Development of an Anti-Discharge Unit (ADU) for gaseous proportional counters at FRIB
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Di Carlo, S. (ORCID:0000000245703135)↗
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Investigating Gadolinium-Lined Sodium-Iodide Neutron Detectors for Mobile Applications
For enhancing the effectiveness of nonproliferation efforts in neutron detection, most portable instruments rely on 6 Li scintillators, 10 B-based detectors, or gas-filled 3 He proportional counters. Additionally, gamma-ray detectors based on scintillators and semiconductors are often employed for search applications to find radioactive material in the field. These systems typically include dedicated detectors along with separate high voltage supplies and processing electronics for the gamma-ray and neutron detectors. Ideally, a portable radiation detection system should be lightweight, compact, and cost-effective. In the field, scintillators can serve a dual purpose: (1) detecting gamma-rays and (2) detecting neutrons. Gamma-ray detection with scintillators is based on the interaction of gamma-rays within the scintillating material, whereas neutron detection depends indirectly on neutron capture events. These capture events generate conversion electrons and gamma-rays, which can interact with the scintillator. For enhancing neutron capture, the scintillator can be surrounded by neutron absorber materials with a high neutron cross section. The resulting secondary electrons and gamma-rays from neutron interactions, depending on the absorber material used, can then be analyzed to detect the presence of neutron sources. Similarly, semiconductor-based detectors can be employed along with neutron absorbers as liners for neutron detection. 158 Gd has a significantly larger neutron cross section than 3 He, commonly used in gas-filled proportional counters, as shown in Figure 1. For thermal (0.025 eV) neutrons, the absorption cross section of 158 Gd is 10,000 times greater than that of 3 He (refer to Figure 1). This feature makes naturally occurring gadolinium, which consists of 24.8% 158 Gd, a promising neutron absorber material for use in combination with gamma-ray detectors–yielding a hybrid detector–for neutron detection.
98 NUCLEAR DISARMAMENT, SAFEGUARDS, AND PHYSICAL P↗
A portable and monoenergetic 24 keV neutron source based on 124Sb-9Be photoneutrons and an iron filter
A portable monoenergetic 24 keV neutron source based on the 124Sb-9Be photoneutron reaction and an iron filter has been constructed and characterized. The coincidence of the neutron energy from SbBe and the low interaction cross-section with iron (mean free path up to 29 cm) makes pure iron specially suited to shield against gamma rays from 124Sb decays while letting through the neutrons. To increase the 124Sb activity and thus the neutron flux, a >1 GBq 124Sb source was produced by irradiating a natural Sb metal pellet with a high flux of thermal neutrons in a nuclear reactor. The design of the source shielding structure makes for easy transportation and deployment. A hydrogen gas proportional counter is used to characterize the neutrons emitted by the source and a NaI detector is used for gamma background characterization. At the exit opening of the neutron beam, the characterization determined the neutron flux in the energy range 20–25 keV to be 6.00±0.30 neutrons per cm2 per second and the total gamma flux to be 245±8 gammas per cm2 per second (numbers scaled to 1 GBq activity of the 124Sb source). A liquid scintillator detector is demonstrated to be sensitive to neutrons with incident kinetic energies from 8 to 17 keV, so it can be paired with the source as a backing detector for neutron scattering calibration experiments. This photoneutron source provides a good tool for in-situ low energy nuclear recoil calibration for dark matter experiments and coherent elastic neutrino-nucleus scattering experiments.
Dark Matter detectors (WIMPs, axions, etc.)↗
Integrating Metal-Hydride and Gas-Detector for Tritium Gas Detection
Detection of trace amounts of environmental tritium is a challenging problem, driving the need for field-deployable systems that offer high sensitivity, selectivity, and minimal false positives. We present a technique for high-sensitivity, high-selectivity tritium measurement, which integrates metal-hydride and gas-detector concepts into a compact field-deployable tritium sensor. A hydrogen-storage metal embedded in a gas proportional counter selectively absorbs protium (1H)/tritium (3H), which are subsequently released into the counter volume with a reduced radiation background. Ionizations induced by 3H beta particles are then measured in proportional counting mode, achieving high detection efficiency. Preliminary studies conducted with palladium (Pd) thin films coated on stainless-steel substrates demonstrated 3H absorption and metal-tritide formation, followed by 3H desorption upon heating the metal-tritide. These processes were confirmed using activity concentrations measured by a commercial tritium monitor and pulse height spectra acquired from a custom-built detector.
