6Li Glass Scintillator Research Articles

Development of a Time-Gated Epithermal Neutron Spectrometer for Resonance Absorption Measurements Driven by a High-Intensity Laser

The advance of laser-driven neutron sources (LDNSs) has enabled neutron resonance spectroscopy to be performed with a single shot of a laser. In this study, we describe a detection system of epithermal (∼eV) neutrons especially designed for neutron resonance spectroscopy. A time-gated photomultiplier tube (PMT) with a high cut-off ratio was introduced for epithermal neutron detection in a high-power laser experiment at the Institute of Laser Engineering, Osaka University. We successfully reduced the PMT response to the intense hard X-ray generated as a result of the interaction between laser light and the target material. A time-gated circuit was designed to turn off the response of the PMT during the laser pulse and resume recording the signal when neutrons arrive. The time-gated PMT was coupled with a 6Li glass scintillator, serving as a time-of-flight (TOF) detector to measure the neutron resonance absorption values of 182W and 109Ag in a laser-driven epithermal neutron generation experiment. The neutron resonance peaks at 4.15 eV of 182W and 5.19 eV of 109Ag were detected after a single pulse of laser at a distance of 1.07 m.

Device and method for low-uncertainty and high-efficiency neutron time-of-flight spectrometry

Neutron detection systems demand precise knowledge of detection efficiency. The goal of this work is to assess the suitability of a neutron detection concept to perform neutron spectrometry by time-of-flight with low systematic uncertainty while attaining high detection efficiency. The device is based on a neutron-to-gamma converter, made of a highly pure 10B disk, coupled to a gamma detector. The behavior of the whole detection system, using the neutron field generated by the 7Li(p,n)7Be reaction at near-threshold energies is studied. Neutron energies ranging from 1 keV up to 250 keV were used. The distribution of TOF events at proton energy equal to 2018 keV was measured and the conversion to neutron spectrum is reported. At around 30 keV of neutron energy, a comparison to a commercially available 6Li-glass scintillator is performed in terms of detection efficiency. An almost three times higher count rate was observed in our detector compared to the commercial one. Using the covariance information on neutron cross section to generate a set of perturbed cross section data, detection efficiency uncertainty ranging from 0.8% to 2.3% was estimated.

The development of spectroscopic personal radiation detectors (SPRDs) for large area deployment

In this paper, a type of spectroscopic personal radiation detectors (SPRDs) is developed, which can be used for large area early warning of radiation. The detector (about 240 g, 130 mm× 80 mm× 24 mm) enable the measurement of neutron and γ energy spectrum, and it can run for 12 h on battery power. The neutron detection module uses a planar 6Li glass scintillation crystal. The optical conversion and transmission of the scintillator is realized by using a wavelength shifting fiber (WLSF), and the photoelectric signal conversion is realized by using SiPM. The energy resolution of the thermal neutron peak is 15.5%. A better n/γ screening effect is obtained by using the amplitude screening method. The γ detection module uses CsI (Tl) scintillation crystals with SiPM to realize the photoelectric signal conversion with an average energy resolution of 6.3% (@661 keV). The detector has realized communication with Android cell phone through Bluetooth communication, and radiation monitoring software has been developed on the Android to transmit real-time monitoring data to the server for analysis and early warning, which realize large area abnormal radiation warning.

Evaluation of resonance parameters for neutron interactions with molybdenum

Resonance parameters for neutron interactions with 92,94,95,96,97,98,100Mo in the energy region below 5 keV were evaluated. The parameters are the result of a compilation of experimental data available in the literature together with a least squares adjustment to transmission data obtained at the time-of-flight facility GELINA. The experiments were performed at a 50 m transmission station using a 6Li glass scintillator as neutron detector and metallic samples of natural Mo with a thickness of 2 mm and 5 mm. The REFIT code was used to adjust the resonance parameters, i.e. resonance energy and strength.

Development of a portable Single Sphere Neutron Spectrometer

A portable Single Sphere Neutron Spectrometer (SSNS) is designed to measure the neutron energy spectrum in the mixed radiation field. The spectrometer consists of a 30 cm diameter single polyethylene moderating sphere with 19 pieces 6Li and 1-piece 7Li glass scintillator detectors located at the orthogonal X/Y/Z axis. The output signals from each detector are processed and readout by the Silicon Photomultipliers (SiPMs) and the Application Specific Integrated Circuit (ASIC) electronic system. The neutron energy responses of portable SSNS are calibrated in the mono-energetic neutron reference fields (144 keV, 1.2 MeV, 5 MeV, 14.8 MeV) based on the 5SDH-2 tandem accelerator in China Institute of Atomic Energy, which are also simulated from 0.0253 eV to 20 MeV by Monte Carlo simulation. The performance of portable SSNS is tested at different distances in 241Am–Be neutron reference radiation field in National Institute of Metrology, China. The fluence response of SSNS is about 0.28 cm2 for 241Am–Be neutron source.

