Comparing Nuclear Radiation

Radioactive nuclei can emit different forms of ionizing radiation. Three of the most important are:

• alpha (α) radiation
• beta (β) radiation
• gamma (γ) radiation

Although all three can transfer energy to matter and cause ionization, they are physically very different. Their differences in mass, charge, penetration and ionization determine both their hazards and their uses.

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Alpha Radiation

An alpha particle is the nucleus of a helium-4 atom.

It contains:

• 2 protons
• 2 neutrons

It can be represented as:

⁴₂He

or:

⁴₂α

Its charge is:

+2e

Its relative mass is approximately:

4 u

Alpha particles are therefore relatively heavy and strongly charged compared with beta particles.

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Beta Radiation

In beta-minus decay, a neutron changes into a proton while an electron and an electron antineutrino are emitted.

The beta-minus particle is:

⁰₋₁e

or:

β⁻

Its charge is:

−e

Its mass is extremely small compared with an alpha particle.

An electron has a mass of approximately:

9.11 × 10⁻³¹ kg

A proton has a mass about:

1836 times greater

than an electron.

Therefore, beta particles are much lighter than alpha particles.

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Gamma Radiation

Gamma radiation consists of high-energy:

electromagnetic photons

Gamma radiation is represented:

γ

or:

⁰₀γ

A gamma photon has:

• no electric charge
• no rest mass
• no mass number
• no atomic number

Gamma radiation is therefore fundamentally different from alpha and beta radiation.

Alpha and beta are material particles.

Gamma is electromagnetic radiation.

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Comparing Their Nature

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Comparing Charge

Alpha particles are positively charged:

q = +2e

Beta-minus particles are negatively charged:

q = −e

Gamma photons are neutral:

q = 0

Therefore:

alpha → positive

beta-minus → negative

gamma → neutral

These differences affect how the radiation behaves in electric and magnetic fields.

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Comparing Mass

Alpha particles have by far the greatest rest mass of the three.

An alpha particle contains four nucleons.

Beta particles are electrons and have a much smaller mass.

Gamma photons have:

zero rest mass

Therefore, in terms of rest mass:

alpha >> beta > gamma

with gamma having no rest mass.

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Electric Fields

A charged particle in an electric field experiences:

F = qE

Therefore:

• alpha particles are deflected
• beta-minus particles are deflected
• gamma photons are not deflected

Because alpha and beta-minus have opposite charges, they deflect in opposite directions in the same electric field.

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Magnetic Fields

A moving charged particle in a magnetic field experiences:

F = |q|vB sin θ

Therefore:

• alpha particles can be deflected
• beta particles can be deflected
• gamma photons are not deflected

Beta particles generally curve much more strongly than alpha particles under comparable conditions because beta particles have much less mass.

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Direction of Deflection

Alpha and beta-minus particles have opposite charges.

Therefore, in the same magnetic field with comparable directions of motion:

alpha and beta-minus curve in opposite directions

Gamma radiation travels straight through because:

q = 0

This provides experimental evidence that gamma radiation is electrically neutral.

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Penetrating Power

One of the most important differences between alpha, beta and gamma radiation is their ability to pass through matter.

The general order of penetrating ability is:

alpha < beta < gamma

Therefore:

alpha → least penetrating

beta → intermediate

gamma → most penetrating

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Alpha Penetration

Alpha radiation has very low penetrating power.

Alpha particles can typically be stopped by:

• paper
• clothing
• outer dead layer of skin
• a few centimetres of air

This means alpha particles cannot normally penetrate deeply into the body from an external source.

However, alpha-emitting material can be very hazardous if it enters the body.

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Beta Penetration

Beta radiation has moderate penetrating power.

Beta particles can pass through paper and can penetrate into skin to some extent.

They can often be substantially reduced or stopped by materials such as:

• plastic
• acrylic
• thin aluminium

The exact range depends on beta-particle energy.

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Gamma Penetration

Gamma radiation has very high penetrating power.

Gamma photons can pass through:

• paper
• clothing
• skin
• significant amounts of body tissue
• thin sheets of metal

Substantial shielding may be required to reduce gamma intensity.

Materials commonly used include:

• lead
• thick concrete
• steel
• water

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Penetration Comparison

A useful simplified picture is:

Alpha:

source → paper → stopped

Beta:

source → paper → passes through → plastic/aluminium → greatly reduced

Gamma:

source → paper → passes → aluminium → passes → thick lead/concrete → substantially reduced

Gamma radiation is usually described as being attenuated rather than simply stopped.

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Why Alpha Radiation Has Low Penetration

Alpha particles are:

• relatively massive
• positively charged +2e

They interact very strongly with atoms in matter.

As they travel, they transfer energy rapidly to nearby atoms.

They therefore lose their kinetic energy over a short distance.

