Types of Radioactive Decay
3. Gamma Radiation
Learning outcomes
- I can describe gamma radiation.
- I can explain how gamma emission differs from alpha and beta decay.
- I can identify situations in which gamma radiation is emitted.
- I can compare the penetrating power of gamma radiation with other forms of radiation.
- I can identify appropriate shielding materials for gamma radiation.
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
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.
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.
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
| Property | Alpha | Beta-Minus | Gamma |
|---|---|---|---|
| Symbol | α | β⁻ | γ |
| Nature | Helium nucleus | Electron | Photon |
| Charge | +2e | −e | 0 |
| Rest mass | Large | Very small | 0 |
| Mass number | 4 | 0 | 0 |
| Atomic-number effect | −2 | +1 | No change |
| Mass-number effect | −4 | No change | No change |
| Speed | Below c | Can approach c | c in vacuum |
| Ionizing ability | Very high | Moderate | Lower per interaction |
| Penetrating ability | Low | Moderate | Very high |
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
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
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
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.
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
- What is gamma radiation?
- What type of particle carries gamma radiation?
- What are the charge and rest mass of a gamma photon?
- Explain why an excited nucleus may emit gamma radiation.
- What happens to mass number during gamma emission?
- What happens to atomic number during gamma emission?
- Write the general equation for gamma emission from an excited nucleus.
- Explain how gamma emission differs from alpha decay.
- Explain how gamma emission differs from beta-minus decay.
- Why can gamma radiation sometimes be emitted immediately after alpha or beta decay?
- Rank alpha, beta and gamma radiation from lowest to highest penetrating power.
- Compare the ionizing and penetrating properties of alpha and gamma radiation.
- Why is gamma radiation not deflected by electric or magnetic fields?
- Name three materials that can be used to reduce gamma radiation.
- Explain why lead is useful for compact gamma shielding.
- Explain why concrete is commonly used in radiation facilities.
- What is meant by a half-value layer?
- A gamma beam passes through four half-value layers. What fraction and percentage of its original intensity remains?
- Explain how gamma radiation can be useful in medical imaging despite being hazardous.
- Compare alpha, beta and gamma radiation in terms of their nature, charge, effect on the nucleus, penetrating ability and appropriate shielding.