Alpha Decay

What Is Alpha Decay?

Alpha decay is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle.

An alpha particle contains:

• 2 protons
• 2 neutrons

It is therefore identical to the nucleus of a helium-4 atom.

An alpha particle can be represented as:

⁴₂He

or:

⁴₂α

Because it contains two protons, an alpha particle has an electric charge of:

+2e

where e is the elementary charge.

 

 

 

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Structure of an Alpha Particle

An alpha particle consists of four nucleons:

2 protons + 2 neutrons

Therefore:

Mass number = 4

Atomic number = 2

The mass number tells us the total number of protons and neutrons.

The atomic number tells us the number of protons.

So:

⁴₂He

means:

4 nucleons

2 protons

and therefore:

2 neutrons.

Why Alpha Decay Occurs

Alpha decay occurs mainly in very heavy, unstable nuclei.

Large nuclei contain many positively charged protons.

These protons repel one another through the electrostatic force.

The strong nuclear force holds nucleons together, but it acts only over very short distances.

In sufficiently large nuclei, the balance between these effects can make the nucleus unstable.

One way for the nucleus to become more stable is to emit an alpha particle.

 

 

 

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The Parent and Daughter Nuclei

The original radioactive nucleus is called the:

parent nucleus

After alpha decay, the remaining nucleus is called the:

daughter nucleus

The process can be represented:

parent nucleus → daughter nucleus + alpha particle

Because the alpha particle carries away two protons and two neutrons, the daughter nucleus is a different element.

What Changes During Alpha Decay?

An alpha particle contains:

• 2 protons
• 2 neutrons

Therefore, when a nucleus emits an alpha particle:

mass number decreases by 4

and:

atomic number decreases by 2

This gives the fundamental alpha-decay rule:

A → A − 4

Z → Z − 2

where:

• A = mass number
• Z = atomic number

General Alpha Decay Equation

Suppose the original nucleus is:

ᴬ_ZX

After alpha decay:

ᴬ_ZX → ᴬ⁻⁴_Z₋₂Y + ⁴₂He

where:

• X = parent element
• Y = daughter element
• A = original mass number
• Z = original atomic number

The new element must be identified using its new atomic number.

Example: Uranium-238

Uranium-238 has:

A = 238

Z = 92

During alpha decay:

Mass number:

238 − 4 = 234

Atomic number:

92 − 2 = 90

Element 90 is thorium.

Therefore:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

 

 

 

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Checking the Equation

A nuclear equation must conserve:

• mass number
• atomic number
• electric charge

For:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

Check mass numbers:

238 = 234 + 4

238 = 238 ✓

Check atomic numbers:

92 = 90 + 2

92 = 92 ✓

The equation is balanced.

Example: Radium-226

Radium has atomic number:

Z = 88

Suppose:

²²⁶₈₈Ra → ? + ⁴₂He

Mass number of daughter:

226 − 4 = 222

Atomic number:

88 − 2 = 86

Element 86 is radon.

Therefore:

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Example: Polonium-210

Polonium has atomic number:

84

During alpha decay:

Mass number:

210 − 4 = 206

Atomic number:

84 − 2 = 82

Element 82 is lead.

Therefore:

²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He

Example: Radon-222

Radon-222 has:

A = 222

Z = 86

After alpha decay:

A = 222 − 4 = 218

Z = 86 − 2 = 84

Element 84 is polonium.

Therefore:

²²²₈₆Rn → ²¹⁸₈₄Po + ⁴₂He

Identifying an Unknown Daughter Nucleus

Suppose:

²⁴¹₉₅Am → ? + ⁴₂He

Subtract 4 from the mass number:

241 − 4 = 237

Subtract 2 from the atomic number:

95 − 2 = 93

Element 93 is neptunium.

Therefore:

²⁴¹₉₅Am → ²³⁷₉₃Np + ⁴₂He

Identifying an Unknown Parent

Sometimes the daughter nucleus is given instead.

Suppose:

? → ²²²₈₆Rn + ⁴₂He

Add the mass numbers:

222 + 4 = 226

Add the atomic numbers:

86 + 2 = 88

Element 88 is radium.

Therefore:

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Alpha Decay Changes the Element

The identity of an element depends on its:

number of protons

Since alpha decay removes two protons, the atomic number decreases by two.

Therefore, alpha decay always changes one element into another.

For example:

uranium → thorium

radium → radon

polonium → lead

This process is called:

nuclear transmutation

Conservation in Nuclear Equations

Nuclear equations can be checked using conservation laws.

For introductory alpha-decay equations, check that both sides have the same total:

Mass number

Total number of nucleons is conserved.

Atomic number

Total proton number/charge number is conserved.

For example:

²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He

Mass:

210 = 206 + 4

Atomic number:

84 = 82 + 2

Both balance.

Alpha Particle Properties

Alpha particles have several important properties.

Alpha Radiation Has Low Penetrating Power

Alpha particles are relatively:

• massive
• slow compared with beta particles and gamma photons
• strongly charged

Their +2 charge causes strong interactions with atoms and molecules in matter.

