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At least 361 records · Page 20

Linear models to perform treaty verification tasks for enhanced information security

Linear mathematical models were applied to binary-discrimination tasks relevant to arms control verification measurements in which a host party wishes to convince a monitoring party that an item is or is not treaty accountable. These models process data in list-mode format and can compensate for the presence of variability in the source, such as uncertain object orientation and location. The Hotelling observer applies an optimal set of weights to binned detector data, yielding a test statistic that is thresholded to make a decision. The channelized Hotelling observer applies a channelizing matrix to the vectorized data, resulting in a lower dimensional vector available to the monitor to make decisions. We demonstrate how incorporating additional terms in this channelizing-matrix optimization offers benefits for treaty verification. We present two methods to increase shared information and trust between the host and monitor. The first method penalizes individual channel performance in order to maximize the information available to the monitor while maintaining optimal performance. Second, we present a method that penalizes predefined sensitive information while maintaining the capability to discriminate between binary choices. Data used in this study was generated using Monte Carlo simulations for fission neutrons, accomplished with the GEANT4 toolkit. Custom models for plutonium inspection objects were measured in simulation by a radiation imaging system. Model performance was evaluated and presented using the area under the receiver operating characteristic curve.

97 MATHEMATICS AND COMPUTING↗

Background studies for the MINER Coherent Neutrino Scattering reactor experiment

The proposed Mitchell Institute Neutrino Experiment at Reactor (MINER) experiment at the Nuclear Science Center at Texas A&M University will search for coherent elastic neutrino-nucleus scattering within close proximity (about 2 m) of a 1 MW TRIGA nuclear reactor core using low threshold, cryogenic germanium and silicon detectors. Given the Standard Model cross section of the scattering process and the proposed experimental proximity to the reactor, as many as 5–20 events/kg/day are expected. In this work, we discuss the status of preliminary measurements to characterize the main backgrounds for the proposed experiment. Both in situ measurements at the experimental site and simulations using the MCNP and GEANT4 codes are described. A strategy for monitoring backgrounds during data taking is briefly discussed.

47 OTHER INSTRUMENTATION↗

A low-background γγ-coincidence spectrometer for radioisotope studies

The performance of a new, low-background NaI(Tl) spectrometer, based on γγ-coincidence counting, is discussed. We present experimental coincidence efficiencies, timing resolutions, background count rates, and minimum detectable activities. The spectrometer has been simulated using Geant4, and the results are used for estimating coincidence efficiencies for volume sources. To test the device, we measured the cosmogenic 26 Al activity in a small (17.7 g) meteorite fragment. Here, we find a value of 52.9±7.8 dpm / kg, in agreement with the activity measured previously in a much larger fragment of the same meteorite using a HPGe detector.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Performance of a gamma-ray tracking array: Characterizing the AGATA array using a 60 Co source

The AGATA (Advanced GAmma Tracking Array) tracking detector is being designed to far surpass the performance of the previous generation, Compton-suppressed arrays. In this paper, a characterization of AGATA is provided based on data from the second GSI campaign. Emphasis is placed on the proper corrections required to extract the absolute photopeak efficiency and peak-to-total ratio. The performance after tracking is extracted and GEANT4 simulations are used both to understand the results and to scale the measurements up to predicted values for the full 4 implementation of the device.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Neutron spectroscopy for pulsed beams with frame overlap using a double time-of-flight technique

A new double time-of- ight (dTOF) neutron spectroscopy technique has been developed for pulsed broad spectrum sources with a duty cycle that results in frame overlap, where fast neutrons from a given pulse overtake slower neutrons from previous pulses. Using a tunable beam at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory, neutrons were produced via thick-target breakup of 16 MeV deuterons on a beryllium target in the cyclotron vault. The breakup spectral shape was deduced from a dTOF measurement using an array of EJ-309 organic liquid scintillators. Simulation of the neutron detection efficiency of the scintillator array was performed using both GEANT4 and MCNP6. The efficiency- corrected spectral shape was normalized using a foil activation technique to obtain the energy-dependent flux of the neutron beam at zero degrees with respect to the incoming deuteron beam. The dTOF neutron spectrum was compared to spectra obtained using HEPROW and GRAVEL pulse height spectrum unfolding techniques. While the unfolding and dTOF results exhibit some discrepancies in shape, the integrated flux values agree within two standard deviations. As a result, this method obviates neutron time-of-flight spectroscopy challenges posed by pulsed beams

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

The impact of incorporating shell-corrections to energy loss in silicon

Modern silicon tracking detectors based on hybrid or fully integrated CMOS technology are continuing to push to thinner sensors. The ionization energy loss fluctuations in very thin silicon sensors significantly deviates from the Landau distribution. Therefore, we have developed a charge deposition setup that implements the Bichsel straggling function, which accounts for shell-effects. This enhanced simulation is important for comparing with testbeam or collision data with thin sensors as demonstrated by reproducing more realistically the degraded position resolution compared with naïve ionization models based on simple Landau-like fluctuation. Our implementation of the Bichsel model agrees well with the multipurpose photo absorption ionization (PAI) model in Geant4 and is significantly faster. The code is made publicly available as part of the Allpix software package in order to facilitate predictions for new detector designs and comparisons with testbeam data.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

