What the three radiations are

Definition

Alpha particle

A particle of two protons and two neutrons emitted from an unstable nucleus, which is the same as a helium nucleus.

Definition

Beta particle

A fast-moving electron emitted from an unstable nucleus.

Definition

Gamma ray

A high-energy electromagnetic wave emitted from an unstable nucleus.

An alpha particle has a relative mass of $4$ and a relative charge of $+ 2$ , and is written $24α^{4}_{2}\alpha$ or $24He^{4}_{2}\text{He}$ .
It is by far the heaviest of the three emissions and leaves the nucleus at roughly $5%5\%$ of the speed of light.
A beta particle has a relative charge of $- 1$ and a relative mass of about $11836\dfrac{1}{1836}$ , and is written $0β^{\;0}_{-1}\beta$ or $0e^{\;0}_{-1}\text{e}$ .
The electron is created inside the nucleus at the moment of decay, so it does not come from the electron shells around the atom.
Beta particles are emitted at speeds up to about $99%99\%$ of the speed of light, which makes them much faster than alpha particles.
A gamma ray has no mass and no charge and is written $00γ^{0}_{0}\gamma$ .
Gamma radiation is part of the electromagnetic spectrum, so it travels at the speed of light, $m/s3\times10^{8}\ \text{m/s}$ .
Alpha and beta emissions are charged particles while gamma is an uncharged wave, and that difference is what makes their behaviour in matter so different.

Ionising power

Definition

Ionising power

A measure of how many atoms a radiation turns into ions along each millimetre of its path.

A charged particle ionises an atom by attracting or repelling an outer electron strongly enough to pull it away as the particle goes past.
Alpha is the most strongly ionising, because its charge of $+ 2$ exerts a large force on electrons and its heavy mass keeps it slow, so it spends longer beside each atom.
A single alpha particle can create tens of thousands of ion pairs before it comes to rest.
Beta is moderately ionising, because its charge is only $- 1$ and its very high speed means it sweeps past each atom in a much shorter time.
Gamma is the least ionising, because it has no charge at all and only occasionally interacts with an atom.
Every ionisation transfers energy out of the radiation, so a strongly ionising radiation runs out of energy over a short distance.

Penetrating power

Definition

Penetrating power

A measure of how far a radiation can travel through a material before it is absorbed.

Definition

Range in air

The distance a radiation travels through air before it has lost all of its energy and can no longer be detected.

Alpha is the least penetrating, with a range in air of only a few centimetres.
A single sheet of paper, or the dead outer layer of skin, absorbs alpha radiation completely.
Beta has a medium penetrating power and travels up to about a metre through air.
Beta passes straight through paper but is absorbed by roughly $mm3\ \text{mm}$ of aluminium.
Gamma is the most penetrating and can travel many metres through air.
Gamma passes through paper and through thin aluminium, so several centimetres of lead or metres of concrete are needed to cut it down substantially.
Gamma radiation is never absorbed completely by a shield, so thicker shielding reduces the intensity that gets through rather than removing it.
Ionising power and penetrating power run in opposite orders, because the radiation that ionises most strongly gives up its energy soonest and therefore stops soonest.
In order of increasing penetrating power the sequence is $α\alpha$ , then $β\beta$ , then $γ\gamma$ .
In order of increasing ionising power the sequence is $γ\gamma$ , then $β\beta$ , then $α\alpha$ .

