Fast-neutron Multiplicity Counter Research Articles

Improving FNMC for the matrix effect of spherical shell plutonium samples

Abstract The fissile mass deduced from fast neutron multiplicity counting (FNMC) measurement is underestimated if the matrix self-absorption effect of the radioactive source is not taken into account. Based on the analysis of FNMC equations, a set of FNMC system was built to simulate and study the mass attribute of the hollow sphere (spherical shell) plutonium under different shapes and different masses conditions. Geant4 simulation shows that an appropriate parameter correction successfully removes the bias because of the matrix effect. Consequently, the self-multiplication factor, α coefficient and scattering crosstalk of the simulation result were corrected after analyzing the detection efficiency and multiplicity counting rate, and the corresponding polynomial fitting equation was obtained. The corrected mass deviation of samples was less than ±1% in this interval. The results show that the combination of the FNMC and parameter correction can accurately measure the sample mass attribute, which provides a new method for solving similar problems.

Application research on neutron-gamma discrimination based on BC501A liquid scintillator

Abstract Liquid organic scintillators are widely used in non-destructive analysis, which plays an important role in nuclear disarmament verification. This paper focused on studying the neutron-gamma discrimination technology in the fast neutron multiplicity measuring counter based on BC501A liquid scintillation detector. First, the charge comparison method, the zero-crossing time method and the rise time method were compared via the Geant4 and Matlab algorithm, and the result shows that charge comparison has the highest Figure of Merit. Then, a neutron-gamma discrimination system based on the six-probe fast neutron multiplicity counter was built and tested with a conclusion that the mean value of Figure of merit is 1.08, which verify the satisfactory neutron-gamma discriminating capability of the system. Finally, for the uranium samples, the mass are detected by fast neutron multiplicity counter, and the enrichment are measured by the characteristic gamma-ray signals using the system. The experimental results are in good agreement with the actual data.

Complexity measures and occurrence of the “breakpoint” in the neutron and gamma-rays time series measured with organic scintillators

This paper deals with the first analysis of the neutron and gamma time series measured with organic scintillators from plutonium samples by using information measures. Fast neutron detection with organic scintillators has been widely used for various nuclear safeguards applications and homeland security. One of the significant attributes of special nuclear materials (SNM) is the high multiplicity events in a short period of time. The time distributions of neutron and gamma-rays events for the plutonium metal plates designed as fuel plates for the Idaho National Laboratory (INL) Zero Power were measured with the Fast Neutron Multiplicity Counter (FNMC) consisting of 8 EJ-309 liquid scintillators and 8 stilbene detectors. Since the neutron correlated counts within the coincidence window of 40 ns are related to 240Pu effective mass of plutonium metal plates, it is of interest to investigate the randomness of the measured neutron and gamma-rays events. To access such information, we resort to complexity measures in the hope of being able to connect complexity values with the reliability of detection. That was done through (i) application of Kolmogorov complexity (KC) and its derivatives [Kolmogorov spectrum and its highest value (KCM) and running complexity (RKC)] and (ii) establishing the “breaking point” after which there exists a sharp drop in the running Kolmogorov complexity of the neutron and gamma-rays time series. It was found that the complexity of all the time series detected from the sample with 5, 9, 11, 13, and 15 plutonium plates had the high almost identical values of KC while the sample with 3 plates had by one-third smaller KC values than all the others. These calculations were supplemented by the Lypaunov exponents for a time series and the National Institute of Standards and Technology (NIST)tests. The low KC values can be addressed to the different sources of uncertainties in measuring procedure with the sample consisting of three plates. The complexity measures applied in this study are capable of revealing aspects of information that would otherwise remain hidden to the one-off complexity estimate.

Design of a simulation platform for fast neutron multiplicity counting measurements

Fast neutron multiplicity counting analysis is a new nondestructive analysis method for nuclear materials. On the basis of the basic theory of fast neutron multiplicity counting technology, an integrated neutron multiplicity simulation platform is designed and set up with the optimized combination of multiple computational tools. The platform consists of four modules: geometric modeling, particle transport, data processing and analysis, and cloud data management modules. It can implement multiple functions, such as automatic modeling, neutron transport, data visual analysis and simulation, and management of cloud platform data. Finally, simulation calculations and an experimental study of fast neutron multiplicity based on a liquid scintillator are performed with this platform. The results show that the deviation between the simulation results and the experimental results is less than 10%, and thus the simulation platform can be used for mutual inspection of mass.

