Thermal Neutron Peak Research Articles

Neutron detection properties of Ti-doped LiGaO2 single crystalline scintillator

Photoluminescence (PL) and scintillation properties of undoped and Ti-doped LiGaO2 single crystals were investigated. In both PL and scintillation, the Ti-doped LiGaO2 crystals showed luminescence due to charge transfer between Ti4+–O2- while the undoped LiGaO2 crystal showed luminescence due to oxygen vacancies. Under thermal neutron irradiation, all the LiGaO2 crystals exhibited a clear thermal neutron peak, and the highest light yield was observed from the 3% Ti-doped LiGaO2 crystal. The light yield of the 3% Ti-doped LiGaO2 crystal was ten times higher than that of the undoped LiGaO2 crystal and comparable with that of a commercial neutron scintillator GS20. In addition, the Ti-doped LiGaO2 crystals showed high α/γ ratios up to about 0.7.

Growth and characterization of 6LiI:Ag crystal scintillators for thermal and epithermal neutron detection on the lunar surface

This paper discusses crystal growth and characterization of 1.0-inch 6LiI:Ag scintillators for thermal and epithermal neutron detection on the lunar surface as part of the Lunar Vehicle Radiation Dosimeter project. Zone purification method was employed to purify the raw material of 6LiI, and the crystals were grown using the Bridgman method. X-ray luminescence spectrum was measured to confirm the presence of Ag+ in the crystal lattice of 6LiI. The feasibility of neutron detection under 252Cf fast neutron irradiation was investigated. A hydrogen-rich paraffin wax moderator was used to slow down fast neutrons to obtain thermal and epithermal neutrons. The charge comparison method was used to perform pulse shape discrimination. The maximum value of the Figure of Merit was obtained to be 1.3, which demonstrates the capability of using 6LiI:Ag crystal scintillators in high neutron and gamma mixed fields. The energy resolution of the thermal neutron peak varied with the thickness of scintillators, achieving 12.0 % for the 0.5 cm thick samples. The potential for further improvement of the scintillation performance of 6LiI:Ag crystal relies on reducing impurities and color centers.

MLEM Neutron Spectra Unfolding in a Radiotherapy Bunker Using Bonner Sphere Spectrometer

High-energy radiotherapy treatments of a medical Linear Accelerator (LinAc) generate secondary neutrons that can produce health damage on the human body as the induction of secondary cancers. The energy spectrum of these neutrons must be determined to estimate the extra dose received by patients inside a radiotherapy room during radiotherapy treatment. To quantify the neutron production, a Ludlum Bonner sphere spectrometer (BSS) is used for measurement at different points of a LinAc bunker at the Hospital Universitari i Politècnic La Fe de València. With the neutron measured data and a set of response detector curves obtained by Monte Carlo simulations with MCNP6.1.1, the Maximum Likelihood Expectation Maximization unfolding method is used to unfold the energy neutron spectrum. Unfolded neutron spectra at different locations were compared to those obtained by Monte Carlo simulation of the same setup, showing the same energetic behavior. The fluence rate decreases with source distance, and the shape changes from a fast neutron peak in the nearest LinAc head location to a prominent thermal neutron peak in the bunker maze region. Moreover, the neutron ambient equivalent dose was obtained from the unfolded spectra and compared to Berthold detector measurements, being consistent.

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.

Radiation damage on the performance of a silicon photomultiplier-coupled Cs2LiYCl6:Ce3+ detector

A silicon photomultiplier (SiPM) coupled Cs2LiYCl6:Ce3+ (CLYC) detector was developed for neutron detection in the narrow, mixed radiation field. Studies show that the SiPM and scintillator will suffer a certain degree of damage in high-intensity irradiation environments. To evaluate the effects of the radiation damage, the detector was exposed to a zero-power research reactor. The thermal neutron exposures ranged from 0 up to 7.60 × 107 n/cm2. The performance specifications on the gamma ray and thermal neutron pulses, 6Li(n, α)3H thermal neutron peaks, energy resolutions, and pulse shape discrimination were analyzed. The results showed that the standard gamma ray and thermal neutron pulses were slightly altered, the thermal neutron peak centroid reduced by 40%, the energy resolution of the 6Li(n, α)3H thermal neutron peak deteriorated from 15% to 21%, and the figure of merit of pulse shape discrimination decreased from 1.41 to 1.16. These variation curves can be applied to evaluate and correct the detector irradiation damage.

