3He Proportional Counter Research Articles

Calibration and validation measurements of the Extended-range Bonner sphere spectrometer

An Extended-range Bonner sphere spectrometer (EBSS) has been developed to investigate the neutron spectra of the China initiative Accelerator Driven System. This paper presents the design, calibration, and validation measurements of the EBSS system using a standard 241Am-Be neutron source and the cosmic ray neutrons. The EBSS system was simulated using the PHITS code, and the geometric structures were designed using the response functions obtained by simulations. The EBSS system comprises of seven polyethylene-only spheres and seven extended-range spheres encased in lead, copper, or tungsten shells along with a bare 3He proportional counter. To verify the accuracy of the simulated response functions, the EBSS system was calibrated with monoenergetic neutron beams of 565 keV at the China Institute of Atomic Energy in Beijing. The EBSS system was used to measure neutron count rates of the 241Am-Be neutron source and cosmic ray neutrons. Subsequently, the measured neutron count rates were converted into neutron spectra by the unfolding software. Experimental findings demonstrate the precise capability of the EBSS system to measure neutron spectra across a wide range of neutron radiation fields.

Observation of neutron emission during acoustic cavitation of deuterated titanium powder

Possibility of nuclear reactions in solid state is intriguing for two reasons: (1) It provides a means of studying nuclear processes in conditions that are much different from traditional plasma-filled reactors or particle accelerators; (2) it dramatically lowers the cost and complexity of the experimental setups by eliminating the highly capital intensive components such as plasma/vacuum systems and particle accelerators. In this article we report the observation of neutron emission coincident with acoustic cavitation of deuterated titanium powder suspended in mineral oil. The resulting neutron emission was detected using an assembly of 3He proportional neutron counters. The peak neutron count rate was in excess of 6500 CPM, more than 10,000 times in excess of background. The observed neutron emission was coincident with the application of acoustic influence. The neutrons were present only when secondary acoustic waves originating from the complex bubble interactions inside the reactor constructively interfered resulting in massive, sharp pressure peaks on the order of a few thousand psi. We were able to sustain the neutron production for several hours and repeated the experiment multiple times under various conditions. We hypothesize that the observed neutrons originate from nuclear fusion of deuterium ions dissolved in titanium lattice due to the mechanical action of the impinging cavitation jets, although other processes (such as spallation) still need to be ruled out.

Development of a silicon-based thermal neutron detection system

Neutron detection systems are of increasing importance in applications from basic science to medical applications and reactors. 3He proportional counters remain the most popular choice for monitoring thermal neutrons with a detection efficiency of around 60%, however, due to 3He global shortages, a new generation of detection technologies will be required to meet the rising demand. As a result, extensive research is being conducted to investigate alternative methods of neutron detection. This work presents such a system and demonstrates its calibration and evaluation using an AmBe neutron source. The detection system involves silicon sensors coated by converter layers to make the detectors sensitive to thermal neutrons via neutron capture and measurement of the resulting secondary charged particles. The detection system is presented in two configurations, a single and a multi-layer configuration, where the latter is used to increase the total detection efficiency. In addition, the system is capable of determining coincident signals from a single neutron capture, a feature which allows background suppression and an increase in the purity of the neutron signal which is particularly useful in mixed radiation environments.

A compact moderator for thermal neutron reference radiation field with an 241Am-Be source

To calibrate thermal neutron detectors, State Key Laboratory of NBC Protection for Civilian plans to establish thermal neutron reference radiation field (TNRRF) based on the existing 7.4 × 1011 Bq 241Am-Be source. An 82 cm (L) × 50 cm (W) × 50 cm (H) moderator was designed through Monte Carlo simulation to thermalize fast neutrons. Each component was determined to optimize thermal neutron fluence rate φ, proportion of thermal neutrons ρ, and uniformity factor μ of the radiation field. A 3He proportional counter was used to evaluate the fabricated moderator. Results show that surrounding reflector is the main origin of thermal neutrons. D2O tank can promote ρ and its length can be utilized to adjust φ. Homogenizing lens will improve μ and reduce φ. Experimental φ, cadmium ratio η, and μ at the reference position are 1808.82 ± 85.96 cm−2 s−1, 315.80, and 5.29 %, respectively, which are consistent with simulated results. Next, TNRRF will be established after long-term stability testing.