Gas-proportional counter↗
Reduction of detection limit and quantification uncertainty due to interferent by neural classification with abstention
Many measurements in the physical sciences can be cast as counting experiments, where the number of occurrences of a physical phenomenon informs the prevalence of the phenomenon's source. Often, detection of the physical phenomenon (termed signal) is difficult to distinguish from naturally occurring phenomena (termed background). In this case, the discrimination of signal events from background can be performed using classifiers, and they may range from simple, threshold-based classifiers to sophisticated neural networks. These classifiers are often trained and validated to obtain optimal accuracy, however we show that the optimal accuracy classifier does not generally coincide with a classifier that provides the lowest detection limit, nor the lowest quantification uncertainty. Here, we present a derivation of the detection limit and quantification uncertainty in the classifier-based counting experiment case. We also present a novel abstention mechanism to minimize the detection limit or quantification uncertainty a posteriori. We illustrate the method on two data sets from the physical sciences, discriminating Ar-37 and Ar-39 radioactive decay from non-radioactive events in a gas proportional counter, and discriminating neutrons from gammas in an inorganic scintillator and report results therefrom.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
High-rate heavy-ion tracking using MWPCs and PPACs with an Anti-Discharge Unit
This work reports on the development of two robust, heavy-ion beam tracking concepts operating at low pressure (< 15 Torr) for high rate applications (> 200 kHz). The first concept consists of a Multi-Wire Proportional Counter (MWPC) with a central anode consisting of 12 μm Au-plated Tungsten wires spaced 1 mm from each other. The anode grid is sandwiched between two segmented cathodes aligned orthogonally in the two dimensions for (x,y) particle localization. The second detector is a Parallel-Plate Avalanche Counter (PPAC) that uses the same readout geometry as the MWPC, but replaces the wire anode with a 150 nm silver layer deposited on both sides of a thin (< 1 mg/cm 2 ) polypropylene foil. Additionally, the bias circuitry for the PPAC anode central foil is equipped with an Anti-Discharge Unit (ADU) to prevent transitions from proportional operation to streamer formation, thereby avoiding damaging discharges. The localization capability of both detectors was tested with a low-rate alpha-particle source (241-Am). A position resolution of < 1 mm (FWHM) was achieved under stable, high-gas-gain (> 1000) operating conditions. Their performance in terms of detection efficiency as a function of the isotope charge (Z) was determined by irradiating the detectors with a cocktail beam (Z ≤ 15) with energy of ∼ 100 MeV/u. Full detection efficiency is maintained for all available fragments under optimal operational conditions (i.e., voltage bias). Full detection efficiency was achieved at rates above 200 kHz by irradiating the detectors with a 1 cm diameter 238 U beam at an energy of 143 MeV/u.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Assessment of Thin Plastic Scintillation Detectors for Beta-Particle Measurements at the Advanced Test Reactor Critical Facility
The Fission Wire Measurement System is a custom measurement system designed in the 1960s to measure the beta-particle activity of irradiated uranium-aluminum fission wires. This measurement is conducted to determine the fission rate profile of the Advanced Reactor Test Critical facility. The Advanced Test Reactor Critical facility is an open-pool, low-power test reactor used to qualify experiment configurations and verify core models prior to full-power experiment irradiations in the Advanced Test Reactor. Power distribution measurements in ATR-C use uranium-aluminum wires that are distributed throughout the core to validate simulation and modeling results. These measurements require from 340 to 1500 wires to be irradiated and measured within a 12-hour window. The system consists of 4 measurement channels and one reference channel, each with a 2-pi proportional gas flow detector and the measurement channels each have an automated sample changer. The gas flow detectors are of a custom design for this detector system that use methane gas with a large anode wire compared to modern proportional counters. These detectors, which are nearly 60 years old are irreplaceable. The measurements from these gas detectors are affected by the gas flow rate, atmospheric and line pressure, and are very sensitive to the applied high voltage. Recent improvements have been made to the control and data acquisition system, but the detectors have remained the same. The nature of the measurement of the fission product decay activity is such that the energy spectrum of the signal is changing with time. Thin, 250-um thick, plastic scintillators were commercially obtained as a potential replacement for the gas flow detectors. The original calibration of the uranium-aluminum fission wires was conducted in 1965 using a series of irradiations of gold foils and the wires in a well-characterized thermal neutron field. These measurements provided a time-dependent fission rate conversion factor from the gold foil data to calibrate the fission wires based on the response from the 2-pi proportional gas detectors. Transitioning to the new detectors requires qualification and testing. The sensitivity of the scintillators to changes in the energy spectrum of the fission wires and translation of the calibration factor have been completed. These measurements indicated that the sensitivity of the scintillators over time changes at a different rate than the sensitivity of the gas flow detectors. However, the inverse activity of measurements of both detector types is linear with time. Initial results indicate that the scintillator detectors will be a sufficient replacement for the gas detectors with minor adjustments to the fission rate conversion factor. Replacement of the detectors will improve the fission wire measurements and provide a more stable and reliable measurement system.