As they travel, they quickly transfer energy to surrounding atoms.

As a result, alpha radiation has:

very low penetrating power

 

 

 

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What Stops Alpha Radiation?

Alpha radiation can usually be stopped by:

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

This means an alpha source outside the body may present a much smaller penetration hazard than a gamma source of comparable activity and energy.

However, this does not mean alpha radiation is harmless.

Alpha Radiation Is Strongly Ionizing

Alpha particles have a charge of:

+2e

and interact strongly with electrons in atoms.

As an alpha particle passes through matter, it can remove electrons from atoms.

This produces:

ions

The process is called:

ionization

Alpha particles deposit their energy over a relatively short distance.

Therefore, alpha radiation has:

high ionizing power

Penetration vs Ionization

It is important to distinguish between:

penetrating power

and:

ionizing power

Alpha radiation has:

low penetration

but:

high ionization

These properties are related.

Because alpha particles interact so strongly with matter, they lose their energy rapidly.

This prevents them from travelling very far.

Comparing Alpha, Beta and Gamma

In simplified terms:

ionizing ability: α > β > γ

while:

penetrating ability: γ > β > α

The exact biological hazard depends on many additional factors, including energy, activity, exposure pathway and tissue involved.

Alpha Radiation Outside the Body

If an alpha-emitting substance remains safely outside the body, the outer layer of skin can usually stop the alpha particles.

Therefore, external exposure to a properly contained alpha source may present relatively limited direct alpha penetration into deeper tissues.

However, safe handling procedures are still required because radioactive materials may present other risks, especially contamination.

Alpha Radiation Inside the Body

Alpha radiation becomes particularly dangerous if an alpha-emitting substance enters the body.

Possible routes include:

• inhalation
• ingestion
• entry through wounds

Once inside the body, the alpha particles can deposit a large amount of energy in a very small volume of tissue.

This can cause substantial biological damage.

 

 

 

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Why Internal Alpha Sources Are Hazardous

Outside the body:

skin can stop alpha particles

Inside the body:

there may be no protective dead skin layer between the radioactive material and living cells.

The short range now becomes especially important.

The alpha particle deposits much of its energy within nearby tissue.

This creates dense ionization along its path.

Therefore:

alpha emitters can be especially hazardous when inhaled or ingested.

Ionization and Biological Damage

Ionizing radiation can damage biological molecules.

Possible effects include:

• breaking chemical bonds
• damaging proteins
• damaging cell structures
• damaging DNA

Cells may:

• repair the damage
• die
• function abnormally
• acquire mutations

The biological effect depends on factors such as:

• radiation type
• radiation energy
• absorbed dose
• exposure duration
• tissue type
• location of radioactive material

Radon and Alpha Radiation

Radon is a naturally occurring radioactive noble gas produced in decay chains involving uranium and radium.

Some radon isotopes and their radioactive decay products emit alpha radiation.

Radon can accumulate in enclosed spaces under certain geological and building conditions.

The major concern is inhalation of radon and especially its radioactive decay products, which can expose respiratory tissues to alpha radiation.

This is an important example of why:

internal alpha exposure can be more hazardous than external alpha exposure.

Alpha Radiation and Smoke Detectors

Some ionization smoke detectors use a very small quantity of the alpha-emitting isotope:

americium-241

Americium-241 emits alpha particles.

The alpha particles ionize air inside an ionization chamber, allowing a small electrical current to flow.

When smoke enters, it disrupts this ionization current.

The detector senses the change and activates the alarm.

 

 

 

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Why Alpha Radiation Can Be Useful in a Smoke Detector

Alpha radiation is useful because it:

• strongly ionizes the air
• has a short range
• is easily contained within the detector's design

The radioactive source is designed to remain securely contained during normal use.

Smoke detectors should not be dismantled or damaged.

Alpha Radiation in Technology and Science

Alpha emitters and alpha particles have applications in:

• smoke detection
• scientific research
• nuclear physics
• some specialized medical treatments
• space exploration power systems involving alpha-emitting radioisotopes

In each application, radioactive materials require appropriate containment and safety procedures.

Alpha Particles in Electric Fields

Because an alpha particle has positive charge:

q = +2e

it experiences force in an electric field:

F = qE

The force acts in the direction of the electric field because the alpha particle is positively charged.

This means alpha particles can be deflected using electric fields.

Alpha Particles in Magnetic Fields

A moving alpha particle also experiences magnetic force:

F = qvB sin θ

Because the alpha particle is positively charged, its force direction follows the positive-charge version of the right-hand rule.

If it moves perpendicular to a magnetic field, it can follow a circular path.

The radius is:

r = mv/(qB)

Because alpha particles are relatively massive, their paths are generally less strongly curved than those of much lighter charged particles under comparable conditions.

Alpha Decay and Energy

Alpha decay releases energy because the total mass of the final products is slightly less than the initial mass of the parent nucleus.