This gives alpha radiation:

low penetration

but:

very high ionization

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Why Beta Radiation Penetrates Farther

Beta particles are:

• much lighter than alpha particles
• charged only ±e

They generally interact less strongly along each unit of path than alpha particles.

Therefore, beta particles usually travel farther before losing their energy.

This gives beta radiation:

moderate penetration

and:

moderate ionization

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Why Gamma Radiation Penetrates Farther

Gamma photons have:

• no charge
• no rest mass

They do not continuously interact with matter in the same way as charged alpha and beta particles.

A gamma photon may travel a substantial distance before interacting.

Therefore, gamma radiation has:

high penetrating power

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Ionization

Ionization occurs when an atom or molecule gains or loses electrons and becomes electrically charged.

Ionizing radiation can transfer enough energy to remove electrons from atoms.

Alpha, beta and gamma radiation are all:

ionizing radiation

However, their ionizing abilities differ.

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Comparing Ionizing Ability

The simplified order is:

alpha > beta > gamma

Therefore:

alpha → very strongly ionizing

beta → moderately ionizing

gamma → less strongly ionizing per unit path

This order is approximately the reverse of their penetrating abilities.

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Penetration vs Ionization

A useful relationship is:

This is not a coincidence.

Radiation that interacts strongly with matter loses its energy quickly.

Therefore:

strong interaction → high ionization → short range

Radiation that interacts less frequently can travel farther.

Therefore:

weaker/fewer interactions → lower ionization density → greater penetration

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Why Alpha Is Strongly Ionizing

Alpha particles have a charge of:

+2e

and move relatively slowly compared with beta particles and gamma photons.

Their strong electric interaction with surrounding electrons causes many ionizations over a short path.

Alpha particles therefore deposit a large amount of energy into a small volume of material.

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Beta Ionization

Beta particles are charged, so they also interact electrically with atoms.

However, their smaller charge magnitude and very small mass produce different interactions from alpha particles.

Beta particles typically cause:

less dense ionization than alpha particles

but more direct ionization than is usually associated with gamma photons along comparable paths.

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Gamma Ionization

Gamma photons are uncharged.

They produce ionization indirectly through interactions such as:

• photoelectric absorption
• Compton scattering
• pair production at sufficiently high energy

These interactions produce energetic charged particles, especially electrons, which then cause further ionization.

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Comparing Radiation Tracks

If alpha and beta radiation are observed in a cloud chamber:

Alpha tracks

Tend to be:

• short
• thick
• relatively straight
• densely ionizing

Beta tracks

Tend to be:

• longer
• thinner
• more irregular
• less densely ionizing

Gamma photons do not usually produce a continuous visible charged-particle track directly. Their interactions can create energetic electrons that then leave tracks.

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Comparing Nuclear Changes

The three forms of radiation also differ in what happens to the nucleus.

Alpha decay

A → A − 4

Z → Z − 2

Example:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

Beta-minus decay

A → A

Z → Z + 1

Example:

¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄ₑ

Gamma emission

A → A

Z → Z

Example:

ᴬ_ZX → ᴬ_ZX + γ*

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Complete Nuclear Comparison

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Shielding Alpha Radiation

Alpha radiation is easy to stop externally.

Suitable barriers include:

• paper
• clothing
• gloves
• outer dead layer of skin

However, the main safety concern with alpha emitters is often:

preventing contamination

because alpha-emitting material can become much more hazardous if inhaled or ingested.

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Shielding Beta Radiation

Suitable beta shielding may include:

• plastic
• acrylic
• thin aluminium

Low-atomic-number materials such as plastic can be particularly useful for energetic beta radiation because they reduce the production of secondary bremsstrahlung compared with using dense high-Z shielding as the first layer.

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Shielding Gamma Radiation

Gamma radiation requires much more substantial shielding.

Common materials include:

• lead
• thick concrete
• steel
• sufficient depth of water

Gamma shielding usually reduces the intensity rather than guaranteeing that every photon is stopped.

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Half-Value Layer

For gamma radiation, shielding can be described using the:

half-value layer (HVL)

The HVL is the thickness of a particular material required to reduce the radiation intensity by half.

After one HVL:

I = I₀/2

After two:

I = I₀/4

After three:

I = I₀/8

This demonstrates why thicker shielding produces progressively greater attenuation.

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External Radiation Hazards

The penetrating ability of radiation strongly affects external hazard.

Alpha

Usually cannot penetrate through the outer dead layer of skin.

Therefore, direct external penetration is relatively limited.

Beta

Can penetrate into skin and nearby tissues.

Therefore, it can be an external hazard.

Gamma

Can penetrate deeply into the body.

Therefore, gamma radiation can be a significant whole-body external hazard.

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Internal Radiation Hazards

If radioactive material enters the body, the situation changes.

