Multiplicity Counting for Thermal Neutrons from Fission Chamber Currents with Multi-point Time Distributions

A new method of multiplicity counting has been recently suggested by the present authors. The method uses the fission detector signals in the current mode to extract the singles, doubles and triples 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. The new method has certain advantages, primarily that it is free from the dead-time problem. In earlier work we showed that if all neutrons originating in one source even and hence leave the item simultaneously, are also detected simultaneously, the S, D and T rates of the traditional multiplicity counting can be extracted from the detector current fluctuations. However, it was also seen that if the detections are not simultaneous, which is always the case in practice, especially if the emitted fast neutrons are first slowed down to thermal energies, 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, so that the pulses belonging to jointly born neutrons do not overlap, 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 then 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. The results suggest that the proposed method has significant potentials as a complement to traditional multiplicity counting.