Thermal Neutron Field Research Articles

Research on perturbation of neutron fluence rate in a closed thermal neutron field due to medium materials

Research on perturbation of neutron fluence rate in a closed thermal neutron field due to medium materials

Microdosimetry for BNCT: First measurements at different site sizes

Microdosimetric techniques are a valuable tool for beam quality monitoring in BNCT, due to their capability to distinguish different contributions to the total dose and provide physics-based quantities related to biological effectiveness of this composite radiation field. To this aim, measurements are generally performed with gas detectors simulating a tissue-equivalent site size between 0.5 and 2 μm. This work presents instead measurements for site sizes up to 10 μm, performed in the thermal neutron field produced by the accelerator-based MUNES source available at INFN-LNL. An avalanche-confinement TEPC with boron doping in the cathode walls was used. Photon and neutron dose fractions were discriminated in the measured dose-weighted distributions based on their different lineal energy range. In the neutron component two separate peaks could be distinguished for site sizes of 5 μm and greater, the origin of which was tentatively related to contributions due to protons and alpha particles. These results allow to assess the impact of increasing site diameter on the measured relative dose contributions and provide valuable reference data for biological modelling and for comparison with solid-state microdosimeters.

Dynamic diffraction of thermal neutrons in crystals with continuous deformation field

The purpose of the work is to consider the basics of the formation of a diffraction wave field of thermal neutrons in the case of Laue transmission geometry of a limited beam of neutrons in the crystal, taking into account the peculiarities of the interaction of thermal neutrons with a continuously and slowly changing deformation field of the crystal net. The conditions of both diffraction blurring and diffraction contraction (focusing) up to the practical restoration of the original shape of the packet will be studied.An integral form for determining the quasi-amplitudes of diffracted waves within the framework of the original spatially inhomogeneous beams is formulated, based on the constructed functions of the influence of a point source of thermal neutron waves for crystals with a continues deformation field. It has been shown that neutron focusing is effective, which is revealed by a smaller half-width of the focal spot, an increase in the resolving power of the energy spectrum and focusing efficiency.

Development of a graphite pile to produce a neutron thermal field at the CIEMAT - Neutron standards laboratory

The CIEMAT Neutron Standards Laboratory (LPN) is the Spanish national reference laboratory in neutron metrology. Within the framework of the LPN expansion project, it has been planned to incorporate a thermal neutron field based on a graphite pile. A study has been carried out with MCNP 6.2 for a graphite pile with dimensions 180 × 150 × 150 cm3, with a 185 GBq Am–Be source (B = 1.1·107 s−1) and the internal cavity with a section of 40 × 40 cm2 and a depth of 50 cm. The main parameters of the graphite pile and the position and emission rate of the Am–Be neutron source have been varied to determine the fluence rates at several points in the pile, their thermal component and homogeneity. The optimal configuration produces a neutron spectrum with more than 98% of thermal component, fluence rates around 400 cm−2 s−1 inside the cavity and 55 cm−2 s−1 outside, and acceptable homogeneity in this thermal neutron field. From a Radiation Protection point of view it is necessary to incorporate borated high-density polyethylene (HDPE-B) around or at least as a shielding wall, and a Cd window to filter the thermal component.

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.

Comparison of H*(10) estimations, due to neutrons and γ-rays, around FANT with two 241Am/9Be sources

FANT (Fuente Ampliada de Neutrones Térmicos; in Spanish) is a thermal neutron irradiation facility with an extended and very uniform irradiation area, which has been developed by the Neutron Measurement Laboratory of the Energy Engineering Department at Universidad Politécnica de Madrid (LMN-UPM). In FANT, an isotopic neutron source (241Am/9Be) produces the primary neutrons. The design and facility optimization were carried out by extensive Monte Carlo calculations. In addition, Monte Carlo methods were used to evaluate the facility's performance to produce a constant and uniform thermal neutron field; these results were validated through experimental methods. FANT is designed to have two neutron sources; the objective of this work is to estimate the ambient dose equivalent due to neutrons and gamma-rays by Monte Carlo methods, and to compare these values with measured experimental doses. Thus, the performance of FANT with the two 241Am/9Be sources of LMN-UPM, with regard to the ambient dose equivalent H*(10) produced by both neutrons and photons around the facility, is analyzed in this work. The results are compared with those previously obtained in the framework of the results obtained with the LB6411 device around FANT.

Thermal neutron detection and track recognition method in reference and out-of-field radiotherapy FLASH electron fields using Timepix3 detectors.

