Monoenergetic Neutron Beams Research Articles

Activation cross section of the (n,2n) reaction on Hf isotopes

Cross sections of the 174Hf(n,2n)173Hf and 176Hf(n,2n)175Hf reactions have been experimentally determined relative to the 27Al(n,α)24Na reference reaction at incident neutron energies of 15.3 and 17.1 MeV by means of the activation technique. The irradiations were carried out at the 5 MV tandem T11/25 Accelerator Laboratory of NCSR "Demokritos" with monoenergetic neutron beams provided via the 3H(d,n)4He reaction, using a new Ti-tritiated target of 373 GBq activity. In the determination of the 176Hf(n,2n)175Hf reaction cross section the contamination of the 174Hf(n,γ)175Hf and 177Hf(n,3n)175Hf reactions has been taken into account. Moreover, the neutron beam energy has been studied by means of Monte Carlo simulation codes and the neutron flux has been determined via the 27Al(n,α)24Na reference reaction.

Activation cross section for the (n,2n) reaction on 197Au

The cross section of the reaction channels 197Au(n,2n)196Aug+m1 and 197Au(n,2n)196Aum2 has been experimentally determined at 15.3 MeV, relative to the 27Al(n,α)24Na reference reaction, using the activation technique. The irradiation was carried out at the 5 MV tandem T11/25 Accelerator Laboratory of NCSR "Demokritos" with monoenergetic neutron beam provided by means of the 3H(d,n)4He reaction, using a new Ti-tritiated target of 373 GBq activity. The induced γ- ray activity was measured with a high resolution HPGe detector.

Application of stilbene crystals in neutron measurement for monoenergetic neutron beam

Stilbene crystals are used as neutron detectors connected with photomultiplier tubes. An online acquisition method assembled by CAMAC standard plug-ins, NIM plug-ins, a neutron-γ (n-γ) discrimination plug-in named Canberra-2160A, and an offline acquisition method, assembled with an oscilloscope, were developed to obtain nuclear signals from neutron detectors. The online and offline acquisition methods collected pulse signals simultaneously while n-γ discrimination and measurement of deuterium-deuterium (DD) and deuterium-tritium (DT) neutron beam were carried out by both acquisition methods. The resulting n-γ discrimination, detection efficiency, and neutron spectra of varying stilbene crystal sizes were obtained. The neutron events accounted for approximately 80% of the total events in this radiation field formed by DD neutron beam and 50% for DT neutron beam. The results of n-γ discrimination of the two acquisition methods matched very well. The figures of merit (FoM) of n-γ discrimination obtained by the online acquisition method were 2.158 for the radiation field formed by DD neutron beam and 1.802 for DT when over 105 pulses were used for analysis. The pulses acquired by the offline acquisition method were repeatedly used to obtain the best discrimination effects, neutron charge-amplitude spectrum, and pulse height spectrum after the particular waveform digitization process. It was shown that the offline acquisition method composed of stilbene crystals, a photomultiplier tube, and a digital oscilloscope could realize the same function and effect as the online acquisition method for monoenergetic neutron beam, but with fewer pulses. A feasible experimental method was obtained which have a good potential for the neutron flux monitoring of the pulsed-neutron radiation fields, in which the total number of events is small or the neutron propagation process is instantaneous.

