High Neutron Flux Research Articles

Reactor activations to constrain astrophysically relevant cross sections

The determination of astrophysically relevant neutron-induced cross sections is particularly difficult when the involved isotopes are radioactive or the cross sections are very small. Activation experiments at reactors offer the possibility to overcome these limitations with high neutron fluxes. The flux determination is typically based on the activation of two monitors with known cross sections to separate the different flux components. The usually applied cadmium difference method allows a distinction between the thermal and the epithermal part. By a combination of two linear functions representing both monitors the neutron flux components can be determined. However, if more than two monitors are used, the linear system of equations is overdetermined, which allows the identification of a probability distribution. In this proceeding, the feasibility and relevance of this method is demonstrated.

First GEM measurements at WEST and perspectives for fast electrons and heavy impurities transport studies in tokamaks

Soft X-ray (SXR) radiation emitted from tokamak plasmas contains very useful information about plasma stability, shape and impurity content, all key parameters to improve plasma performance. In the deuterium-tritium phase of ITER, the high neutron fluxes, gamma and hard X-ray emission will constitute too harsh an environment to permit the use of classical semiconductor detectors. New SXR detector technologies, more robust to such environments, should thus be investigated. First GEM (Gas Electron Multiplier) measurements performed at WEST were successful and showed that both spatially and spectrally resolved calibrated data could be acquired. Strategies to reconstruct tungsten (W) impurity radiation synthetic diagnostics, modelling and real measurements based on multiple diagnostics are proposed.

Источник ультрахолодных нейтронов на основе сверхтекучего гелия для реактора ПИК

The PIK reactor at NRC "Kurchatov Institute" - PNPI is going to be equipped with a high-flux Ultra Cold Neutron (UCN) source for fundamental physics researches. The UCN source will use superfluid helium, which will make possible to achieve the density of UCN 2.2·10^3 cm-3, that has not yet been achieved anywhere in the world. The UCN source will be installed on the GEK-4 channel, which will make possible to obtain a low value of heat influx to cryogenic vessels from reactor radiation. The heat removal from the UCN source vessel will be implemented by using a heat exchanger. The calculated UCN density in the EDM spectrometer chamber at the PIK is going to be 200 cm-3, which is 20 times higher than the existing UCN densities in the world. For a new UCN source based on superfluid helium, an extensive research program has been developed in field of the physics of fundamental interactions, including the search for a nonzero neutron EDM, precision measurement of the neutron lifetime, and search for mirror dark matter.

Superfluid helium based ultracold neutron source for the PIK reactor

The PIK reactor at NRC "Kurchatov Institute"-PNPI is going to be equipped with a high-flux Ultra Cold Neutron (UCN) source for fundamental physics researches. The UCN source will use superfluid helium, which will make possible to achieve the density of UCN 2.2·10^3 cm-3, that has not yet been achieved anywhere in the world. The UCN source will be installed on the GEK-4 channel, which will make possible to obtain a low value of heat influx to cryogenic vessels from reactor radiation. The heat removal from the UCN source vessel will be implemented by using a heat exchanger. The calculated UCN density in the EDM spectrometer chamber at the PIK is going to be 200 cm-3, which is 20 times higher than the existing UCN densities in the world. For a new UCN source based on superfluid helium, an extensive research program has been developed in field of the physics of fundamental interactions, including the search for a nonzero neutron EDM, precision measurement of the neutron lifetime, and search for mirror dark matter. Keywords: ultracold neutrons, neutron sources, superfluid helium, neutron EDM, neutron decay.

