Thermal Neutron Flux Measurements Research Articles

CLYC as a neutron detector in low background conditions

We report on the thermal neutron flux measurements carried out at the Laboratorio Subterráneo de Canfranc (LSC) with two commercial 2″×2″\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2^{\\prime \\prime } \ imes 2^{\\prime \\prime }$$\\end{document} CLYC detectors. The measurements were performed as part of an experimental campaign at LSC with 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document}He detectors, for establishing the sensitivity limits and use of CLYCs in low background conditions. A careful characterization of the intrinsic α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} and γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-ray background in the detectors was required and done with dedicated measurements. It was found that the α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} activities in the two CLYC crystals differ by a factor of three, and the use of Monte Carlo simulations and a Bayesian unfolding method allowed us to determine the specific α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} activities from the 238\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{238}$$\\end{document}U and 232\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{232}$$\\end{document}Th decay chains. The simulations and unfolding also revealed that the γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-ray background registered in the detectors is dominated by the intrinsic activity of the components of the detector such as the aluminum housing and photo-multiplier and that the activity within the crystal is low in comparison. The data from the neutron flux measurements with the two detectors were analyzed with different methodologies: one based on an innovative α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document}/neutron pulse shape discrimination method and one based on the subtraction of the intrinsic α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} background that masks the neutron signals in the region of interest. The neutron sensitivity of the CLYCs was calculated by Monte Carlo simulations with MCNP6 and GEANT4. The resulting thermal neutron fluxes are in good agreement with complementary flux measurement performed with 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document}He detectors, but close to the detection limit imposed by the intrinsic α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha $$\\end{document} activity.

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.

TENIS — ThErmal Neutron Imaging System for use in BNCT

A thermal neutron flux measurement tool with two perpendicular sets of plastic scintillator arrays was designed and simulated (Ghal-Eh and Green, 2016) with the MCNPX code (Version 2.6.0, with ENDF/B-VII cross section library (ENDF, 2011)). The proposed system aimed to provide a thermal neutron map based on the detection of 2.22 MeV gamma-rays resulting from 1H(nth, γ)2D reactions. In the present work, using Monte Carlo code FLUKA and its scintillation light transport capability, several important upgrades were carried out to include the light transport modeling in the response of plastic scintillators, analyze the cross-talk phenomenon, optimize the system geometry, and also provide a new approach in thermal neutron image reconstruction. The results showed that the last two cases played a significant role in improving the longitudinal profile of thermal neutron flux.

Efficiency limitation of thin film coated planar semiconductor thermal neutron detector

Semiconductor thermal neutron detectors are increasingly been used in in-core thermal neutron flux measurements in nuclear reactors. One limitation of these detectors is that they suffer from low detection efficiency. In this work, the maximum efficiency of a planar structure thermal neutron detector was determined using two widely used computer codes: Geant4 and MCNP6. Diamond and SiC are used as based materials in this work because of their large electron-hole pair production efficiency which generally translates to high detection efficiency. The electron-hole pair production efficiency is the fraction of energy that goes into electron-hole pair creation and depends on the band-gap energy and the W-values. These two materials are also not susceptible to radiation damage which makes them suitable for high radiation environments such as nuclear reactors. Thermal neutron detection is achieved using 10B and 6LiF conversion layers coated on the surface of the detector. The maximum efficiency for 10B conversion layer was achieved at a thickness of 2 μm. The efficiency at this thickness is 5.57 ± 0.09% and 5.49±0.09% for diamond and silicon carbide, respectively. When 6LiF was used as a thermal neutron conversion layer, the maximum thickness of the conversion layer was determined to occur at 17 μm. The efficiency at this thickness is 5.47 ±0.06% and 5.38±0.06% for diamond and SiC, respectively.

