Measurement Of Fast Neutron Flux Research Articles

A background suppression detector array for fast neutron measurement in space science study

A compact detector array with a symmetrical structure was designed and developed for the measurement of fast neutron flux in space radiation environments by the recoil proton method. It consists of seven solid-state detectors and a hydrogen-containing conversion layer, which can effectively eliminate the background from the charged particles and gamma rays. The performances for fast neutron measurements in the low Earth orbit and on the lunar surface have been simulated by Geant4 software. The signals from recoil protons can be well distinguished in a complex radiation environment. The accuracy of fast neutron flux measurement can be improved with a higher signal-to-noise ratio. Utilizing this array, a neutron spectrometer for low Earth orbit applications was constructed and was launched on board the “Weiming-1” CubeSat at the beginning of 2024.

Feasibility study of a new method to measure fast neutron flux by neutron-induced X-ray fluorescence method

A new method is proposed to detect fast neutron flux: the neutron-induced X-ray fluorescence method. In this method, the neutrons are firstly converted to charged particles, and then the charged particles interact with the target to generate the characteristic X-ray fluorescence. Finally, the Kα X-ray emission from the target is detected for measurement of the neutron flux. On the basis of this method, the structure of the detection device is designed: it is composed mainly of a layer of a fast neutron converter and a layer of the target. The Geant4 Monte Carlo simulation toolkit is used to optimize the detection device. According to the simulation results, the detection device is manufactured so as to perform the feasibility verification experiments. In the experiments, a D-T neutron generator is used as the neutron source, and the number of neutrons is changed in two ways: by change of the working voltage of the neutron generator or by change of the measurement time. The experimental results show that there is a good linear relationship between the number of neutrons and the Kα X-ray counts when the measurement time is changed. When the working voltage of the neutron generator is changed, a 3He proportional counter is used as a reference detector. The results show that the trend of the Kα X-ray counts is in good agreement with the trend for the 3He proportional counter. It is proved that this method is feasible for fast neutron flux measurement.

In-Pile Qualification of a Fast-Neutron-Detection-System

In order to ensure the quality and the relevance of irradiation programs in the future Jules Horowitz Reactor (JHR), the French Alternative Energies and Atomic Energy Commission (CEA) has significantly increased its research and development effort in the field of in-pile instrumentation during the last decade. Major progress has been achieved in the capability to perform accurate in-pile measurements using reliable and updated techniques. A significant part of this effort has been conducted in the framework of the Joint Instrumentation Laboratory between the CEA and the Belgian Nuclear Research Centre (SCK $\cdot$ CEN). In order to improve measurement techniques for neutron flux assessment, a unique system for online measurement of fast neutron flux has been developed and recently qualified in-pile. The fast-neutron-detection-system (FNDS) has been designed to monitor accurately high-energy neutron flux (E > 1 MeV) in typical material testing reactor (MTR) conditions, where neutron flux levels can be as high as $10^{15}\,\,\text {N}\cdot \text {cm}^{-2}\cdot \text {s}^{-1}$ and are generally dominated by thermal neutrons. Moreover, the neutron flux is coupled with a high gamma flux of typically a few $10^{15} \gamma \cdot \text {cm}^{-2}\cdot \text {s}^{-1}$ , which can be highly disturbing for the online measurement of neutron fluxes. The patented FNDS system is based on two detectors allowing the simultaneous detection of both thermal- and fast-neutron flux. Thermal neutrons can be measured using a self-powered neutron detector or a 235U miniature fission chamber, while fast neutron detection requires a miniature fission chamber with a special fissile material presenting an energy threshold near 1 MeV, which can be 242Pu for MTR conditions. Fission chambers are operated in Campbelling mode for an efficient gamma rejection. FNDS also includes a specific software that processes measurements to compensate online the fissile material depletion and to adjust the sensitivity of the detectors, in order to produce a precise evaluation of both thermal and fast neutron flux even after long-term irradiation. FNDS has been validated through a two-step experimental program. A first set of tests was performed at BR2 reactor operated by SCK $\cdot$ CEN in Belgium. Two FNDS prototypes were operated in-pile during nearly 1000 h. These tests exhibited the consistency of the measurement of thermal to fast neutron flux ratio with Monte Carlo neutron and photon transport calculations, as well as the right compensation of fissile material depletion. This paper describes a second test recently completed at ISIS reactor operated by CEA in France. For this irradiation, FNDS signal was compared to reference thermal- and fast-neutron flux measurements using activation dosimeters analyzed under COFRAC Quality Certification. During this latter test, FNDS proved its ability to measure online the fast neutron flux with an overall accuracy better than 5%. FNDS is now operational and is assumed to be the first and unique acquisition system able to provide an online measurement of the fast neutron flux in MTR conditions. This system will be used to perform spectral neutron characterization of JHR channels, but it may also be implemented in future irradiation experiments, for a better and real-time evaluation of the fast neutron flux received by material and fuel samples.

