Alpha and Beta Particles

Shielding

Shielding is used extensively to reduce radiation exposures to acceptable levels at TRIUMF. The installation of additional shielding between a source of radiation and personnel working nearby is often a practical means of reducing external dose. The material used as the absorber and the thickness required depend on the type of radiation and its energy.

Alpha particles have relatively small penetrating power. The range in air of the most energetic alpha particles is only about 10 cm. They are completely stopped by the epidermis (outer dead layer of the skin) and because of this they do not present an external radiation hazard. Shielding against alpha particles is therefore unnecessary. However, as the body has no epidermis over internal tissues to protect them and because of the high quality factor (Q = 20) of alpha particles they are a very serious internal hazard and great care must be exercised to ensure that alpha radioactivity is not taken into the body.

Figure 7.5: Range of beta rays in various materials

Beta radioactivity does not generally present an external exposure hazard at proton accelerators especially at the facility. Radioactivity tends to be deeply buried in the activated components or shielding and only the gamma radiation penetrates to the surface. Exceptions to this general rule are those instances where the radioactivity is deposited in a thin material like a vacuum window or where it is deposited in a thin surface layer as loose contamination. Also materials bombarded by the proton beam of the radioisotope production cyclotrons may have the radioactivity located very close to the surface because of the small range of the protons produced by these machines. An external hazard could then exist because beta particles have a considerable range in air. The skin and the lens of the eye are the most susceptible organs and serious damage can result to both from large exposures. The wearing of safety glasses under these circumstances can reduce the hazard to the eye considerably. The gonads are also susceptible if the beta energy is very high.

Beta particles, however, are readily absorbed and beta sources should not present a problem when proper techniques are applied by anyone working with them. Fig. 7.5 shows the range of beta radiation in various materials. If work is to be done on a piece of contaminated equipment, decontamination of the unit to reduce the extremity dose may also be appropriate.

Дата добавления: 2015-06-27