Alpha emitters

Can be particularly hazardous internally because alpha particles deposit large amounts of energy over very short distances.

Beta emitters

Can irradiate tissues around the radioactive material over a greater range.

Gamma emitters

Can irradiate tissues over still greater distances and gamma photons may also leave the body.

The actual risk depends on:

• isotope
• activity
• energy
• exposure time
• location
• biological behaviour

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Example: Choosing Shielding

Suppose you have three radioactive sources.

Source A emits alpha radiation

Suitable simple barrier:

paper

Source B emits beta radiation

Suitable material:

plastic or acrylic

Source C emits gamma radiation

Suitable shielding:

thick lead or concrete

The shielding must always be appropriate for the particular source energy and activity in real radiation-safety situations.

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Example: Identifying Radiation from Penetration

A radioactive source is tested.

The radiation:

• passes through paper
• is stopped by a few millimetres of aluminium

This is most consistent with:

beta radiation

Why?

Alpha would have been stopped by paper.

Gamma would generally require much more substantial shielding for strong attenuation.

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Example: Identifying Radiation from Fields

Unknown radiation passes between charged plates.

It is strongly deflected toward the positive plate.

The radiation is likely:

beta-minus

because it carries negative charge.

If it were alpha radiation, it would deflect toward the negative plate.

If it were gamma radiation, it would not be deflected.

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Example: No Deflection

Unknown radiation passes through both:

• electric field
• magnetic field

without deflection.

It also penetrates deeply through matter.

The radiation is most likely:

gamma

because gamma photons:

• have no charge
• are highly penetrating

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Example: Strong Ionization but Short Range

A radiation source produces particles that:

• travel only a few centimetres through air
• are stopped by paper
• produce dense ionization

The radiation is:

alpha

These characteristics result from its relatively large mass and +2 charge.

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Penetration Experiment

A simple classroom investigation can compare radiation penetration using a suitable simulated activity or professionally managed sealed-source demonstration.

Measurements could be taken:

1. without absorber
2. with paper
3. with plastic or aluminium
4. with thicker shielding

Typical observations might show:

paper strongly reduces alpha

plastic/aluminium strongly reduces beta

dense thick materials are required for substantial gamma reduction

Radioactive-source experiments must follow the institution's radiation-safety procedures.

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Background Radiation

Radiation detectors normally register some radiation even when no laboratory source is nearby.

This is called:

background radiation

Possible sources include:

• cosmic rays
• rocks and soil
• radon and its decay products
• building materials
• naturally occurring radioactive isotopes
• medical exposures

When conducting experiments, background count rate should be measured and considered.

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Corrected Count Rate

Suppose:

measured source count = 520 counts/min

background count = 40 counts/min

Then:

corrected count = measured count − background count

corrected count:

= 520 − 40

= 480 counts/min

This gives a better estimate of the count associated with the source under those measurement conditions.

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Distance and Radiation Exposure

Increasing distance from a source can reduce radiation exposure.

For an approximately point-like source in open space:

I ∝ 1/r²

Therefore, doubling the distance can reduce intensity to approximately:

1/4

and tripling distance can reduce it to approximately:

1/9

This is especially important for penetrating radiation such as gamma radiation.

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Time, Distance and Shielding

Three major principles of external radiation protection are:

Time

Minimize unnecessary exposure time.

Distance

Increase distance from the source where appropriate.

Shielding

Place suitable material between the source and people.

The correct shielding depends on the radiation type.

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Why One Shield Does Not Suit Every Radiation Type

The densest material is not automatically the best first shielding material for every situation.

For example:

• alpha needs little external shielding, but containment is important
• beta may be better initially shielded using lower-Z materials such as plastic
• gamma generally requires substantial dense shielding

Radiation protection therefore depends on understanding how each type interacts with matter.

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Uses and Properties

The different properties of nuclear radiation make different forms useful for different purposes.

Alpha

Strong ionization and short range can be useful in applications such as ionization smoke detectors.

Beta

Moderate penetration can be useful in some thickness-monitoring systems.

Gamma

High penetration can be useful for:

• medical imaging
• radiotherapy applications
• sterilization
• industrial radiography

The same properties that make radiation useful can also create hazards.

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Identifying Unknown Radiation

A useful identification strategy is to ask:

Is it stopped by paper?

Yes → likely alpha

Does it pass paper but get strongly reduced by plastic or thin aluminium?

Likely beta

Does it require substantial dense shielding for strong attenuation?

Likely gamma

Then check field behaviour:

Strongly deflected and negative?

β⁻

Less strongly deflected and positive?

α

Not deflected?

γ

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Common Mistakes

Mistake: Saying alpha radiation is the most penetrating.
Alpha is the least penetrating of the three.

