Monoenergetic Neutron Fields Research Articles

Fluence measurement of monoenergetic neutrons from D(d,n) and T(d,n) reactions at the KRISS

A Cockcroft–Walton accelerator from High Voltage Engineering Europa BV was installed at the Korea Research Institute of Standards and Science in June 2022 to generate monoenergetic neutron fields. In this study, the fluences of monoenergetic neutron fields with energy peaks at 2.5, 2.8, and 3.2 MeV from the D(d,n) reaction and at 14.8 MeV from the T(d,n) reaction were measured. To measure neutron fluence, Bonner spheres with diameters of 17.78, 20.32, and 25.40 cm were placed at a distance of 1.50 m from the target. Additionally, a small size long counter was used separately as a neutron reference detector to monitor the stability of neutron production rate. For a deuteron beam current of 1 μA, the neutron fluences of monoenergetic neutron fields with energy peak at 2.5, 2.8, 3.2, and 14.8 MeV were determined to be 0.71 ± 0.03, 1.10 ± 0.04, 13.9 ± 0.5, and 258.0 ± 8.1 cm−2s−1μA−1, respectively.

EURADOS intercomparison for whole-body neutron dosemeters from 2012 to 2022

Within the EURADOS (European Radiation Dosimetry Group) self-sustained programme of regular intercomparisons (ICs) for individual monitoring, three campaigns had been performed in 2012, 2017 and 2022 to assess the performance of neutron personal dosemeters routinely used to measure personal dose equivalent, Hp(10). The irradiation schemes were established to provide participants with useful information regarding linearity, reproducibility and response of their dosimetry systems at different energies and angles of incidence. Small alterations were made in irradiation plans for subsequent exercises. Combinations of standard calibration fields and simulated workplace fields were employed, using bare and moderated neutron sources as well as monoenergetic and thermal neutron fields. Neutron energies used to evaluate dosemeter performance ranged from thermal to several MeV. Dose values varied between 0.3 mSv and 15 mSv. The number of participating laboratories in the three campaigns was stable: 31 in IC2012n, 32 in IC2017n and 29 in IC2022n. The dosimetry systems could have been categorized according to their detection principle into albedo and track dosemeters, or a combination of both. In 2012, a category ‘other’ had to be introduced to consider active personal dosemeters and fission track detectors. In 2012, participants were asked to provided results in a two-step procedure: (i) with no information on the radiation fields, and (ii) after receiving additional simplified a priori information on the energy distribution of the neutron fields to enable correction for dosemeter response. In 2017 and 2022, based on a one-step procedure, however, participants might have requested a priori information upon registration, which was reported in the certificate of participation. Intercomparison results were analysed against the acceptance criteria of the ISO 14146:2018 standard, although there were no standardized performance requirements for neutron personal dosemeters at the time of IC2012n. The paper compares the results of the three exercises, identifies major problems for individual monitoring services and shows trends in dosimetric performance. In all three intercomparisons, a few systems had shown problems related to calibration. Data do not allow to draw a universally applicable conclusion with regard to potentially superior performance of one particular dosimetry system.

Quantitative investigation of gamma radiation in accelerator produced ISO neutron fields

In standard monoenergetic ISO neutron fields, the neutron yield of neutron-producing reactions was measured in combination with the prompt photon yield, including photon energies up to 10 MeV, for the purpose of comparing the two yields. Separating the photons produced by the target (direct photons) from those generated by secondary neutron reactions was achieved using the time-of-flight method. Photon and neutron ambient dose equivalent values were calculated from measured spectral energy distributions. Quasi monoenergetic neutron fields are needed to systematically test the response of measuring instruments to neutron radiation. For this reason, ISO has defined a number of reference neutron radiation fields covering a wide energy range up to 19 MeV. Because neutron detectors may also be affected by photon radiation, the photon fluence in the ISO neutron fields has to be known. This work focuses on quasi monoenergetic accelerator-produced neutron fields in the energy range of 24 keV to 19 MeV.

Photon spectra in NPL standard monoenergetic neutron fields.

