Dead-Time Measurement for Radiation Counters by Variance-to-Mean Method.

The variance-to-mean ratios of counts accumulated during a time-gate were measured as a function of gate width for several Geiger-Mueller(GM) and neutron proportional counters. The measured ratios asymptotically decreased with an increase in gate width. The ratio for a GM counter was saturated in about 400 mu s of gate width, while that for a neutron counter was done in about 40 mu s. Applying the first- and second-order formulae derived from the Muller's expression for the variance-to-mean ratio, the dead times of these counters were inferred from these saturated ratios. In the count-rate range above 1,000 cps for the GM counter and above 12,000 cps for the neutron counter, the dead times obtained by the first-order formula apparently depended on count rate. On the contrary, the application of the second-order formula sufficiently reduced the count-rate dependence. The dead times obtained by the second-order formula were consistent with those by the previous two-source or reactor-power variational methods.