Practical

Identifying radiation using absorbers

Aim: to find which types of radiation a sealed source emits by measuring how the corrected count rate changes when different absorbers are placed between the source and the detector.
Apparatus: Geiger-Muller tube, digital counter or ratemeter, stand, clamps and boss, metre rule, stopwatch, sealed source, source tongs, lead-lined store, sheets of paper, aluminium sheets about $mm3\ \text{mm}$ thick and lead sheets about $cm2\ \text{cm}$ thick.
Variables: the absorbing material is the independent variable, the corrected count rate is the dependent variable, and the source, the source-to-tube distance, the counting time and the tube itself are kept the same throughout.
Method, measuring the background:
- Clamp the tube at bench height with its mica window facing along the bench, and keep every source locked in the lead-lined store on the far side of the room.
- Record the count over $s300\ \text{s}$ and divide by $s300\ \text{s}$ to obtain the background count rate.
Method, testing each absorber:
- Use tongs to clamp the source with its front face exactly $cm3.0\ \text{cm}$ from the mica window, pointing away from everyone in the room.
- With nothing between the source and the tube, record the count over $s60\ \text{s}$ , repeat twice more and take a mean.
- Clamp a single sheet of paper midway between the source and the window, making sure it covers the whole window, and repeat the three timed counts.
- Replace the paper with $mm3\ \text{mm}$ of aluminium and repeat the three timed counts.
- Replace the aluminium with $cm2\ \text{cm}$ of lead and repeat the three timed counts.
- Leave the source and the tube clamped in exactly the same positions for every reading, then return the source to the store as soon as the last count is finished.
Results: a large fall when the paper is added shows alpha is present, a further large fall when the aluminium is added shows beta is present, and a count rate still clearly above background behind the lead shows gamma is present.
Maths: subtract the background count rate from every mean count rate before comparing them, and work out what percentage of the unshielded corrected count rate each absorber lets through.
Watch out: use ordinary paper rather than thick card, cover the whole window or radiation will reach the tube around the edge of the absorber, and remember that the readings vary between repeats because decay is random.
Safety: handle the source with tongs only, keep it at arm's length and pointing away from people, never bring it near your eyes, keep the time out of store short and wash your hands afterwards.

Example

Identifying an unknown source

The background count rate is measured as $0.4$ counts per second.
With no absorber the measured count rate is $60.4$ counts per second, so the corrected count rate is $60.0$ counts per second.
With paper in place the corrected count rate falls to $40.0$ counts per second, and paper absorbs only alpha radiation, so the source emits alpha.
With $mm3\ \text{mm}$ of aluminium the corrected count rate falls to $5.0$ counts per second, and that further fall of $35.0$ counts per second is caused by beta being absorbed.
With $cm2\ \text{cm}$ of lead the corrected count rate falls to $0.5$ counts per second, so a penetrating radiation had still been getting through the aluminium and the source also emits gamma.

Exam technique

Comparing the radiations

Write about all three radiations when the command word is compare, using words such as most, least and intermediate.
Give both properties, ionising power and penetrating power, unless the question names only one of them.
Name the absorbing material and its thickness, so write $mm3\ \text{mm}$ of aluminium rather than a piece of metal.
Explain a link rather than stating it: alpha ionises strongly, so it loses energy quickly, so its range is short.
Say that lead reduces gamma radiation rather than stopping it.

Common Mistake

Do not call an alpha particle a helium atom, because it is a bare nucleus with no electrons.
Do not write that a beta particle comes from an electron shell, because it is created inside the nucleus.
Do not describe gamma radiation as a light particle with a small mass, because it is an electromagnetic wave with no mass and no charge.
Do not state that lead blocks gamma radiation, because thick lead only reduces its intensity.
Do not mix up the two orders: alpha is the most ionising but the least penetrating.

Self review

State what an alpha particle, a beta particle and a gamma ray each consist of, with the relative charge of each.
Put alpha, beta and gamma in order of increasing penetrating power and name a material that absorbs each one.
Explain why alpha radiation is the most strongly ionising.
Explain why a strongly ionising radiation has a short range.
A source is unaffected by paper but its corrected count rate falls almost to zero behind $mm3\ \text{mm}$ of aluminium, so name the radiation it emits.

What the three radiations are

Definition

Alpha particle

A particle of two protons and two neutrons emitted from an unstable nucleus, which is the same as a helium nucleus.

Definition

Beta particle

A fast-moving electron emitted from an unstable nucleus.

Definition

Gamma ray

A high-energy electromagnetic wave emitted from an unstable nucleus.

