Radioprotection neutrons

A rare radiation, dangerous, penetrating, difficult to absorb

Editorial review 2026

The neutron radiation is more penetrating than alpha and beta . It is more dangerous than gamma rays. It is fortunately short-lived and rarely encountered. Its exposure is exceptional: intervention in the core of a reactor, critical accident, and at a different scale, exploding atomic bomb or hydrogen bomb (in an hydrogen bomb neutrons are produced through the deuterium-tritium reaction).

Under normal circumstances, exposure is reduced to a few neutrons produced by cosmic radiation. It is very low.

In an atomic explosion, the neutron radiation is particularly harmful. But the neutronic flash does not last. Free neutrons have an half-life of 12 minutes. They are generally absorbed by matters before decaying.

In the 80s, low-power atomic bombs were developed, without blast – so not destructing – but releasing an instantaneous neutron flux of fatal intensity. These « neutron bombs » were intended to annihilate enemy combatants, while allowing to occupy the ground shortly after. The subject of numerous public protests at the time, these weapons have, in principle, been abandoned.

In practice, it is in the vicinity of reactors and accelerators in certain research laboratories that protection against neutrons needs to be implemented because of high neutron fluxes.

Neutrons, slowed down by multiple collisions with the encountered material nuclei, are quickly captured. Generally capture is followed by a de-excitation gamma radiation of which we must be protected. It should also be remembered that part of the captures produced radioactive nuclei. The effects of this radioactivity diluted over time, continue to occur after neutron absorption.

Neutrons eventually end up being captured by nuclei. To protect against them, the most effective method is to promote these captures by incorporating nuclei with a very high affinity for neutrons, such as boron-10 or cadmium, into the shielding material. The capture probabilities for slow neutrons become very high for these neutron-hungry nuclei, which could be compared to a kind of goalkeeper with tentacle-like arms.

For external protection, neutron shielding is provided by concrete walls containing boron. Concrete contains water, and therefore hydrogen, which effectively slows down neutrons. The boron incorporated into the concrete of the enclosures contains 20% boron-10, which is very effective at capturing neutrons. For slow neutrons, this nucleus appears 60 times larger than it actually is.

The probability (or cross section) of slow-neutron capture by boron-10 is 3,800 barns. By comparison, the fission cross section of uranium-235 nuclei is 650 barns (1 barn = 10⁻²⁴ cm²).

For reactor control rods, cadmium is used, a metallic element whose capture probability of 2,000 barns is 200 times that of iron (10 barns).

In the case of fissile materials, precautions must be taken against the risk of criticality, that is, the unexpected development of chain reactions. For example, in storage pools, spent fuel assemblies that still contain 1% fissile uranium are spaced apart and placed in borated steel racks.