46 - INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AN↗
Creating high-precision reference gas standards of 85Kr for groundwater age-dating
Absolute gas counting (AGC) was applied to two gas blends of 85Kr in argon-methane (P10) counting gas to establish a high-precision specific activity (Bq/cm3) reference value for characterizing 85Kr detection efficiency for groundwater age dating measurements. The AGC or length-compensated technique has been utilized by the metrology community for decades and is an accepted method for developing radioactive gas standards. The AGC capability at Pacific Northwest National Laboratory (PNNL) uses a set of nine unequal-length proportional counters with precisely-measured internal volumes, and a gas loading system with high-precision pressure and temperature sensors. A series of AGC measurements were collected at multiple pressures to determine the inverse pressure relationship (1/P) for 85Kr and define a wall-effect correction that accounts for events decaying into the detector wall and not depositing sufficient energy in the gas to be detected. In addition to the wall-effect, two additional corrections were evaluated and are discussed in detail. Specifically, the threshold effect which accounts for events deposited below the analysis threshold and a detection efficiency as a function of detector volume effect that was observed during analysis. A robust uncertainty model was developed using the Guide to the expression of Uncertainty in Measurements (GUM) approach. The combination of carefully scrutinized correction factors, precise measurements of pressure, temperature and detector volume, and robust counting statistics resulted in the determination of high-precision specific activity values with 0.50% or less total combined uncertainty for two Kr-in-P10 reference gas standards (KP10) that will enable new groundwater age-dating measurements at PNNL.
Kr-85↗
The development of a 6 Li-based pixelated neutron detector for neutron reflectometry at the Spallation Neutron Source
We present a high-rate 6 Li-based pixelated neutron detector developed for neutron reflectometry instruments at the Spallation Neutron Source (SNS). The neutron detector has a pixelated design: each 6 Li scintillator element has its own photosensor and independent channel readout. This paper focuses on the general overview of the detector design and construction, the characterization of the pixelated detector, and the results of the first neutron reflectivity experiments conducted using the pixelated neutron detector at the SNS Liquid Reflectometer (BL-4B). The pixelated neutron detector demonstrated a global time-average count rate of ≥ 1.8 x 10 6 cps, at least 3 orders of magnitude higher than that of the existing neutron detector ( 3 He-based Multi-Wire Proportional Counter), and a local instantaneous count rate of ≥ 1.73 x 10 6 cps / cm 2 . The maximum counting rate of the detector has not yet been determined as the detector is capable of handling the maximum flux available at the beamline. Furthermore, the outcome of the neutron reflectivity experiments showed that the pixelated neutron detector is a promising candidate for next-generation neutron reflectometry instruments at the SNS.