Scintillation properties of lithium-6 salicylate-loaded liquid scintillators.

The limited availability of conventional 3He proportional counters provides impetus for developing novel neutron detectors. As a candidate, lithium-6-loaded liquid scintillators with neutron/gamma pulse shape discrimination (n-γ PSD) capabilities have been developed. However, the trade-off relationship between the 6Li-loading amount and scintillation light yield is a significant problem. This is because 6Li-loading involves the addition of non-luminescent materials, which cause non-radiative relaxation of the excited states. Therefore, aiming to reduce non-radiative relaxation, we chose lithium-6 salicylate (6LiSal), which shows fluorescence in the visible light region, as a chemical for 6Li-loading. In this study, we analyzed the photoluminescence/scintillation properties based on the Förster resonance energy transfer and investigated the optimal content for obtaining a high light yield. By maximizing the sequential energy transfer from the solvent (toluene) to the phosphor (POPOP), a high light yield 6Li-loaded liquid scintillator (4220 photons per MeV under gamma-ray irradiation) with a 6Li concentration of approximately 0.1 wt% was developed. Thermal neutron events were successfully detected with a light yield of 3970 photons per neutron, which is more than three times higher than those of other organic scintillators. In addition, focusing on the triplet-triplet annihilation process and further optimizing the component for the n-γ PSD, the thermal neutron and gamma-ray events were successfully separated. The developed high light yield 6Li-loaded liquid scintillators show n-γ PSD capabilities and can be promising candidates as alternative detectors to the 3He proportional counter.

Measurement of neutron spectra for various thicknesses of concrete and steel shielding at 24-GeV/c proton beam facility using Bonner sphere spectrometer

ABSTRACT Neutron energy spectra down to thermal energy were measured using a Bonner sphere spectrometer (BSS) for various thicknesses of concrete and steel shielding at the CERN/CHARM facility, where high-energy neutrons were produced by 24-GeV/c protons incident on a thick copper target. The thicknesses of the concrete and steel shielding blocks ranged from 40 cm to 200 cm and from 20 cm to 80 cm, respectively. The BSS consisted of a spherical 3He proportional counter and five polyethylene moderators with diameters of 7.62 cm, 10.2 cm, 12.7 cm, 17.8 cm, and 24.1 cm, respectively. In addition, polyethylene moderators combined with a lead or copper inner shell were used to increase the sensitivity to high-energy neutrons. The neutron energy spectra were deduced using an unfolding method. The initial guesses were obtained using the PHITS code for each experimental geometry. The response function for the BSS was determined using the MCNP6.2 code with JENDL-4.0/HE. The neutron energy spectra over the entire energy region from 10−4 eV to 10 GeV were successfully obtained for the different shielding conditions. The validity of the response function and the contribution of each moderator are discussed referring to previous studies and tests at the standard neutron fields of AIST.

Comparative investigation of detector responses and measurement uncertainties in fast-neutron coincidence counting for nuclear material accountancy

Fast-neutron measurement with organic scintillators has been considered one of the alternative techniques to thermal neutron measurement based on He-3 proportional counters, due to their economic and measurement-uncertainty potential for nuclear material accountancy. Nevertheless, highly suppressed neutron detection efficiencies and nuclide-dependent features indicated in the fast system as well as the utilization of pulse-shape discrimination which necessitates a significant energy threshold could lead to challenges in terms of characterizing nuclear material samples. This work confirmed a reasonable similarity of simulated results to experimental results concerning the non-linear light-output function of proton recoils. Consequently, nuclide-dependent detector responses and measurement uncertainty were investigated with varying diverse measurement parameters (the number of detectors, measurement time, and sample quantity). Also, comparative studies were conducted for the ACP Safeguards Neutron Counter, a developed thermal system, and the Fission Informative Neutron-based ACcounting, the proposed fast system currently under development by Korea Atomic Energy Research Institute, for passive non-destructive assay of special nuclear materials.

Proof-of-concept study on a water phantom-based neutron spectrometer: Experimental test with 252Cf and 241Am-Be sources

Boron neutron capture therapy (BNCT) is a promising cancer treatment that uses energetic ions released from 10B(n, α)7Li reactions. Accurate assessment of neutron energy spectra is important for simulation-based evaluation of neutron doses during BNCT. In this study, a proof-of-concept study was conducted for a neutron spectrometry technique that involves the use of a water phantom, which is commonly used for quality assurance in BNCT, as a moderator. The technique involves applying unfolding to the count rate distribution of the thermal neutron counter measured within the phantom to derive the energy spectrum. We performed experiments using a spherical 3He proportional counter in neutron fields generated by 252Cf and 241Am-Be sources. The results demonstrated that the spectrometer reasonably reproduced neutron spectra and showed the potential of using a water phantom as a moderator for such a technique.

