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539.125.42 Neutron-multiplicity counters are an important tool for the precise determination of plutonium in a wide range of plutonium content, isotopic and chemical composition, and density and mechanical structure of the compositions analyzed. The main negative aspect, which is holding back the use of such counters for international guarantees, is that neutron detectors are large, they have a large mass, and they are expensive.A portable neutron-multiplicity counter, whose characteristics and cost would be acceptable for purposes of guarantees for IAEA, is being developed on the basis of a contract with IAEA [1].High-density ultrapure polyethylene was chosen as the neutron moderator. The measuring well is 170 mm in diameter (just as in IILNC-2), taking into account the 1.5-mm-thick cadmium sleeve. The moderated neutrons are recorded using 72 SNM-33a counters with a 32 mm in diameter housing and a 550-mm-long active part. The counters are filled with 3He under a pressure of 4.105 Pa with an addition of 0.7% argon. The SNM-33a counters are designed for efficient operation in the proportional mode. The use of domestic counters made it possible to reduce substantially (as compared to ~he foreign analogs) the total cost of the setup without degrading the quality of the operation.The geometry of the detection block was optimized using a version of the BRAND-PC program complex [2], intended for Monte Carlo modeling of the response of the neutron and photon detectors in implementing nondestructive methods of analysis. The placement of 72 SNM-33 counters in ultrapure high-density polyethylene in the form of a threering assembly with 16, 24, and 32 counters in the first, second, and third rings, respectively, concentrically around a central well, was modeled. The position of the counters was varied in order to obtain a short neutron lifetime with high and quasiconstant detection efficiency over the volume of the well and a neutron energy range from 1.4 to 2.8 MeV. The calculations showed that the thickness of the ring layer, in which neutrons are recorded efficiently with a three-ring assembly of SNM-33a counters, is 130-150 mm, which is the same as the thickness of the layer covered by the four-ring assembly of counters used in [3]. The contribution of the fourth ring of SNM-33a counters would only be 5-7% of the total efficiency of the neutron detector. For this reason there was no point in using it.Graphite was used for the covers covering the measuring well at the top and bottom. The maximum thickness of the graphite layer was 150 ram.The computed efficiency of the neutron detector is 44 + 4% with a neutron lifetime of 57 + 8/~sec. The outer shielding of the detection block was made of borated polyethylene, screened on both sides by cadmium with a thickness of 1.5 mm. The assembled detection unit is 650 mm wide and 950 mm high, taking into account the dimensions of the transport wagon, and its mass does not exceed 240 kg. Charge-sensitive A-Ill preamplifiers-discriminators, manufactured by the AMPTEK Company (USA...

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...

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