3He proportional counter development for thermal neutron detection

Neutron detection is used in a wide range of applications in nuclear physics, radiation protection, nuclear fuel cycle, reactor instrumentation, security and industrial measurement. Among the detectors used in this field, we notice the gas-filled 3He proportional counters which have been one of the main detecting tools for thermal and cold neutron detection for many years. This last characteristic has ensured that this detector is one of the best tools for thermal neutron flux measurements in a nuclear reactor control. In the instrumentation and detection laboratory of the Nuclear Research Center, of Birine, we are working toward the design and the development of this type of detectors, indeed, several prototypes of neutron detectors have been realized including the 3He proportional counters. Through this paper, we will present the experimental steps and the obtained results to carry out a 3He proportional counter prototype that was fully developed and tested in our laboratory. A comparison study was made between our detector and a commercial cylindrical 3He neutron detector which was considered in this work as a reference detector: LND252 (3He)-PC. The results showed that the characteristics of the gas amplification and the counting plateau for the two counters reference LND 252 (3He)-PC and our prototype was of the same order of scale. The experimental tests show that our developed prototype perfect fit with the standard International Electrotechnical Commission (IEC, www.iec.ch ) in the operating principle, the technology adopted and obtained technical specifications.

First experimental verification of the neutron field of Nagoya University Accelerator-driven neutron source for boron neutron capture therapy

As the commissioning phase of the Nagoya University Accelerator-driven Neutron Source for boron neutron capture therapy, in-phantom thermal neutron flux measurements were conducted using a small ⁶LiF/Eu:CaF₂ scintillator detector and activation foils. The spatial distribution of the measured thermal neutron flux agreed with the Monte Carlo simulation results. Based on this comparison, the free-in-air neutron spectrum, calculated at the exit aperture, was verified and the epithermal neutron flux, at a 0.25 mA proton current, was evaluated as (1.49 ± 0.10) × 10⁷ n/(cm2 s).

Natural background radiation at Lab 2 of Callio Lab, Pyhäsalmi mine in Finland

In operating mines, as well as in deep locations for planned scientific activities, it is essential to recognize the natural background radiation from the point of view of both occupational hazard and experimental background. Callio Lab, located in the Pyhäsalmi Mine, Finland, is one of the underground laboratories participating in the Baltic Sea Underground Innovation Network (BSUIN). The characterization of the natural background radiation was done at the Lab 2, which is the deepest located in Callio Lab. It involved in-situ gamma spectrometry, thermal neutron flux measurements, radon concentration determination, and α∕β laboratory spectrometry of water and rock samples. At a depth of 1436 m (∼4000 m w.e.) within the felsic volcanic bedrock occurs a volcanogenic massive sulphide deposit, wherein a thermal neutron flux of (1.73 ± 0.10) × 10−5 cm−2s−1, a gamma-ray flux of 12.7 ± 1.5 cm−2s−1, a gamma-ray dose of 0.158 ± 0.029 μ Sv/h and a radon concentration of 213.3 Bq/m3 ± 11% were determined.

Thermal neutron intensity measurement with fission chamber in current, pulse and Campbell modes

In thermal nuclear reactors, most of the power is generated by thermal neutron induced fission. Therefore, fission chambers with targets that respond directly to slow neutrons are of great interest for thermal neutron flux measurements due to relatively low sensitivity to gamma radiation. However, the extreme conditions associated with experiments at very low cross section demand highly possible thermal neutron flux, leading often to substantial design changes. In this paper we report design of a fission chamber for wide range (from 10 to 1012 n/cm2 sec) measurement of thermal neutron flux. Test experiments were performed at the first beam of IBR2 pulsed reactor using digital pulse processing (DPP) technique with modern waveform digitizers (WFD). The neutron pulses detected by the fission chamber in each burst (5 Hz repetition rate) of the reactor were digitized and recorded to PC memory for further on-line and off-line analysis. New method is suggested to make link between the pulse counting, the current mode and the Campbell technique.

The BSUIN project – overview and same results.

The Baltic Sea Underground Innovation Network (BSUIN) [1] project aims to make the underground laboratories in the Baltic Sea region more accessible for innovation, business development and science by improving the information about the underground laboratories, the operation, user experiences and safety. BSUIN is EU funded by Interreg Baltic Sea [2] funding cooperation. The BSUIN consortium has 14 members from eight Baltic Sea countries. Six underground labs are looking for new collaboration in the project. One of the goals of the BSUIN project is to propose standard methods for the characterization of underground laboratories, including natural radiation background characterisation. We have proposed scheme for thermal neutron flux measurement: simple and low-cost but still very reliable. A pilot measurements were made in mines in Freiberg (Germany) Data analysis is in progress

Neutron flux measurements in the Gran Sasso national laboratory and in the Slanic Prahova Salt Mine