Objective.This work presents a method for enhanced detection, imaging, and measurement of the thermal neutron flux.Approach. Measurements were performed in a water tank, while the detector is positioned out-of-field of a 20 MeV ultra-high pulse dose rate electron beam. A semiconductor pixel detector Timepix3 with a silicon sensor partially covered by a6LiF neutron converter was used to measure the flux, spatial, and time characteristics of the neutron field. To provide absolute measurements of thermal neutron flux, the detection efficiency calibration of the detectors was performed in a reference thermal neutron field. Neutron signals are recognized and discriminated against other particles such as gamma rays and x-rays. This is achieved by the resolving power of the pixel detector using machine learning algorithms and high-resolution pattern recognition analysis of the high-energy tracks created by thermal neutron interactions in the converter.Main results. The resulting thermal neutrons equivalent dose was obtained using conversion factor (2.13(10) pSv·cm2) from thermal neutron fluence to thermal neutron equivalent dose obtained by Monte Carlo simulations. The calibrated detectors were used to characterize scattered radiation created by electron beams. The results at 12.0 cm depth in the beam axis inside of the water for a delivered dose per pulse of 1.85 Gy (pulse length of 2.4μs) at the reference depth, showed a contribution of flux of 4.07(8) × 103particles·cm-2·s-1and equivalent dose of 1.73(3) nSv per pulse, which is lower by ∼9 orders of magnitude than the delivered dose.Significance. The presented methodology for in-water measurements and identification of characteristic thermal neutrons tracks serves for the selective quantification of equivalent dose made by thermal neutrons in out-of-field particle therapy.

Characterization of thermal neutron distribution of an Am–Be neutron source setup by CdZnTe detector

Owing to the developments in prompt gamma neutron activation analysis (PGNAA) and prompt gamma ray activation imaging (PGAI), it is necessary to develop an online thermal neutron distribution measurement method. The CdZnTe detector is considered as an alternative thermal neutron detector because of its high thermal neutron capture cross section. In this study, the thermal neutron field of an 241Am–Be neutron source was determined by CdZnTe detector. The intrinsic neutron detection efficiency of CdZnTe detector was calculated by using indium foil activation, and the value was 3.65%. Then the characteristics of neutron source were conducted with the calibrated CdZnTe detector. The thermal neutron fluxes in front of beam port were measured at several distances ranging from 0 cm up to 28 cm. Thermal neutron field at distances of 1 cm and 5 cm were also measured. The experimental data were then compared with Monte Carlo simulation. The results demonstrated that the simulated data agree well with experimental measurements.

Characterization of the collimated output fluence , in a thermal neutron field, in terms of ambient equivalent dose H*(10,0)

The Brazilian Neutron Laboratory at the Institute of Radioprotection and Dosimetry (IRD) developed and built a new standard facility for thermal neutron flux, seeking to provide a new homogeneous fluence to calibrate neutron detectors and personal dosimeters. The purpose of this work is therefore to characterize the collimated output fluence, in a thermal neutron field, in terms of ambient equivalent dose H*(10,0) for calibration and irradiation of personal monitors and neutron survey meters. The neutron fluence in the system irradiation field needs to be well known for personal monitors and neutron survey meters calibrated used in nuclear reactors in addition to studies and development of new materials for neutron dosimetry. The characterization of neutron thermal energy fields at the usual calibration points is one of the recommendations of the International Organization for Standardization ISO 8529, 2001 for international intercomparison of neutron fluence measurements. The determination of the ambient equivalent dose of the new facility was estimated based on a secondary transfer standard internationally known as Long Counter and properly characterized and traced back to the Neutron Laboratory primary standards.

Conceptual design of a target station using a 30-MeV cyclotron accelerator for the basic study of boron neutron capture therapy at KURNS

A new target station that generates a thermal neutron field based on a 30-MeV proton accelerator was designed for the basic study of boron neutron capture therapy (BNCT). The calculated thermal neutron fluxes at the sample regions under free air conditions reached 2.84 × 109 [n/cm2/s]. The fast neutron and gamma ray dose rates per thermal neutron flux in several regions were on the order of 10−13 [Gy⋅cm2]. The beam characteristics were found to be closer to those of the heavy-water neutron irradiation facility of Kyoto University Research Reactor, which is an established facility in BNCT research. In addition, the shielding size for the new target station was realistically estimated for long-term beam operation.

Characterization of Thermoluminescent Dosimeters for Neutron Dosimetry at High Altitudes.