English

Response matrices for 3He proportional neutron counters of spherical and cylindrical form are simulated. It is shown that the latter can be converted to the former with a simple linear operation, which points to tube counters as a potential device for neutron spectrum observation.

Fast neutron background characterization of the future Ricochet experiment at the ILL research nuclear reactor

The future Ricochet experiment aims at searching for new physics in the electroweak sector by providing a high precision measurement of the Coherent Elastic Neutrino-Nucleus Scattering (CENNS) process down to the sub-100 eV nuclear recoil energy range. The experiment will deploy a kg-scale low-energy-threshold detector array combining Ge and Zn target crystals 8.8 m away from the 58 MW research nuclear reactor core of the Institut Laue Langevin (ILL) in Grenoble, France. Currently, the Ricochet Collaboration is characterizing the backgrounds at its future experimental site in order to optimize the experiment’s shielding design. The most threatening background component, which cannot be actively rejected by particle identification, consists of keV-scale neutron-induced nuclear recoils. These initial fast neutrons are generated by the reactor core and surrounding experiments (reactogenics), and by the cosmic rays producing primary neutrons and muon-induced neutrons in the surrounding materials. In this paper, we present the Ricochet neutron background characterization using ^3He proportional counters which exhibit a high sensitivity to thermal, epithermal and fast neutrons. We compare these measurements to the Ricochet Geant4 simulations to validate our reactogenic and cosmogenic neutron background estimations. Eventually, we present our estimated neutron background for the future Ricochet experiment and the resulting CENNS detection significance. Our results show that depending on the effectiveness of the muon veto, we expect a total nuclear recoil background rate between 44 ± 3 and 9 ± 2 events/day/kg in the CENNS region of interest, i.e. between 50 eV and 1 keV. We therefore found that the Ricochet experiment should reach a statistical significance of 4.6 to 13.6 sigma for the detection of CENNS after one reactor cycle, when only the limiting neutron background is considered.

Evaluation of delayed neutron yields and time spectra from photofission of 238U induced by Bremsstrahlung photons below 9 MeV

Few data exist regarding the yields and time spectra of delayed neutrons from the photofission reaction on actinides. In this paper, we report the characteristics of both six-group and eight-group delayed neutron time models for the photofission of 238U induced by a Bremsstrahlung photon spectrum with an end-point energy at 9 MeV. This study was conducted using a 9 MeV linear electron accelerator housed at the SAPHIR platform located at CEA Paris-Saclay, France. A reference sample of depleted uranium was positioned inside a neutron detection block, previously designed and optimized by Monte Carlo simulation, made of high-density polyethylene covered by cadmium and including ten one meter-long 3He proportional counters. Based on the consistent set of half-lives proposed by Spriggs et al. as early as 1999 for neutroninduced fission, our description in eight delayed neutron groups for 238U at 9 MeV is the first published for photofission. These new results bring valuable information to the scarce data available in the literature and represent useful inputs for the development of photofission-based nuclear measurement systems as they meet a significant interest in the fields of nuclear data, radioactive waste package characterization and for security-related applications.

Thermal neutron background at Laboratorio Subterráneo de Canfranc (LSC)

The thermal neutron background at Laboratorio Subterráneo de Canfranc (LSC) has been determined using several 3He proportional counter detectors. Bare and Cd shielded counters were used in a series of long measurements. Pulse shape discrimination techniques were applied to discriminate between neutron and gamma signals as well as other intrinsic contributions. Montecarlo simulations allowed us to estimate the sensitivity of the detectors and calculate values for the background flux of thermal neutrons inside Hall-A of LSC. The obtained value is (3.5±0.8)×10−6 n/cm2s, and is within an order of magnitude compared to similar facilities.