The difference is called the:

mass defect

The released energy is related to the mass difference by:

E = Δmc²

Much of this released energy appears as kinetic energy of:

• the alpha particle
• the recoiling daughter nucleus

Nuclear Recoil

Momentum must be conserved during alpha decay.

Before decay, suppose the parent nucleus is approximately at rest.

Its total momentum is approximately:

0

After decay, the alpha particle moves in one direction.

The daughter nucleus therefore recoils in the opposite direction.

Their momenta are equal in magnitude and opposite in direction:

p_alpha = −p_daughter

Because the daughter nucleus has much greater mass, its recoil speed is much smaller.

Alpha Decay Is Spontaneous

It is impossible to predict exactly when a particular unstable nucleus will decay.

Alpha decay is:

spontaneous and random

However, for a very large number of identical radioactive nuclei, the overall statistical behaviour is predictable.

This leads to concepts such as:

• activity
• decay constant
• half-life

which describe radioactive samples statistically.

Alpha Decay Chains

One alpha decay may not produce a stable daughter nucleus.

The daughter may also be radioactive.

It can undergo another decay.

This can produce a:

radioactive decay chain

For example, heavy naturally occurring nuclei can pass through several alpha and beta decays before eventually reaching a stable nucleus.

 

 

 

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Effect of Multiple Alpha Decays

Each alpha decay causes:

A → A − 4

Z → Z − 2

After two alpha decays:

A → A − 8

Z → Z − 4

After three alpha decays:

A → A − 12

Z → Z − 6

In general, after n alpha decays:

A_new = A_original − 4n

Z_new = Z_original − 2n

Example: Two Alpha Decays

Suppose a nucleus initially has:

A = 238

Z = 92

After two alpha decays:

A = 238 − 2(4)

A = 230

Z = 92 − 2(2)

Z = 88

Therefore, after two alpha decays:

A = 230

Z = 88

Element 88 is radium.

Example: Finding Number of Alpha Decays

A nucleus changes from:

A = 232

to:

A = 220

Difference:

232 − 220 = 12

Each alpha decay reduces A by 4.

Number of alpha decays:

12/4 = 3

The atomic number should therefore decrease by:

3 × 2 = 6

This provides a useful way to check decay-chain calculations.

Detecting Alpha Radiation

Alpha radiation can be detected using suitable instruments such as:

• Geiger-Müller detectors with appropriate thin windows
• semiconductor detectors
• scintillation detectors
• cloud chambers

Because alpha particles have a short range, detector geometry is particularly important.

The source must often be relatively close to the detector.

Alpha Tracks in a Cloud Chamber

Alpha particles strongly ionize the material through which they pass.

In a cloud chamber, this can produce visible condensation tracks.

Alpha tracks tend to be:

• relatively thick
• short
• strongly ionizing

compared with tracks from lighter charged particles.

This provides visual evidence of their strong interaction with matter.

 

 

 

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Safe Handling Principles

Radioactive materials should be handled according to established radiation-safety procedures.

Three general principles for reducing external radiation exposure are:

Time

Reduce unnecessary exposure time.

Distance

Increase distance from the source where appropriate.

Shielding

Use suitable shielding and containment.

For alpha emitters, preventing:

contamination, inhalation and ingestion

is particularly important because of the internal hazard.

External Exposure vs Contamination

These terms are different.

Exposure means radiation reaches a person from a source.

Contamination means radioactive material is deposited on or enters a person, object or environment.

For alpha emitters, contamination can be particularly important because radioactive material entering the body can continue irradiating nearby tissue.

Example: Writing an Alpha Decay Equation

Suppose thorium-232 undergoes alpha decay.

Thorium:

A = 232

Z = 90

Subtract:

A = 232 − 4 = 228

Z = 90 − 2 = 88

Element 88 is radium.

Therefore:

²³²₉₀Th → ²²⁸₈₈Ra + ⁴₂He

Check:

Mass:

232 = 228 + 4 ✓

Atomic number:

90 = 88 + 2 ✓

Example: Complete the Equation

Complete:

²²⁶₈₈Ra → ²²²₈₆Rn + ?

Mass number of missing particle:

226 − 222 = 4

Atomic number:

88 − 86 = 2

Therefore, the missing particle is:

⁴₂He

an alpha particle.

Problem-Solving Strategy

When solving an alpha-decay equation:

Identify the parent

Write its mass number A and atomic number Z.

Subtract four from A

A_new = A − 4

Subtract two from Z

Z_new = Z − 2

Identify the daughter element

Use the new atomic number.

Add the alpha particle

⁴₂He

Check conservation

Make sure both sides have equal total:

• mass number
• atomic number

Common Mistakes

Mistake: Saying an alpha particle is a helium atom.
It is a helium nucleus. It does not initially contain electrons.

Mistake: Saying an alpha particle contains four protons.
It contains two protons and two neutrons.

Mistake: Decreasing mass number by 2.
Alpha decay decreases mass number by 4.