Towards radiation hard converter material for SiC-based fast neutron detectors

In the present work, Geant4 Monte-Carlo simulations have been carried out to study the neutron detection efficiency of the various neutron to other charge particle (recoil proton) converter materials. The converter material is placed over Silicon Carbide (SiC) in Fast Neutron detectors (FNDs) to achieve higher neutron detection efficiency as compared to bare SiC FNDs. Hydrogenous converter material such as High-Density Polyethylene (HDPE) is preferred over other converter materials due to the virtue of its high elastic scattering reaction cross-section for fast neutron detection at room temperature. Upon interaction with fast neutrons, hydrogenous converter material generates recoil protons which liberate e-hole pairs in the active region of SiC detector to provide a detector signal. The neutron detection efficiency offered by HDPE converter is compared with several other hydrogenous materials viz., 1) Lithium Hydride (LiH), 2) Perylene, 3) PTCDA . It is found that, HDPE, though providing highest efficiency among various studied materials, cannot withstand high temperature and harsh radiation environment. On the other hand, perylene and PTCDA can sustain harsh environments, but yields low efficiency. The analysis carried out reveals that LiH is a better material for neutron to other charge particle conversion with competent efficiency and desired radiation hardness. Further, the thickness of LiH has also been optimized for various mono-energetic neutron beams and Am-Be neutron source generating a neutron fluence of 109 neutrons/cm2. The optimized thickness of LiH converter for fast neutron detection is found to be ∼ 500 μm. However, the estimated efficiency for fast neutron detection is only 0.1%, which is deemed to be inadequate for reliable detection of neutrons. A sensitivity study has also been done investigating the gamma background effect on the neutron detection efficiency for various energy threshold of Low-Level Discriminator (LLD). The detection efficiency of a stacked structure concept has been explored by juxtaposing several converter-detector layers to improve the efficiency of LiH-SiC-based FNDs . It is observed that approximately tenfold efficiency improvement has been achieved—0.93% for ten layers stacked configuration vis-à-vis 0.1% of single converter-detector layer configuration. Finally, stacked detectors have also been simulated for different converter thicknesses to attain the efficiency as high as ∼ 3.25% with the help of 50 stacked layers.

Activation cross section and isomeric cross section ratio for the 76Ge(n,2n)75m,gGe process

.We measured neutron-induced reaction cross sections for the 76Ge(n,2n)75m,gGe reactions and their isomeric cross section ratios sigma_{m}/sigma_{g} at three neutron energies between 13 and 15MeV by an activation and off-line gamma-ray spectrometric technique using the K-400 Neutron Generator at the Chinese Academy of Engineering Physics (CAEP). Ge samples and Nb monitor foils were activated together to determine the reaction cross section and the incident neutron flux. The monoenergetic neutron beams were formed via the 3H(d,n)4He reaction. The pure cross section of the ground state was derived from the absolute cross section of the metastable state and the residual nuclear decay analysis. The cross sections were also calculated using the nuclear model code TALYS-1.8 with different level density options at neutron energies varying from the reaction threshold to 20MeV. Results are discussed and compared with the corresponding literature data.

Use of SRIM and Garfield with Geant4 for the characterization of a hybrid 10B/3He neutron detector

This paper describes a method for more complete neutron detector characterization using Geant4’s Monte Carlo methods for characterizing overall detector response rate and Garfield interfaced with SRIM for the simulation of the detector’s raw pulses, as applied to a hybrid10B/3He detector. The Geant4 models characterizing the detector’s interaction with a 252Cf point source and parallel beams of mono-energetic neutrons (assuming ISO 8529 reference energy values) compare and agree well with calibrated252Cf measurements to within 6.4%. Validated Geant4 model outputs serve as input to Garfield+SRIM calculations to provide meaningful pulse height spectra. Modifications to Garfield for this work were necessary to account for simultaneous tracking of electrons resulting from proton and triton reaction products from a single3He neutron capture event, and it was further necessary to interface Garfield with the energy loss, range, and straggling calculations provided by SRIM. Individual raw pulses generated by Garfield+SRIM are also observed to agree well with experimentally measured raw pulses from the detector.

Activation cross section and isomeric cross-section ratio for the 151Eu(n,2n)150m,gEu process

The cross sections of 151Eu(n,2n)150m,gEu reactions and their isomeric cross section ratios σm/σt have been measured experimentally. Cross sections are measured, relative to a reference 93Nb(n,2n)92mNb reaction cross section, by means of the activation technique at three neutron energies 13.5, 14.1, and 14.8 MeV. Monoenergetic neutron beams were formed via the 3H(d,n)4He reaction and both Eu2O3 samples and Nb monitor foils were activated together to determine the reaction cross section and the incident neutron flux. The activities induced in the reaction products were measured using high−resolution gamma ray spectroscopy. Cross sections were also evaluated theoretically using the numerical nuclear model code, TALYS-1.8 with different level density options at neutron energies varying from the reaction threshold to 20 MeV. Results are discussed and compared with the corresponding literature.

DETERMINATION OF NEUTRON EFFECTIVE DOSES IN WHOLE BODY POINT SOURCE EXPOSURES.

