An extremely low-level X-ray spectrometer

Abstract

An extremely low-level X-ray counter system has been constructed with heavy-mass and anticoincidence shieldings. The detector is made of oxygen-free copper and of cylindrical form. It operates as a proportional counter of gas-flow type (argon-methane), inside of which are put samples. It is surrounded by 7mm thick purified mercury, multiwire GM anticoincidence counters, 1 cm thick oxygen-free copper, 12 cm thick iron, and finally 10 cm lead. The whole assembly is built in an underground cell heavily shielded from major cosmic ray components with a rock cover of at least 100 mwe. After careful measurements of the background counting rate, it was found that oxygen-free copper still contains some radioactive impurities, constituting a considerable portion of background count. By coating the inside wall of sample holders with purified nickel or copper, the background counting rate was reduced to a great extent. The definitive background counting rate is 0.1 cpm/keV in the energy region of 4–5 keV for 940 cm 2 of sample area (2π geometry).