Improvement of plutonium sample property measurement based on fast neutron multiplicity counting

Fast neutron multiplicity counting have (FNMC) is a new method of non-destructive analysis for nuclear materials. The new measurement equations of the fourth-order FNMC was established for plutonium samples. Based on the analysis of equations, a set of FNMC with three layers and six liquid scintillations in each layer is set up considering crosstalk, and the single crosstalk rate κ is determined to be 5.68‰. In addition, the quality of Pu and the measurement deviation correction under the different shell materials conditions were analyzed respectively. Finally, the third-order polynomial fitting equations under the condition of multiplication coefficient and (α, n) reaction rate are obtained by modifying the solution quality of Pu. Within the mass range of 500 g Pu, the corrected deviation was less than ±6%. The results show that the combination of FNMC and the coefficient correction method can achieve a more accurate measurement of Pu properties.

Analysis and improvement of the influence of sample shape on Pu measurement results

ABSTRACT The analysis method of Fast Neutron Multiplicity Counting (FNMC) plays an increasingly important role in the measurement of nuclear material properties. Based on the assumption of point model, fast neutron multiplicity measurement equation is derived which can be used to measure the mass of Pu sample. However, the deviation of the simulated measurement of 1 kg Pu sample reaches 16.6% and increases with mass. Because nonpoint source samples of different shapes do not fully satify the hypothesis. To correct this deviation, a set of fast neutron multiplicity counters was built by Geant4 to simulate and study the mass attribute of Pu samples.The cylindrical sources of different shapes and different masses were simulated, the self-multiplication factor and α coefficient were corrected.And the corresponding third-order polynomial fitting equation was obtained, the goodness of fit was greater than 0.970. In the same way, the spherical and spherical shell source samples in the mass range of 0–5 kg were analyzed, the corrected mass deviation of samples was less than 10% in this interval. The results show that the combination of the fast neutron multiplicity counter and parameter correction can accurately measure the sample mass attribute.

A 235U mass measurement method for UO2 rod assembly based on the n/γ joint measurement system

Fast-Neutron Multiplicity Counter based on Liquid Scintillator Detector can directly measure the fast neutron multiplicity emitted by UO 2 rod. HPGe gamma spectrometer; which has superior energy resolution; is routinely used for the gamma energy spectrum measurement. Combing Fast-Neutron Multiplicity Counter and HPGe γ-spectrometer, the n/γ joint measurement system is developed. The fast neutron multiplicity and gamma energy spectrum of UO 2 rod assemblies under different conditions are measured by the n/γ joint measurement system. The induced fission rate and the 235 U abundance, thereby the 235 U mass; can be obtained for UO 2 rod assemblies. The 235 U mass deviation of the measured value from the reference value is less than 5%. The results show that the n/γ joint measurement system is effective and applicable in the measurement of the 235 U mass in samples.

High order fast neutron multiplicity measurement equations based on liquid scintillation detector

Fast neutron multiplicity counting (FNMC) analysis method is a new nondestructive analysis method for nuclear materials. It uses a fast neutron detector array to detect the neutrons emitted by the sample. The quality of plutonium in the sample is obtained by recording the neutron coincidence counting of fast neutron multiplicity. At present, the first three multiplets widely studied including singlets, doublets and triplets can no longer meet the needs of the research. In this paper, The derivation process of fast neutron multiplicity measurement equation for liquid scintillation detector is studied in detail based on the basic principle, neutron counting method, by using the methods of the factorial moments, probability generating functions and parameter estimation method, and considering the influence of scattering crosstalk. Finally, the fast neutron multiplicity measurement equation including singlets, doublets, triplets, quadruplets and pentuplets are established according to the parameter estimation method and the two kinds of solution methods of this equation are given. The work in this paper lays a theoretical and analytical foundation for the research of fast neutron multiplicity measurement system based on liquid scintillation detector.