A High Purity Germanium (HPGe) detector has been used to measure the photon spectra in the majority of monoenergetic neutron fields produced at NPL (0.144, 0.250, 0.565, 2.0, 5.0 and 16.5MeV). The HPGe was characterised and then modelled to produce a response matrix. The measured pulse height spectra were then unfolded to produce photon fluence spectra. The new spectra were used to improve the photon to neutron dose equivalent ratios from some earlier work at NPL with Geiger-Muller tubes and electronic personal dosemeters.

Calibration of the W-PIE neutron spectrometer at CERF reference facility.

The W-PIE is a cosmic neutron spectrometer used for environmental measurements, developed by Politecnico di Milano. The instrument is based on the Artkis M800 thermal neutron detector and works as a 4-channel spectrometer for detecting neutrons in the energy range of 0.01eV-1GeV. After the spectrometer had been calibrated in some monoenergetic neutron fields at the PIAF facility of PTB, the instrument was tested in the neutron field generated at the CERF facility, whose spectrum fairly resembles the cosmic one. This measurement is a precious tool for predicting the performance of the instrument in a realistic situation.

Development of A 14.8-MEV Mono-Energetic Neutron Field in Korea Research Institute of Standards and Science.

A standard irradiation field for 14.8-MeV neutrons is under development for mono-energetic neutron standards research in the Korea Research Institute of Standards and Science, Republic of Korea. We developed a target chamber with the associated alpha particle (AAP) system for 14.8-MeV mono-energetic neutrons by a T(d,n)4He reaction. We designed the target chamber and the AAP detector system using a two-body kinematic calculation. We conducted simulations of the T(d,n)4He reaction on a tritiated target to determine a specification of the target and the AAP detector. This paper will discuss the simulation and calculation results for the associated particle system design.

Experimental characterization of the LUPIN Rem counter in monoenergetic neutron fields

The LUPIN is a Rem counter for neutron dosimetry in pulsed radiation fields, i.e., those fields whose intensity varies greatly on short timescales with respect to the characteristic time of the utilized detector. This work describes the characterization of the energy response of the instrument. The response function was calculated with the Monte Carlo code MCNP6, representing the geometry and material composition of the LUPIN and simulating an irradiation in expanded and aligned monoenergetic neutron fields. The calculated response was validated in the monoenergetic fields of the National Physical Laboratory. The agreement between the calculated and measured responses is satisfactory, with a maximum discrepancy of 5%.

Neutron spectrometry of D2O-moderated 252Cf with Bonner sphere spectrometer

For neutron spectrometry of the D2O-moderated 252Cf source with a Bonner sphere spectrometer (BSS), it is difficult to use the large and heavy shadow cone to correct the neutron scattering effect. To overcome this problem, Monte Carlo (MC) simulation method was applied to calculate the neutron scattering ratio and to establish the BSS response functions. The simulated response functions were verified by experimental measurements in reference mono-energetic neutron fields. MC simulation based scattering-correction was validated by measurement of 252Cf neutron field. The measured and simulated values of the neutron scattering ratio were very close with relative errors within ±6%. Finally, the neutron spectrum and the spectrum averaged conversion coefficients of the D2O-moderated 252Cf were measured using BSS after scattering-correction by MC simulation, and the results agreed with the values recommended by ISO 8529–1:2021. It shows that the MC simulation can be a useful substitute to shadow cones method for neutron scattering-correction.

The NCT-WES directional neutron spectrometer: validation of the response with monoenergetic neutron fields

A directional neutron spectrometer named NCT-WES (Neutron Capture Therapy Wide Energy Spectrometer) was developed for quality assurance of the therapeutic neutron beam in Neutron Capture Therapy (NCT). NCT-WES operates as a “parallelized” Bonner spheres spectrometer, embedding six semiconductor-based thermal neutron detectors in a collimated cylindrical moderator. With the objective of validating the simulation model used to derive its response matrix, irradiations in reference monoenergetic fields were performed at National Physical Laboratory (UK). As the energy distributions of neutron beams in NCT extend from keV to a few MeV, monoenergetic fields in this domain were chosen, namely 71.5 keV, 144.2 keV, 565.1 keV, 841.9 keV and 1200.4 keV. The results of the experiment confirm the correctness of the NCT-WES simulation model, within an overall uncertainty lower than ± 2%.