Construction and test of a single sphere neutron spectrometer based on pairs of 6Li-and 7Li-glass scintillators

A single sphere neutron spectrometer (SSNS) was designed to measure the neutron energy spectrum in mixed radiation fields. The spectrometer consisted of a 30 cm diameter single polyethylene moderating sphere with 19 pairs of 6Li- and 7Li-glass scintillator detectors located at the orthogonal x-, y-, z-axis. The glass scintillator detector was composed of glass scintillator, light guide and photomultiplier tube. The output signals from each detector were processed and read out by a charge division resistance network and an electronic system. The thermal neutron counts were discriminated by subtracting the signal counts of 7Li-glass scintillator detector from the signal counts of 6Li-glass scintillator detector at the same position. The neutron energy response of the SSNS was calibrated in mono-energetic neutron fields (144 keV, 250 keV, 565 keV, 1.2 MeV, 2.5 MeV, 14.8 MeV) based on the 5SDH-2 tandem accelerator in China Institute of Atomic Energy, which was also simulated from 0.0253eV to 19 MeV by the Monte Carlo method. Finally, a241Am–Be neutron energy spectrum and the ambient dose equivalent values were measured using the SSNS. The deviation between the experimental result of the SSNS and the reference value was 2.6%.

Light propagation in a neutron detector based on 6Li glass scintillator particles in an organic matrix

Composite materials consisting of 6Li scintillator particles in an organic matrix can enable thermal neutron detectors with excellent rejection of gamma-ray backgrounds. The efficiency of transporting scintillation light through such a composite is critical to the detector performance. This optical raytracing study of a composite thermal neutron detector quantifies the various sources of scintillation light loss and identifies favorable photomultiplier tube (PMT) readout schemes. The composite material consisted of scintillator cubes within an organic matrix shaped as a right cylinder. The cylinder surface was surrounded by an optical reflector, and the light was detected by PMTs attached to the cylinder end faces. A reflector in direct contact with the composite caused 53% loss of scintillation light. This loss was reduced 8-fold by creating an air gap between the composite and the reflector to allow a fraction of the scintillation light to propagate by total internal reflection. Replacing a liquid mineral oil matrix with a solid acrylic matrix decreased the light transport efficiency by only ∼10% for the benefit of creating an all-solid-state device. The light propagation loss was found to scale exponentially with the distance between the scintillation event and the PMT along the cylinder main axis. This enabled a PMT readout scheme that corrects for light propagation loss on an event-by-event basis and achieved a 4.0% energy resolution that approached Poisson-limited performance. These results demonstrate that composite materials can enable practical thermal neutron detectors for a wide range of nuclear non-proliferation and safeguard applications.

The Prompt Fission Neutron Spectrum of 235U(n,f) below 2.5 MeV for Incident Neutrons from 0.7 to 20 MeV

New prompt fission neutron spectrum measurements are reported for 235U(n,f) reactions induced by neutrons with energies from 0.7 to 20 MeV. These measurements cover outgoing neutron energies from 2.5 MeV down to 10 keV, using an array of 6Li-glass scintillators for neutron detection and a double time-of-flight technique. The neutrons were produced at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center. A detailed MCNP® model of the experimental equipment and the surrounding room was used to interpret the experimental results. Backgrounds were measured in situ, making use of the time-dependent singles rates of the various detectors with asynchronous readout from waveform digitizers. The results presented here have been included in a re-evaluation of the fission neutron spectra for this fissioning system, a description of which is presented elsewhere in this issue.

Measurements of the Prompt Fission Neutron Spectrum at LANSCE: The Chi-Nu Experiment

The goal of the Chi-Nu experiment at the Los Alamos Neutron Science Center is to measure the prompt fission neutron spectra from major actinides using a double time-of-flight method with a pulsed, white incoming neutron source. Fission events are detected with a parallel-plate avalanche counter and outgoing neutrons are detected with either a 6Li-glass or liquid scintillator detector array for low- or high-energy neutrons, respectively. A detector response matrix for the interaction of neutrons with the experimental environment for neutrons measured with the Chi-Nu 6Li-glass detector array has been calculated to obtain a full understanding of the measured Chi-Nu data and also to allow for nearly instantaneous production of simulated Chi-Nu data spectra. Prompt fission neutron spectra corresponding to 19 incoming neutron energy ranges from 0.7-20 MeV have been extracted using the ratio-of-ratios method with Chi-Nu 6Li-glass data on the neutron-induced fission of 235U.