Neutrons constitute a significant component of the secondary cosmic rays and are one of the most important contributors to natural cosmic ray radiation background dose. The study of the cosmic ray neutrons’ contribution to the dose equivalent received by humans is an interesting and challenging task for the scientific community. In addition, international regulations demand assessing the biological risk due to radiation exposure for both workers and the general population. Because the dose rate due to cosmic radiation increases significantly with altitude, the objective of this work was to characterize the thermoluminescent dosimeter (TLDs) from the perspective of exposing them at high altitudes for longtime neutron dose monitoring. The pair of TLD-700 and TLD-600 is amply used to obtain the information on gamma and neutron dose in mixed neutron-gamma fields due to the present difference in 6Li isotope concentration. A thermoluminescence dosimeter system based on pair of TLD-600/700 was characterized to enable it for neutron dosimetry in the thermal energy range. The system was calibrated in terms of neutron ambient dose equivalent in an experimental setup using a 241Am-B radionuclide neutron source coated by a moderator material, polyethylene, creating a thermalized neutron field. Afterward, the pair of TLD-600/700 was exposed at the CERN-EU High-Energy Reference Field (CERF) facility in Geneva, which delivers a neutron field with a spectrum similar to that of secondary cosmic rays. The dosimetric system provided a dose value comparable with the calculated one demonstrating a good performance for neutron dosimetry.

Microdosimetry of an accelerator based thermal neutron field for Boron Neutron Capture Therapy

The MUNES project (MUltidisciplinary NEutron Source) aims at the realization of an intense accelerator-based source of thermal neutrons, suitable for Boron Neutron Capture Therapy (BNCT). This exploits the interaction of 5 MeV protons onto a beryllium target, producing a fast neutron spectrum, which is moderated to the thermal range by a large assembly made of a Polytetrafluoroethylene (PTFE) tank filled with heavy water, surrounded by graphite blocks. The thermal neutron field is extracted through a bismuth beam port. The microdosimetric characterization of this field was performed using a cylindrical avalanche-confinement Tissue Equivalent Proportional Counter (TEPC) equipped with interchangeable cathode walls, positioned in front of the beam port. Measurements were taken both with a boron-doped wall and with an undoped one. The comparison of the two microdosimetric distributions allows to distinguish the relative dose contribution due to alpha particles and lithium ions from the BNC reaction from that of photons and other particles from neutron interactions on the cathode walls. The Relative Biological Effectiveness (RBE) was also calculated from the convolution of the measured spectra with a biological weighting function. This paper describes the experimental microdosimetric approach and the results of measurements with a boron-loaded cathode performed for the first time at an accelerator-based BNCT source.

Plans for neutron metrology at NPL

The National Physical Laboratory has provided industry and research with neutron fluence and dose standard neutron fields since the 1960’s. Sectors such as civil nuclear power, defence, radiation-protection and fusion generation rely on NPLs facilities to operate safely and ensure traceability back to an internationally recognised UK primary standard. A central part of the capability is the 3.5 MV Van de Graaff accelerator which, using light ions, generates monoenergetic fields of neutrons ranging from a few keV to 19 MeV and intense thermal neutron fields. This accelerator is reaching the end of its operational life and a project to replace it is underway.

Experimental characterization of FANT, a new thermal neutron source

FANT is the acronym of Enhanced Thermal Neutron Source (Fuente Ampliada de Neutrones Térmicos, in Spanish). This is a parallelepiped box of high-density polyethylene moderator and an isotopic neutron source. The moderator has a cylindrical irradiation chamber where a rather uniform thermal neutron flux is obtained. The FANT design was previously optimized and the neutron spectra were estimated by Monte Carlo calculations with the MCNP6.1 code. To check the characteristics of the FANT thermal neutron field, measurements have been performed at the reference point inside the irradiation chamber with a Bonner sphere spectrometer holding a small 6LiI(Eu) thermal neutron detector. To unfold the neutron spectrum BUNKIUT with UTA4 response matrix and NSDann Ver 4.0 codes were used. Some issues have been found and recommendations are made about the use of large BSS inside narrow spaces, and about the capacity of NSDann code to unfold these kind of spectra. However, the results confirm that the moderation process in FANT is very effective and allows obtaining useful thermal neutron fluence rates.

Characteristic evaluation of the thermal neutron irradiation field using a 30 MeV cyclotron accelerator for basic research on neutron capture therapy

The development of a new boron compound for boron neutron capture therapy (BNCT) requires a high-intensity thermal neutron beam and the construction of thermal neutron fields in place of research reactors scheduled for suspension in the future. A thermal neutron irradiation field was produced by combining a cyclotron-based epithermal neutron source with a moderator to form an irradiation field for mice and cells, and its characteristics were evaluated.Our study confirmed that the lower limbs of the mouse could be sufficiently irradiated with a thermal neutron intensity of 4.83 × 108 n cm−2 s−1. The neutron dose rate and gamma ray dose rate per thermal neutron flux were 8.3 × 10−13 and 1.4 × 10−12 Gy cm2, respectively. In addition, by using a water phantom, irradiation with a maximum thermal neutron flux of 1.35 × 109 n cm−2 s−1 was achieved. In this case, the neutron dose rate and gamma ray dose rate per thermal neutron flux were 2.7 × 10−13 and 8.3 × 10−13 Gy cm2, respectively. An irradiation field capable of irradiating mice and cells was successfully produced and compared favorably with the irradiation fields from research reactors currently in use.

