771. αバックグラウンド除去によるトリチウムガスモニタの高感度化

抄録

A cancellation circuit was designed and constructed in order to eliminate signals, induced by α-particles from Rn in air, from output signals of a flow-through ion chamber used in a tritium gas monitor. This cancellation treatments can cause a high sensitivity in tritium gas monitoring.
The circuit consists of a differentiating circuit, discrimator, adding circuit, peak-hold circuit, sample-hold circuit and differential amplifier. Pulsed α-signals are separated from tritium β-signals by the differentiating circuit and discriminator. After the shaping of α-signals are formed in the peak-hold circuit with the same decay time constant as that of the initlal α-signals, the α-signals are subtracted from initial signals in the differential amplifier.
The test for the system showed that the cancellation cireuit eliminated successfully the effect of α-signals. The cancellation circuit functioned very well not only for isolated α-signals, but also for successive α-signals coming within their pulse widths. The long term stability of the system was obtained by combining the cancellation circuit with a temperature-controlled small-current amplifier. The system can offer the means to detect the tritium-gas concentration below the maximum permissible concentration for exhaust gas.