Underground Low Flux Neutron Background Measurements in LSM Using a Large Volume (1m3) Spherical Proportional Counter

A large volume (1m3) spherical proportional counter has been developed at CEA/Saclay, for low flux neutron measurements. The high voltage is applied to a small sphere 15mm in diameter, located in the center of the counter and the wall of the counter is grounded. Neutrons can be measured successfully, with high sensitivity, using 3He gas in the detector. The proton and tritium energy deposition in the drift gaseous volume, from the reaction 3He(n,p)3H, can provide the neutron spectra from thermal neutrons up to several MeV.The detector has been installed in the underground laboratory in Modane (LSM) to measure the neutron background. The sphere has been has been filled with gas mixture of Ar + 2% CH4 +3gr He-3, at 275 mbar. The thermal neutron peak is well separated from the cosmic ray and gamma background, permitting of neutron flux calculation. Other potential applications requiring large volume of about 10 m in radius are described in detail in reference

Neutronic Study of Fuel Depletion for the MNSR Research Reactor Using DRAGON5 Code

The aim of this work is to study the fuel depletion of 30 kW miniature neutron source reactor (MNSR). Under operating conditions of two hours per day for five days a week at a peak thermal neutron flux of 1.0E+12 n/cm2 s, the estimated core life is 10 years (200 days). The DRAGON5 code is utilizing to generate the fuel group constants and the infinite multiplication factor versus the MNSR reactor operating time, at different burn-up values. The amounts of uranium burnt up and plutonium produced in the reactor core are calculated using the DRAGON5 transport lattice code. The results are in good agreement with previous studies.

Analysis of Depletion and Inventory of the Fuel for the MNSR Research Reactor Using the Deterministic Code DRAGON5

The Syrian miniature neutron source reactor (MNSR), a 30-kW, 90.0% highly enriched uranium fueled (U-Al) MNSR-type reactor has gone critical. Under operating conditions of 2 h per day for 5 days a week at a peak thermal neutron flux of 1.0 × 1012 n/cm2·s, the estimated core life is 10 years. After the fuel is depleted, the full spent-fuel assembly will be replaced with new low-enriched uranium. This study presents the results of a multigroup fuel burnup and depletion analysis of the MNSR fuel lattice using the DRAGON5 transport lattice code. Furthermore, infinite multiplication factor k∞ and several two-group macroscopic parameters, including scattering cross section, fission cross section, total cross section, and diffusion coefficient, and the transport mean free path have been studied. In addition to this, fuel isotopic composition dependency on burnup was calculated as a part of this study. The results contained in this study can be used as a microscopic database for performing criticality safety analysis and shielding computations for the design of a spent-fuel storage cask for the MNSR reactor core.

Neutronics Analyses for the ORNL’s Spallation Neutron Source Second Target Station

This paper presents an update of the status of the neutronics analyses performed for the Second Target Station (STS). The target station is driven with short (less than 1 micro second long) proton pulses at 15 Hz repetition rate and 700 kW proton beam power. The target will be optimized for high intensity and high resolution long wavelength neutron applications. The STS will accommodate 22 beamlines and will expand and complement the current national neutron scattering capabilities. The proton beam footprint as small as acceptable from the mechanical and heat removal aspects is planned to generate a compact-volume neutron production zone in the target, which is essential for tight coupling of the target and the moderators and for achieving high-intensity peak thermal and cold neutron fluxes. Present efforts to develop high fidelity engineering models for neutronics analyses with automatic CAD-to-MCNP conversion are described. Heating rates and radiation damage, which provide input in the engineering design are presented and the performance of the moderators is briefly addressed.

Characterization of the internal background for thermal and fast neutron detection with CLLB

We report on a set of experiments conducted to determine what effects, if any, the internal background in the CLLB scintillation detector has on the thermal neutron detection performance. We conducted source measurements using an unmoderated and moderated 252Cf neutron/γ-ray source and long (48-h), unshielded and shielded, background measurements to characterize the internal background with and without a source present. These measurements allowed us to determine the 2-d event selections needed to isolate the thermal neutron peak observed in pulse shape vs. energy space and apply those selections to our background measurements. Our results indicate that the thermal neutron detection capabilities of the CLLB are marginally affected by the presence of internal background. An unmoderated 113-µCi 252Cf source at 15cm from the detector yields a thermal neutron rate of 8×10−2/scm3, while moderating the source with 5cm of polyethylene yields a thermal neutron rate of 5.5×10−1/scm3. The measured background rate for events that fall within the selected thermal neutron region is 1.2×10−3/scm3. Lastly, the potential for CLLB for detecting fast neutrons was investigated.

