Penetrating radiation, difficult to absorb
Editorial review 2026
Exponential attenuation of gamma rays in air
The curve represents, for 1,000 gamma rays with an energy of 1 MeV, the number of gamma rays that pass through a certain thickness of air without interacting. Every 90 m — a distance called the attenuation length — the number of gamma rays is divided by 2. It is divided by 4, 8, 16 after 2, 3, 4 times this distance. This exponential decay law resembles the decay law of the activity of a radioactive isotope, with thickness replacing time and attenuation length replacing the half-life. The value of 90 m is specific to 1 MeV gamma rays. At lower energies, the decrease is faster, and at higher energies, the decrease is slower.
© IN2P3
Gamma rays are much less dangerous than alpha rays and beta rays in the event of ingestion or inhalation of radioactive products. More penetrating than X-rays, a large proportion of these rays will leave the affected part of the body without having deposited energy there.
On the other hand, gamma rays are the main concern in the case of exposure to an external source of radiation. Protection is then sought by shielding the source, placing screens between the source and the person, or moving away from it. Exposure decreases with the square of the distance from the source: this is the solid angle effect. With regard to the dose rates received at different distances, the following formula can be written:
Ḋ1 · (d1)2 = Ḋ2 · (d2)2
Solid angle effect
Protection from a gamma source is achieved by moving away from it. Since gamma rays are emitted equally in all directions, the probability that a gamma ray will travel towards a target decreases rapidly with the distance (d) between the source and the target. The portion of space, called the solid angle, within which this target is seen decreases as the inverse square of the distance. For the distances of a few tens of metres shown in the figure (A, B, C), the probability of a gamma ray reaching a 1 m² target is very small.
© IN2P3
Radiation is attenuated more rapidly by shielding or a screen when the material it passes through contains heavy atoms whose nuclei have a very high electric charge. This is why lead bricks are used for protection in laboratories. This material, whose atoms are particularly heavy (A=208), is also inexpensive.
The intensity of the radiation remaining after passing through a certain thickness of shielding decreases in the same way as the activity of a radioactive source decreases over time: according to an exponential law defined by a length called the attenuation length. Whatever the thickness passed through, a fraction of the radiation remains that has not interacted. In practice, the radiation is greatly attenuated. It is divided by 1,000 after ten attenuation lengths.
Interaction modes and stopping power
The figure shows how the probability of photon interaction with carbon atoms (left) or lead atoms (right) varies with energy, in other words, their stopping power. Since this probability, also called the cross section, varies by more than six orders of magnitude between 100 and 10 million electronvolts (MeV), logarithmic scales had to be used. It can be seen that for carbon, the Compton effect is the interaction mechanism that predominates in the energy range of gamma rays from radioactivity (from 0.01 to 2 MeV). Heavy atoms such as lead interact much more strongly and are much more effective at attenuating gamma rays.
© IN2P3
Attenuation and absorption
Attenuation does not mean absorption. Radiation that interacts does not disappear completely. It gives rise to secondary radiation, X-rays or gamma rays produced by the Compton effect or the photoelectric effect. These secondary rays inherit part of the energy and in turn propagate. The accompanying electrons will be absorbed after travelling a short distance, especially if the material is dense.
The phenomenon becomes complex. For radiation protection, it is appropriate to consider the energy absorbed rather than the attenuation of the primary rays. If a sufficient thickness of shielding is added, the secondary radiation is also absorbed.
The counterpart of penetrating power is lower toxicity. Gamma rays do not ionise the medium they pass through but set electrons in motion at the point where they interact, transferring part of their energy to them. Since they rely on these intermediaries to deposit their energy, the effects of gamma rays are not localised like those of charged radiation, which are concentrated along trajectories.
This explains why an intense cobalt-60 source can be observed safely through lead glass about twenty centimetres thick, or nuclear fuel assemblies through three metres of water in a reactor pool.
Examples of mean paths of gamma photons
A characteristic of their attenuation in matter, the mean path of gamma rays emitted by fission fragments in a reactor varies from a few millimetres to 50 centimetres depending on the material, with uranium and water representing the extremes, as shown by the two curves. The decrease is very rapid. Three metres of water are sufficient to protect power plant personnel from gamma irradiation from irradiated fuel.
© Source Clés CEA
In the case of medical diagnostics, a greater or lesser fraction of the gamma rays leaves the body before interacting: these gamma rays cause no harm and can reach a detector. This is why gamma rays are used for medical diagnostic procedures such as scintigraphy: the most commonly used radionuclide is technetium-99m, which emits only gamma rays. During these scintigraphy examinations, the emitter is internal, but irradiation of the tissue or organ being examined is minimal.
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