Mistake: Decreasing atomic number by 4.
Alpha decay decreases atomic number by 2.

Mistake: Keeping the same element after alpha decay.
Losing two protons changes the element.

Mistake: Saying alpha radiation has high penetrating power.
Alpha radiation has very low penetrating power.

Mistake: Saying low penetration means alpha radiation is harmless.
Alpha emitters can be particularly hazardous when radioactive material enters the body.

Mistake: Confusing irradiation with contamination.
A person can be exposed to radiation without radioactive material being deposited on or inside them.

Key Terms

Alpha particle — helium-4 nucleus containing two protons and two neutrons.

Alpha decay — radioactive decay in which an unstable nucleus emits an alpha particle.

Parent nucleus — original radioactive nucleus before decay.

Daughter nucleus — nucleus produced after radioactive decay.

Mass number (A) — total number of protons and neutrons in a nucleus.

Atomic number (Z) — number of protons in a nucleus.

Nucleon — proton or neutron.

Radioactivity — spontaneous emission of radiation from unstable nuclei.

Ionization — removal or addition of electrons, producing ions.

Ionizing radiation — radiation capable of causing ionization in matter.

Penetrating power — ability of radiation to travel through matter.

Transmutation — conversion of one element into another through a nuclear process.

Contamination — unwanted radioactive material present on or within a person, object or environment.

Irradiation — exposure to ionizing radiation.

Decay chain — sequence of radioactive decays leading through several daughter nuclei.

Mass defect — difference between the mass of a system before and after a nuclear process, associated with released or absorbed energy.

Key Takeaways

• Alpha decay occurs when an unstable nucleus emits an alpha particle.
• An alpha particle contains 2 protons and 2 neutrons.
• It is represented as ⁴₂He or ⁴₂α.
• An alpha particle has mass number 4.
• It has charge +2e.
• Alpha decay decreases mass number by 4.
• Alpha decay decreases atomic number by 2.
• The daughter nucleus is therefore a different element.
• Alpha-decay equations must conserve mass number and atomic number.
• Alpha radiation has low penetrating power.
• Paper, a short distance of air or the outer layer of skin can stop alpha particles.
• Alpha radiation has very high ionizing ability.
• Alpha particles lose energy rapidly because they interact strongly with matter.
• External alpha exposure generally has limited penetration through intact skin.
• Alpha-emitting materials can be particularly hazardous if inhaled, ingested or introduced through wounds.
• Radon and its decay products are an important example of an internal alpha-radiation hazard.
• Alpha particles can be deflected by electric and magnetic fields because they are charged.
• Alpha decay releases energy associated with a small change in mass.
• The daughter nucleus recoils in the opposite direction to the emitted alpha particle.
• Alpha decay is spontaneous and random for individual nuclei.

Check Your Understanding

1. What is an alpha particle?
2. How many protons and neutrons are in an alpha particle?
3. What are the mass number and atomic number of an alpha particle?
4. What electric charge does an alpha particle have?
5. Describe what happens to an unstable nucleus during alpha decay.
6. How does alpha decay change the mass number?
7. How does alpha decay change the atomic number?
8. Complete: ²³⁸₉₂U → _____ + ⁴₂He.
9. Complete: ²²⁶₈₈Ra → _____ + ⁴₂He.
10. Write the alpha-decay equation for polonium-210.
11. Explain how you can check that an alpha-decay equation is balanced.
12. Why does alpha decay change one element into another?
13. Compare the ionizing ability and penetrating power of alpha radiation.
14. Why can alpha particles be stopped relatively easily by matter?
15. Explain why an alpha source outside the body may present a smaller direct penetration hazard than the same radioactive material inside the body.
16. Identify three ways an alpha-emitting substance could enter the body.
17. Explain why inhaled alpha-emitting material can be hazardous.
18. A nucleus undergoes three alpha decays. By how much do its mass number and atomic number change?
19. A nucleus changes from mass number 240 to 224 entirely through alpha decay. How many alpha decays occurred?
20. Explain how alpha decay demonstrates conservation of mass number, atomic number, energy and momentum.

Gamma Radiation

What Is Gamma Radiation?

Gamma radiation is a form of very high-energy electromagnetic radiation emitted by an unstable or excited atomic nucleus.

A gamma ray is made of photons, not particles containing protons, neutrons, or electrons.

Gamma radiation is represented by:

γ

or in nuclear equations as:

⁰₀γ

A gamma photon has:

• no electric charge
• no rest mass
• no mass number
• no atomic number
• very high energy
• speed equal to the speed of light in a vacuum

c ≈ 3.00 × 10⁸ m/s

 

 

 

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Gamma Radiation Is Electromagnetic Radiation

Gamma radiation belongs to the electromagnetic spectrum.

The electromagnetic spectrum includes:

• radio waves
• microwaves
• infrared
• visible light
• ultraviolet
• X-rays
• gamma rays

All electromagnetic waves consist of oscillating electric and magnetic fields.

