α, β and γ radiation (A-level only) Flashcards – AQA A-level Physics (7408)

What is an alpha particle made of?

A helium nucleus containing two protons and two neutrons.

What does beta-minus radiation consist of?

Fast electrons.

What does gamma radiation consist of?

High-energy electromagnetic photons.

What are the charges of alpha particles, beta-minus particles and gamma photons?

• Alpha: $+2e$
• Beta-minus: $-e$
• Gamma: uncharged

How do alpha, beta and gamma radiation compare in ionising strength?

Alpha is strongly ionising, beta is less strongly ionising, and gamma is generally weakly ionising.

What is the range of alpha radiation in air?

It has a short range.

What can stop alpha radiation?

Paper or the outer layer of skin.

What absorber can stop many beta particles?

A few millimetres of aluminium.

What effect does thick lead or concrete have on gamma radiation?

It reduces its intensity, but does not necessarily stop it completely.

In an absorption experiment, what count-rate patterns suggest alpha, beta and gamma radiation?

• Alpha: a large reduction with paper.
• Beta: little effect from paper, then a reduction with aluminium.
• Gamma: remains detectable through aluminium but is reduced by thick shielding.

What must be accounted for when interpreting count rates in an absorption experiment?

The background count rate.

What harm can ionising radiation cause to cells and DNA?

It can damage cells and DNA, creating a risk of tissue damage or cancer.

What three factors affect the relative hazard of radiation exposure?

The radiation type, the amount of exposure, and whether the source is outside or inside the body.

Why is an inhaled or ingested alpha-emitting substance especially hazardous?

Alpha radiation strongly ionises, depositing energy over a short distance in living tissue.

How do alpha and beta radiation differ in their ability to penetrate skin?

External alpha radiation is largely stopped by skin; beta radiation can penetrate it.

From where can gamma radiation irradiate body tissues?

From sources outside or inside the body.

What three measures reduce exposure when handling radioactive sources?

Limit time near the source, increase distance from it, and use suitable shielding.

How do tongs reduce exposure when handling a radioactive source?

They increase the handling distance from the source.

How does containment reduce the hazard from radioactive material?

It reduces the risk of the material entering the body.

For a gamma source that approximates a point source, how does intensity vary with distance?

According to the inverse-square law: $I \propto \frac{1}{r^2}$.

What is the inverse-square law for the intensity of gamma radiation spreading uniformly from an approximate point source?

$I = \frac{k}{x^2}$, where $I$ is intensity, $x$ is distance from the source, and $k$ is constant for the source and measurement conditions.

Why does gamma intensity decrease with distance from an approximate point source?

The radiation spreads over an increasingly large area.

What happens to gamma intensity when the distance from an approximate point source doubles, if other relevant conditions remain unchanged?

It falls to one quarter of its original value.

How can detector readings be collected to test the inverse-square law for gamma radiation?

Measure count rates at several distances, keeping the source and detector arrangement consistent.

How is a gamma detector reading corrected for background radiation?

Subtract the background count rate measured without the source.

If corrected count rate is proportional to gamma intensity, what plot should test the inverse-square law, and what shape should it have?

Plot corrected count rate against $1/x^2$. It should be approximately a straight line through the origin.

What factors can affect agreement with the inverse-square law in a gamma radiation experiment?

Finite source and detector size, absorption, and counting fluctuations.

What is background radiation?

Ionising radiation present in the environment independently of the source being investigated.

What are four origins of background radiation?

• Cosmic radiation
• Radioactive substances in rocks and soil
• Radon gas
• Artificial sources, such as medical uses of radioactive materials

Why must a measured count rate be corrected for background radiation?

The detector records events from both the source and the background.

How is a source’s count rate corrected for background radiation?

Measure the background count with the source absent. Convert both background and source-present counts to rates, then subtract the background rate from the source-present rate.

Why do longer counting periods generally give a more reliable estimate of the mean count rate?

Radioactive counts fluctuate.

What are the two main medical benefits of ionising radiation?

It can provide diagnostic information or treat disease.

What are two risks of medical exposure to ionising radiation?

Damage to healthy cells and an increased risk of later cancer.

What factors affect the balance of benefit and risk in a medical use of ionising radiation?

Clinical need, likely benefit, amount and location of exposure, and availability of alternatives.

When is a medical use of ionising radiation justified?

When its expected benefit outweighs the associated risk for the patient.

How should exposure to ionising radiation be limited in medical use?

Keep it as low as reasonably practicable while still achieving the medical purpose.