Neutron/[formula omitted]-ray discrimination based on the property and thickness controls of scintillators using Li glass and LiCAF (Ce) in a [formula omitted]-ray field

Recently, various neutron detectors, such as scintillation detectors, are being developed as alternatives to 3He proportional counters. Here we focus on the thickness and property controls of inorganic scintillators used for thermal neutron detection in intense γ-ray fields without considering pulse shape discrimination (PSD) techniques. GS20TM (a lithium glass) and LiCaAlF6 crystals activated with Ce (LiCAF:Ce) scintillators with thicknesses of 0.5 and 1.0 mm, respectively, have been employed because they have faster decay times than other inorganic scintillators. Pulse signals generated by photomultiplier tubes, to which the scintillators were coupled, were inserted into a digital pulse processing unit with a sampling rate of 1 GHz, and the areas of waveforms were integrated for 360 ns. Neutron/γ-ray detections and discriminations were performed via pulse integrations without using PSD techniques. For GS20TM, the relative energy resolution values of the thermal neutron peaks were 11% and 12% in 0.5 mm and 1.0 mm thicknesses, respectively, and those for LiCAF:Ce were 18% and 36% in 0.5 mm and 1.0 mm thicknesses, respectively. In a 60Co γ-ray field, the neutron detection for GS20TM in a 0.5 mm thickness was possible at dose rates of up to 0.919 Gy/h; however, for LiCAF:Ce, neutron detection was possible at 0.473 Gy/h, and it failed at 0.709 Gy/h. The neutron/γ-ray discrimination of GS20TM was better than that of LiCAF:Ce due to its better energy resolution and higher detection efficiency (6Li content).

Neutron yield measurement system of HL-2A tokamak

This research presents the development of HL-2A neutron yield measurement which includes 235U fission chamber and BF3 and 3He proportional counters. Equivalent noise formula of the radiation detection signal amplification system was derived to guide the development of the signal amplification system. Then all detectors were calibrated in situ by using the 252Cf neutron source. The neutron yield of the HL-2A during neutral beam heating was analyzed. These results indicate that the developed neutron flux diagnostic system can obtain neutron yield results under various experimental conditions of the HL-2A tokamak, and can provide information on neutron yield.

Understanding the Radiation Soaking Effect in Neutron Survey Instruments.

The drift in response of neutron survey meters that employ moderated 3He and BF3 proportional counters has been reported and investigated by instrument calibration laboratories for over 20 y. When within a constant radiation rate field of high intensity, dose rate readings of an instrument may steadily increase with a rate proportional to the field intensity before reaching a relatively stable plateau. This phenomenon, commonly referred to as the radiation soaking effect, has been attributed to the accumulation of charge on insulating surfaces inside the detector. This paper provides results of a study specifically conducted to better understand the effect and, hence, to possibly identify mitigation techniques. The magnitude of the effect in different instruments tested under the same conditions has been assessed, and a reason for the detector response variance has been clearly identified. In addition to measures used in calibration practice to compensate for the radiation soaking effect, a potential method to return an instrument sensitivity to its baseline level is also discussed.

Research on angle sensitivity of the boron-lined multilayer converter neutron detector

In our previous work, a new design for a gaseous neutron detector composed of a boron-lined multilayer converter was proposed to replace the 3He proportional counter, and a prototype detector was constructed to test its performance and characteristics. The neutron detection efficiency varies for different incident angles of neutrons to the converter surface, mainly due to the effective area of the converter and the emitting probability of the alpha and 7Li. Hence, the variation of detection efficiency with the incident angle is analyzed theoretically in this work, and then the GEANT4 simulations and preliminary experimental tests are conducted to verify the calculation results. The highest detection efficiency is obtained at 1.72° (100 mm length, 3 mm gap, and θ = atan (3/100) ≈ 1.72°), the detection efficiency decreases sharply when the angle approaches 0°, and the detection efficiency decreases gradually when the incident angle is larger than 1.72°. The simulation results and experimental results are consistent with the trend of theoretical calculation. Therefore, the proposed detector is suitable for angle-sensitive applications, such as small angle neutron scattering and locating the isotopic neutron sources.