6Li, silicon photomultiplier↗
Characterization of 235 U, 238 U and 239 Pu fission ionization chamber foils by α and γ -ray spectrometry
The masses of thin, electroplated deposits of 235 U, 238 U and 239 Pu on titanium backings have been determined to better than 1.3% precision by employing three complementary measurement techniques: $\alpha$ spectrometry, 2$\pi$ gas counting, and $\gamma$-ray spectrometry. A dedicated $\alpha$ spectrometer was designed and calibrated to allow for repeatable, high-precision measurements of the absolute $\alpha$ activity of a sample. The isotopic composition of the sample was determined by fitting the peaks in the $\alpha$ spectrum, a technique which was demonstrated to be generally consistent with mass spectrometry. Independently, the total activity of the deposits were measured in a 2$\pi$ gas flow proportional counter. A detailed study of the effects of $\alpha$ particle straggling in the actinide deposit and $\alpha$ particle recoil off the sample backing material was performed. The masses of these actinide deposits were also measured via $\gamma$-ray spectrometry. Here, we found that all three systematically independent techniques produced consistent results, and could be combined into a weighted average with further reduced uncertainty. The mass ratios of these actinide deposits were determined to better than 1% precision via $\alpha$ spectrometry, enabling high-precision fission cross section ratio measurements.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Large area position sensitive detector for thermal neutrons
Large area thermal neutron detectors are applied in many fields including industrial imaging, nuclear safeguarding, neutron scattering, and fundamental science. Historically, these detectors were based on 3 He gas proportional counters despite the limitations of 3 He detectors such as high cost, limited supply, non-uniform spatial resolution, and depth of absorption problems. Two alternatives to 3 He detectors are 6 Li-loaded glass scintillators, and powdered ZnS(Ag) scintillators mixed with 6LiF neutron converters. The 6 LiF/ZnS(Ag) scintillator has advantages over 6 Li glass as it is less expensive and can be produced in larger areas, although its self-absorption presents a problem. In this work, we developed a large area thermal neutron detector based on 6 LiF/ZnS(Ag) scintillator coupled with wavelength shifting fibers. The detector uses resistive charge divider-based position encoding. We further modified and improved the method by 2D segmentation of the detector using modular multichannel readout electronics. This segmentation approach allows for a combination of large detector area, improved spatial resolution, and increased count rate. Furthermore, spatial resolution can be variable across the detector area by adjusting the segment size.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Ground calibration and in-flight commissioning of the Gamma-Ray and Neutron Spectrometer (GRNS) for NASA’s Psyche mission
NASA's Psyche spacecraft is currently enroute to the asteroid 16 Psyche, where it will perform an orbital investigation focused on determining the nature of the asteroid. A key part of this investigation is measurements of elemental composition to determine if the asteroid is rich in iron-nickel metal. To that end, the Psyche spacecraft payload includes a Gamma-Ray and Neutron Spectrometer (GRNS). GRNS is composed of two subsystems; a Gamma-Ray Spectrometer (GRS) with a high-purity germanium γ-ray sensor surrounded by a borated plastic scintillator Anti-Coincidence Shield (ACS), and a Neutron Spectrometer (NS) with three 3 He-filled gas proportional counters that are sensitive to different neutron energy regimes. We describe results from the pre-launch calibration campaign, along with early in-flight results from post-launch instrument commissioning. The information detailed here is focused on providing future users of GRNS data with the information needed to properly interpret the observations from the instrument.
Peplowski, Patrick N. [Johns Hopkins Univ., Baltim↗
Next-generation neutron detection using a 6 Li glass scintillator composite
Neutron detectors are crucial in fundamental science, nuclear security, safeguards, and civil applications. 3 He-filled gas proportional counters are the gold standard for thermal neutron detection, prized for their efficiency, neutron/gamma discrimination, and stability; however, the scarcity of 3 He has prompted the search for alternatives. Here, we introduce a neutron detector design based on a scintillating composite consisting of 6 Li glass scintillator particles dispersed in an organic matrix. A detector consisting of this scintillating composite, photomultiplier tubes (PMTs) for optical detection, and electronics for reading out the PMT signal in both pulse and current modes was prototyped and characterized using various neutron and gamma sources. The prototype achieves a measured intrinsic detection efficiency of 6.70 ± 0.01%, a die-away time of 10.3 ± 0.1 μs, a negligible gamma misidentification probability, and response linearity up to at least 3.7 × 10 6 incident neutrons/s established via a cross-calibration technique. This detector holds the potential to outperform traditional 3 He-gas-based neutron detection systems, offering a viable alternative amidst the ongoing 3 He shortage and promising advancements in neutron detection technology.
47 OTHER INSTRUMENTATION↗
Alternatives to NIST Cf-252 Iirradiations for Transfer Calibration of S-32 Neutron Monitors
Gas-flow proportional counting systems are used by the Radiation Metrology Laboratory (RML) at Sandia National Laboratories for reactor fluence monitoring with the 32 S(n,p) 32 P reaction. Calibration of these systems has traditionally been accomplished by fluence-transfer irradiations at the NIST 252 Cf facility. Such calibrations have become increasingly difficult as the NIST 252 Cf source decayed to unusable levels. To minimize the risk to the testing programs from an inability to properly calibrate these systems, the RML has developed two alternative calibration techniques: 1) development and implementation of certified 32 P sources for activity calibrations and subsequent calculation of neutron fluence, and 2) direct counting of non-certified reactor-irradiated sulfur pellets by liquid scintillation counting to determine 32 P activity for the subsequent calibration of gas-flow proportional counters. Preliminary comparisons show that the several calibration methods are capable of overall uncertainties within about 5 percent.
