Pulsed, Photonuclear-induced, Neutron Measurements of Nuclear Materials with Composite Shielding

Active measurements were performed using a 10-MeV electron accelerator with inspection objects containing various nuclear and nonnuclear materials available at the Idaho National Laboratory’s Zero Power Physics Reactor (ZPPR) facility. The inspection objects were assembled from ZPPR reactor plate materials to evaluate the measurement technologies for the characterization of plutonium, depleted uranium or highly enriched uranium shielded by both nuclear and non-nuclear materials. A series of pulsed photonuclear, time-correlated measurements were performed with unshielded calibration materials and then compared with the more complex composite shield configurations. The measurements used multiple 3He detectors that are designed to detect fission neutrons between pulses of an electron linear accelerator. The accelerator produced 10-MeV bremsstrahlung X-rays at a repetition rate of 125 Hz (8 ms between pulses) with a 4-us pulse width. All inspected objects were positioned on beam centerline and 100 cm from the X-ray source. The time-correlated data was collected in parallel using both a Los Alamos National Laboratory-designed list-mode acquisition system and a commercial multichannel scaler analyzer. A combination of different measurement configurations and data analysis methods enabled the identification of each object. This paper describes the experimental configuration, the ZPPR inspection objects used, and the various measurement and analysis results for each inspected object

An absolute measurement of the 7^7Li(n,n’t)4^4He reaction cross-section between 5.0 and 14.0 MeV by tritium assaying

In view of the world’s energy situation, it is necessary to investigate all possible means of supply. The principles of controlled thermonuclear fusion are described, together with the role of lithium and the need for nuclear data. Information on $^7$Li and the $^7$Li(n,n’t)$^4$He reaction is given. The conclusion is that the state of the data is inadequate for the CTR development programme. In the experiment, the tritium is produced in samples- of lithium hydroxide, which are then dissolved, leaving the tritium as tritiated water. This is then measured by liquid scintillation counting in the normal way. The counting efficiency is measured as a function of quenching, with standard tritiated water. As a check on this method the $^6$Li(n,$\alpha$ t) cross-section was measured in a reactor irradiation. The result (including a self-shielding correction) was 900$\pm$52 barns, in agreement with the accepted value of 940$\pm$5 barns. The neutron dose was measured with, an NE213 liquid scintillation detector, the efficiency of which was measured as a function of neutron energy from 1.5 to 25 MeV, using the associated particle technique. Irradiations were carried out between 4.7 and 14.1 MeV, using the Harwell Tandem Van de Graaff and Cock-croft Walton generator. The cross-section obtained for the $^7$Li(n,n’t)$^4$He was 28% lower than the ENDF/B-IV evaluation, with standard deviation of 5%. The cross-section of $^{27}$AI(n,$\alpha$)$^{24}$ Na was measured concurrently and good agreement was obtained with the accepted value. Other evidence is presented for this result; el astic cross section measurements from the Triangle Universities (USA), and tritium production measurements in an integral assembly at Karlsruhe (West Germany). The results from the present experiment may also improve the overall fitting of a recent R-matrix analysis to the $^7$Li nucleus. Finally, the conclusions are that, at least, a new evaluation is required, and also more measurements are required to resolve the discrepancies especially at 14 MeV

Results of the Esarda Multiplicity Benchmark Exercise

The ESARDA NDA working group has run a benchmark in order to compare the different algorithms and codes used in the simulation of neutron multiplicity counters. In order to derive the maximum of information and at the same time to allow a large participation, the working group decided to split the exercise in two parts with two participation levels: a full simulation exercise where participants were asked to compute the count rates starting from the basic technical specifications and/or a partial exercise involving the processing of the pulse trains produced by a single laboratory. The results of participants performing the entire exercise allows us to make a comparison among the different Monte Carlo codes for the simulation of neutron multiplicity counters. The results of the partial exercise help to test the available algorithms for pulse train analysis and to derive some important information about the models applied for dead-time correction.JRC.G.8-Nuclear safeguard