In-Pile Qualification of the Fast-Neutron-Detection-System

In order to improve measurement techniques for neutron flux assessment, a unique system for online measurement of fast neutron flux has been developed and recently qualified in-pile by the French Alternative Energies and Atomic Energy Commission (CEA) in cooperation with the Belgian Nuclear Research Centre (SCK•ECEN). The Fast-Neutron-Detection-System (FNDS) has been designed to monitor accurately high-energy neutrons flux (E > 1 MeV) in typical Material Testing Reactor conditions, where overall neutron flux level can be as high as 1015 n.cm-2.s-1 and is generally dominated by thermal neutrons. Moreover, the neutron flux is coupled with a high gamma flux of typically a few 1015 γ.cm-2.s-1, which can be highly disturbing for the online measurement of neutron fluxes. The patented FNDS system is based on two detectors, including a miniature fission chamber with a special fissile material presenting an energy threshold near 1 MeV, which can be 242Pu for MTR conditions. Fission chambers are operated in Campbelling mode for an efficient gamma rejection. FNDS also includes a specific software that processes measurements to compensate online the fissile material depletion and to adjust the sensitivity of the detectors, in order to produce a precise evaluation of both thermal and fast neutron flux even after long term irradiation. FNDS has been validated through a two-step experimental program. A first set of tests was performed at BR2 reactor operated by SCK•CEN in Belgium. Then a second test was recently completed at ISIS reactor operated by CEA in France. FNDS proved its ability to measure online the fast neutron flux with an overall accuracy better than 5%.

Fast neutron flux analyzer with real-time digital pulse shape discrimination

Investigation of subthermonuclear plasma confinement and heating in magnetic fusion devices such as GOL–3 and GDT at the Budker Institute (Novosibirsk, Russia) requires sophisticated equipment for neutron-, gamma- diagnostics and upgrading data acquisition systems with online data processing. Measurement of fast neutron flux with stilbene scintillation detectors raised the problem of discrimination of the neutrons (n) from background cosmic particles (muons) and neutron-induced gamma rays (γ). This paper describes a fast neutron flux analyzer with real-time digital pulse-shape discrimination (DPSD) algorithm FPGA-implemented for the GOL–3 and GDT devices. This analyzer was tested and calibrated with the help of 137Cs and 252Cf radiation sources. The Figures of Merit (FOM) calculated for different energy cuts are presented.

Monte Carlo simulations for high-rate fast neutron flux measurements made at the RAON neutron science facility by using MICROMEGAS

RAON is a Korean heavy-ion accelerator complex that is planned to be built by 2021. Deuterons (53 MeV) and protons (88 MeV) accelerated by using a low-energy driver linac (SCL1) are delivered to the neutron production target in the Neutron Science Facility (NSF) to produce high-energy neutrons in the interval from 1 to 88 MeV with high fluxes of the order of 1012 n/cm2-sec. The repetition rate of the neutron beam ranges from 1 kHz to 1 MHz, and the maximum beam current is ~12 μA at 1 MHz. The beam width is 1 ~ 2 ns. The high-energy and high-rate fast neutrons are used to estimate accurate neutron-induced cross sections for various nuclides at the NSF. A MICROMEGAS (MICRO Mesh Gaseous Structure), which is a gaseous detector initially developed for tracking in high-rate, high-energy physics experiments, is tentatively being considered as a neutron beam monitor. It can be used to measure both the energy distribution and the flux of the neutron beam. In this study, a MICROMEGAS detector for installation at the NSF was designed and investigated. 6Li, 10B, 235U and 238U targets are being considered as neutron/charged particle converters. For the low-energy region, 6Li(n,α)t and 10B(n,α)7Li are used in the energy range from thermal to 1 MeV. 235U(n,f) and 238U(n,f) reactions are used for high-energy region up to 90 MeV. All calculations are performed by using the GEANT4 toolkit.

