A Proportional Counter for Space and High Altitude Dosimetry Applications

Recent studies investigating the energy deposited by ionizing radiation in microscopic absorption sites have led to a program at Battelle-Northwest to develop a portable tissue equivalent proportional counter for use in mixed radiation fields. In the previous investigations, a 10 cm diameter spherical counter was used. This counter is very sensitive to mechanical shock due largely to a spiral grid surrounding the anode. The grid is necessary to produce uniform gas multiplication regardless of the location of ionizing events within the cavity. The wall thickness is sufficient to assure charged particle equilibrium when using the counter for gamma-rays or neutrons. The present development program is based on a much simpler and more rugged proportional counter. This device is 5 cm in diamter and uses a single 0.001 inch diameter stainless steel wire as the anode without a spiral grid. Uniform gas multiplication is obtained by field shaping electrodes near the end of the wires. The small counter is more stable and much less susceptible to mechanical shock. To simulate spherical absorption sites on the order of 1¿ diameter, the counters must be operated at pressures of a few mm Hg of tissue-equivalent gas. This special gas is composed of nitrogen, methane, and carbon dioxide in percentages of 3.2%, 64.4% and 32.4%, respectively. With a gas pressure equivalent to a 2 micron absorption site, the count rate is 2,000 to 3,000 counts per millirad when exposed to 0.75 MeV monoenergetic neutrons.