Improvements in Dead-time Correction Using List-mode Neutron Counters

Neutron multiplicity counting is a widely used technique in safeguards to determine the mass of fissile material. Unfortunately the multiplicity measurement is disturbed by the dead-time effect, which prevents pulses from being detected by the electronic chain on the same channel when they follow shortly in time after another pulse. This is particularly important since this loss due to dead-time especially affects neutrons originating from multiple-emission events, which in fact carry a lot of information about the sample and so should be measured. There exist a number of different dead-time correction methods. The most widely used (INCC) makes use of a semi-empirical correction for the single and double neutron emissions (Singles, Doubles) and another method (of Dytlewski) for Triples. They require the application of a pre-delay and of dedicated calibration measurements in order to determine the necessary parameters. Other correction methods have had little in-field use because of the complexity of their formulation. In this paper we present a new approach for correction, based rather on mathematical-combinatorial theory than on heuristic formulas suggested by the physical principles. In contrast to other existing methods this correction is applied to the multiplicity distributions of measured “Reals plus Accidentals” and “Accidentals”. Test results for this new method using simulations are presented as well as comparisons to other, well established methods for dead-time correction. This new technique requires the deployment of new, so-called multi-channel list mode counters. This new approach will result in a number of advantages over the heuristic one: since the Singles, Doubles, Triples, etc. are calculated later from the multiplicity distribution, this method will be suitable to correct also for higher number of simultaneous neutron emissions than Triples (Quadruples, Quintuples). Since this method is based on the collected data itself, no prior calibration will be necessary. Furthermore use of a pre-delay may not be necessary, leading to an increased gate-fraction and – in principle – to increased quality of the results. Although this method is still under development, it has already demonstrated good results for low and medium count-rates.JRC.E.8-Nuclear securit

Dead Time Correction for any Multiplicity using List Mode Neutron Multiplicity Counters: A new Approach - Low and medium Count-Rates

In the field of neutron multiplicity counting, modern list mode counters provide increased possibilities for neutron data analysis. Here a new method to correct dead time using a multi-channel list mode neutron counter is described. As it will become clear in this article, the data analysis can be done "on the fly" without further data storage. This will allow an instrument to be built having this method implemented, which will give directly dead time corrected results. In practice, a classical theory of dead time correction is applied to the final Totals, Reals (or Doubles) and possibly Triples, whereas for higher multiplicities no dead time correction has been implemented so far. In contrast to that, this approach directly corrects the multiplicity distributions of the Reals plus Accidentals (R+A) and Accidentals (A) obtained by multiplicity counting. Hence this dead time correction holds for any kind of multiplicity (Totals, Doubles, Triples, Quadruples, Quintuples, etc.) because the calculation of these values can then be derived directly from the corrected multiplicity distribution of R+A and A.JRC.DG.E.9-Nuclear security (Ispra

Results of phase III and IV of the ESARDA Multiplicity Benchmark

In 2003 the ESARDA NDA working group launched a benchmark exercise in order to compare the different algorithms and codes used in the simulation of neutron multiplicity counters. The results of the 1st and 2nd phase of the ESARDA Multiplicity Benchmark, based on synthetic cases, have been published in the ESARDA Bulletin number 34. Notwithstanding the satisfactory conclusion that all the algorithms developed by the different participants in the first two phases and used to analyse the pulse trains have proven to be satisfactory, the working group felt that an extension to real experimental cases would have added a supplementary value to the exercise that brought to the organisation of phases III and IV. This paper summarises the outcomes of the benchmark, whose full report will soon be made available on the ESARDA Bulletin.JRC.G.8-Nuclear securit

New Trends in Neutron Coincidence Counting: Digital Signal Processing

Neutron coincidence counting is the reference NDA technique used in nuclear safeguards to measure the fissile mass in nuclear material samples. Nowadays most of the neutron counting systems are based on the original shift register technology, like the (ordinary or Multiplicity) Shift Register Analyser. The analogue signal from the He-3 tubes is processed by amplifier/SCA producing a train of logical TTL pulses that are fed into a neutron analyser performing the time correlation analysis. In the future these systems could be replaced by high-speed PC’s equipped with pulse acquisition cards, providing a time stamp (LIST mode acquisition) for every digital pulse. The time stamp data can be processed directly during acquisition or saved on a hard disk. The latter method has the advantage that measurement parameters, like for instance the pre-delay and gate-width, can be modified without repeating the acquisition. The use of PC based instruments, also called virtual instruments, could be the future major development in practical neutron correlation analysis. Under a push from the main inspection authorities (IAEA, Euratom and French Ministry of Industry) several research laboratories have started to study and develop prototypes of neutron counting systems with PC-based processing. A collaboration in this field among JRC, IRSN and LANL has been established within the framework of the ESARDA-NDA working group. Joint testing campaigns have been performed in the JRC PERLA laboratory, using different equipment provided by the three partners. The paper will describe the rationale for changing to the new technology, give an overview of the hardware and software tools available today and a feedback of the experience gained in the first tests. Associated to the experimental tests, the ESARDA-NDA working group is also performing an inter-comparison benchmark exercise on the analysis software for pulse processing.JRC.G.8-Nuclear safeguard

Modeling and Simulation Optimization and Feasibility Studies for the Neutron Detection without Helium-3 Project

This report details the results of the modeling and simulation work accomplished for the ‘Neutron Detection without Helium-3’ project during the 2011 and 2012 fiscal years. The primary focus of the project is to investigate commercially available technologies that might be used in safeguards applications in the relatively near term. Other technologies that are being developed may be more applicable in the future, but are outside the scope of this study