Influence of thermal and resonance neutron on fast neutron flux measurement by 239Pu fission chamber

The 239Pu fission chambers are widely used to measure fission spectrum neutron flux due to a flat response to fast neutrons. However, in the meantime the resonance and thermal neutrons can cause a significant influence on the measurement if they are moderated, which could be eliminated by using 10B and Cd covers. At a column enriched uranium fast neutron critical assembly, the fission reaction rates of 239Pu are measured as 1.791×10−16, 2.350×10−16 and 1.385×10−15 per second for 15 mm thick 10B cover, 0.5 mm thick Cd cover, and no cover respectively, while the fission reaction rate of 239Pu is rapidly increased to 2.569×10−14 for a 20 mm thick polythene covering fission chamber. The average 239Pu fission cross-section of thermal and resonance neutrons is calculated to be 500 b and 24.95 b with the assumption of 1/v and 1/E spectra respectively, then thermal, resonance and fast neutron flux are achieved to be 2.30×106, 2.24×106 and 1.04×108 cm−2·s−1.

Study of neutron radiation behavior for RF heating on HT-7

Neutron radiation was specially correlated with the type and the power of the radio frequency (RF) heating for deuterium plasma on HT-7 tokamak. We hare researched the production mechanism of the fast neutron under different RF heating conditions by the fast time-resolved neutron flux measurement system make up of BF3 and 3He proportional counters. Lower power of LHW heating was apt to bring abont runaway discharge and produce a mass of photoneutrons. The rate of fusion neutron and the total neutron yield will rapidly increase with the ion cyclotron resonance heating at the special frequencies of (27 MHz and 24 MHz).

Innovations for In-Pile Measurements in the Framework of the CEA-SCK•CEN Joint Instrumentation Laboratory

Optimizing the life cycle of nuclear systems under safety constraints requires high-performance experimental programs to reduce uncertainties on margins and limits. In addition to improvement in modeling and simulation, innovation in instrumentation is crucial for analytical and integral experiments conducted in research reactors. The quality of nuclear research programs relies obviously on an excellent knowledge of their experimental environment which constantly calls for better online determination of neutron and gamma flux. But the combination of continuously increasing scientific requirements and new experimental domains -brought for example by Generation IV programsnecessitates also major innovations for in-pile measurements of temperature, dimensions, pressure or chemical analysis in innovative mediums. At the same time, the recent arising of a European platform around the building of the Jules Horowitz Reactor offers new opportunities for research institutes and organizations to pool their resources in order to face these technical challenges. In this situation, CEA (French Nuclear Energy Commission) and SCK'CEN (Belgian Nuclear Research Centre) have combined their efforts and now share common developments through a Joint Instrumentation Laboratory. Significant progresses have thus been obtained recently in the field of in-pile measurements, on one hand by improvement of existing measurement methods, and on the other hand by introduction in research reactors of original measurement techniques. This paper highlights the state-of-the-art and the main requirements regarding in-pile measurements, particularly for the needs of current and future irradiation programs performed in material testing reactors. Some of the main on-going developments performed in the framework of the Joint Instrumentation Laboratory are also described, such as: - a unique fast neutron flux measurement system using fission chambers with 242Pu deposit and a specific online data processing, - an optical system designed to perform in-pile dimensional measurements of material samples under irradiation, - an acoustical instrumentation allowing the online characterization of fission gas release in Pressurized Water Reactor fuel rods. For each example, the obtained results, expected impacts and development status are detailed.

