MULTIPLICITY COUNTING FOR THERMAL NEUTRONS FROM FISSION DETECTOR CURRENTS WITH MULTI-POINT TIME DISTRIBUTIONS

摘要

A new method of multiplicity counting has been recently suggested by the present authors [1,2]. The method uses the fission detector signals in the current mode to extract the singles, doubles and triples (S, D and T) rates. Instead of the statistics of discrete pulses, the new method is based on the first three central moments of the time-resolved signals of one or more (up to three) fission chambers. This has certain advantages, primarily that it is free from the dead-time problem. In earlier work we showed that if all neutrons originating from one source event are detected simultaneously, the S, D and T rates of the traditional multiplicity counting can be extracted from the detector current fluctuations. However, if the detections are not simultaneous, which is always the case in practice, the performance of the method deteriorates. If the spread of the time difference between the detection of neutrons of common origin is larger than the width of the detector pulse, the coefficients of the D and T rates become negligibly small, hence only the singles rates can be extracted from the measurements. The present work eliminates this shortcoming, by considering temporally two- and three-point distributions (covariance and bi-covariance functions) for the second and third central moment of the detector current, and integrating the covariance functions with respect to the time delay parameters. It will be seen that the density function of the time delay of the detections disappears from the equations, and the coefficients of all multiplicity rates are of the same order. This means that the S, D and T rates can be extracted from the covariance and bi-covariance of the currents of one to three detectors even for thermalized neutrons. Hence the proposed method has good potentials as a complement to traditional multiplicity counting.