We performed measurements of thermal and non-thermal neutron flux in underground laboratories in Gran Sasso, Italy and in Slanic Prahova, Romania. The measurements were made with a set of helium counters: bare for thermal neutrons and covered by blocks of moderator for higher energy neutrons. We found that thermal neutron flux in Slanic is four times smaller than in Gran Sasso, probably due to lack of alpha-radioactive sources in the surrounding rocks. In contrast, the flux of higher energy neutrons in Slanic proved to be 1.4 times higher than in Gran Sasso, which can be explained by higher cosmic muon flux in Slanic.Our result for thermal neutron flux is lower than results of most of the previous measurements carried out in LNGS laboratory but thanks to a different method of analysis the reliability of our results is very high. The result for higher energy neutrons agrees with average energy spectrum resulting from previous measurements.Our measurements in Slanic were the first ones ever performed in this laboratory, and can be used as a reference by future experiments.

A large area detector for thermal neutron flux measurements at the KamLAND site

A large area (0.5 m × 0.5 m) thermal neutron detector was developed based on 0.32 mm-thick 6LiF:ZnS(Ag) scintillator sheets. The detector was constructed for measurements of seasonal variations in low intensity thermal neutron flux at deep underground sites. The detector response to neutrons was studied using a low intensity 241Am/9Be(α,n) neutron source. The digital pulse shape discrimination technique was applied to separate neutron capture events from the background caused by traces of α-emitters from Uranium and Thorium decay chains in scintillator components. The thermal neutron detection efficiency was 0.368 ± 0.018 (0.155 ± 0.009) before (after) event selection based on the digital pulse shape discrimination method. The measured α-particle background event rate in the 6LiF:ZnS(Ag) scintillator was (4.88 ± 0.06(stat))×10−6α cm−2 s−1. As a test of detector performance we measured the thermal neutron flux at the KamLAND area of the Kamioka neutrino observatory and compared it with result of the neutron flux measurement carried out at the Super-Kamiokande site. The thermal neutron flux at KamLAND was (6.43 ± 0.50)×10−6 n cm−2 s−1 that included ∼10% contribution from neutrons with energies above 1 eV. During the detector construction we identified a source of high amplitude noise pulses generated by 5-inch R1250 photomultipliers manufactured by Hamamatsu Photonics K. K.

Activation analysis of tibetan coins and thermal neutron flux measurement at the VR-1 training reactor

The VR-1 training reactor of Czech Technical University in Prague serves mostly to train students; however, owing to the presence of the experimental channels, it can be utilized for particular research tasks such as irradiation experiments, neutron radiography, and neutron activation analysis (NAA). Within the presented experiments, eight samples of historical Tibetan coins were irradiated in the experimental channel at maximum power, and the amount of the copper, silver, gold, and zinc was investigated using the NAA. The irradiated samples were repeatedly measured by the calibrated semiconductor HPGe detector; saturated activities were obtained by means of the γ-ray spectrometry, and subsequently the composition of the samples was determined. In investigated Tibetan coins, the determined concentration of silver ranged from 47.9% to 54.5%, concentration of copper ranged from 13.5% to 99.3%, concentration of zinc ranged from 0.6% to 4.0%, and abundance of gold was 0.03–0.2%. Moreover, the activation measurement of neutron field parameters using the dosimetry foils was performed, and the reaction rates in the vertical experimental channel were determined utilizing the bare and cadmium covered gold foils. Afterwards, the thermal neutron flux for the relevant position was extracted employing the Westcott formalism and activation cross-section from the ENDF/B-VII.1 database. The results presented in this paper show clearly that the low-power VR-1 training reactor (ca. 80 W) is suitable for the neutron activation analysis of historical samples containing the noble metals.

Thermal neutron flux measurement using self-powered neutron detector (SPND) at out-core locations of TRIGA PUSPATI Reactor (RTP)

The thermal neutron flux measurement has been conducted at the out-core location using self-powered neutron detectors (SPNDs). This work represents the first attempt to study SPNDs as neutron flux sensor for developing the fault detection system (FDS) focusing on neutron flux parameters. The study was conducted to test the reliability of the SPND’s signal by measuring the neutron flux through the interaction between neutrons and emitter materials of the SPNDs. Three SPNDs were used to measure the flux at four different radial locations which located at the fission chamber cylinder, 10cm above graphite reflector, between graphite reflector and tank liner and fuel rack. The measurements were conducted at 750 kW reactor power. The outputs from SPNDs were collected through data acquisition system and were corrected to obtain the actual neutron flux due to delayed responses from SPNDs. The measurements showed that thermal neutron flux between fission chamber location near to the tank liner and fuel rack were between 5.18 × 1011 nv to 8.45 × 109 nv. The average thermal neutron flux showed a good agreement with those from previous studies that has been made using simulation at the same core configuration at the nearest irradiation facilities with detector locations.

