The Design of Detector Shielding

Photon energy (keV) Absorber element Mass attenuation coefficient (m kg ) Density (kgm- ) Thickness for 1000-fold attenuation (mm) 

You should be aware that cadmium has a high cross-section for the absorption of thermal neutrons. During this absorption, or thermal neutron capture reaction, gamma radiation is emitted, the most noticeable of which is at an energy of 558 keV. If a detector system is to be used in a neutron field, then cadmium in the graded shield should be avoided. Fortunately, tin has a much lower thermal neutron cross-section and can be used instead. Difficulties due to neutron capture might be expected, and indeed have been observed, when operating a detector close to a nuclear reactor but the problem can also occur in environmental measurements (see Chapter 13, Section 13.3.4.2.) 

Cadmium is a toxic metal and should not be handled without actively considering the hazards involved. Under no circumstances should cadmium be flame-cut or soldered unless proper ventilation is provided. Once the cadmium has been formed to the correct shape for the shield, coating it with a varnish wiU reduce the handling hazard. 

Apart from commenting on the possible presence of Co in steel we have said nothing about the contribution to detector background from impurities in the shielding and construction materials. These are of particular importance 

---