A spurious-count effect observed in Rossi-alpha and Feynman-alpha analysis using fission counter for Kyoto University Reactor

For a subcritical state of Kyoto University Reactor (KUR), Rossi-alpha and Feynman-alpha analyses were carried out using a water-surrounded fission counter assigned as reactor startup channel, where the Rossi-alpha analysis had a long time-interval range from 0.5 mu s to 2 ms and the Feynman-alpha analysis had a wide gate-width range from 1 mu s to 5 ms. The result in a shorter time range than several microseconds shows clearly a rise in a positive correlation amplitude resulting from spurious count, which is a well-known characteristic of fission counter. A time constant of the generating process of the spurious count and a probability of occurrence of a spurious count subsequently to a neutron count can be determined from these analyses. The time constant is around 1 mu s, and the occurrence probability is around 0.1%, which is one digit smaller than that evaluated from a previous Feynman-alpha analysis using a graphite-surrounded fission counter. The considerable difference between the present and the previous occurrence probability indicates that the probability must strongly depend on surroundings of fission counter.