Measurement of low-level tritium by gas-counting technique

Measurement of natural tritium has been one of the most difficult works because its concentration in nature is extremely low. Though the advent of thermonuclear weapon test increased tritium content in the upper atmosphere, the measurement is still a difficult task because of its low β energy and low level. The authors are now aiming at developing the method by which the accurate and rapid measurement of such low level tritium is feasible. The main equipment used consists of combustion unit, sample converter, detector assembly and electronics. The detector assembly is essentially a proportional gas counter with anticoincidence guard detector. The aim of the present study is to use the radiocarbon counter for routine measurements with high accuracy. First, the plateau characteristics were measured by using different internal quenching gas, i.e. methane and propane. For the gas mixture of 10:1 H2:CH4, an excellent plateau of ca 300 volts was obtained. Methane was selected as the quenching gas because of its good reproducibility and stability in counting characteristics. Then tritium sample preparation and measurement on some environmental samples are described. For routine measurements, the time required to achieve precision from 3% to 20% was calculated for different tritium concentrations. Since the tritium level in most of environmental samples exceeds 10 tritium ratios, the system is capable to yield reliable results in relatively short time. (Wakatsuki, Y.)