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Criteria for the Selection of a Shield Material

Theoretically, almost any material can be used for radiation shielding if employed in a thickness sufficient to attenuate the radiation to safe limits. However, due to certain characteristics discussed below, lead and concrete are among the most commonly used materials. The choice of the shield material is dependent upon many varied factors, such as final desired attenuated radiation levels, ease of heat dissipation, resistance to radiation damage, required thickness and weight, multiple use considerations (e.g., shield and/or structural), uniformity of shielding capability, permanence of shielding, and availability.

Gamma Rays and X-Rays

Their attenuation is dependent upon the density of the shielding material; it can be shown that a dense shield material with a higher atomic number is a better attenuator of x-rays. Lead enjoys the advantage of being the densest of any commonly available material. Where space is at a premium and radiation protection is important, lead is often prescribed. It is recognized that lead is not the densest element (e.g., tantalum, tungsten, and thorium are higher on the density scale), but lead is readily available, easily fabricated and the lowest cost of these materials.

Neutrons

In shielding against neutron particles, it is necessary to provide a protective shield that will attenuate both the neutron particles and the secondary gamma radiation.

When applied as part of a neutron particle shielding system, lead has an extremely low level of neutron absorption and, hence, practically no absorption of secondary gamma radiation.

If the shield material has a high rate of neutron capture, it will in time become radioactive, sharply reducing its effectiveness as a shield material. Pure lead itself cannot become highly radioactive under bombardment by neutrons. Therefore, lead shielding, even after long periods of neutron exposure, emits only insignificant amounts of radiation due to activation.

It cannot be stressed too strongly that before a radiation protection shielding system is considered, contact should be made with the radiation control officer of the local municipality having jurisdiction, as well as a registered specialist in the field of radiological shielding.

The main use of lead as a shield is against x-ray and gamma radiation, where the presence of other elements (as impurities or deliberate alloying additions) will have a minor effect, depending only on the degree of dilution of the lead. Where neutrons are also present, however, impurities or additions which would become radioactive must be avoided.

The properties of lead which make it an excellent shielding material are its density, high atomic number, high level of stability, ease of fabrication, high degree of flexibility in application, and its availability.

Lead is heavier than roughly 80 percent of the elements in the periodic table. It could be assumed, therefore, that shield constructions making use of lead will tend to be heavier than constructions making use of lighter elements. This concept may be true in static shielding structures where weight and volume restrictions are of lesser importance. In mobile shielding, however, where weight and volume reductions are at a premium, the selection of the lighter materials would have quite the opposite effect on reducing radiation to the levels intended.

The remaining elements which are heavier than lead could contribute to even greater weight savings, although the use of such materials as depleted uranium and tungsten is usually prohibitive in cost.

The traditional concept of lead being heavy must be re-evaluated in terms of providing a highly effective shield structure, with the lowest volume and weight of the commonly available material.

Also, being a metal, lead has an advantage over various aggregate materials, such as concrete, being more uniform in density throughout. In addition, because commonly used forms of lead exhibit smooth surfaces, lead is less likely to become contaminated with dirt or other materials which, in turn, may become radioactive.