The objective of this work is to obtain fluence to effective dose conversion coefficients for neutron point sources, using the GEANT4 toolkit. These calculations aim to investigate the aspects of neutron transport in the human body through Monte Carlo simulation using the International Commission on Radiological Protection (ICRP) voxel phantoms, described in its publication 110. A benchmarking of the code was made for the case of monoenergetic plane parallel neutron beam in the antero-posterior (AP) irradiation geometry and organ absorbed dose conversion coefficients were compared with those found in the ICRP publication 116. The results showed good agreement with ICRP results in the studied energy range. Conversion coefficients were presented for specific conditions with 241Am-Be and 252Cf point neutron sources 1 m away from the phantom in the AP geometry.

Characterization of EJ-200 plastic scintillators as active background shield for cosmogenic radiation

This paper describes the characterization of commercially available plastic scintillation detectors to be used as an active shield or veto system to reduce the neutron background resulting from atmospheric muon interactions in low-level nuclear waste assay systems. The shield consists of an array of scintillation detectors surrounding a neutron detection system. Scintillation detectors with different thicknesses are characterized for their response to gamma rays, neutrons, and muons. Response functions to gamma rays were determined and measured in the energy range from 0.6 MeV to 6.0 MeV using radionuclide sources. Neutron response functions were derived from results of time-of-flight measurements at the Van de Graaff accelerator of the INFN Legnaro and from measurements with quasi mono-energetic neutron beams produced at the Van de Graaff accelerator of the JRC Geel. From these data, the light output and resolution functions for protons and electrons were derived. The response to muons was verified by background measurements, i.e. without the presence of any neutron or gamma source. It was found that the muon peak is more pronounced when the detectors are placed horizontally. The results indicate that a scintillator with a minimum thickness of 20 mm is needed to separate events due to atmospheric muons from natural gamma ray background, and contributions due to neutron production in nuclear waste based on only the total energy deposition in the detector. In addition, it was shown that muons can be identified with a coincidence pattern when the detectors are stacked. The effectiveness of the proposed system was demonstrated based on muon induced spallation reactions in a lead sample.

Accurate U238 ( n, 2n ) U237 reaction cross-section measurements from 6.5 to 14.8 MeV

The cross section for the $^{238}\mathrm{U}(n,2n)^{237}\mathrm{U}$ reaction has been measured in the incident neutron energy range from 6.5 to 14.8 MeV in small energy steps using an activation technique. Monoenergetic neutron beams were produced via the $^{2}\mathrm{H}(d,n)^{3}\mathrm{He}$ and $^{3}\mathrm{H}(d,n)^{4}\mathrm{He}$ reactions. $^{238}\mathrm{U}$ targets were activated along with Au and Al monitor foils to determine the incident neutron flux. The activity of the reaction products was measured in TUNL's low-background counting facility using high-resolution $\ensuremath{\gamma}$-ray spectroscopy. The results are compared with previous measurements and latest data evaluations. Statistical-model calculations, based on the Hauser-Feshbach formalism, have been carried out using the ${\mathrm{CoH}}_{3}$ code and are compared with the experimental results. The present self-consistent and high-quality data are important for stockpile stewardship and nuclear forensic purposes as well as for the design and operation of fast reactors.

Monte-Carlo modeling and simulation of neutrons detection process using Boron-10

The interaction of a mono-energetic neutron beam with a target layer of Boron-10 is studied to characterize the behavior of neutrons and produced charged particles inside the layer. The detection system is modeled through a Monte-Carlo simulation. The quality of the simulation method and controlling parameters is assessed with the response function. The Von Neumann rejection sampling method is used to model the interaction process. The obtained results show high agreement with previous results.

Activation Cross Section and Isomeric Cross-Section Ratio for the (n,2n) Reaction on 113,115In

Cross sections of the 113In(n,2n)112m,gIn and 115In(n,2n)114m,gIn reactions and their isomeric cross-section ratios σm/σg have been measured by means of the activation technique at three neutron energies in the range 13 to 15 MeV. Indium samples and niobium monitor foils were activated together to determine the reaction cross section and the incident neutron flux. The monoenergetic neutron beam was produced via the 3H(d,n)4He reaction at the Pd-300 Neutron Generator of the Chinese Academy of Engineering Physics. The activities induced in the reaction products were measured using high-resolution gamma-ray spectroscopy. The pure cross section of the ground state was derived from the absolute cross section of the metastable state and the residual nuclear decay analysis. Cross sections were also evaluated theoretically using the numerical nuclear model code TALYS-1.8 with different level density options at neutron energies varying from the reaction threshold to 20 MeV. Results are discussed and compared with the corresponding literature.