High-energy radiographic imaging performance of LYSO

Here, a comprehensive comparison of the dominant sources of radiation-induced blur for radiographic imaging system performance is made. End-point energies of 6, 10, 15, and 20 MeV bremsstrahlung photon radiation produced at the Los Alamos National Laboratory Microtron facility were used to examine the performance of large-panel cerium-doped lutetium yttrium silicon oxide (LYSO:Ce) scintillators 3, 5 and 10 mm thick. The system resolution was measured and compared between the various end-point energies and scintillator thicknesses. Contrary to expectations, it is found that there was only a minor dependence of system resolution on scintillator thickness or beam end-point energy. This indicates that increased scintillator thickness does not have a dramatic effect on system performance. The data are then compared to Geant4 simulations to assess contributions to the system performance through the examination of modulation transfer functions. It was determined that the low-frequency response of the system is dominated by the radiation-induced signal, while the higher-frequency response of the system is dominated by the optical imaging of the scintillation emission.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

An event-triggered coincidence algorithm for fast-neutron multiplicity assay corrected for cross-talk and photon breakthrough

Here, a model quantifying detector cross-talk and the misidentification of events in fast neutron coincidence distributions is described. This is demonstrated for two experimental arrangements comprising rings of 8 and 15 organic liquid scintillation detectors. Correction terms developed as part of this model are tested with 252 Cf and a relationship is developed between the 235 U enrichment of U 3 O 8 and the order of correlated, fast neutron multiplets induced by an americium–lithium source. The model is also supported by Geant4 simulations. The results suggest that a typical assay, for experimental arrangements that are similar to the examples investigated in this research, will exhibit cross-talk for less than 1% of all detected fast neutrons but, if not accounted for, this can bias the numerical analysis by a margin of 10% and 35% in second- and third-order coincidences (i.e. couplet and triplet counts), respectively. Further, for the case of 252 Cf, it is shown that a relatively low proportion of 4% breakthrough by rays (that is, photons misidentified as neutrons by the pulse-shape discrimination process) can lead to an erroneous increase of 20% in total neutron counts in the assay of a mixed-field, in this case of 252 Cf. These findings will help direct the developments needed to enable organic scintillation detectors with pulse shape discriminators to be applied reliably to nuclear safeguards and non-proliferation verification tasks.

42 ENGINEERING↗

Commissioning of the LaBr 3 (Ce) detector array at the National Superconducting Cyclotron Laboratory

The timing and energy resolution properties of LaBr 3 (Ce) scintillators are well suited for use in γγ fast-timing experiments. Here, using standard 60 Co, 88 Y, and 152 Eu sources, the 16-element array of 1.5 inch by 1.5 inch right-cylindrical LaBr 3 (Ce) detectors and the associated readout electronics at the National Superconducting Cyclotron Laboratory were commissioned by measuring the lifetimes of excited states and the angular correlations of γ rays emitted in cascade. Excited-state lifetimes in 152 Sm and 152 Gd were measured using the exponential slope of the decay and a method based on the centroids of time-difference spectra. Results for the lifetimes are consistent with the well-known literature values which cover a range from a few picoseconds to several nanoseconds. Finally, the γγ angular correlations for the 4 + → 2 + → 0 + cascade in 60 Ni and the 3 - → 2 + → 0 + cascade in 88 Sr were measured and shown to agree with GEANT4 simulations that take into account the expected angular correlations within the cascades as well as the geometry of the array.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Development of a neutron spectrometer utilizing rubberized Eu:LiCAF wafers

A portable ten-layer neutron spectrometer has been developed using a novel material, LiCAF. The spectrometer consists of rubber matrix wafers with embedded europium doped lithium calcium aluminum uoride (Eu:LiCAF) crystals and crosshatched wave shifting fibers (WSF). Layers of high density polyethylene (HDPE) separating each Eu:LiCAF wafer serve as neutron moderating material. Neutrons entering the spec- trometer react in the Eu:LiCAF crystal, creating scintillation photons. The photons travel down the WSFs to silicon photomultipliers (SiPMs) that convert the photons to an electrical signal. Custom electronics are used to gather, amplify, process, and readout the signal. A library of several neutron response curves was created in Geant4 and MCNP. The experimental output of the spectrometer was unfolded using a maximum entropy algorithm, MAXED. The spectrometer was tested using two separate DD generators. These two tests resulted in unfolded neutron spectra with average neutron energies of 2.71 and 2.78 MeV and chi-squared values of 0.88 and 1.51 χ 2 /D.O.F. respectively, when fit to a 2.45 MeV monoenergetic energy spectrum.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Background discrimination for neutrinoless double beta decay in liquid xenon using Cherenkov light