An alpha particle has a relative mass of $4$ and a relative charge of $+ 2$ , and is written $24α^{4}_{2}\alpha$ or $24He^{4}_{2}\text{He}$ .
It is by far the heaviest of the three emissions and leaves the nucleus at roughly $5%5\%$ of the speed of light.
A beta particle has a relative charge of $- 1$ and a relative mass of about $11836\dfrac{1}{1836}$ , and is written $0β^{\;0}_{-1}\beta$ or $0e^{\;0}_{-1}\text{e}$ .
The electron is created inside the nucleus at the moment of decay, so it does not come from the electron shells around the atom.
Beta particles are emitted at speeds up to about $99%99\%$ of the speed of light, which makes them much faster than alpha particles.
A gamma ray has no mass and no charge and is written $00γ^{0}_{0}\gamma$ .
Gamma radiation is part of the electromagnetic spectrum, so it travels at the speed of light, $m/s3\times10^{8}\ \text{m/s}$ .
Alpha and beta emissions are charged particles while gamma is an uncharged wave, and that difference is what makes their behaviour in matter so different.

Ionising power

Definition

Ionising power

A measure of how many atoms a radiation turns into ions along each millimetre of its path.

A charged particle ionises an atom by attracting or repelling an outer electron strongly enough to pull it away as the particle goes past.
Alpha is the most strongly ionising, because its charge of $+ 2$ exerts a large force on electrons and its heavy mass keeps it slow, so it spends longer beside each atom.
A single alpha particle can create tens of thousands of ion pairs before it comes to rest.
Beta is moderately ionising, because its charge is only $- 1$ and its very high speed means it sweeps past each atom in a much shorter time.
Gamma is the least ionising, because it has no charge at all and only occasionally interacts with an atom.
Every ionisation transfers energy out of the radiation, so a strongly ionising radiation runs out of energy over a short distance.

Penetrating power

Definition

Penetrating power

A measure of how far a radiation can travel through a material before it is absorbed.

Definition

Range in air

The distance a radiation travels through air before it has lost all of its energy and can no longer be detected.

Alpha is the least penetrating, with a range in air of only a few centimetres.
A single sheet of paper, or the dead outer layer of skin, absorbs alpha radiation completely.
Beta has a medium penetrating power and travels up to about a metre through air.
Beta passes straight through paper but is absorbed by roughly $mm3\ \text{mm}$ of aluminium.
Gamma is the most penetrating and can travel many metres through air.
Gamma passes through paper and through thin aluminium, so several centimetres of lead or metres of concrete are needed to cut it down substantially.
Gamma radiation is never absorbed completely by a shield, so thicker shielding reduces the intensity that gets through rather than removing it.
Ionising power and penetrating power run in opposite orders, because the radiation that ionises most strongly gives up its energy soonest and therefore stops soonest.
In order of increasing penetrating power the sequence is $α\alpha$ , then $β\beta$ , then $γ\gamma$ .
In order of increasing ionising power the sequence is $γ\gamma$ , then $β\beta$ , then $α\alpha$ .

Practical

Identifying radiation using absorbers

Aim: to find which types of radiation a sealed source emits by measuring how the corrected count rate changes when different absorbers are placed between the source and the detector.
Apparatus: Geiger-Muller tube, digital counter or ratemeter, stand, clamps and boss, metre rule, stopwatch, sealed source, source tongs, lead-lined store, sheets of paper, aluminium sheets about $mm3\ \text{mm}$ thick and lead sheets about $cm2\ \text{cm}$ thick.
Variables: the absorbing material is the independent variable, the corrected count rate is the dependent variable, and the source, the source-to-tube distance, the counting time and the tube itself are kept the same throughout.
Method, measuring the background:
- Clamp the tube at bench height with its mica window facing along the bench, and keep every source locked in the lead-lined store on the far side of the room.
- Record the count over $s300\ \text{s}$ and divide by $s300\ \text{s}$ to obtain the background count rate.
Method, testing each absorber:
- Use tongs to clamp the source with its front face exactly $cm3.0\ \text{cm}$ from the mica window, pointing away from everyone in the room.
- With nothing between the source and the tube, record the count over $s60\ \text{s}$ , repeat twice more and take a mean.
- Clamp a single sheet of paper midway between the source and the window, making sure it covers the whole window, and repeat the three timed counts.
- Replace the paper with $mm3\ \text{mm}$ of aluminium and repeat the three timed counts.
- Replace the aluminium with $cm2\ \text{cm}$ of lead and repeat the three timed counts.
- Leave the source and the tube clamped in exactly the same positions for every reading, then return the source to the store as soon as the last count is finished.
Results: a large fall when the paper is added shows alpha is present, a further large fall when the aluminium is added shows beta is present, and a count rate still clearly above background behind the lead shows gamma is present.
Maths: subtract the background count rate from every mean count rate before comparing them, and work out what percentage of the unshielded corrected count rate each absorber lets through.
Watch out: use ordinary paper rather than thick card, cover the whole window or radiation will reach the tube around the edge of the absorber, and remember that the readings vary between repeats because decay is random.
Safety: handle the source with tongs only, keep it at arm's length and pointing away from people, never bring it near your eyes, keep the time out of store short and wash your hands afterwards.