Bright [formula omitted]-activated polycrystalline ceramic neutron scintillators

Scintillation properties of Eu2+-doped CaF2-AlF3-6LiF (Eu:CALF) polycrystalline ceramic thermal-neutron scintillators as a function of AlF3 concentration have been studied. The emission band peaked at a wavelength of 425–431 nm is due to the presence of Eu:CaF2 micro-crystallites. The highest light output from these samples is approximately 20,000 photons per thermal neutron, which is 3 times that of a GS20 6Li-glass scintillator. The pulse-decay lifetime and light output vs. AlF3 concentration may be understood using a radiation trapping model and the formation of a Li3AlF6 phase. At lower AlF3 concentration, Al3+ ions in Eu:CaF2 passivate the hole-trapping defects and enhance the light output; whereas at higher AlF3 concentration, Al3+ ions lead to the formation of electron trapping centers in Eu:CaF2 and the Li3AlF6 phase is formed, which reduces the light output. A neutron–gamma-discrimination (NGD) ratio of 9×108 was obtained from Principal Component Analysis (PCA) of digital waveforms, while Fisher Linear Discriminant Analysis (FLDA) can completely separate the thermal neutrons from 60Co gamma rays within the limit of gamma event statistics used in this work. Our results suggest that Eu:CALF scintillators can potentially replace the GS20 scintillator used for thermal and cold neutron detection systems.

Rhenium resonance parameters from neutron capture and transmission measurements in the energy range 0.01 eV to 1 keV

Rhenium is a refractory metal with potential uses in nuclear reactor applications, particularly those at very high temperatures. Measurements have been made using natural samples. Natural rhenium consists of two isotopes: 185Re (37.40%) and 187Re (62.60%). The electron linear accelerator (LINAC) at the Rensselaer Polytechnic Institute (RPI) Gaerttner LINAC Center was used to explore neutron interactions with rhenium in the energy region from 0.01 eV to 1 keV. Neutron capture and transmission measurements were performed by the time-of-flight technique. Two transmission measurements were performed at flight paths of 15 m and 25 m with 6Li glass scintillation detectors. The neutron capture measurements were performed at a flight path of 25 m with a 16-segment sodium iodide multiplicity detector. Resonance parameters were extracted from the data using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented. The uncertainties in resonance parameters were propagated from a number of experimental quantities using a Bayesian analysis. Uncertainties were also estimated from fitting each Re sample measurement individually. The measured neutron capture resonance integral for 185Re is (4 ± 1)% larger than ENDF/B-VII.1. The capture resonance integral for 187Re is (3 ± 1)% larger than ENDF/B-VII.1. Other findings from these measurements include: a decrease in the thermal capture cross section for 185Re of (2 ± 2)% from ENDF/B-VII.1; a decrease in the thermal capture cross section for 187Re of (3 ± 4)% from ENDF/B-VII.1; a decrease in the thermal total cross section for 185Re of (2 ± 2)% from ENDF/B-VII.1; and a decrease in the thermal total cross section for 187Re of (6 ± 5)% from ENDF/B-VII.1. Considering the uncertainties, none of the indicated changes in thermal cross sections represents a statistically significant change from ENDF/B-VII.1.

Development of the high-energy neutron fluence rate standard field in Japan with a peak energy of 45 MeV using the 7Li(p,n)7Be reaction at TIARA

ABSTRACTIn this study, we developed a 45 MeV neutron fluence rate standard of Japan. Quasi-monoenergetic neutrons with a peak energy of 45 MeV in the neutron standard field were produced by the 7Li(p,n)7Be reaction using a 50-MeV proton beam from an azimuthally varying field (AVF) cyclotron of the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA). The neutron energy spectrum was measured using an organic liquid scintillation detector and a 6Li-glass scintillation detector by the time-of-flight method, and using a Bonner sphere spectrometer by the unfolding method. The absolute neutron fluence was determined using a proton recoil telescope (PRT) composed of the liquid scintillation detector and a Si(Li) detector that was newly developed in the present study. The detection efficiency of the PRT was obtained using the MCNPX code. The peak neutron production cross section for the 7Li(p,n)7Be reaction was also derived from the neutron fluence in order to confirm the neutron fluence of the TIARA high-energy neutron field. The peak neutron production cross section obtained in the present study was in good agreement with those of previous studies. The characteristics of the 45-MeV neutron field in TIARA were successfully evaluated in order to calibrate high-energy neutron detectors and high-energy neutron dosimeters.