Monte Carlo Modeling of the Portuguese Research Reactor Core and Comparison with Experimental Measurements

The Monte Carlo code MCNP-4C was used to calculate the effective multiplication coefficient of a core configuration of the Portuguese Research Reactor (RPI) and the neutron fluxes in the core and in the reflector region.A comparison of the results obtained with MCNP and with the deterministic codes WIMSD-5 and CITATION was made. Consistent deviations of 2% for the effective multiplication constant and 8 to 28% for the neutron flux, depending on the energy range, were observed.Thermal, epithermal, and fast neutron flux measurements were performed using activation detectors. The calculations agree with experimental values within <15%; therefore, the Monte Carlo results can be used to predict the neutron field in other locations and irradiation facilities of the RPI.

High-sensitive spectrometer of fast neutrons and the results of fast neutron background flux measurements at the Gallium–Germanium Solar Neutrino Experiment

The principle of operation, design, registration system and main characteristics of a fast neutron spectrometer are described. The spectrometer is intended for direct measurements of ultra low fluxes of fast neutrons. It is sensitive to neutron fluxes of 10 −7 cm −2 s −1 and lower. The detection efficiency of fast neutrons with simultaneous energy measurement was determined from Monte-Carlo simulation to be equal to 0.11±0.01. The background counting rate in the detector corresponds to a neutron flux of (6.5±2.1)×10 −7 cm −2 s −1 in the range 1.0–11.0 MeV. The natural neutron flux from the surrounding mine rock at the depth of 4600 m of water equivalent was measured to be (7.3±2.4)×10 −7 cm −2 s −1 in the range 1.0–11.0 MeV. The flux of fast neutrons in the SAGE main room was measured to be <2.3×10 −7 cm −2 s −1 in 1.0–11.0 MeV energy range.

Underground neutron flux measurement

We report the measurement of thermal, epithermal and fast neutron flux performed in an underground laboratory excavated under about 2000 m of rock; they are compared with measurements in open air. A set of three large 3He proportional counters in parallel was used.

核分裂片飛跡検出法による速中性子測定のためのトリウム薄膜ターゲットの作製

The measurement of fast neutron flux was made by means of electrical spark counting of etched nuclear tracks on polycarbonate film (Makrofol KG) of 10μm thickness. 232Th electrodeposited disk was used as a detector target. By this method the upper limit of spark counts was about 660/cm2 of detector film. A linear relation was obtained from 5×106 to 4×1010n/cm2 of neutron fluence for 199μg/cm2 thorium target. Detection sensitivity (the ratio of spark counts per unit area to fast neutron fluence) was estimated to be 3.8×10-8 counts/n, hence one spark count equivalent to 189 mrem (1.89mSv).Process of thorium electroplating on a stainless steel disk as a target for fast neutron detection was described in detail. Thorium nitrate dissolved in 0.035M ammonium oxalate [(NH4)2C2O4] was used as electrolyte. Maximum electroplating efficiency obtained was about 70% with electric current density 22mA/cm2 and pH 6-8 at 80°C for 3hr.

The Use of the Reaction93Nb(n,n′)93mNb for Fast Neutron Flux Measurement

A method for high fast neutron flux measurement using the reaction 93Nb(n,n′)93mNb was tested. The method is based on measurement of niobium x rays. An average cross section over a measured 252Cf n...

A concentric ring technique for the measurement of fast neutron fluxes and energies

The limitations imposed by fixed foil, mixed powder and sandwich foil techniques for fast neutron flux measurement led the authors previously to suggest a concentric ring monitoring technique. This technique can be extended to measure neutron energies if excitation functions of the two materials are known. The method is verified by using concentric copper monitoring rings with aluminium discs. The ratios of activities of copper and aluminium enable the neutron energy to be measured to ±1·2 percent. The advantages and applications of the technique are discussed and a modified design is suggested for very short target to specimen separation. An antimony disc with concentric copper and aluminium rings was used to measure the 121Sb(n, 2n) 120 g Sb excitation function, the copper and aluminium rings being used to measure both fast neutron flux and energy. It is suggested that the 121Sb(n, 2n) 120 g Sb reaction would be very useful as a standard for flux monitoring in the 14 MeV region since its excitation function is relatively flat between 13·5 and 15 MeV.