Example

Identifying an unknown source

The background count rate is measured as $0.4$ counts per second.
With no absorber the measured count rate is $60.4$ counts per second, so the corrected count rate is $60.0$ counts per second.
With paper in place the corrected count rate falls to $40.0$ counts per second, and paper absorbs only alpha radiation, so the source emits alpha.
With $mm3\ \text{mm}$ of aluminium the corrected count rate falls to $5.0$ counts per second, and that further fall of $35.0$ counts per second is caused by beta being absorbed.
With $cm2\ \text{cm}$ of lead the corrected count rate falls to $0.5$ counts per second, so a penetrating radiation had still been getting through the aluminium and the source also emits gamma.

Exam technique

Comparing the radiations

Write about all three radiations when the command word is compare, using words such as most, least and intermediate.
Give both properties, ionising power and penetrating power, unless the question names only one of them.
Name the absorbing material and its thickness, so write $mm3\ \text{mm}$ of aluminium rather than a piece of metal.
Explain a link rather than stating it: alpha ionises strongly, so it loses energy quickly, so its range is short.
Say that lead reduces gamma radiation rather than stopping it.

Common Mistake

Do not call an alpha particle a helium atom, because it is a bare nucleus with no electrons.
Do not write that a beta particle comes from an electron shell, because it is created inside the nucleus.
Do not describe gamma radiation as a light particle with a small mass, because it is an electromagnetic wave with no mass and no charge.
Do not state that lead blocks gamma radiation, because thick lead only reduces its intensity.
Do not mix up the two orders: alpha is the most ionising but the least penetrating.

Self review

State what an alpha particle, a beta particle and a gamma ray each consist of, with the relative charge of each.
Put alpha, beta and gamma in order of increasing penetrating power and name a material that absorbs each one.
Explain why alpha radiation is the most strongly ionising.
Explain why a strongly ionising radiation has a short range.
A source is unaffected by paper but its corrected count rate falls almost to zero behind $mm3\ \text{mm}$ of aluminium, so name the radiation it emits.

6.2.4 Properties of alpha, beta and gamma radiation

What the three radiations are

Ionising power

Penetrating power

Identifying radiation using absorbers

Identifying an unknown source

Comparing the radiations

Review 6.2.4 Properties of alpha, beta and gamma radiation by teaching Genie

6.2.4 Properties of alpha, beta and gamma radiation

What the three radiations are

Ionising power

Penetrating power

Identifying radiation using absorbers

Identifying an unknown source

Comparing the radiations

Review 6.2.4 Properties of alpha, beta and gamma radiation by teaching Genie

6.2.4 Properties of alpha, beta and gamma radiation

What the three radiations are

Ionising power

Penetrating power

Identifying radiation using absorbers

Identifying an unknown source

Comparing the radiations

Review 6.2.4 Properties of alpha, beta and gamma radiation by teaching Genie

6.2.4 Properties of alpha, beta and gamma radiation Revision Guide

6.2.4 Properties of alpha, beta and gamma radiation

What the three radiations are

Ionising power

Penetrating power

Identifying radiation using absorbers

Identifying an unknown source

Comparing the radiations

Review 6.2.4 Properties of alpha, beta and gamma radiation by teaching Genie

6.2.4 Properties of alpha, beta and gamma radiation Revision Guide