Gamma rays generally occupy the very high-frequency, high-energy region of the electromagnetic spectrum.

 

 

 

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Energy of a Gamma Photon

The energy of a photon is:

E = hf

where:

• E = photon energy (J)
• h = Planck's constant
• f = frequency (Hz)

Planck's constant is:

h = 6.63 × 10⁻³⁴ J·s

Since:

c = fλ

gamma photons have very high frequencies and correspondingly very short wavelengths.

Because:

E = hf

higher frequency means higher photon energy.

Gamma Emission

Gamma emission occurs when an atomic nucleus changes from a:

higher-energy nuclear state

to a:

lower-energy nuclear state

The excess energy is released as a gamma photon.

The nucleus itself does not lose a proton or neutron during this process.

Therefore:

mass number does not change

and:

atomic number does not change

Excited Nuclei

An excited nucleus can be represented with an asterisk:

X*

The asterisk indicates that the nucleus contains excess energy.

Gamma emission can then be represented:

X → X + γ*

The nucleus remains the same element and isotope, but moves into a lower-energy state.

 

 

 

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General Gamma Equation

A nuclear gamma transition can be written:

ᴬ_ZX → ᴬ_ZX + ⁰₀γ*

Notice that:

A remains unchanged.

Z remains unchanged.

Therefore:

gamma emission does not change the identity of the nuclide.

It changes only its nuclear energy state.

Example: Gamma Emission

Suppose an excited cobalt-60 nucleus emits gamma radiation.

A simplified representation is:

⁶⁰₂₇Co → ⁶⁰₂₇Co + ⁰₀γ*

Before emission:

A = 60
Z = 27

After emission:

A = 60
Z = 27

The element remains cobalt.

Only the nuclear energy decreases.

Gamma Radiation Often Follows Other Nuclear Decays

Gamma radiation is frequently emitted after another radioactive process such as:

• alpha decay
• beta decay

The daughter nucleus produced by the first decay may be left in an excited state.

It can then release excess energy by emitting one or more gamma photons.

So a sequence may look like:

parent → excited daughter + alpha/beta

followed by:

excited daughter → lower-energy daughter + gamma

Example: Beta Decay Followed by Gamma Emission

Cobalt-60 undergoes beta-minus decay to an excited state of nickel-60.

A simplified sequence is:

⁶⁰₂₇Co → ⁶⁰₂₈Ni + ⁰₋₁e + ν̄ₑ*

The excited nickel nucleus then releases gamma radiation:

⁶⁰₂₈Ni → ⁶⁰₂₈Ni + γ*

The beta decay changes the atomic number.

The gamma emission does not.

 

 

 

Gamma Emission After Alpha Decay

The same principle can occur after alpha decay.

First:

parent → daughter + alpha*

Then:

daughter → daughter + gamma*

The alpha decay changes:

• mass number
• atomic number

The subsequent gamma emission changes neither.

Gamma emission simply removes excess nuclear energy.

Comparing Nuclear Changes

The three major types of radiation affect nuclei differently.

Alpha decay

The nucleus loses:

• 2 protons
• 2 neutrons

Therefore:

A → A − 4

Z → Z − 2

Beta-minus decay

A neutron changes into a proton.

Therefore:

A → A

Z → Z + 1

Gamma emission

The nucleus loses energy.

Therefore:

A → A

Z → Z

This is one of the most important distinctions among alpha, beta and gamma radiation.

Alpha, Beta and Gamma Compared

Gamma Radiation Has High Penetrating Power

Gamma radiation has much greater penetrating ability than alpha or beta radiation.

A simplified ranking is:

alpha < beta < gamma

for penetrating power.

Alpha particles can be stopped by paper or skin.

Beta particles can often be significantly reduced by plastic or thin metal.

Gamma radiation may require substantial thicknesses of dense shielding material to reduce its intensity.

 

Why Gamma Radiation Is Highly Penetrating

Gamma photons:

• have no electric charge
• have no rest mass
• interact differently with matter than charged alpha and beta particles

Charged particles continually interact with electrons and nuclei as they travel.

Gamma photons can travel significant distances between interactions.

Therefore, gamma radiation can penetrate deeply into materials and biological tissue.

Gamma Radiation Is Ionizing

Although gamma radiation is highly penetrating, it is still:

ionizing radiation

Gamma photons can transfer energy to electrons in matter.

These energetic electrons can then cause further ionization.

Therefore, gamma radiation can damage:

• atoms
• molecules
• cells
• DNA

Its high penetrating power means that an external gamma source can irradiate tissues throughout the body.

Gamma Interactions with Matter

Gamma photons can interact with matter through several processes.

Three important mechanisms are:

• photoelectric absorption
• Compton scattering
• pair production

Which process dominates depends strongly on:

• gamma energy
• shielding material
• atomic number of the material

Photoelectric Absorption

In the photoelectric effect, a gamma photon transfers essentially all of its energy to an electron.

The photon disappears.