Last Improvements of the CALMOS Calorimeter Dedicated to Thermal Neutron Flux and Nuclear Heating Measurements inside the OSIRIS Reactor

Nuclear heating inside an MTR reactor needs to be known in order to design and to run irradiation experiments which have to fulfill target temperature constraints. To improve the nuclear heating knowledge, an innovative calorimetric system CALMOS has been studied, manufactured and tested for the 70MWth OSIRIS reactor operated by CEA. This device is based on a mobile calorimetric probe which can be inserted in any in-core experimental location and can be moved axially from the bottom of the core to 1000 mm above the core mid-plane. Obtained results and advantages brought by the first CALMOS-1 equipment have been already presented. However, some difficulties appeared with this first version. A thermal limitation in cells did not allow to monitor nuclear heating up to the 70 MW nominal power, and some significant discrepancies were observed at high heating rates between results deduced from the calibration and those obtained by the “zero method”. Taking this feedback into account, the new CALMOS-2 calorimeter has been designed both for extending the heating range up to 13W.g-1 and for improving the “zero method” measurement thanks to the implementation of a 4-wires technique. In addition, the new calorimeter has been designed as a real operational measurement system, well suited to characterize and to follow the radiation field evolution throughout the reactor cycle. To meet this requirement, a programmable system associated with a specific software allows automatic complete cell mobility in the core, the data acquisition and the measurements processing. This paper presents the analysis of results collected during the 2015 comprehensive measurement campaign. The 4-wires technique was tested up to around a 4 W.g-1 heating level and allowed to quantify discrepancies between “zero” and calibration methods. Thermal neutron flux and nuclear heating measurements from CALMOS-1 and CALMOS-2 are compared. Thermal neutron flux distributions, obtained with the Self-Power Neutron Detector suited to the CALMOS-2 calorimetric probe, are compared with those obtained with current devices. This campaign allowed to highlight advantages brought by the human machine interface automation, which deeply refined the profiles definition. Finally, the decay of the reactor residual power after shutdown could be performed after shutdown, demonstrating the ability of such type of calorimeter to follow the heating level whatever the thermohydraulic conditions, forced or natural convection regimes.

Neutron measurement at the thermal column of the Malaysian Triga Mark II reactor using gold foil activation method and TLD

In order to design facilities for boron neutron capture therapy (BNCT), the neutron measurement must be considered to obtain the optimal design of BNCT facility such as collimator and shielding. The previous feasibility study showed that the thermal column could generate higher thermal neutrons yield for BNCT application at the TRIGA MARK II reactor. Currently, the facility for BNCT are planned to be developed at thermal column. Thus, the main objective was focused on the thermal neutron and epithermal neutron flux measurement at the thermal column. In this measurement, pure gold and cadmium were used as a filter to obtain the thermal and epithermal neutron fluxes from inside and outside of the thermal column door of the 200kW reactor power using a gold foil activation method. The results were compared with neutron fluxes using TLD 600 and TLD 700. The outcome of this work will become the benchmark for the design of BNCT collimator and the shielding

Measurement and simulation of thermal neutron flux distribution in the RTP core

The in-core thermal neutron flux distribution was determined using measurement and simulation methods for the Malaysian’s PUSPATI TRIGA Reactor (RTP). In this work, online thermal neutron flux measurement using Self Powered Neutron Detector (SPND) has been performed to verify and validate the computational methods for neutron flux calculation in RTP calculations. The experimental results were used as a validation to the calculations performed with Monte Carlo code MCNP. The detail in-core neutron flux distributions were estimated using MCNP mesh tally method. The neutron flux mapping obtained revealed the heterogeneous configuration of the core. Based on the measurement and simulation, the thermal flux profile peaked at the centre of the core and gradually decreased towards the outer side of the core. The results show a good agreement (relatively) between calculation and measurement where both show the same radial thermal flux profile inside the core: MCNP model over estimation with maximum discrepancy around 20% higher compared to SPND measurement. As our model also predicts well the neutron flux distribution in the core it can be used for the characterization of the full core, that is neutron flux and spectra calculation, dose rate calculations, reaction rate calculations, etc.

