Gas proportional counter with a large volume for measurement of small tritium concentrations

The design of a multisection counter with a large sensitive volume (2l), which ensures operation at a voltage of 2000-4000 V, is described. The cylindrical volume of the counter is divided by earthed filaments into 55 cells; each cell has 0.74 l volume. To reduce the background, each internal section of the counter is switched into an anticoincidence circuit with respect to the others and, besides, all the external sections make up an additional ring of anticoincidence counters. For a methane-filled counter surrounded by 5 cm Pb layer from the sides and from below, the background in the energy range of 1.5-20 keV for a voltage of 2500 V is 0.34+-0.02 imp/s for a methane pressure of 0.5 kgf/cm and O.46+-0.01 imp/s on increase in pressure to 1 kgf/cm2. The counter design suggested makes it possible to detect a specific activity of tritium of the order of 1x10-13 Ci/l and higher; an activity of 1x10-13 Ci/l can be measured in 200 min with an error of 10%