The electron is ejected from the atom.

A simplified energy relationship is:

E_photon = binding energy + electron kinetic energy

This process can contribute to the absorption of gamma radiation.

Compton Scattering

In Compton scattering, a gamma photon collides with an electron.

The photon transfers some of its energy to the electron and changes direction.

After the interaction:

• electron moves away with kinetic energy
• gamma photon continues with lower energy

 

 

 

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Pair Production

A sufficiently energetic gamma photon passing near a nucleus can produce:

• an electron
• a positron

The gamma photon disappears and its energy becomes particle rest energy and kinetic energy.

The minimum photon energy required is:

1.022 MeV

because the rest-energy equivalent of one electron is approximately:

0.511 MeV

and two particles are produced:

2 × 0.511 MeV = 1.022 MeV

Gamma Radiation Is Not Easily Stopped Completely

For gamma radiation, it is often more accurate to talk about:

reducing intensity

rather than completely stopping every photon.

As gamma radiation passes through matter, some photons interact and are removed from the original beam.

Others may pass through.

Therefore, increasing shielding thickness progressively reduces the radiation intensity.

Exponential Attenuation

Gamma intensity approximately follows:

I = I₀e^(−μx)

where:

• I = transmitted intensity
• I₀ = initial intensity
• μ = linear attenuation coefficient
• x = shielding thickness

The attenuation coefficient depends on:

• photon energy
• shielding material

Therefore, different gamma sources may require different shielding designs.

Half-Value Layer

A useful shielding quantity is the:

half-value layer (HVL)

The HVL is the thickness of material needed to reduce gamma intensity to:

one-half

of its original value.

After one HVL:

I = I₀/2

After two HVLs:

I = I₀/4

After three HVLs:

I = I₀/8

After n HVLs:

I = I₀(1/2)ⁿ

Example: Half-Value Layers

Suppose the gamma intensity is initially:

800 counts/min

One HVL of shielding reduces this to:

400 counts/min

A second HVL gives:

200 counts/min

A third gives:

100 counts/min

Therefore, three HVLs reduce the intensity to:

1/8 of the original value

or:

12.5%

Gamma Shielding

Because gamma radiation is highly penetrating, shielding often uses materials with substantial density and thickness.

Common shielding materials include:

• lead
• concrete
• steel
• water

The appropriate material depends on:

• gamma energy
• source activity
• required attenuation
• available space
• structural requirements
• cost
• radiation environment

 

 

 

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Why Lead Is Used

Lead has:

• high density
• high atomic number
• strong gamma attenuation per unit thickness for many gamma energies

Therefore, relatively compact lead shielding can substantially reduce gamma intensity.

Lead is often used in:

• radiation containers
• laboratory shielding
• medical facilities
• nuclear applications

However, shielding design depends on the radiation energy and application.

Why Concrete Is Used

Concrete is widely used because it can provide:

• thick structural shielding
• relatively low cost
• large-scale protection
• attenuation of gamma radiation

It is commonly found in:

• nuclear facilities
• radiation-treatment rooms
• research laboratories
• accelerator facilities

Very thick concrete walls can substantially reduce radiation levels outside controlled areas.

Water as Radiation Shielding

Water can also provide radiation shielding when sufficient thickness is used.

For example, water surrounding stored radioactive material can:

• absorb radiation
• provide distance from the source
• allow visual inspection in some facilities

Large pools are therefore used in parts of the nuclear industry.

Gamma Radiation Outside the Body

Gamma radiation is particularly important as an external radiation hazard.

Unlike alpha particles, gamma photons can penetrate the skin and travel into deeper tissues.

Therefore, a gamma source does not need to enter the body to expose internal organs.

Radiation protection is consequently important even when the source remains outside the body.

Gamma Radiation Inside the Body

Gamma-emitting radioactive material can also present an internal hazard if it enters the body.

The emitted gamma photons can travel away from the radioactive material and expose surrounding tissues.

Some gamma-emitting isotopes are deliberately used in medicine because their radiation can be detected outside the body.

Gamma Radiation in Medical Imaging

Gamma-emitting radioactive tracers can be introduced into the body in carefully controlled medical procedures.

A detector outside the body detects gamma photons leaving the patient.

This allows doctors to investigate biological processes and organ function.

One example is:

gamma-camera imaging

 

 

 

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Why Gamma Radiation Is Useful for Imaging

For an internal tracer to be detected externally, its radiation must escape the body.

Alpha radiation would be unsuitable for this purpose because its range in tissue is extremely short.

Gamma radiation is useful because:

• it can leave the body
• detectors can measure it externally
• its location can help determine tracer distribution

The isotope must also have suitable:

• half-life
• photon energy
• chemical properties

Gamma Radiation in Cancer Treatment

High-energy photons can also be used to damage cancer cells.

Radiation therapy aims to deliver a controlled dose to a tumour while minimizing unnecessary exposure to healthy tissue.

Modern systems can direct radiation from carefully selected directions and shapes.