Tests of a solution-grown stilbene scintillator in mono-energetic neutron beams of 565 keV and 5 MeV

The results of measurements performed with a solution-grown stilbene scintillator placed in reference mono-energetic neutron fields are presented. The ∅ 25 mm organic scintillator was positioned in 5 MeV and 565 keV neutron fields delivered by the AIFIRA facility at CENBG. The goal of the experiment was to assess the performance of the solution-grown stilbene crystal (n-γ discrimination, response, anisotropy, sensitivity) relative to that of a BC501A liquid scintillator of larger size. Neutron pulse height spectra after gamma discrimination are compared. The results show that the stilbene crystal not only has a better discrimination capability than the BC501A (35% higher FoM) at 5 MeV, but is also able to separate neutrons from gamma-rays at 565 keV and below, a range where the BC501A is inoperative. This study also confirms the anisotropy of the crystal response, as already observed by other groups at different energies.

Experimental determination of the response functions of a Bonner sphere spectrometer to monoenergetic neutrons

A Bonner sphere spectrometer (BSS) plays an important role in characterizing neutron spectra and determining their neutron dose in a neutron-gamma mixed field. A BSS consisting of a set of nine polyethylene spheres with a 3He proportional counter was developed at Peking University to perform neutron spectrum and dosimetry measurements. Response functions (RFs) of the BSS were calculated with the general Monte Carlo code MCNP5 for the neutron energy range from thermal up to 20 MeV, and were experimentally calibrated with monoenergetic neutron beams from 144 keV to 14 MeV on a 4.5 MV Van de Graaff accelerator. The calculated RFs were corrected with the experimental values, and the whole response matrix was completely established. The spectrum of a 241Am-Be source was obtained after unfolding the measurement data of the BSS to the source and in fair agreement with the expected one. The integral ambient dose equivalent corresponding to the spectrum was 0.95 of the expected value. Results of the unfolded spectrum and the integral dose equivalent measured by the BSS verified that the RFs of the BSS were well established.

Neutron-induced fission cross-section measurement of 234U with quasi-monoenergetic beams in the keV and MeV range using micromegas detectors

Accurate data on neutron-induced fission cross-sections of actinides are essential for the design of advanced nuclear reactors based either on fast neutron spectra or alternative fuel cycles, as well as for the reduction of safety margins of existing and future conventional facilities. The fission cross-section of $^{234}$U was measured at incident neutron energies of 560 and 660 keV and 7.5 MeV with a setup based on ‘microbulk’ Micromegas detectors and the same samples previously used for the measurement performed at the CERN n_TOF facility (Karadimos et al., 2014). The $^{235}$U fission cross-section was used as reference. The (quasi-)monoenergetic neutron beams were produced via the 7Li(p,n) and the 2H(d,n) reactions at the neutron beam facility of the Institute of Nuclear and Particle Physics at the ‘Demokritos’ National Centre for Scientific Research. A detailed study of the neutron spectra produced in the targets and intercepted by the samples was performed coupling the NeuSDesc and MCNPX codes, taking into account the energy spread, energy loss and angular straggling of the beam ions in the target assemblies, as well as contributions from competing reactions and neutron scattering in the experimental setup. Auxiliary Monte-Carlo simulations were performed with the FLUKA code to study the behaviour of the detectors, focusing particularly on the reproduction of the pulse height spectra of α-particles and fission fragments (using distributions produced with the GEF code) for the evaluation of the detector efficiency. An overview of the developed methodology and preliminary results are presented.

