Multiplicity Counter

Description

There is an increasing demand for measurement systems that can determine the mass of fissile materials in safeguards and nuclear material. Since the mass of the material is proportional to the spontaneous fission rate, the mass can be determined. Multiple neutrons are emitted within a short time (i.e. coincident) of each other during the fission process. The number of these neutrons emitted in coincidence determines the multiplicity of the event.

The limitation of classical coincidence counting has been that only two independent quantities are measured (total counts and real coincidences). Corrections must be applied to get accurate assays, except when the material is pure metal or pure oxides, with the low Z material well characterized. The problem is that the characterization of the samples measured is often not well known. Both spontaneous and induced fissions contribute to the rate of the coincident neutrons from the sample. The induced fissions are caused by neutrons from outside the nucleus. The possible causes for these neutrons to be given off are a previous spontaneous or induced fission and alpha-neutron reactions if the sample contains low Z element impurities. The induced fission neutrons are classified as multiplication neutrons. The 2150 Multiplicity Counter can be used to determine the spontaneous fission rate from the measured neutron multiplicity distributions.

Operation

The 2150 is a single width NIM used for multiplicity neutron counting. It is intended for use with passive thermal-neutron counters. The 2150 allows measurement of three quantities (total count rate, coincidence count rate and a higher order coincidence count rate).