Neutron irradiation facilities, neutron measurement system, and mono-energetic neutron fields at KRISS

Neutron irradiation facilities, neutron measurement system, and mono-energetic neutron fields at KRISS

The methodology for validation of cross sections in quasi monoenergetic neutron field

Dosimetry cross sections are fundamental quantities for proper determination of the neutron fluences in points of interest under heavy radiation load. One of the critical applications is the Reactor Pressure Vessel aging management, related to the correct estimation of its residual lifetime or for many non-reactor applications including neutron dosimetry of accelerator-based fields or space applications. The neutron flux of neutrons above 12 MeV in reference to fission spectra is below 1%. Therefore, new reference neutron fields with average energy above 5 MeV should be developed for the validation of neutron dosimetry cross sections up to 60 MeV. This paper presents the testing of a new methodology for the use of quasi monoenergetic neutron fields, where different sensitivity allows validations of the dosimetry cross section at energies much higher than the average energy of around 2 MeV typical of fission spectra. The exact shape of the neutron spectrum in the tested fields is measured by stilbene spectrometry. The total flux is determined from Ni and Al flux monitors. The developed methodology was applied to the validation of selected reactions from the IRDFF-II library showing satisfactory agreement.

Calibration of a large-size wide-range neutron spectrometer

Over the last year, Politecnico di Milano has developed the W-PIE, a cosmic neutron spectrometer employed for environmental measurements. The W-PIE is based on the Artkis M800 thermal neutron detector and works as a 4-channel spectrometer for detecting neutrons in the energy range 0.01eV – 1GeV. In this paper, the calibration of the instrument in monoenergetic neutron fields at the PIAF facility of PTB is discussed. These measurements were intended to calibrate the detector and validate the response functions. Despite obtaining satisfactory results in terms of fluence and dose calculations, the irradiation in the monoenergetic fields suggested that the detector sensitivity to fast neutrons was greater than expected by the calculated response functions. This discrepancy was investigated and a reasonable explanation for it was proposed.

Development of personal neutron dosimeter with energy resolution ability

The prototype neutron personal dosimeter consists of three passive implanted planar silicon detectors. Polyethylene, B4C-containing polyethylene, and 6LiF act as neutron moderators, absorbers, and converters in front of each detector. This personal dosimeter has two modes of operation: real-time mode and unfolding mode. In real-time mode, it shows three neutron personal dose equivalent values, which represent the personal neutron equivalent of high energy (1–15 MeV), intermediate energy (1 × 10−3 MeV –1MeV), and low-energy (2.53 × 10−8 MeV −1 × 10−3 MeV), respectively. Unfolding method based on Genetic Algorithm was used to deduce neutron spectra from pulse height spectra of recoil proton. The dosimeter was calibrated in monoenergetic neutron fields and was tested in an 241Am–Be neutron field.

Dark Matter Directionality Detection performance of the Micromegas-based [formula omitted]TPC-MIMAC detector

Directional Dark Matter Detection (DDMD) can open a new signature for Weakly Massive Interacting Particles (WIMPs) Dark Matter. The directional signature provides in addition, an unique way to overcome the neutron and neutrino backgrounds. In order to get the directional signature, the DDM detectors should be sensitive to low nuclear energy recoils in the keV range and have an angular resolution better than 20°. We have performed experiments with low energy (<30keV) ion beam facilities to measure the angular distribution of nuclear recoil tracks in a MIMAC detector prototype. In this paper, we study angular spreads with respect to the electron drift direction (0° incident angle) of Fluorine nuclear tracks in this low energy range, and show nuclear recoil angle reconstruction produced by a monoenergetic neutron field experiment. We find that a high-gain systematic effect leads to a high angular resolution along the electron drift direction. The measured angular distribution is impacted by diffusion, and space charge or ion feedback effects, which can be corrected for by an asymmetry factor observed in the flash-ADC profile. The estimated angular resolution of the 0° incident ion is better than 15° at 10 keV kinetic energy and agrees with the simulations within 20%. As it was not possible to inject ions at angles different from zero with respect to the electric drift field, we have performed experiments with monoenergetic neutrons producing nuclear recoils at all angles. The distributions from the nuclear recoils have been compared with simulated results based on a modified Garfield++ code. Our study shows that protons would be a more adapted target than heavier nuclei for DDMD of light WIMPs. We demonstrate that directional signature from the Galactic halo origin of a Dark Matter WIMP signal is experimentally achievable, with a deep understanding of the operating conditions of a low pressure detector with its diffusion mechanism.