Neutrinoless double beta decays in liquid xenon produce a significant amount of Cherenkov light, with a photon number and angular distribution that distinguishes these events from common backgrounds. A GEANT4 simulation was used to simulate Cherenkov photon production and measurement in a liquid xenon detector and a multilayer perceptron was used to analyze the resulting distributions to classify events based on their Cherenkov photons. Our results show here that a modest improvement in the sensitivity of neutrinoless double beta decay searches is possible using this technique, but the kinematics of the neutrinoless double beta decay and electron scattering in liquid xenon substantially limit this approach.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Measurements of the muon flux produced by 10.6 GeV electrons in a beam dump

This paper proposes the results of an experiment to assess the muon flux produced by the interaction of a 10.6 GeV electron beam with the Hall-A beam dump at Jefferson Lab (JLab). The goal was to benchmark Monte Carlo simulations that are an essential tool for estimating beam-related backgrounds in beam-dump experiments aimed at searching for rare events, such as the Beam Dump eXperiment (BDX) planned at JLab. Beam-produced muons were measured with a CsI(Tl) crystal sandwiched between a set of segmented plastic scintillators placed at two different distances from the dump: 25.7 m and 28.8 m. At each location the muon flux was sampled at different vertical positions with respect to the beam height. Data have been compared with detailed Monte Carlo simulations using FLUKA for the muon production in the dump and propagation to the detector, and GEANT4 to simulate the detector response. The good agreement between data and simulations, within the uncertainties of the soil composition and density, exhibit the validity of our simulation tools to predict the beam-related muon background in electron beam-dump experiments at ~10 GeV.

47 OTHER INSTRUMENTATION↗

Refining instrument response functions with 3-D Monte Carlo simulations of differential hard x-ray spectrometers

Three x-ray spectrometers – the hard x-ray detector (HXRD), the bremsstrahlung MeV x-ray spectrometer (BMXS), and the hard x-ray image plate (HXIP) – areused here to estimate hot-electron bremsstrahlung spectrum parameters for inertial confinement fusion experiments on OMEGA. Using 1-D deterministically calculated response functions, they show significantly different estimates of spectrum slope temperature. For all three detectors, 3-D Geant4 Monte Carlo simulations are developed and used to compute instrument response functions. Simulations explain features in raw image-plate data, but only significantly improve systematic errors in predicted detector channel signals for BMXS and do not entirely eliminate the discrepancy between estimates of slope temperature.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

A new Transition Radiation detector based on GEM technology

Transition Radiation Detectors (TRD) have the potential feature of separating particles by their gamma factor. Classical TRDs are based on Multi-Wire Proportional Chambers (MWPC) or straw tubes, using a Xenon based gas mixture to efficiently absorb transition radiation photons. These detectors operate well in experiments with relatively low particle multiplicity. The performance of MWPC-TRD in experiments with luminosity of order 10 34 cm 2 s –1 and above, is significantly deteriorated due to the high particle multiplicity and channel occupancy. Replacing MWPC or straw tubes with a high granularity Micro Pattern Gas Detectors (MPGD) like Gas Electron Multipliers (GEMs), could improve the performance of the TRD. In addition, GEM technology allows one to combine a tracker with TRD identification (GEM-TRD/T). This report presents a new TRD development based on GEM technology for the future Electron Ion Collider (EIC). The first beam test was performed at Jefferson Lab (Hall-D) using 3–6 GeV electrons. A GEM-TRD/T module has been exposed to electrons with and without a fiber radiator. First results of test beam measurements and comparison with Geant4 Monte Carlo are introduced here.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Design and performance of a silicon–tungsten calorimeter prototype module and the associated readout

Here, we describe the details of a silicon–tungsten prototype electromagnetic calorimeter module and associated readout electronics. Detector performance for this prototype has been measured in test beam experiments at the CERN PS and SPS accelerator facilities in 2015/16. The results are compared to those in Monte Carlo Geant4 simulations. This is the first real-world demonstration of the performance of a custom ASIC designed for fast, lower-power, high-granularity applications.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Development and validation of a gamma-ray detection response model for neutron-diagnosed subcritical experiments

In a neutron-diagnosed subcritical experiment, a highly sensitive and fast detector is required to accurately determine the true fission γ-ray emission rate from an object composed of special nuclear material under interrogation, in addition to a proper detector response model for use in forward modeling. We present the development of a detector response model that is based in GEANT4 and uses post-processing techniques to include photomultiplier tube time and pulse height response effects. We also define a set of detector performance evaluation metrics that emphasized our experimental objectives. A suite of time and pulse height response measurements for several diverse detector assemblies are described. These assemblies were simulated with this response model and benchmarked against the laboratory measurements. This response model reproduces the pulse height curve shapes and performance hierarchy of the measured detectors well. Additionally, the simulated time response curves exhibit reasonable agreement with counterpart measurements. These measurements and simulation results together suggest that a large volume EJ-299-49 plastic scintillator that is mated to an ultraviolet transmitting plastic light guide and relies on total internal reflection for light transport to the photocathode will provide significantly faster timing, increase pulse height by a factor of 3, and require 25% less γ rays to meet experimental needs, compared to the candidate detector design developed prior to the availability of this response model. Application of this response model and associated performance metrics in the context of large-scale design sensitivity studies is also discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