Fast-neutron flux measurement with a lithium-drifted germanium detector

The use of lithium-drifted germanium detectors for fast-neutron flux measurement has been investigated. The proposed detection mechanism is observation of the internal conversion electrons emitted during the de-excitation of the 691 KeV 0 + state in 72Ge following excitation via fast-neutron scattering. A 4.0 cm 3 (approx.) planar Ge(Li) detector has been calibrated for neutron flux measurements over the energy range E n = 700−5000 keV. The absolute efficiency for this detector (ratio of neutrons detected to neutrons incident on the detector) has been determined to be (8.2±0.5) × 10 −4 at E n = 1500 keV which is about 10% of the full-energy peak efficiency for 1332 keV 60Co gamma rays. Cross-sections for excitation of the 691 keV 0 + state in 72Ge via neutron scattering have been extracted from the measured efficiency function. These cross-sections can be used to calculate efficiency functions for other detectors. Insensitivity to thermal neutrons, gamma-ray discrimination capability and stability are among the desirable features of this type of fast-neutron detector. Submicrosecond timing is not possible because of the isomeric nature of the 691 keV 0 + state in 72Ge. While Ge(Li) detectors suffer radiation damage from fast-neutron bombardment, the detector used for these measurements withstood an integrated dose exceeding 10 10 neutrons/cm 2 without serious deterioration of its performance.

The importance of excitation functions in fast neutron flux measurement

The importance of a knowledge of the excitation function of reaction used as monitors for 14 MeV flux measurements is discussed. The information on excitation functions of frequently used reactions is summarised and some new measurements are reported.

Corrections for distortion effects in fast neutron flux measurements with a stilbene-scintillator

If the method of recoil detection in organic scintillators shall be used for measuring the flux of fast neutrons with good accuracy corrections have to be applied for accounting for various effects distorting the recoil spectrum. This problem is investigated experimentally as well as theoretically for a 1“ dia. ×1“ stilbene scintillator for neutrons in the energy region 2.6−14.1 MeV. A Monte Carlo method is used for calculating the distortion of the recoil proton spectrum by multiple neutron scattering, edge-effects and nonlinear response of the scintillator. Theoretical and experimental corrections to be applied to the integral recoil proton spectrum used for the flux measurement are given. An accuracy of better than 4% for the flux determination is claimed.

531. 高速中性子束測定用&lt;SUP&gt;232&lt;/SUP&gt;Th小型核分裂計数管

A 232Th miniature fission counter probe was perfected with use of the Th depositing technique. A sensitivity of approximately 1×10-6 counts/n/cmcm2 was obtained for reactor fast neutron flux with a typical cylindrical counter (6mm O.D., 10mm sensitive length). The counter was applied to the measurement of the fast neutron flux distribution within a research reactor core. The fast neutron flux distribution thus measured with the counter provided results identical with that obtained from the activation of threshold foils. Depleted U fission counters with Cd-cover have in the past been used for fast neutron flux measurements. The 232Th fission counter can, however, be used without Cd-filter, with excellent resolution of thermal and epithermal neutrons.

Cosmic-ray fast-neutron flux measurements in the atmosphere at various latitudes

We have extended in time our series of balloon flights, made at several latitudes between Hyderabad, India, and Ft. Churchill, Manitoba, to altitudes close to the top of the atmosphere. In these flights the neutrons generated by the cosmic radiation in the energy interval between 1 and 10 MeV are measured. The first set of measurements was made during the period of the minimum of solar activity, and the more recent flights carry the work into the start of the next solar cycle. A decrease in intensity at high elevations with the onset of the present solar cycle has been noted. Further data were also obtained on an airplane flight around the world over both poles, thus covering the full range of latitudes at two opposite longitudes. The relationship between the observed neutron spectrum and that derived by the use of a neutron transport code will be discussed. We shall also discuss other factors emerging from this analysis, including the numbers for radiocarbon production and the leakage flux.