Lithium Alkaline Halides—Next Generation of Dual Mode Scintillators

We report on a new family of scintillators - Lithium alkaline halides , developed based on the alkaline halides by introducing lithium for dual mode gamma-neutron detection. Many different compositions were grown, among which ${\rm LiSr}_{2}{\rm I}_{5}$ (LSI), ${\rm LiCa}_{2}{\rm I}_{5}$ (LCI), ${\rm LiSr}_{2}{\rm Br}_{5}$ (LSB) activated with divalent Europium show good gamma and neutron detection properties. LSI shows the main emission at ${\sim}$ 497 nm under X-ray excitation. It also shows good proportionality, which in combination with the light yield as high as 60000 photons/MeV, results in an energy resolution of 3.5% at 662 keV. The electron or gamma equivalent energy (GEE) of the thermal neutron peak due to the $^6{\rm Li}$ neutron capture is 4.1 MeV, which amounts to a very high neutron light yield of ${\sim}$ 245000 photons. The decay times for neutrons are faster compared to that for gamma-rays, hence we achieved good pulse shape discrimination (PSD) between gamma and neutron events. Our initial studies on the effects of Eu concentration on the properties of LSI show that 3%–4% Eu concentration is optimal for the best performance in terms of gamma and neutron light yields and pulse shape discrimination. LCI shows the main emission at ${\sim}$ 475 nm under X-ray excitation and a very high gamma light yield of ${\sim}$ 90000 photons/MeV. The measured energy resolution is 6% at 662 keV. The electron equivalent energy for neutron detection has been measured to be around 3 MeV, which gives a neutron light yield of ${\sim}$ 270000 photons. The measured decay times for neutrons are faster compared to gamma decays and the PSD between the gamma-rays and neutrons is not as good as LSI. LSB shows two emissions at ${\sim}$ 410 and 475 nm under X-ray excitation. The measured light yield is ${\sim}$ 32000 ph/MeV gamma-ray with an energy resolution of 6% at 662 keV. The electron equivalent energy of the $^6{\rm Li}$ capture peak was measured to be 3.3 MeV.

Integrated Neutron Detector for Handheld Systems

This paper describes detectors comprised of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Ce (CLYC) scintillators coupled to silicon photomultipliers (MPPC, Hamamatsu). CLYC has been developed for dual gamma ray and thermal neutron detection. MPPCs are compact detectors with a very thin profile (~2 mm), but also a small active area (6 mm × 6 mm). The combination of both can create a compact replacement of a He-3 tube. Three different crystal sizes were tested: a 5 mm × 5 mm × 5 mm cube, a 1 cm × 1 cm × 4 cm cuboid, and a Ø1 in × 1 in cylinder. All three detectors showed a well-resolved thermal neutron peak with energy resolution of 5.4%, 7%, and 11% (FWHM), respectively. The degradation in larger crystals is due to a mismatch between the coupled crystal face and the active area of the light detector. All detectors showed the 662 keV gamma ray peak from a Cs-137 source, indicating capability for limited gamma ray spectroscopy, with energy resolutions of 10%, 17%, and 26%, respectively. The capability for pulse shape discrimination was shown as well. The efficiency of the detectors was measured against a 10 atm He-3 tube (9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> volume). The best absolute efficiency was shown by the cylinder detector, next by the He-3 tube (almost the same as the cylinder detector), and finally by the cuboid detector. The latter showed the best neutron count per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , twice as high as the two other detectors.

Design studies of a typical PWR core using advanced computational tools and techniques

