539.125.053 Nondestructive methods for detection of plutonium in nuclear materials by analysis of the time correlations of spontaneous-fission neutrons, now called neutron-coincidence methods, are widely employed by IAEA for performing inspections [1, 2]. These methods are evolving in the directions of development and improvement of measuring apparatus, the production and certification of standard samples and calibration procedures, as well as the construction of models for describing the response of apparatus as a function of influencing factors [3] -multiplication effect in the characteristic neutron field of the composite materials being analyzed, fission induced by neutrons from (tx, n) reactions, neutron-energy dependence of the detection efficiency, the dead time of the measuring apparatus in the sum and coincidence channels, the radiation background, and others.In the last few years significant progress has been made in the development of measuring apparatus. The neutroncoincidence apparatus developed at the LOs Alamos National Laboratory (USA) [4][5][6][7][8] has high neutron-detection efficiency (up to 30%), a short dead time (-0.15 t~sec), high operating stability, and scale linearity. Neutron correlations are analyzed using the most widely used logic in the world -shift register logic, which remains stable with total loading of up to 1.5.106 see-1 in the sum channel.Sets of standard samples and the corresponding calibration procedures, based on different principles, are being developed for the metrological support for the measurements. In constructing these samples and calibration procedures every effort is made to reduce to a minimum, when possible, the required number of standard samples and to unify the calibration procedures [8][9][10].The present paper is concerned with the development of a model which gives a better description of the response of neutron-coincidence apparatus using Monte-Carlo estimates of the multiplicity of leakage neutrons. The workability of the model was checked on a collection of standard samples which are part of the VI~T 10-5-88 working unit standards of neutron-flux units and VI~T 6-16-88 standards of spontaneous-fission activity of radionuclides [11][12][13]. The measurements were performed on an AWCC apparatus with a shift-register-type coincidence selector during the IAEA School for Inspectors during the fall of 1989 at the Scientific-Research Institute of Atomic Reactors.
Construction of a Model for Describing the Response of Neutron-CoincidenceApparatus Taking into Account the Actual Multiplicity of Leakage Neutrons. In the absence of multiplication in the intrinsic neutron field of the composite material being analyzed, as is observed, for example, in californium sources with low neutron flux, the neutron counting rate n~ in the sum channel is n E = e~sAs(1 + c~), where e is the neutron detection efficiency (it is assumed that fission-spectrum neutrons are detected with approximately the same efficiency as the sum-spectrum neutrons from (c~, n) reactions); ~s i...