DOSE CONVERSION COEFFICIENTS FROM MONOENERGETIC NEUTRONS COMPUTED WITH THE MCNP6.2 CODE IN THE VIP-MAN PHANTOM.

Computational estimates of dosimetric quantities from external exposures to neutron fields are valuable to ongoing radiation protection studies that are of interest to US defense-focused organizations. Conversion coefficients for fluence-to-absorbed dose and fluence-to-effective dose were computed using version 6.2 of the Monte Carlo N-Particle® code (MCNP6.2) in the visible photographic man (VIP-Man) numerical model for 45 monoenergetic neutron fields between 10-9 and 20MeV under six irradiation geometries. Comparisons to the formative VIP-Man study by Bozkurt etal. revealed excellent agreement. The mean relative difference between our VIP-Man/MCNP6.2-computed and the International Commission on Radiological Protection (ICRP) 116-published absorbed dose conversion coefficients in 14 organs and remainder tissues under all geometries was 4.7%, which is within ICRP-reported variations. The effective dose coefficients computed with VIP-Man/MCNP6.2 were strongly similar to ICRP 116 values in all geometries except for posterior-anterior, where the difference was attributed to shielding of anterior organs by the relatively fatty composition of the VIP-Man phantom.

Tests of Various Scintillator Detectors in Selected Mono-Energetic Neutron Beams

In this paper we study quality of three types of organic scintillators stilbene, p-terphenyl and EJ-299-33. We used monoenergetic neutron fields with a wide range of neutron energies in PTB Braunschweig. All the tests were carried out with NGA-01 spectrometer. The results of the measurements are evaluated spectra from the spectrometer. We discuss the quality of each scintillator, such as the FWHM of the peaks.

Investigation of the neutron-gamma ray discrimination performance at low neutron energy of a solution-grown stilbene scintillator

The results of experiments performed with a Ø25 x 25 mm solution-grown stilbene crystal in mono-energetic neutron fields in the 80-to-230 keV energy range are presented. The goal of the measurements, performed at the AMANDE facility, was to explore the capabilities of this organic scintillator to measure neutrons at the lowest possible energy with good pulse shape discrimination (PSD). The time of flight (TOF) technique was used in order to help with the neutron-gamma discrimination. The data are collected via a programmable digital acquisition (DAQ) system CAEN DT7530 using the software CoMPASS with the charge comparison method (CCM). The data are analysed using post-processing codes developed in the ROOT environment. The results show that the stilbene detector has discrimination capabilities for energies as low as 80 keV.

Correction and verification of HL-2A Tokamak Bonner sphere spectrometer in monoenergetic neutron fields from 100 keV to 5 MeV

A real-time Bonner sphere spectrometer (BSS) has been developed for spectral neutron measurements with the HL-2A Tokamak. To correct and verify the accuracy of the neutron spectrum from the BSS, the BSS system was calibrated using monoenergetic neutron beams in the energy range of 100 keV–5 MeV. The response function of the BSS was corrected based on the calibration results, and the corrected BSS system was verified by unfolding monoenergetic neutron spectra. Fusion neutron spectra on the HL-2A have been obtained from the calibrated BSS system for the first time.

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%.

Performance test of liquid-moderator-based neutron spectrometer for BNCT: Mono-energetic and spontaneous fission spectrum neutron measurements

Abstract Boron neutron capture therapy (BNCT) is an effective cancer therapy that uses neutron irradiation; it is necessary to evaluate the neutron spectrum in the treatment field as precisely as possible. To conduct such evaluations, we developed a new device, called a liquid-moderator-based neutron spectrometer. In the present study, the performance of the developed spectrometer was tested by measuring a mono-energetic neutron field and a fast neutron field generated by a 252Cf neutron source. Finally, we revealed that the response function of the spectrometer could be appropriately calculated and confirmed the capability of the device to measure the neutron energy spectra between 1 keV and 10 MeV.