Gamma-emitting radioactive sources have historically and currently been used in certain radiotherapy technologies, alongside X-ray and particle-beam systems.

Sterilization

Gamma radiation can be used to sterilize certain:

• medical equipment
• laboratory materials
• packaged products

The radiation penetrates packaging and damages microorganisms.

This can be useful for items that cannot easily be sterilized using high temperatures.

Food Irradiation

Ionizing radiation can also be used under controlled conditions to:

• reduce microorganisms
• control insects
• delay some spoilage processes

Food irradiation does not automatically make the food radioactive.

Radiation passing through a material and radioactive contamination are different concepts.

Industrial Gamma Radiography

Gamma radiation can be used to inspect the internal structure of manufactured components.

A gamma source is placed on one side of an object and a detector on the other.

Differences in transmitted radiation can reveal:

• cracks
• voids
• defects
• variations in thickness

This technique is similar in principle to X-ray imaging.

Measuring Material Thickness

Gamma radiation can also be used to monitor thick materials.

A source and detector are placed on opposite sides.

If the material becomes thicker:

less gamma radiation reaches the detector

If it becomes thinner:

more reaches the detector

This can be incorporated into industrial control systems.

Gamma-Ray Astronomy

Gamma rays are also produced naturally in extremely energetic astronomical environments.

Sources include phenomena associated with:

• neutron stars
• black-hole environments
• supernova remnants
• energetic particle interactions
• gamma-ray bursts

Because Earth's atmosphere absorbs much high-energy gamma radiation, gamma-ray observatories are often placed on spacecraft.

 

 

 

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Detecting Gamma Radiation

Gamma radiation can be detected using devices such as:

• Geiger-Müller detectors
• scintillation detectors
• semiconductor detectors

Because gamma photons are uncharged, detection generally depends on their interactions with matter producing charged particles or ionization that can be measured.

Gamma Radiation in a Magnetic Field

Gamma radiation has:

q = 0

Therefore, magnetic force:

F = qvB

is zero.

So gamma radiation is:

not deflected by a magnetic field

Gamma Radiation in an Electric Field

Similarly:

F = qE

Since:

q = 0

gamma radiation experiences no electric force.

Therefore:

gamma rays are not deflected by electric fields

This differs from both alpha and beta radiation.

Radiation Deflection Comparison

In suitable electric or magnetic fields:

• alpha radiation is deflected
• beta radiation is deflected more strongly because of its much smaller mass
• gamma radiation is not deflected

Alpha and beta also deflect in opposite directions because their charges have opposite signs.

Alpha, Beta and Gamma Shielding

A useful simplified comparison is:

Alpha

Can be stopped by:

• paper
• outer dead layer of skin
• a short distance of air

Beta

Can often be significantly reduced by:

• plastic
• acrylic
• thin aluminium

Gamma

Requires much more substantial shielding, such as:

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

Shielding should always be selected for the particular radiation source and energy.

Radiation Protection

Three fundamental principles are used when controlling external radiation exposure:

Time

Reduce unnecessary exposure time.

Distance

Increase distance from the radiation source where practical.

Shielding

Place suitable material between the source and people.

For highly penetrating gamma radiation, all three can be important.

Why Distance Helps

Radiation spreading outward from a small source approximately follows the:

inverse-square law

under suitable conditions.

Intensity is approximately:

I ∝ 1/r²

If distance doubles:

r → 2r

then:

I → I/4

If distance triples:

r → 3r

then:

I → I/9

This makes distance a powerful radiation-protection tool.

Example: Inverse-Square Law

A detector measures:

900 counts/min

at:

1.0 m

Ignoring background and assuming a point-source geometry, what would be expected at 3.0 m?

Use:

I₂/I₁ = r₁²/r₂²

I₂ = 900 × (1²/3²)

I₂ = 900/9

I₂ = 100 counts/min

Example: Gamma Equation

An excited nucleus is:

¹³⁷₅₆Ba*

It emits a gamma photon.

The equation is:

¹³⁷₅₆Ba → ¹³⁷₅₆Ba + ⁰₀γ*

Notice:

Mass number:

137 → 137

Atomic number:

56 → 56

No element change occurs.

Example: Comparing Decay Types

Suppose three nuclei initially have:

A = 100

Z = 50

Alpha decay

A = 96

Z = 48

Beta-minus decay

A = 100

Z = 51

Gamma emission

A = 100

Z = 50

This comparison clearly shows how differently the three processes affect a nucleus.

Example: Shielding Calculation

A particular shielding material has an HVL of:

2.0 cm

How much radiation remains after:

6.0 cm?

Number of HVLs:

n = 6.0/2.0

n = 3

Therefore:

I = I₀(1/2)³

I = I₀/8

So:

12.5% of the original intensity remains

under the assumptions of the HVL model.

Choosing Shielding

When selecting gamma shielding, engineers consider more than simply density.

