Neutrons

While there are few sources of neutrons at Ames, researchers should still have a general familiarization with this type of radiation.  Neutrons are emitted by only the heaviest elements such as uranium. These heavy atoms are comparatively large and tend to split spontaneously down the middle creating two new elements with nuclei that are very nearly equal in size. The emitted neutrons are merely part of the debris left over from the splitting event. The emitted neutrons carry enough energy to trigger a second nucleus into splitting if another large nucleus is nearby. The emitted neutrons can cause the number of splitting nuclei to multiply rapidly in what is called a chain reaction if there are enough such nuclei nearby. Such reactions are used in nuclear reactors to heat water for generating electricity.

Neutrons can also be artificially generated by instruments utilizing alpha emitters, such as radium, if the alpha emitter is surrounded by a metal such as beryllium. The action of the alpha particle on the beryllium nucleus is to induce it to split and give off a neutron in the process. The neutron has no charge and therefore can be very penetrating. The amount of shielding necessary for reducing a neutron stream to a safe level is directly dependent on the energy of the neutrons in the stream. A few centimeters of shielding will provide adequate protection from slow neutrons, but several feet of shielding may be needed for the most energetic neutrons. Shielding is best achieved by using hydrogen-rich materials like water, Plexiglas, or paraffin.

Interaction of Neutrons with Matter

The way in which neutrons  interact with matter depends to a large extent on their energies, which can range from hundreds of MeV down to fractions of an eV.  Neutrons are uncharged particles and do not interact with atomic electrons in the matter through which they are passing, but they do interact with the nuclei of these atoms .  The nuclear force, which leads to these interactions, is very short ranged which means the neutrons have to pass close to a nucleus for an interaction to take place. Because of the small size of the nucleus in relation to the atom   as a whole, the neutrons will have a low probability of interaction, and could thus travel considerable distances in matter.

The most common neutron reactions are the ones listed below:

Inelastic Scattering

A neutron may strike a nucleus and form a compound nucleus  instead of bouncing off as in elastic scattering. This nucleus is unstable and emits a neutron of lower energy together with a gamma photon  which takes up the remaining energy. This process, called inelastic scattering, is most effective at high neutron energies in heavy materials, but at lower energies (a few MeV) elastic scattering becomes a more important reaction for energy loss provided there are light nuclei present.

Elastic Scattering

This is analogous to a billiard ball type of collision. The neutron collides with a nucleus and rebounds in a different direction. The energy the neutron loses is gained by the target nucleus which moves away at an increased speed. If the neutron collides with a massive nucleus it rebounds with almost the same speed and loses very little energy. Light nuclei, on the other hand, will gain a lot of energy from such a collision and will therefore be more effective for slowing down neutrons.

Elastic scattering is not effective in slowing down neutrons with very high energy (above 150 MeV).

Radiative Capture

 This is one of the most common neutron reactions . The neutron  is again captured by a nucleus   which emits only a gamma photon . This reaction, which occurs in most materials, is the most important one for neutrons with very low energy. The product nuclei of (n,) reactions are usually radioactive and are beta and gamma emitters  .

Two of the neutron capture reactions which are important from the radiological standpoint at most accelerator and nuclear reactor facilities are the (n,) reaction in Co-59, which is normal stable cobalt metal and quite commonly occurs in steel, to produce Co-60 , which is radioactive. The cobalt readily captures neutrons, and Co-60 has a half-life of about 5 years. The other is the neutron capture in Na-23, which is normal, stable sodium. In this case the product is the radioisotope Na-24 . Traces of sodium are present in the concrete shielding.

Protection

Interaction of Neutrons with Tissue

The human body is composed largely of water, about 60% by weight, which contains many hydrogen nuclei . Elastic scattering  of the neutrons with the hydrogen nuclei will cause the protons  to recoil violently. Similarly elastic collisions of neutrons with carbon, oxygen or other heavier nuclei will cause these to recoil. Because the mass of protons  and the other recoiling nuclei  is much greater than that of electrons, they generate a much denser ion path resulting in more damage to the tissue. Once neutrons have been slowed down by elastic collisions to thermal energy, 0.025 eV, they are readily captured by some of the reactions described above.

A very common reaction is the (n,) reaction, particularly with hydrogen. The gamma photon produced in this reaction always has an energy of 2.2 MeV and will cause indirect ionization  as described previously. When neutrons are absorbed by an (n, ) reaction in the body the tissues will be further damaged by gamma radiation  in addition to damage which they receive in slowing down the neutrons.

Other radionuclides  may be formed in the body by the interaction of slow neutrons  with stable nuclei . However, the dose contributed by these radionuclides is usually insignificant compared to the dose the neutrons themselves contribute.

Our normal method of protection from this external radiation hazard is shielding.  Shielding will consist of concrete, water, and/or poly-type materials because of their high water content and possibly a borated material to absorb the thermalized neutrons.