Evaluation of the dark signal performance of different SiPM-technologies under irradiation with cold neutrons

In this paper we report the results of the assessment of changes in the dark signal delivered by three silicon photomultiplier (SiPM) detector arrays, fabricated by three different manufacturers, when irradiated with cold neutrons (wavelength λn=5Å or neutron energy of En=3.27meV) up to a neutron dose of 6×1012n/cm2. The dark signals as well as the breakdown voltages (Vbr) of the SiPM detectors were monitored during the irradiation. The system was characterized at room temperature. The analog SiPM detectors, with and without a 1mm thick Cerium doped 6Li-glass scintillator material located in front of them, were operated using a bias voltage recommended by the respective manufacturer for a proper detector performance. Iout-Vbias measurements, used to determine the breakdown voltage of the devices, were repeated every 30s during the first hour and every 300s during the rest of the irradiation time. The digital SiPM detectors were held at the advised bias voltage between the respective breakdown voltage and dark count mappings repeated every 4min. The measurements were performed on the KWS-1 instrument of the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. The two analog and one digital SiPM detector modules under investigation were respectively fabricated by SensL (Ireland), Hamamatsu Photonics (Japan), and Philips Digital Photon Counting (Germany).

Reliability of Monte Carlo simulations in modeling neutron yields from a shielded fission source

Using the combination of a neutron-sensitive 6Li glass scintillator detector with a neutron-insensitive 7Li glass scintillator detector, we are able to make an accurate measurement of the capture rate of fission neutrons on 6Li. We used this detector with a 252Cf neutron source to measure the effects of both non-borated polyethylene and 5% borated polyethylene shielding on detection rates over a range of shielding thicknesses. Both of these measurements were compared with MCNP calculations to determine how well the calculations reproduced the measurements. When the source is highly shielded, the number of interactions experienced by each neutron prior to arriving at the detector is large, so it is important to compare Monte Carlo modeling with actual experimental measurements. MCNP reproduces the data fairly well, but it does generally underestimate detector efficiency both with and without polyethylene shielding. For non-borated polyethylene it underestimates the measured value by an average of 8%. This increases to an average of 11% for borated polyethylene.

236U resonance parameters at 5.467 eV from neutron transmission measurements using thin liquid samples

The 236U isotope is an important buildup product that is generated in the 235U fuel cycle and influences reactor neutronic calculations. The aim of the present work is to improve upon the existing neutron total cross section data for the strong 236U resonance at 5.467 eV.High accuracy neutron transmission measurements were performed using the time-of-flight technique at the Rensselaer Polytechnic Institute linear accelerator. An approach was used to fabricate thin 236U samples using liquid, allowing for non-saturated resonances. The transmission measurements were made at the 15 m flight station with a 6Li glass scintillation detector. Methods to characterize the background and experimental resolution function were developed. The 236U resonance parameters and their uncertainties at 5.467 eV were determined by fitting the transmission data using a Monte Carlo approach with the SAMMY multi-level R-matrix Bayesian code.The resonance parameters determined in this work are: energy, E, equal to 5.467 ± 0.005 eV; neutron width, Γn, equal to 2.13 ± 0.04 meV; and radiation width, Γγ, equal to 27 ± 1 meV. The fission width, Γf, was not fitted and was fixed to the ENDF-7.1 value of 0.290 meV. These parameters gave a neutron capture resonance integral of 330 ± 5 b that is lower than all of the selected evaluations: Mughabghab by 4.7%, ENDF-7.1 by 3.7%, JEFF-3.1 by 4.8%, and JENDL-4.0 by 7.2%.

Neutron detector based on Particles of 6Li glass scintillator dispersed in organic lightguide matrix

Most 3He replacement neutron detector technologies today have overlapping neutron–gamma pulse-height distributions, which limits their usefulness and performance. Different techniques are used to mitigate this shortcoming, including Pulse Shape Discrimination (PSD) or threshold settings that suppress all gammas as well as much of the neutrons. As a result, count rates are limited and dead times are high when PSD is used, and the detection efficiency for neutron events is reduced due to the high threshold. This is a problem in most applications where the neutron–gamma separation of 3He detectors had been essential. This challenge is especially severe for neutron coincidence and multiplicity measurements that have numerous conflicting requirements such as high detection efficiency, short die-away time, short dead time, and high stability. 6Li-glass scintillators have excellent light output and a single peak distribution, but they are difficult to implement because of their gamma sensitivity. The idea of reducing the gamma sensitivity of 6Li-glass scintillators by embedding small glass particles in an organic light-guide medium was first presented by L.M. Bollinger in the early 60s but, to the best of our knowledge, has never been reduced to practice. We present a proof of principle detector design and experimental data that develop this concept to a large-area neutron detector. This is achieved by using a multi-component optical medium (6Li glass particles attached to a glass supporting structure and a mineral oil light guide) which matches the indices of refraction and minimizes the absorption of the 395nm scintillator light. The detector design comprises a 10in. long tube with dual end readout with about 3% volume density of 6Li glass particles installed. The presented experimental data with various neutron and gamma sources show the desired wide gap between the neutron and gamma pulse height distributions, resulting in a true plateau in the counting characteristics similar to that of 3He detectors.