Vehar, David W.↗
Solid, structured composite neutron detectors with high dynamic range capability
Neutron detectors are essential across disciplines such as fundamental science, nuclear security, safeguards, and civilian applications. While 3 He-filled gas proportional counters have long been revered for their efficacy in detecting thermal neutrons and praised for their efficiency, neutron/gamma discrimination, and stability, the scarcity of 3He has spurred a search for alternatives. Here, we explore a solid structured scintillating particle composite (SPC) consisting of 6 Li-containing scintillating glass particles within an acrylic matrix as a neutron detector for high dynamic range applications. We show for the first time that an SPC neutron detector can boast an intrinsic detection efficiency of 0.261% for pure 252 Cf fission neutrons and an overall neutron detection efficiency of (0.546 ± 0.003)% at the Neutron Free-in-Air facility while being able to function in an intense gamma-ray environment. We also show that the SPC neutron detector supports fast neutron capture times and enables a dual-readout scheme that extends the detector dynamic range to high incident neutron fluxes. A scalable fabrication process allows for tailoring the SPC detector properties to the requirements of specific applications. Good agreement is found between the experimental results taken with a National Institute of Standards and Technology traceable 252 Cf source and the coupled MCNP6 and optical-ray-tracing simulations.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
The NEWS-G detector at SNOLAB
The New Experiments With Spheres-Gas (NEWS-G) collaboration intends to achieve sub-GeV/c 2 Weakly Interacting Massive Particles (WIMPs) detection using Spherical Proportional Counters (SPCs). SPCs are gaseous detectors relying on ionisation with a single ionization electron energy threshold. The latest generation of SPC for direct dark matter searches has been installed at SNOLAB in Canada in 2021. This article details the different processes involved in the fabrication of the NEWS-G experiment. Also outlined in this paper are the mitigation strategies, measurements of radioactivity of the different components, and estimations of induced background event rates that were used to quantify and address detector backgrounds.
47 OTHER INSTRUMENTATION↗
Quenching factor measurements of neon nuclei in neon gas
The NEWS-G collaboration uses spherical proportional counters (SPCs) to search for weakly interacting massive particles (WIMPs). Here, in this paper, we report the first measurements of the nuclear quenching factor in neon gas at 2 bar using an SPC deployed in a neutron beam at the TUNL facility. The energy-dependence of the nuclear quenching factor is modeled using a simple power law: $αE^{β}_{nr}$; we determine its parameters by simultaneously fitting the data collected with the detector over a range of energies. We measured the following parameters in Ne : CH 4 at 2 bar: α= 0.2801±0.0050 (fit) ±0.0045 (sys) and β=0.0867±0.020 (fit) ±0.006 (sys). Our measurements do not agree with expected values from SRIM or Lindhard theory. We demonstrated the feasibility of performing quenching factor measurements at sub-keV energies in gases using SPCs and a neutron beam.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Review of Direct Neutron Conversion and Detection Processes
Commercial and laboratory neutron detection systems use indirect neutron response of materials like pressurized helium-3 (via nuclear reaction 3He(n, p)3H) to measure and count neutrons emanating from a source. Recently a host of semiconductors, especially a ternary semiconductor of lithium indium diselenide (6LiInSe2) and a quaternary alloy of enriched lithium-6, indium, phosphorous, and selenium (6LiInP2Se6), have shown promising neutron counting possibilities by directly converting neutrons into charge-carrying elements within the body of the semiconductor. These semiconductors have high thermal neutron capture cross sections, suitable energy bandgaps (~2.0 electron volts) for room-temperature operations, and a favorable electronic band structure for efficient electron charge transport. The article examines the semiconductor properties of these compounds in terms of their neutron counting capabilities and possible ways to extract neutron energy information from them. Lithium-6 and boron-10 (with thermal neutron absorption cross sections of 938 ± 6 and 3855 ± 26 barns, respectively) produce charged particles to be measured via indirect neutron interactions. The efficiency of indirect conversion neutron detectors is limited because of the inefficiencies in conversion mechanism. In case of direct conversion, the neutrons create charged particles in a single material for neutron capture and charge collection, increasing detection efficiency. Unlike 3He proportional counters, which provide no neutron energy information, the semiconductors can be used as neutron energy spectrometer. Fully resolved neutron energy by 6LiInP2Se6 from a plutonium-beryllium source has been reported in the literature. We will discuss the influence of these multilayered semiconductors’ crystallographic structures and growth techniques on neutron energy determination.
61 RADIATION PROTECTION AND DOSIMETRY↗