Important factors include:

• gamma photon energy
• required reduction
• shielding thickness
• material density
• structural strength
• cost
• secondary radiation
• available space
• source geometry

For example:

lead can provide compact shielding.

concrete can provide economical large-scale structural shielding.

water can provide both shielding and useful separation from radioactive materials.

Common Mistakes

Mistake: Saying gamma radiation consists of high-speed electrons.
Those are beta-minus particles. Gamma radiation consists of photons.

Mistake: Saying gamma emission changes atomic number.
Gamma emission does not change Z.

Mistake: Saying gamma emission changes mass number.
Gamma emission does not change A.

Mistake: Saying gamma emission changes one element into another.
The nucleus remains the same nuclide but moves to a lower-energy state.

Mistake: Saying gamma radiation is not ionizing.
Gamma radiation is ionizing, although its interactions differ from those of charged alpha and beta particles.

Mistake: Saying gamma radiation is easily stopped by paper or aluminium foil.
Gamma radiation is highly penetrating.

Mistake: Saying gamma radiation can always be completely stopped by a fixed thickness of lead.
Shielding generally attenuates gamma radiation; thicker shielding progressively reduces intensity.

Mistake: Saying gamma rays are deflected by magnets.
Gamma photons are uncharged and are not deflected by magnetic fields.

Key Terms

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

Gamma photon — quantum of gamma electromagnetic radiation.

Excited nucleus — nucleus containing more energy than its lower-energy or ground state.

Ground state — lowest available energy state of a nucleus.

Nuclear transition — change from one nuclear energy state to another.

Electromagnetic radiation — energy transmitted through oscillating electric and magnetic fields in the form of photons.

Photon — quantum of electromagnetic radiation.

Ionizing radiation — radiation capable of causing ionization in matter.

Penetrating power — ability of radiation to travel through matter.

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

Attenuation coefficient — quantity describing how strongly a material reduces radiation intensity.

Half-value layer (HVL) — material thickness that reduces radiation intensity to half its original value.

Photoelectric absorption — interaction in which a photon transfers its energy to an electron and is absorbed.

Compton scattering — interaction in which a photon transfers some energy to an electron and changes direction.

Pair production — conversion of a sufficiently energetic photon into an electron and positron near a nucleus.

Shielding — material used to reduce radiation exposure.

Key Takeaways

• Gamma radiation consists of high-energy electromagnetic photons.
• Gamma photons have no electric charge and no rest mass.
• Gamma radiation travels at the speed of light in a vacuum.
• Gamma emission occurs when an excited nucleus moves to a lower-energy state.
• Gamma emission does not change mass number.
• Gamma emission does not change atomic number.
• Therefore, gamma emission does not change the identity of the nuclide.
• Gamma emission often follows alpha or beta decay when the daughter nucleus is left excited.
• Gamma radiation is ionizing.
• Gamma radiation is much more penetrating than alpha or beta radiation.
• Gamma rays are not deflected by electric or magnetic fields.
• Gamma radiation interacts with matter through processes including photoelectric absorption, Compton scattering and pair production.
• Gamma shielding reduces intensity rather than necessarily stopping every photon.
• Lead is useful when compact, dense shielding is required.
• Thick concrete is useful for large-scale structural shielding.
• Water can also provide substantial shielding when used in sufficient depth.
• Half-value layer describes the thickness needed to reduce intensity by half.
• Time, distance and shielding are fundamental radiation-protection principles.
• Gamma radiation has important applications in medicine, industry and scientific research.
• Its high penetrating power makes gamma radiation useful but also creates a significant external exposure hazard.

Check Your Understanding

1. What is gamma radiation?
2. What type of particle carries gamma radiation?
3. What are the charge and rest mass of a gamma photon?
4. Explain why an excited nucleus may emit gamma radiation.
5. What happens to mass number during gamma emission?
6. What happens to atomic number during gamma emission?
7. Write the general equation for gamma emission from an excited nucleus.
8. Explain how gamma emission differs from alpha decay.
9. Explain how gamma emission differs from beta-minus decay.
10. Why can gamma radiation sometimes be emitted immediately after alpha or beta decay?
11. Rank alpha, beta and gamma radiation from lowest to highest penetrating power.
12. Compare the ionizing and penetrating properties of alpha and gamma radiation.
13. Why is gamma radiation not deflected by electric or magnetic fields?
14. Name three materials that can be used to reduce gamma radiation.
15. Explain why lead is useful for compact gamma shielding.
16. Explain why concrete is commonly used in radiation facilities.
17. What is meant by a half-value layer?
18. A gamma beam passes through four half-value layers. What fraction and percentage of its original intensity remains?
19. Explain how gamma radiation can be useful in medical imaging despite being hazardous.
20. Compare alpha, beta and gamma radiation in terms of their nature, charge, effect on the nucleus, penetrating ability and appropriate shielding.

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.

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.

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.

Comparing Their Nature

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.

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.

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.

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.

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.

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.

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

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.

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

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

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

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.

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.

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

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.

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.

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*

Complete Nuclear Comparison

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.