The Board's low background facilities

A description is given of the low background facilities available in the National Radiological Protection Board's new building at Harwell. The lay-out of the suite, its structural shielding (provided by steel sheet 15cm thick made from steel plates salvaged from a 1915 naval wreck) and the use of specially selected building materials have had the combined effect of minimizing the ambient radiation level in the two steel chambers (one about 5m x 2m x 2m high and the other about 2m x 2m x 2m high). The equipment in the larger chamber is used for whole-body monitoring, thyroid monitoring and low-energy photon counting. The whole-body monitor consists of six sodium iodide crystals on a fixed ring around a moving stretcher. Low energy photon detectors have been developed and used for such measurements as plutonium in lungs and lead-210 in bones. An existing lead shielded sample counter is installed near the body measurements chamber for quick and approximate measurements of miscellaneous gamma ray emitting nuclides. Measurements on environmental and biological samples are made in the smaller chamber. The major piece of equipment here is a low-level gamma ray spectrometer consisting of a high efficiency germanium detector shielded by a cylindrical anti-coincidence guard of plastic scintillator. There is also a low level beta ray measuring device with some spectrometric capability, and alpha emitting radionuclides are measured by silicon surface-barrier detectors with good energy resolution. A central spectrum analysis system based on PDP 11 small computer is situated alongside the steel chambers for data processing of the multiplicity of inputs generated by long counting periods and simultaneous operation of different detectors. Special precautions are taken to avoid contamination of the low background laboratory by patients and staff. (U.K.)