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.

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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.

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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

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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.

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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.

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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

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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

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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

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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

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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

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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

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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.

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Alpha Particle Properties

Alpha particles have several important properties.

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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.

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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

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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 ✓

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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.

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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

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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.

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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.

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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.