Investigation of CLYC-6 for thermal neutron detection and CLYC-7 for fast neutron spectrometry

Highly enriched 6Li and 7Li Cs2LiYCl 6 (CLYC) scintillators were investigated in view of their application as thermal neutron counter (CLYC-6) and as fast neutron spectrometer (CLYC-7) in the recently developed B-RAD radiation survey metre, an instrument designed to work in regions of high magnetic fields, currently equipped with a probe for photon dose rate measurements and γ-spectrometry. Both crystals were irradiated with a 2.5 MeV mono-energetic neutron beam from a Deuterium–Deuterium (D–D) generator and characterised in terms of neutron/γ-ray (n/γ) discrimination capability by the Pulse Shape Discrimination (PSD) method, energy resolution and quenching factor. Both crystals exhibit an excellent n/γ discrimination, quantified by a Figure of Merit higher than 2. The measured thermal energy resolution of CLYC-6 is 6%. The quenching factor for the 6Li(n,t)α thermal neutron reaction is 0.65 and for the 35Cl(n,p)35S reaction is 0.91. The measured neutron detection efficiency per unit volume of CLYC-6 is 60% higher than that of a 3He proportional counter. CLYC-7 was tested as a fast neutron spectrometer below 10 MeV, when irradiated with continuum neutron spectra from Am–Be and 252Cf sources. A proton/α-particle (p/α) discrimination was performed exploiting the PSD technique, without any unfolding procedure. The resulting spectra match the corresponding ISO spectra better than the spectra without the p/α discrimination, especially in the range 3 MeV to 5 MeV. A more sensitive PSD algorithm might help to improve particle discrimination at lower energies, while different reaction channels arise at higher energies preventing a spectrometry analysis.

Design of a neutron shielding performance test system base on Am–Be neutron source

A system for neutron shielding performance test was designed based on an Am–Be neutron source and 3He proportional counter. The Am–Be neutron source was shielded by paraffin wax block with a thickness of dozens of centimeters and collimated by polyethylene rod. The neutron energy spectra in front of the collimator were simulated by MCNP. The detector system was designed based on 3He proportional counter and different response tuning assembly (RTA) to optimize the neutron energy region of interest. A method for testing neutron shielding performance for thermal, epithermal, fast neutrons and neutron ambient dose equivalent rate was proposed by employing different response tuning assembly (RTA). Common radiation shielding materials including polyethylene (PE), borated polyethylene (BPE) and graphite were conducted with the designed system. The results demonstrated that the proposed method is efficient for investigating the neutron shielding performance of materials.

Design and verification of a multi-layer single-sphere neutron spectrometer with water as the moderator

Measuring the neutron energy spectrum is important in nuclear radiation detection and protection. Common neutron spectrometers include the Bonner sphere spectrometer (BSS), time-of-flight neutron spectrometer, and plastic scintillation detector. Among them, the BSS is the most widely used for its wide measurement range and simple operation. A BSS usually occupies a large space because it contains several independent spheres working at the same time, leading to poor consistency. This paper proposes a multi-layer single-sphere spectrometer using water as the moderator. The spectrometer includes a multi-layered sphere that can be filled with water or air and a ^3He proportional counter placed in the center of the sphere. To verify the feasibility of this design, we use Geant4 to calculate the moderating ability of water and the response functions of the multi-layer single-sphere spectrometer. Additionally, several standard neutron energy spectra (from IAEA) are used to test the response characteristics of this spectrometer through simulation. The simulation results prove the feasibility of the design. This research provides a theoretical basis for a multi-layer single-sphere neutron spectrometer with water as the moderator.

A thermal neutron source for the CERN radiation Calibration Laboratory

This work presents the design, construction and experimental characterisation of a lightweight and low-cost thermal neutron assembly, to be used with the existing Am–Be source irradiator of CERN radiation Calibration Laboratory (Cal Lab). The assembly consists of a cylindrical moderator (18 cm diameter, 25.5 cm height and 5.5 kg weight) and an optional reflector box (5 cm thick walls, 20 kg weight). The moderator is tailored to fit on the Am–Be source in its irradiation position, while the box encloses the detector under test during the irradiation. The exposure volume delimited by the box is 30 × 30 × 30 cm3. The thermal neutron fluence at the exposure location, i.e., 30 cm from the source, was optimized by FLUKA Monte Carlo (MC) simulations. The simulations were validated with measurements performed with a bare 3He proportional counter. The thermal neutron fluence at the nominal irradiation position is 7.43 × 102 cm−2s−1 with the cylindrical moderator only, and 5.75 × 103 cm−2s−1 with the cylinder and the reflector box, with the detector placed at the centre of the box. The thermal neutron fluence inside the box is rather uniform (variation <5%).