Fast-neutron multiplicity counter for active measurements of uranium oxide certified material

We developed a fast-neutron multiplicity counter (FNMC) based on stilbene and EJ-309 organic scintillators. The system can detect and discriminate correlated photon and neutron multiplets emitted by fission reactions. We used the system to estimate the fissile mass of uranium oxide samples in active interrogation mode at the Zero Power Physics Reactor of Idaho National Laboratory (INL). Two sets of certified reference material (CRM) samples were characterized. The U-235 enrichment of the first set is constant at 93.2 wt%, and the UO2 mass ranges between 0.5 and 4 kg. The second set includes samples of increasing enrichment (from 20 wt% to 97 wt%) and constant UO2 mass of 230 g. We used two AmLi sources to induce fission reactions in the samples. Despite the intense gamma-ray background of the UO2 and interrogating sources, the system could measure induced fission neutrons emerging from the interrogated samples without additional shielding surrounding the sample and only relying on pulse shape discrimination to classify neutron and gamma-ray pulses. The overall neutron count rate and time-correlated counts are well correlated with the sample fissile mass. We also proved that CRM samples can be used to build a calibration curve to assay the U-235 mass of unknown samples of different mass, geometry and enrichment, with an average bias error of 8%, for U-235 mass higher than 390 g.

Simulation study on neutron multiplicity of plutonium based on liquid scintillation detector

Fast neutron multiplicity counting (FNMC) is a new method of non-destructive analysis for nuclear materials. Using fast neutron detector array to detect the neutrons emitted from a sample, it obtains measurements according to the multiple counting method and analyzes the mass of plutonium in the sample. In this paper, A FNMC measuring device model has been constructed for simulation study in reference to the most advanced FNMC experimental device in the world. A series of typical plutonium sample models, including samples of plutonium metal and samples of plutonium oxide with different isotopic abundance and in different mass, have been designed. The correctness of the newly established fast neutron multiplicity measuring equation and the necessity of modifying the classical measuring equation have been verified by the computer simulated measurements. Compared with the calculation result from the equation developed in this paper and the calculation result from the classical NMC model, the simulated result shows that the new method is better than the classic method in result deviation.

Passive assay of plutonium metal plates using a fast-neutron multiplicity counter

We developed a fast-neutron multiplicity counter based on organic scintillators (EJ-309 liquid and stilbene). The system detects correlated photon and neutron multiplets emitted by fission reactions, within a gate time of tens of nanoseconds. The system was used at Idaho National Laboratory to assay a variety of plutonium metal plates. A coincidence counting strategy was used to quantify the 240Pu effective mass of the samples. Coincident neutrons, detected within a 40-ns coincidence window, show a monotonic trend, increasing with the 240Pu-effective mass (in this work, we tested the 0.005–0.5kg range). After calibration, the system estimated the 240Pu effective mass of an unknown sample (240Pueff >50g) with an uncertainty lower than 1% in a 4-min assay time.

Fast-neutron multiplicity analysis based on liquid scintillation

In this study, according to the establishment of the classical neutron multiplicity measurement equation, a fast-neutron multiplicity analysis and measurement equation is established, considering the influence of neutron scattering cross-talk, by means of theoretical analysis and computer simulation. Moreover, the fission rate F, multiplication M, and (α, n) reaction rate α in the established equation were solved. A new measurement method of scattering cross-talk was established and the established equation was validated using Geant4 simulation. The fast-neutron multiplicity counting equation has only a smaller deviation from the fast-neutron multiplicity counting system based on liquid scintillation detector, and it has a wider application prospect.

A neutron multiplicity analysis method for uranium samples with liquid scintillators

A new neutron multiplicity analysis method for uranium samples with liquid scintillators is introduced. An active well-type fast neutron multiplicity counter has been built, which consists of four BC501A liquid scintillators, a n/γdiscrimination module MPD-4, a multi-stop time to digital convertor MCS6A, and two Am–Li sources. A mathematical model is built to symbolize the detection processes of fission neutrons. Based on this model, equations in the form of R=F*P*Q*T could be achieved, where F indicates the induced fission rate by interrogation sources, P indicates the transfer matrix determined by multiplication process, Q indicates the transfer matrix determined by detection efficiency, T indicates the transfer matrix determined by signal recording process and crosstalk in the counter. Unknown parameters about the item are determined by the solutions of the equations. A 252Cf source and some low enriched uranium items have been measured. The feasibility of the method is proven by its application to the data analysis of the experiments.

Plutonium measurements with a fast-neutron multiplicity counter for nuclear safeguards applications

Measurements were performed at the Joint Research Centre in Ispra, Italy to field test a fast-neutron multiplicity counter developed at the University of Michigan. The measurements allowed the assessment of the system׳s photon discrimination abilities, efficiency when measuring neutron multiplicity, ability to characterize 240Pueff mass, and performance relative to a currently deployed neutron coincidence counter. This work is motivated by the need to replace and improve upon 3He neutron detection systems for nuclear safeguards applications.