NG-TRAP: Measuring neutron capture cross-sections of short-lived fission fragments

We lack significant nuclear physics input to understand the rapid-neutron capture (r-)process fully. The r-process is the source of half the elements heavier than iron and the only way to produce the long-lived actinides we find on earth. This process’s key nuclear physics inputs are nuclear masses, cross-sections of (n,γ) and (γ,n), and decay half-lives and branching ratios of neutron-rich isotopes. However, there is currently no method to directly measure neutron-induced reaction rates on short-lived nuclides, so there is no experimental data for the primary nuclear reaction that drives the r-process. We show here a conceptual design of a novel approach to access this information experimentally. The idea is to form a target of short-lived isotopes by confining them as ions in a radio-frequency (RF) trap. Next, they are irradiated with an intense neutron flux, and the reaction products are identified by mass spectrometry. The chosen method is a two-stage process in the presence of high neutron fluxes. The first process is neutron-induced fission in a thin actinide foil to create fission fragments. These fragments are slowed down in a cryogenic stopping cell before being filtered through a radio frequency quadrupole (RFQ) system. The RFQ system selects fission fragments of a specific atomic mass number A and confines them to a small volume in an RF trap, where they are irradiated for a second time in a controlled manner. The resultant A+1 isotopes are mass-selectively transported to a multiple-reflection time-of-flight mass spectrometer, where the reaction products are identified and counted.

Two Design Options for Compact Linear Accelerators for High Flux Neutron Source

We describe and compare two optimized design options of RF linear accelerators with different resonant frequencies at 162.5 MHz (f0) and 325 MHz (2∙f0). The RFQ + DTL linacs have been designed to provide 13 MeV acceleration to a proton beam for achieving a fast neutron yield of not lower than 1013 n/s via 9Be(p, n)9B reaction in pulsed-mode operation. Our design studies show that none of the two options is better than the other, but that the choice of operating frequency will mainly be determined by the accelerator length and RF cost consideration. This study can serve as a basis for the design of an initial stage of a new high brilliance Compact Accelerator-driven Neutron Source (CANS), aiming to use neutron scattering techniques for studying material properties in fundamental physics, materials science, nuclear energy, as well as for industries and societal challenges.

Void swelling of conventional and composition engineered HT9 alloys after high-dose self-ion irradiation

Ferritic/martensitic (F/M) steels are being considered as potential structural materials for next generation nuclear reactors, and variants of the alloy HT9 are some of the most promising candidates. In this study, two conventional and two composition engineered HT9 alloys were irradiated using 3.5 MeV Fe2+ up to 600 peak displacement-per-atom (dpa) at 450 ℃. Void swelling and microstructure evolution were characterized for each alloy and compared. The two conventional HT9 alloys (INL and ACO3) showed similar void swelling behavior due to their similar elemental composition and processing conditions. The INL HT9 exhibited a maximum of 2.4% swelling and the ACO3 HT9 showed a maximum of 2.8% swelling at 342 and 393 average local dpa, respectively. On the other hand, the two-composition engineered HT9 alloys with varying N contents (10 ppm for low N and 440 ppm for high N) showed disparate swelling behavior. The low N HT9 exhibited a maximum of 4.6% swelling, while the high N HT9 showed a maximum of 0.7% swelling at 342 average local dpa. Changes in the N content also affected Ni/Si rich G-phase formation. The low N HT9 showed a larger size and lower density of G-phase precipitates compared with the high N HT9 after 600 peak dpa irradiation. This study compares the void swelling behavior of the ion irradiated four current HT9 alloys to extremely high doses, with the void swelling data from neutron irradiated HT9 alloys. The comparison lends critical insights into how well these current alloys can withstand high neutron fluxes in future reactors, especially since the low N and high N HT9 alloys have never been exposed to such high doses before.