This study deals with the design and development of calculational techniques and evaluation of key neutronic parameters of a typical PWR core having a total reactor power of 2652 MW t (890 MW e). The PWR core consists of 157 fuel assemblies containing a total of ∼72 tons of uranium arranged vertically in a concentric square array within the core shroud. Each fuel assembly contains 264 UO 2 fuel pins with various enrichments (2.1, 2.6 and 3.1%), 24 control rods of Gd 2O 3 and one central water channel and all are arranged in a 17 × 17 array of matrix. Different computer codes including WIMS, TWOTRAN, CITATION and MCNP have been employed to develop a versatile and accurate reactor physics model of the PWR core. The computational methods, tools and techniques, customization of cross section libraries, various models for cells and super cells, and a lot of associated utilities have been standardized and established/validated for the overall core analysis. The analyses were performed in 3 steps: firstly for fuel pincells, then for the fuel assemblies and finally for the whole core. The WIMS and MCNP calculated infinite multiplication factors for fuel pincells having 2.1% enriched 235U were found to be 1.23393 and 1.23654, for 2.6% enrichment 1.28635 and 1.28887, and finally for 3.1% enrichment 1.32481 and 1.32812, respectively. For fuel assembly, WIMS and MCNP calculated infinite multiplication factors having 2.1% enrichment were found to be 1.24853 and 1.25445, for 2.6% enrichment 1.30372 and 1.30992, and for 3.1% enrichment 1.34424 and 1.35041, respectively. The effective multiplication factor calculated by CITATION, TWOTRAN and MCNP for whole core were found to be 1.25580, 1.25909 and 1.26382, respectively. The peak thermal neutron flux in the core calculated by MCNP was found to be 5.0298 × 10 14 neutrons/cm 2 s and the average core power density was 17.1 kW/cm 3. The calculated results from different codes were found to be very good agreement for different moderator conditions. The choice of computer codes like WIMSD, TWOTRAN, CITATION and MCNP which are being used in nuclear industry for many years were selected to identify and develop new capabilities needed to support PWR analysis. The ultimate goal of the validation of the computer codes for PWR applications is to acquire and reinforce the capability of these general purpose computer codes to perform the core design and optimization study.

Temperature Dependence of Li-Glass Scintillator Response to Neutrons

A Li-glass scintillator was placed in the slow neutrono eld at temperatures of 300 K and 77 K. An avalanche photodiode (APD) was employed for converting scintillation photons into voltage signal pulses. A peak corresponding to the Q-value of the 6 Li(n, α)T reaction appeared in both pulse height distributions obtained at temperatures of 300 K and 77 K. By using the relationship between the Compton-edge energy and the pulse height in response of the Li-glass scintillator to γ-rays radiated from 137 Cs, the values of the gamma equivalent energy of the thermal neutron peak were evaluated to be 1.60 ± 0.09 MeV and 1.16 ± 0.06 MeV at temperatures of 300 K and 77 K, respectively. For the Li-glass scintillator, the ratio of the light yield in thermal neutron detection to that in γ-ray detection was found to decrease with temperatures.

Selected Properties of Cs$_{2}$LiYCl$_{6}$, Cs$_{2}$LiLaCl$_{6}$, and Cs$_{2}$LiLaBr$_{6}$ Scintillators

Homeland security applications often require detection of both neutron and gamma radiation. A combination of two detectors registering neutrons and gammas separately is typically used. Recently, a number of scintillators from the elpasolite crystal family were proposed, that provide detection of both types of radiation. The most promising are Cs2LiYCl6, Cs2LiLaCl6, and Cs2LiLaBr6. All are doped with Ce3+. They are capable of providing very high energy resolution. The best values achieved for each material are 3.9%, 3.4%, and 2.9% at 662 keV (FWHM), respectively. Since 6Li has an acceptable cross-section for thermal neutron capture, these materials also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak typically appears above 3 MeV gamma equivalent energy. Thus very effective pulse height discrimination can be implemented with these materials. The CLLC and CLYC emissions consist of two main components: Core-to-Valence Luminescence (CVL; 220 nm to 320 nm) and Ce emission (350 to 500 nm). The former is of particular interest since it appears only under gamma excitation. It is also very fast and decays with less than 2 ns time constant. The CVL provides a significant difference to temporal responses under gamma and neutron excitation thus it may be used for effective pulse shape discrimination.

Evaluation of a LiI(Eu) neutron detector with coincident double photodiode readout

Previous work showed that enriched 6Li halide scintillation crystal is a good candidate for portable neutron-sensitive detectors. Photodiode readout is a good alternative to PMT in compact devices. These detectors are often required to work in presence of a strong gamma background. Therefore, great discrimination against gamma rays is crucial. Because of the high Q-value of the 6Li(n,α)3H reaction, the light yield of a neutron capture signal corresponds to 3–4 MeV gamma equivalent in spite of the quenching effect of heavily charged particles. As a result, energy discrimination is quite effective against gamma signals generated in thin crystals. However, direct gamma interactions inside the photodiode can create pulses whose amplitude is large enough to interfere with thermal neutron peak. This study shows an innovative design based on coincident readout to solve this problem. In this design, two photodiodes are attached on both sides of the LiI crystal. The output signal is only accepted when both photodiodes give out coincident output. The method is proved to effectively suppress background in the neutron window in a 420 mR/h 137Cs field down to the level of natural background.

