Gamma-ray Build-up

Gamma-Ray Shielding

The effectiveness of gamma-ray shielding is frequently described in terms of the half value layer (HVL) or the tenth value layer (TVL). These are the thicknesses of an absorber that will reduce the gamma radiation to half, and one tenth of its intensity respectively. Fig. 7.6 shows the half-value layers for some common materials as a function of gamma-ray energy.

The most effective gamma shields are materials which have a high density and high atomic number, such as lead, tungsten, and uranium among others. Generally speaking these materials are expensive, so, in situations where space is not a constraint and where structural strength is required, concrete is used even though it is a less effective shielding material. Lead shields are frequently used where space is limited or where only a small area of absorber is required.

Figure 7.6: Half value layers for some materials as a function of gamma-ray energy.

The HVL values given so far apply only to narrow beams of radiation and where the number of HVLs is not too large. In practice it is more likely that the gamma radiation encountered will be divergent. In this case, 3 HVLs for example, would not reduce the field by a factor of 8, but by rather less. This is because some photons which would have missed the detector had there been no shielding are now scattered in the shield to interact with the detector. The result is less attenuation than would be expected. The effect becomes bigger as the thickness of shielding is increased, and is called gamma build up. Fig. 7.7 shows the way in which this occurs.

Figure 7.7: Build-up of 0.5 MeV gamma rays in iron.

In practical work situations, you should never base your plans for radiation protection on shielding calculations alone. The shielding should always be verified by a measurement.

Дата добавления: 2015-06-27