Characterization of the PTW 34031 ionization chamber (PMI) at RCNP with high energy neutrons ranging from 100 – 392 MeV

Radiation monitoring at high energy proton accelerators poses a considerable challenge due to the complexity of the encountered stray radiation fields. These environments comprise a wide variety of different particle types and span from fractions of electron-volts up to several terra electron-volts. As a consequence the use of Monte Carlo simulation programs like FLUKA is indispensable to obtain appropriate field-specific calibration factors. At many locations of the LHC a large contribution to the particle fluence is expected to originate from high-energy neutrons and thus, benchmark experiments with mono-energetic neutron beams are of high importance to verify the aforementioned detector response calculations. This paper summarizes the results of a series of benchmark experiments with quasi mono-energetic neutrons of 100, 140, 200, 250 and 392 MeV that have been carried out at RCNP - Osaka University, during several campaigns between 2006 and 2014.

Recoil Directionality Studies in Two-Phase Liquid Argon TPC Detectors

Projects attempting the direct detection of WIMP dark matter share the common problem of eliminating sources of background or using techniques to distinguish background events from true signals. Although experiments such as DarkSide have achieved essentially background free exposures through careful choice of materials and application of efficient veto techniques, there will still be a high burden of proof to convince the greater scientific community when a discovery is claimed. A directional signature in the data would provide extremely strong evidence to distinguish a true WIMP signal from that of an isotropic background. Two-phase argon time projection chambers (TPCs) provide an experimental apparatus which can both be scaled to the ton-scale size required to accommodate the low cross-section expected for WIMP interactions and have an anisotropy that could be exploited to evaluate the polar angles of the resulting nuclear recoils from WIMP collisions with target nuclei. Our studies show that even a modest resolution in the polar angle reconstruction would offer a powerful tool to detect a directional signature. In this contribution, the status of the ReD experiment, which is under construction at Naples University, will be also shown. The aim of the project is to assess and enhance the directionality of two-phase argon TPCs. ReD will use a small TPC exposed to a beam of mono-energetic neutrons to study the so called “columnar recombination” in liquid argon. This development could have high impact on the future experiments in the field, opening up the potential to find conclusive evidence for dark matter or disprove the WIMP hypothesis at and above the mass range explored by planned accelerator experiments.

Calibration of the radiation monitors from DESY and SPring-8 at the quasi-mono-energetic neutron beams using 100 and 300 MeV7Li(p,n) reaction at RCNP in Osaka Japan in November 2014

At the ring cyclotron facility of the Research Center for Nuclear Physics (RCNP) Osaka University, Osaka, Japan a series of measurement campaigns had been continued with quasi mono-energetic neutron beams in November 2014. A 7 Li target was bombarded with 100 and 300 MeV protons and the generated neutron beams were directed into a long time-of-flight tunnel at 0 and 25 degrees deflection angle with respect to the proton beam. At a distance of 41 m the cross section of the neutron beam was large enough for the illumination of square meter sized objects like extended range rem-counters. The research institutes SPring-8/RIKEN, Japan, and DESY, Germany, participated in this campaign for the calibration of 4 different types of active ambient dose rate monitors: LB 6411, LB 6411-Pb, LB 6419 and LB 6420. The measurements of their responses are reported and compared with the calculated values.

Activation Cross Section for Reactions Induced by d-T Neutrons on Natural Yttrium

Cross sections for (n,2n), (n,α), (n,p), and (n,t) reactions were measured on yttrium isotopes at neutron energies ranging from 13.5 to 14.8 MeV using the activation technique in combination with high-resolution gamma-ray spectroscopy. The monoenergetic neutron beam was produced via the 3H(d,n)4He reaction using solid T-Mo. Data are reported for the following reactions: 89Y(n,2n)88(m+g)Y, 89Y(n,α)86(m+g)Rb, 89Y(n,p)89Sr, and 89Y(n,t)87mSr. The cross sections were also estimated with the TALYS-1.8 nuclear model code using different level density options, at neutron energies varying from the reaction threshold to 20 MeV. Results are also discussed and compared to corresponding values found in the literature.

Neutron beam test of barium fluoride crystal for dark matter direct detection

In order to test the capabilities of Barium Fluoride (BaF2) crystal for dark matter direct detection, nuclear recoils are studied with mono-energetic neutron beam. The energy spectra of nuclear recoils, quenching factors for elastic scattering neutrons and discrimination capability between neutron inelastic scattering events and γ events are obtained for various recoil energies of the F content in BaF2.