Measurement of Average Thermal Neutron Flux for PGNAA Setup

Measurement of Average Thermal Neutron Flux for PGNAA Setup

Thermal neutron flux measurements in self-serve and tray rod facilities of Dhruva reactor

Dhruva is a 100 MW tank-type research reactor with natural metallic uranium as fuel and heavy water as coolant, moderator, and reflector. The reactor is utilized for production of a large variety of radioisotopes for fulfilling growing demands of various applications in industrial, agricultural, and medicinal sectors, and neutron beam research in condensed matter physics. The core consists of two on-power tray rods for radioisotope production and 15 experimental beam holes for neutron beam research. Recently, a self-serve facility has also been commissioned in one of the through tubes in the reactor for carrying out short-term irradiations. To get accurate information about thermal neutron flux, measurements have been carried out in self-serve facility and on-power tray rod K-09 of Dhruva reactor. The present report describes measurement method, analysis technique, and results. Theoretical estimations for neutron flux were also carried out and a comparison between theoretical and experimental results is made.

First In-Core Simultaneous Measurements of Nuclear Heating and Thermal Neutron Flux Obtained With the Innovative Mobile Calorimeter CALMOS Inside the OSIRIS Reactor

Nuclear heating inside a MTR reactor has to be known in order to design and run irradiation experiments which have to fulfill target temperature constraints. This measurement is usually carried out by calorimetry. The innovative calorimetric system, CALMOS, has been studied and built in 2011 for the 70MWth OSIRIS reactor operated by CEA. Thanks to a new type of calorimetric probe, associated to a specific displacement system, it provides measurements along the fissile height and above the core. Calorimeter working modes, measurement procedures, main modeling and experimental results and expected advantages of this new technique have been already presented in previous papers. However, these first in-core measurements were not performed beyond $6~\mathrm {W} \cdot \mathrm {g} ^{-1}$ , due to an inside temperature limitation imposed by a safety authority requirement. In this paper, we present the first in-core simultaneous measurements of nuclear heating and conventional thermal neutron flux obtained by the CALMOS device at 70 MW nominal reactor power. For the first time, this experimental system was operated in nominal in-core conditions, with nominal neutron flux up to 2.7 $10 ^{14} ~\mathrm {n}\cdot \mathrm {cm}^{-2} \cdot \mathrm {s} ^{-1}$ and nuclear heating up to $12~\mathrm {W} \cdot \mathrm {g} ^{-1}$ . After a brief reminder of the calorimetric cell configuration and displacement system specificities, first nuclear heating distributions at nominal power are presented and discussed. In order to reinforce the heating evaluation, a comparison is made between results obtained by the probe calibration coefficient and the zero methods. Thermal neutron flux evaluation from SPND signal processing required a specific TRIPOLI-4 Monte Carlo calculation which has been performed with the precise CALMOS cell geometry. In addition, the Finite Element model for temperatures map prediction inside the calorimetric cell has been upgraded with recent experimental data obtained up to $12~\mathrm {W} \cdot \mathrm {g} ^{-1}$ . Finally, the experience feedback led us to improvement perspectives. A second device is currently under manufacturing and main technical options are presented.

Thermal neutron background measurement in CJPL

This paper describes the measurement of thermal neutron flux in the CJPL underground laboratory in the proximity of the CDEX experiment. A low background thermal neutron detection system is designed which applies a combination of a 3He proportional tube and a 4He proportional tube as the detector. Thermal neutrons can be captured by the 3He proportional tube while the 4He proportional tube is for the purpose of background measurement. The tube wall is made up of oxygen-free copper to reduce the background due to radioactivity of the wall material. The electronics readout system has been developed to store triggered events׳ waveforms so as to get the amplitude spectrum and monitor the data quality. We observed an average thermal neutron flux of Φ=4.00±0.08×10−6/cm2s in the CJPL experiment hall in the proximity of CDEX experiment and the neutron and background events both distribute uniformly along the tube.