Detail analysis of fusion neutronics benchmark experiment on beryllium

Our previous analysis of the integral experiments (in situ and TOF experiments) on beryllium with DT neutrons at JAEA/FNS pointed out two problems by using MCNP4C and the latest nuclear data libraries; one was a strange larger neutron peak around 12 MeV appearing in the TOF experiment analysis with JEFF-3.1 and the other was an overestimation on law energy neutrons in the in situ experiment analyses with all the nuclear data libraries. We investigated reasons for these problems in detail. It was found out that the official ACE file MCJEFF3.1 of JEFF-3.1 had an inconsistency with the original JEFF-3.1, which caused the strange larger neutron peak around 12 MeV in the TOF experiment analysis. We also found out that the calculated thermal neutron peak was probably too large in the in situ experiment. On trial we examined influence of the thermal neutron scattering law data of beryllium metal in ENDF/B-VI. The result pointed out that the coherent elastic scattering cross-section data in the thermal neutron scattering law data of beryllium metal were probably too large.

Underground low flux neutron background measurements in LSM using a large volume (1m3) spherical proportional counter

A large volume (1m3) spherical proportional counter has been developed at CEA/Saclay, for low flux neutron measurements. The high voltage is applied to a small sphere 15mm in diameter, located in the center of the counter and the wall of the counter is grounded. Neutrons can be measured successfully, with high sensitivity, using 3He gas in the detector. The proton and tritium energy deposition in the drift gaseous volume, from the reaction 3He(n,p)3H, can provide the neutron spectra from thermal neutrons up to several MeV.The detector has been installed in the underground laboratory in Modane (LSM) to measure the neutron background. The sphere has been has been filled with gas mixture of Ar + 2% CH4 +3gr He-3, at 275 mbar. The thermal neutron peak is well separated from the cosmic ray and gamma background, permitting of neutron flux calculation. Other potential applications requiring large volume of about 10 m in radius are described in detail in reference

${\rm Cs}_{2}{\rm LiYCl}_{6}:{\rm Ce}$ Scintillator for Nuclear Monitoring Applications

This paper summarizes the initial investigation of large diameter (2-inch) Cs2LiYCl6:Ce (CLYC) crystals grown at RMD. Although the crystals had some cracks, the tested sample provided adequate results: It produced a clear thermal neutron peak, a difference in time profiles under gamma and neutron excitation was observed, and pulse shape discrimination (PSD) based on two integration windows and their ratios was achieved. The PSD provided an excellent separation between gamma and neutron events. The discrimination ratio was better than 1:1000 (based on 15,000 events). We have also tested a relatively smaller sample with excellent crystal quality for the energy resolution. The results surpassed those previously obtained. The recorded energy resolution at 662 keV was 4.7% (FWHM) using a standard bialkali PMT. This is better than the previously measured 5.1% value. A 4.3% energy resolution was obtained with a super bialkali PMT.

Cosmic-ray Neutron Spectrometry and Dosimetry

When primary cosmic rays consisting of galactic cosmic rays and solar particles come into the earth’s atmosphere, secondary neutrons generated through nuclear reactions with atmospheric atoms reach the ground. Over the past 10 years, there has been increasing concern about the exposure of air crews to atmospheric cosmic radiation. At aviation altitudes, the neutron component of the secondary cosmic radiation contributes about half of the dose equivalent. Recently, an accumulation of the semiconductor device greatly increases and the soft-errors of SRAM and DRAM on the ground level caused by high-energy cosmic-ray neutrons become a serious problem in the world. Under these circumstances, cosmic-ray neutron spectrometry and dosimetry are presented here in the terrestrial and space environments where neutrons and protons coexist.The neutron detection methods for use in this mixed field are described; 1) multi-moderator spectrometer (Bonner Ball), 2) organic liquid scintillation spectrometer, 3) dose-equivalent counter (rem counter) and 4) Phoswich-type detector. Using these detectors, neutron energy spectra and dose-equivalent rates have been measured on the ground at sea level and at mountain level, aboard an airplane and in space. The space experiments were done using a balloon, space shuttle and space station. The neutron spectrum on the ground has three major peaks, thermal energy peak, evaporation peak around 1 MeV and cascade peak around 100 MeV. While on the other hand, the neutron spectrum apart from the ground has no thermal neutron peak that comes from the albedo neutron effect backscattered from the terrestrial surface. The time-sequential experimental results in Japan, Europe and U.S.A. are described with the experimental procedures by paying attention to variation with latitude, altitude and solar activity.