{e}$ .

Key termsdecay equation

Common mistake

In beta decay the mass number does not change, but the atomic number goes up by 1, not down.

Must Know

Decay is spontaneous and random.
Alpha: helium nucleus, +2, strongly ionising, stopped by paper. Beta: fast electron, -1, stopped by aluminium. Gamma: EM wave, 0, reduced by thick lead.
Alpha and beta are deflected in opposite directions; gamma is not deflected.
Subtract background radiation from GM tube readings.
Alpha: A - 4, Z - 2. Beta: A same, Z + 1.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Radioactive decay and types of radiation

A hospital physicist in Manchester stores three radioactive sources, one that emits alpha particles, one that emits beta particles and one that emits gamma rays. She must choose suitable shielding for each source and check which types of radiation are most harmful to living cells.
Compare the ionising power and the penetrating power of alpha particles and gamma rays.2 marks
Uranium-238 has atomic number 92. Its nuclei are unstable. A uranium-238 nucleus decays by emitting an alpha particle and forms a nucleus of thorium (symbol Th). The thorium-234 nucleus then decays by beta emission to form protactinium (symbol Pa).
State the mass number and the atomic number of the thorium nucleus formed when uranium-238 decays by alpha emission.2 marks
A student investigates how well two materials absorb gamma radiation. She holds a Geiger-Muller tube 10 cm from a gamma source and records the number of counts in 60 seconds. With the source removed she records a background count of 12 in 60 seconds. With nothing between the source and the tube she records 612 counts. With a 5 mm sheet of aluminium in the way she records 598 counts, and with a 5 mm sheet of lead in the way she records 214 counts.
Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).