often, such a system is housed in a low background counting chamber whose thick walls will be made of low-background steel (~20 cm thick) and will be lined with ~1 cm of lead, then perhaps thin layers of cadmium, or tin, with a final layer of copper. the purpose of the lead, cadmium (or tin), and copper is to reduce the background in the low energy region of a gamma spectrum (typically less than 200 kev)