Europium resonance parameters from neutron capture and transmission measurements in the energy range 0.01–200 eV

Europium is a good absorber of neutrons suitable for use as a nuclear reactor control material. It is also a fission product in the low-yield tail at the high end of the fission fragment mass distribution. Measurements have been made of the stable isotopes with natural and enriched samples.The linear electron accelerator center (LINAC) at the Rensselaer Polytechnic Institute (RPI) was used to explore neutron interactions with europium in the energy region from 0.01 to 200eV. Neutron capture and transmission measurements were performed by the time-of-flight technique. Two transmission measurements were performed at flight paths of 15 and 25m with 6Li glass scintillation detectors. The neutron capture measurements were performed at a flight path of 25m with a 16-segment sodium iodide multiplicity detector.Resonance parameters were extracted from the data using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented.To prevent air oxidation metal samples were sealed in airtight aluminum cans in an inert environment. Metal samples of natural europium, 47.8atom% 151Eu, 52.2atom% 153Eu, as well as metal samples enriched to 98.77atom% 153Eu were measured.The measured neutron capture resonance integral for 153Eu is (9.9±0.4)% larger than ENDF/B-VII.1. The capture resonance integral for 151Eu is (7±1)% larger than ENDF/B-VII.1.Another significant finding from these measurements was a significant increase in thermal total cross section for 151Eu, up (9±3)% from ENDF/B-VII.1. The thermal total cross section for 153Eu is down (8±3)% from ENDF/B-VII.1, but it is larger than that of ENDF/B-VII.0.The resolved resonance region has been extended from 100eV to 200eV for both naturally-occurring isotopes. Uncertainties in resonance parameters have been propagated from a number of experimental quantities using a Bayesian analysis. Uncertainties have also been estimated from fitting each Eu sample measurement individually.

Results of total cross section measurements for 197Au in the neutron energy region from 4 to 108 keV at GELINA

Transmission measurements have been performed to determine the total cross section for neutron-induced reactions with 197Au in the energy region from 4keV to 108keV. The experiments were carried out at a 50m measurement station of the time-of-flight facility GELINA using a 6Li glass scintillator. The average total cross section as a function of neutron energy was derived after correcting the observed average transmission for cross section fluctuations due to resonance structures. The results have been compared with literature data and with the total cross section resulting from a dispersive coupled-channel optical model potential. The neutron strength function for s -wave neutrons and an orbital-independent scattering radius, together with their covariance matrix, have been derived in the neutron energy region between 4keV and 108keV through a parameterization of the total cross section based on a nuclear reaction theory.

Multichannel down-scattered neutron detector for areal density measurement

A down-scattered neutron detector operating in the multichannel counting mode was developed for areal density (ρR) measurement. Equipped with a newly developed 6Li glass scintillator (APLF80), the detector was tested in a fusion experiment at the GEKKO XII facility, Osaka University, Osaka, Japan. For a low-ρR fusion shot, the detector clearly discriminated the γ-rays, primary neutrons, γ-rays produced via (n, γ) reactions from the target chamber, and neutrons scattered by the target chamber. Furthermore, the observed signal was in good agreement with predictions made by Monte Carlo simulation.

Li-glass detector response study with a 252Cf source for low-energy prompt fission neutrons

Prompt-fission-neutron spectra for neutron-induced fission reactions on uranium and plutonium isotopes are important for nuclear applications. We have used 6Li-glass scintillation detectors to measure outgoing neutron energies in the range from 10keV to 1MeV, where there is currently large uncertainty in nuclear data. To better understand the response of 6Li-glass detectors in this energy range, measurements of well-known spontaneous-fission neutrons from a 252Cf source were done in the neutron-beam flight path. Results were compared with Monte Carlo simulations and they show good agreement. Similar measurements with a 7Li-glass detector were used to assess gamma-ray background yields.