Investigation of mammoth remains using the neutron activation analysis at the Training Reactor VR-1

At the time when the importance of the interdisciplinary research increases, the nuclear analytical techniques supported by the small research reactors represent a useful tool for investigation of human society, culture, history etc. The historical, archaeological, and palaeontological samples and objects of cultural heritage can be easily studied using the radioanalytical methods such as the neutron activation analysis. This paper deals with the detailed investigation of fragments of mammoth remains from the Mid-Upper Palaeolithic site Pavlov VI by means of the instrumental neutron activation analysis at the Training Reactor VR-1 of the Czech Technical University in Prague. Six mammoth hard tissue samples (fragments of bones, tusk, and molar) from the Institute of Archaeology of the Czech Academy of Sciences were irradiated in the dry vertical irradiation channel with thermal neutron field (φ=2×109cm−2s−1) at maximum reactor power (80 W). The activated mammoth samples were analysed employing the nuclear γ-spectrometry and semiconductor HPGe detector, and the composition of the remains was determined (qualitative and quantitative analysis). The presence of Na, Cl, K, As, Fe, Sr, Mn, Br, I, Ba, and U was revealed in studied mammoth samples. Based on obtained production rates, the concentrations of Fe, Sr, Na, K, As, and U were determined. The results presented in this paper show clearly that the low-power Training Reactor VR-1 is excellent tool for the neutron activation analysis experiments within the interdisciplinary research and can provide experimental data important for archaeologists and palaeontologists.

Development of gamma insensitive silicon carbide diagnostics to qualify intense thermal and epithermal neutron fields

The e\\_LiBANS project aims at creating accelerator based compact neutron facilities for diverse interdisciplinary applications. After the successful setting up and characterization of a thermal neutron source based on a medical electron LINAC, a similar assembly for epithermal neutrons has been developed. The project is based on an Elekta 18 MV LINAC coupled with a photoconverter-moderator system which deploys the (γ,n) photonuclear reaction to convert a bremsstrahlung photon beam into a neutron field. This communication describes the development of novel diagnostics to qualify the thermal and epithermal neutron fields that have been produced. In particular, a proof of concept for the use of silicon carbide photodiodes as thermal neutron rate detector is presented.

ON THE MULTIPURPOSE USE OF A PORTABLE NEUTRON SOURCE

The possibility of creating a multipurpose complex for generating a reference field of thermal neutrons based on a portable neutron source (PNS) is considered. It has been shown that our method can be used to detect fissile materials without determining their isotopic composition during an inspection of the hand luggage of passengers, mail, etc. The PNS multipurpose complex will allow to indirectly indicating the possible presence of chemical explosives in the test samples, as well as cadmium and boron, which possibly hide fissile elements from detection. The paper gives recommendations on the use of the most effective instruments and equipment.

Polyethylene moderator optimized for increasing thermal neutron flux in the NASBEE accelerator-based neutron field

Neutron exposure Accelerator System for Biological Effect Experiments (NASBEE) at National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology (NIRS-QST) achieved thermal neutron flux of a therapeutic intensity of (1.75 ± 0.13) × 109 cm−2 s−1 by optimizing polyethylene (PE) moderator in the beam port. The dose contamination of gamma rays was 2.37 × 10−12 Gy cm2 (the absorbed dose of 14.9 Gy h−1). The improved thermal neutron field at the NASBEE facility will be useful in the research and development for boron neutron capture therapy (BNCT).

Water radiolysis with thermal neutrons, fast neutrons and contamination γ rays in the accelerator based thermal neutron field: Time dependence of hydroxyl radical yields

Radiation chemical yields (G value) of OH radicals created in the accelerator-based neutron field are evaluated using coumarin-3-carboxilic acid (C3CA) solution to understand the role of the indirect action induced by radiations emitted as a result of 10B(n,α)7Li reactions for boron neutron capture therapy. The G values for ionizing radiations emitted as a result of 10B(n,α)7Li reactions, the contamination (ionizing) radiations (i.e., fast neutrons and γ rays) in the accelerator-based neutron field are discriminately assessed. We have followed the recombination process of OH radicals by adjusting the concentration of C3CA solution from 0.5 to 20 mM. The G values for ionizing radiations emitted as a result of 10B(n,α)7Li reactions are the lowest ones. This finding indicates that the indirect action induced by 10B(n,α)7Li reactions is not effective to kill tissues. In comparison to this, a significant influence would be expected by the contamination (ionizing) radiations not only on cancer tissues but also on healthy tissues.