Experimental investigation on secondary-count effect in Feynman- measurement by fission counter

The Feynman-a (variance-to-mean ratio) measurements were carried out by using fission counters. which contain fissile material of around 2 g. to investigate experimentally the secondary-count effect caused by fission events in the counter. The secondary-count effect was observed when the fission counter was covered with graphite, whereas it was not observed when the bare counter was used and when the counter was surrounded by light-water. Therefore, it was concluded that the secondary-count effect depends not only on fission events in the counter but also on the surrounding material of the counter. A formula to correct the secondary-count effect was derived, and it was applied to obtain the inherent decay constant a. The validity of this formula was demonstrated through the comparison with the results obtained by using 3He and BF 3 counters which are free from the secondary-count effect