Gamma Radiation: Emission and Properties
Gamma radiation is high-energy electromagnetic radiation emitted by an excited nucleus when it drops to a lower energy state; it has no charge and no mass, and the nucleus keeps the same A and Z. In brief: cobalt-60 emits gamma rays of 1.17 and 1.33 MeV after its beta decay, and about 3 cm of lead reduces the intensity of 662 keV gamma rays from cesium-137 to roughly 4 %.
Gamma rays are the most penetrating of the three types of radiation emitted by radioactive nuclei. They are used to sterilize medical equipment, to treat cancer and to image organs, and they require careful shielding. This article explains how gamma rays arise, what their properties are, how they interact with matter and how to calculate shielding, with a comparison to alpha and beta radiation and worked examples.
How is gamma radiation emitted?
After alpha or beta decay, the daughter nucleus is often left in an excited state, in the same way as an atom with an electron in a higher orbital. The nucleus drops to a lower level by emitting a photon with an energy equal to the difference between the levels, typically 0.01 to 10 MeV. This photon is a gamma ray (γ). In the nuclear equation it appears without a charge or a mass number: 6028Ni* → 6028Ni + γ. The asterisk shows the excited state. The sequence for cobalt-60 is: 60Co → 60Ni* + e− + ν̄ (β−), then 60Ni* → 60Ni + γ (1.17 MeV) and a second gamma ray (1.33 MeV).
Some excited states live for a long time (from seconds to years) and are called metastable (isomeric), and denoted by “m”: technetium-99m has a half-life of 6.01 hours and decays to technetium-99 by emitting a 140 keV gamma ray, with no change in Z or A. An alternative process, internal conversion, transfers the energy to an orbital electron, which is ejected. Gamma rays also come from the annihilation of positrons (511 keV), from nuclear reactions and from cosmic sources.
What are the properties of gamma rays?
- Nature: electromagnetic radiation with wavelengths shorter than about 0.01 nm. E = hc/λ: a photon of 662 keV has a wavelength of 1.87 pm.
- Charge and mass: none. They are not deflected by electric or magnetic fields.
- Speed: the speed of light.
- Penetration: high. They can pass through the body, and are reduced only by thick layers of dense material such as lead or concrete.
- Ionization: indirect and weaker per unit length than for alpha or beta particles. Gamma rays interact with matter by the photoelectric effect (at low energies), Compton scattering (intermediate) and pair production (above 1.022 MeV), and the electrons released produce the ionization.
- Attenuation: exponential: I = I0e−μx, where μ is the attenuation coefficient and x is the thickness. They are never completely stopped.
Alpha, beta and gamma radiation compared
| Property | Alpha (α) | Beta (β−) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus | Electron | Photon |
| Charge | +2 | −1 | 0 |
| Mass | 4 u | 1/1836 u | 0 |
| Range in air | 3–7 cm | Up to a few meters | Very large |
| Stopped by | Paper, skin | A few mm of aluminum | Centimeters of lead, meters of concrete |
| Ionizing power | Very high | Moderate | Low (per path length) |
| Main hazard | Internal (inhaled, swallowed) | Skin and eyes; internal | External, whole body |
How do you shield against gamma rays?
Dense materials with a high atomic number are the most effective, because they absorb photons by the photoelectric effect. The thickness that cuts the intensity by half is the half-value layer (HVL): HVL = ln 2 / μ = 0.693 / μ. After n half-value layers the intensity is (½)n of the original.
| Source | Photon energy | HVL in lead | HVL in concrete |
|---|---|---|---|
| 99mTc | 140 keV | about 0.03 cm | about 2.5 cm |
| 137Cs | 662 keV | about 0.65 cm | about 4.8 cm |
| 60Co | 1.17 and 1.33 MeV | about 1.2 cm | about 6.2 cm |
The values depend on the source geometry and the density of the material, and are given only for guidance. Radiation workers also use the principles of time (short exposures), distance (the intensity from a point source falls with the square of the distance) and shielding.
Worked examples
Example 1: shielding a cesium-137 source
How much is the intensity of 662 keV gamma rays reduced by 3.0 cm of lead (HVL = 0.65 cm)? The number of half-value layers is 3.0 / 0.65 = 4.6. The transmitted fraction is (½)4.6 = 0.041, so the intensity falls to about 4 %.
Example 2: thickness for a given reduction
What thickness of lead reduces the intensity of cobalt-60 gamma rays (HVL = 1.2 cm) to 1 % of the original? (½)n = 0.01, so n = log(0.01) / log(0.5) = 6.64. The thickness is 6.64 × 1.2 = 8.0 cm.
Example 3: the inverse square law
A small cobalt-60 source gives a dose rate of 4.0 mSv/h at 1.0 m. At 4.0 m, the dose rate is 4.0 / 4.02 = 0.25 mSv/h. Combining distance and shielding gives a large reduction.
Example 4: the wavelength of a gamma ray
The 1.33 MeV gamma ray from cobalt-60 has an energy of 1.33 × 106 × 1.602 × 10−19 = 2.13 × 10−13 J. λ = hc/E = 1.988 × 10−25 / 2.13 × 10−13 = 9.3 × 10−13 m = 0.93 pm, and ν = c/λ = 3.2 × 1020 Hz.
Example 5: the decay of technetium-99m
A technetium-99m dose of 600 MBq is injected for a bone scan. After 12.0 hours (two half-lives of 6.01 h) the activity is 600 × (½)12.0/6.01 = 600 × 0.2506 = 150 MBq, so most of the activity has decayed within a day, and the radiation dose to the patient is low.
Where are gamma rays used?
- Medical imaging: technetium-99m, with its 140 keV gamma ray and short half-life, is the most commonly used medical radioisotope, and is used for tens of millions of diagnostic procedures a year.
- Cancer therapy: cobalt-60 teletherapy units and the Gamma Knife focus beams of gamma rays on tumors.
- Sterilization: medical supplies, and some foods and spices, are irradiated with gamma rays to kill microorganisms, with no radioactivity left in the products.
- Industry: gamma radiography to inspect welds and castings, level and density gauges.
- Science: gamma-ray spectroscopy identifies radionuclides by their characteristic photon energies, and gamma-ray astronomy studies cosmic explosions.
How were gamma rays discovered?
The French chemist Paul Villard discovered gamma radiation in 1900, when he studied the radiation from radium and found a component that was not deflected by a magnetic field and was more penetrating than the alpha and beta rays that had been described by Rutherford. Rutherford named it gamma radiation in 1903, after the third letter of the Greek alphabet, and in 1914 he and Edward Andrade showed, by diffraction from a crystal, that it is electromagnetic radiation of a very short wavelength.
Which mistakes are common?
- Showing a change in A or Z for gamma emission: neither changes.
- Assuming that gamma rays can be stopped completely: shielding only reduces the intensity.
- Thinking that irradiated food becomes radioactive: gamma rays of these energies do not make materials radioactive.
- Confusing gamma rays and X-rays: they differ in origin (nucleus versus electrons), not in nature.
- Using a wrong number of half-value layers: the transmission is (½)n, not 1/n.