Mistake: Saying gamma radiation is not ionizing.
Gamma radiation is ionizing, although it produces less dense ionization than alpha radiation in typical comparisons.

Mistake: Assuming high penetration means high ionization density.
Alpha demonstrates the opposite: very high ionization but very low penetration.

Mistake: Saying gamma rays have a small positive charge.
Gamma photons have zero charge.

Mistake: Saying beta-minus particles are positively charged.
β⁻ particles are electrons and have negative charge.

Mistake: Saying alpha particles are just protons.
An alpha particle contains two protons and two neutrons.

Mistake: Saying paper is suitable shielding for beta radiation.
Beta particles can generally pass through paper.

Mistake: Saying aluminium foil is sufficient protection from all gamma radiation.
Gamma radiation is highly penetrating and usually requires substantial shielding for significant attenuation.

Mistake: Assuming alpha radiation is harmless because skin stops it.
Alpha-emitting material can be particularly hazardous if it enters the body.

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Key Terms

Alpha radiation — radiation consisting of helium nuclei containing two protons and two neutrons.

Beta radiation — radiation consisting of high-speed electrons or positrons produced in nuclear decay.

Gamma radiation — high-energy electromagnetic radiation emitted during nuclear transitions.

Ionizing radiation — radiation capable of producing ions in matter.

Ionization — process in which atoms or molecules gain or lose electrons.

Penetrating power — ability of radiation to pass through matter.

Ionizing power — ability of radiation to cause ionization.

Shielding — material placed between a radiation source and people or equipment to reduce radiation exposure.

Attenuation — reduction in radiation intensity as radiation passes through matter.

Half-value layer — thickness of a material required to reduce radiation intensity by half.

Bremsstrahlung — electromagnetic radiation produced when a charged particle, especially a fast electron, is rapidly decelerated.

Background radiation — ionizing radiation continually present in the environment.

Contamination — radioactive material being deposited where it is not intended, including on or inside a person.

Irradiation — exposure of an object or organism to ionizing radiation.

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Key Takeaways

• Alpha, beta and gamma are three important forms of nuclear radiation.
• Alpha particles contain two protons and two neutrons.
• Alpha particles have charge +2e and relatively large mass.
• Beta-minus particles are electrons with charge −e and very small mass.
• Gamma radiation consists of photons with zero charge and zero rest mass.
• Alpha and beta radiation can be deflected by electric and magnetic fields.
• Gamma radiation is not deflected because it is electrically neutral.
• Penetrating power increases approximately alpha → beta → gamma.
• Ionizing ability decreases approximately alpha → beta → gamma in the usual introductory comparison.
• Alpha radiation is stopped easily by paper or the outer layer of skin.
• Beta radiation can often be shielded using plastic, acrylic or thin metal.
• Gamma radiation requires substantial shielding such as lead or thick concrete for strong attenuation.
• Alpha radiation is highly ionizing because it interacts strongly with matter.
• Gamma radiation is highly penetrating because photons can travel significant distances between interactions.
• Alpha radiation is generally a smaller external penetration hazard but can be a serious internal contamination hazard.
• Beta radiation can present both external and internal hazards.
• Gamma radiation can penetrate deeply into the body from an external source.
• The appropriate shielding depends on radiation type and energy.
• Time, distance and shielding are fundamental radiation-protection principles.
• Understanding the differences between alpha, beta and gamma radiation allows their behaviour, hazards and applications to be predicted.

Check Your Understanding

1. What is an alpha particle made from?
2. What is a beta-minus particle?
3. What is gamma radiation?
4. State the charge of an alpha particle.
5. State the charge of a beta-minus particle.
6. State the charge of a gamma photon.
7. Rank alpha, beta and gamma from greatest to smallest rest mass.
8. Rank alpha, beta and gamma from lowest to highest penetrating ability.
9. Rank alpha, beta and gamma from lowest to highest ionizing ability.
10. Explain why alpha radiation has low penetrating power.
11. Explain why gamma radiation has high penetrating power.
12. Which radiation can normally be stopped by paper?
13. Which radiation can often be significantly reduced using plastic or thin aluminium?
14. Which radiation generally requires substantial lead or concrete shielding?
15. Explain why alpha and beta radiation are deflected in electric fields but gamma radiation is not.
16. Explain why beta radiation is usually deflected more strongly than alpha radiation in a magnetic field.
17. Unknown radiation passes through paper but is strongly reduced by a thin aluminium sheet. Identify the most likely radiation and explain your reasoning.
18. Unknown radiation passes through an electric field without deflection and requires thick dense shielding for substantial attenuation. Identify the radiation.
19. Explain why an alpha emitter can be more hazardous inside the body than outside it.
20. Compare alpha, beta and gamma radiation in terms of nature, mass, charge, ionizing ability, penetrating ability and suitable shielding.