Radioactive Decay Flashcards

Alpha, beta minus ( $\beta^-$ ), positron ( $\beta^+$ ), gamma rays and neutron radiation.

Alpha, beta minus, positron (beta plus) and gamma are all ionising.

A helium nucleus: 2 protons and 2 neutrons, written $^{4}_{2}\alpha$ (or $^{4}_{2}\mathrm{He}$ ).

A beta particle is an electron emitted from the nucleus; a gamma ray is electromagnetic radiation.

False. Alpha, beta and gamma emissions are ionising and are emitted by unstable nuclei.

Alpha: a sheet of paper. Beta: a few mm of aluminium. Gamma: thick lead (or concrete).

Alpha, then beta, then gamma.

Alpha a few cm; beta a few metres; gamma hundreds of metres (effectively unlimited).

The count rate increases: there is no aluminium to absorb the beta particles, so more beta particles reach the GM tube.

False. Gamma is the most penetrating but the least ionising; alpha is the most ionising but least penetrating.

The low-level ionising radiation that is around us all the time, from natural and man-made sources.

Radon gas from rocks, rocks and soil (e.g. granite), food and drink, and building materials.

Cosmic rays — high-energy radiation from the Sun and other stars reaching the Earth's atmosphere.

False. Background radiation is everywhere, mostly from natural sources such as radon gas, rocks, food and cosmic rays.

The film darkens (becomes fogged) when exposed to ionising radiation; the more radiation, the darker it becomes.

The count rate — the number of radioactive emissions detected per second (counts/s).

So it can be subtracted from each reading, leaving the count rate due to each source alone.

False. The distance must be kept the same for a fair comparison.

A neutron becomes a proton plus an electron ( $n \rightarrow p$ ), and the electron is emitted as the beta particle.

A proton becomes a neutron ( $p \rightarrow n$ ), and a positron is emitted.

False. The electron is created in the nucleus when a neutron becomes a proton.

Mass number decreases by 4; atomic number decreases by 2.

Mass number stays the same; atomic number increases by 1.

Mass number stays the same; atomic number decreases by 1.

Neutron emission: mass number decreases by 1, atomic number unchanged. Gamma: both unchanged.

The nucleus rearranges and loses energy as gamma radiation to become more stable.

Beta minus: $^{0}_{-1}\beta$ (electron). Positron: $^{0}_{+1}\beta$ .

Mass (nucleon) number and charge (atomic/proton number).

The number of nuclear decays (emissions) per second.

The becquerel, Bq. 1 Bq = 1 decay per second.

False. Count rate is only the radiation reaching the detector, and it includes background radiation.

The time taken for half the undecayed nuclei to decay, or for the activity of a source to halve.

Final : initial $= 1 : 2^n$ .

After 3 half-lives this is $1 : 8$ .

Radioactive decay is a random (unpredictable) process.

Decay is random, so the count rate is not constant — there are variations with each reading.

The activity (or number of undecayed nuclei) of a very large number of nuclei during the decay process.

True. It registers a count whenever any of these ionising radiations enters the tube.

Find the time taken for the activity to fall to half its starting value. That time is the half-life.

Radiation with enough energy to remove electrons from atoms, so that those atoms become ions.

Each decay leaves one fewer unstable nucleus, so as time passes there are fewer nuclei left that can decay.

It drops much faster, so the source falls to a low level of activity within a short time.

Fallout from nuclear weapons tests or accidents, or nuclear waste.

Key Concepts of Physics

Motion & Forces

Conservation of Energy

Waves

Light & the Electromagnetic Spectrum

Radioactivity

Energy – Forces Doing Work

Forces & their Effects

Electricity & Circuits

Magnetism & the Motor Effect

Electromagnetic Induction

Particle Model

Forces & Matter