Accelerator Shielding

Neutron Shielding

Fast neutrons must be slowed down before they can be captured. This can be accomplished by inelastic scattering with heavy elements, such as iron, and by elastic scattering with light nuclei. The resulting slow neutrons are then readily captured in (n, ) reactions. Gamma radiation is produced as a result of these capture reactions and shielding must be provided to absorb this radiation.

Materials containing a high proportion of hydrogen such as water, paraffin wax, polyethylene and concrete are the most effective for elastic scattering at all energies. Concrete contains hydrogen atoms because water, which is chemically bound to the cement contains hydrogen (they constitute about 17% of the atoms).

The shielding around proton accelerators is designed to protect personnel from intense fluxes of secondary particles and gamma radiation created when protons are lost or spilled during the acceleration process, and when the proton beam is used and eventually dumped. The maximum intensity of radiation fields outside the accelerator shielding at TRIUMF is specified in the Operating Licence in the following way: `Unless otherwise indicated by boundaries and warning signs, maximum equivalent dose rates in unsupervised and uncontrolled areas within the TRIUMF exclusion area shall not exceed those in Table 7.2 '.

Shielding against neutrons emerges as the main consideration in the design and choice of material for this shielding. This is because neutrons interact only with the nuclei of atoms and therefore penetrate material to great depths. In designing shielding for accelerators it must be realized that the shielding problems for low-energy proton accelerators (proton energy less than about ) are qualitatively different from those of higher energy proton accelerators. This is because at the lower energy accelerators neutrons are produced with energies not exceeding about . These neutrons are very effectively scattered and slowed down by elastic collisions with light nuclei, especially hydrogen. The strategy therefore is to use shielding material which contains a high fraction of hydrogen. A costly solution is to use a material such as polyethylene, but a cheaper solution is to use concrete which contains a high fraction of hydrogen in the form of water chemically bound to the cement.

Table 7.2: Maximum equivalent dose rates in unmarked areas at TRIUMF.

At high-energy accelerators neutrons with energies up to the energy of the proton beams may be produced. These high-energy neutrons do not lose much energy nor change their direction very much when they scatter off nuclei and therefore penetrate shielding to a much greater depth. They do undergo inelastic interactions with nuclei and these result in a cascade of lower energy particles including more lower energy neutrons. The ideal shielding solution would be a material containing a mixture of heavy nuclei and hydrogen. A material approximating this ideal is so-called `heavy' concrete which is made with an iron bearing aggregate. The strategy most often employed is to use dense material near the source to reduce the flux of high-energy neutrons which propagate the cascade and then a concrete outer layer to reduce the lower energy component. The most inexpensive dense material in this case is iron or mild steel. Often the relative amounts of iron and concrete are a compromise based on cost.

Openings in the shielding must be provided for access and for services. These openings may provide paths through which radiation can stream and are usually designed with suitable bends or dog-legs to reduce this leakage.

Дата добавления: 2015-06-27