Study of rod ejection accident at hot zero power condition in a PWR using RELAP5

The core reactivity and power distribution within the core of a pressurized water reactor (PWR) are controlled for safe and efficient operation of the reactor. Several postulated events, however, can alter the core reactivity and power distribution and disrupt reactor operation. Such those events are the reactivity accidents. Among the different expected reactivity accidents, the control rod ejection accident (REA) is of a particular concern. This accident can occur in PWRs by mechanical failure of the control rod drive mechanism or due to inadvertent operator action. There are two cases of REA; REA at hot zero power condition, and REA at power condition. This accident results in reactivity addition, and based on the rate of reactivity addition, the reactor power may increases to levels large enough to endanger the fuel assemblies faster than the automatic protective actions.In the current work, RELAP5 code is used to study the consequences of REA at Hot Zero Power Condition in a standard four loop Westinghouse PWR reactor. A reactivity insertion of up to 75 pcm/sec is assumed in the analysis due to the rod movement. This value is consistent with the withdrawal of two control rod banks. A RELAP5 input file is prepared to simulate the reactor core and primary system. In this file, the core is simulated by two channels, a hot channel represents a one fuel assembly and an average channel represents the rest of the core. RELAP5 reactor kinetics model is used to determine the power generation with time including the core feedback effects; i.e., Doppler reactivity and moderator reactivity. The reactor is set to trip when it reaches 35% of full power, the low setting reactor trip assumed to be initiated by power range high neutron flux. The initial power level was assumed to be below the power level expected for any shutdown condition (10−9 of nominal power).Results of the study including the peak power, maximum fuel pellet temperature and fuel pellet enthalpy are presented. The effects of the rod ejection rate on REA consequences are also studied.

Collimated neutron beam design for TRR thermal column

Tehran Research Reactor (TRR) is the main neutron source in Iran which can be used for different applications of neutrons such as neutron radiography and neutron therapy. TRR has a thermal column which can provide high intensity flux of thermal neutrons for users. The aim of this study is to design a neutron collimator for TRR thermal column to produce parallel neutron beam with suitable intensity of thermal neutrons. To achieve this goal, Monte Carlo code of MCNX has been used to evaluate different configurations, geometries and materials of neutron collimator. The results show that the final selected configuration can provide a uniform thermal neutron beam with a flux of 1.21E+13 (cm-2·s-1) which is suitable for many different neutron applications.

Improving the sensitivity to light dark matter with the Migdal effect

We studied, using a Bayesian approach, how the Migdal effect can be exploited in experiments based on liquid argon detectors. In particular we develop a simulated experiment to show how the Migdal electron allows to push the experimental sensitivity down to masses of 0.1 GeV/c 2, extending the search region for dark matter particles of previous results. Finally, given the relevance of the Migdal effect to the search for low mass Dark Matter, we discuss some new ideas on how to possibly measure such an effect with detectors based on a Time Projection Chamber exposed to an high neutron flux.

A stopping layer concept to improve the spatial resolution of gas-electron-multiplier neutron detector

In recent years, gas electron multiplier (GEM) neutron detectors have been developing towards high spatial resolution and high dynamic counting range. We propose a novel concept of an Al stopping layer to enable the detector to achieve sub-millimeter (sub-mm) spatial resolution. The neutron conversion layer is coated with the Al stopping layer to limit the emission angle of ions into the drift region. The short track projection of ions is obtained on the signal readout board, and the detector would get good spatial resolution. The spatial resolutions of the GEM neutron detector with the Al stopping layer are simulated and optimized based on Geant4GarfieldInterface. The spatial resolution of the detector is 0.76 mm and the thermal neutron detection efficiency is about 0.01% when the Al stopping layer is 3.0 μm thick, the drift region is 2 mm thick, the strip pitch is 600 μm, and the digital readout is employed. Thus, the GEM neutron detector with a simple detector structure and a fast readout mode is developed to obtain a high spatial resolution and high dynamic counting range. It could be used for the direct measurement of a high-flux neutron beam, such as Bragg transmission imaging, very small-angle scattering neutron detection and neutron beam diagnostic.

The effect of temperature and cascade collision on thermal conductivity of 3C-SiC: A molecular dynamics study

The cubic silicon carbide 3C-SiC, the significant nuclear material used to manufacture the nuclear fuel in High Temperature Gas-cooled Reactor (HTGR), operates under a high temperature and neutron flux condition. The effect of temperature and irradiation on thermal conductivity of 3C-SiC were investigated with the molecular dynamics method. The equilibrium and non-equilibrium methods were utilized to calculate the thermal conductivity of 3C-SiC and the results of tersoff/zbl and MEAM interatomic potential were compared. Exponential decay of thermal conductivity with increasing temperature was illustrated. The irradiation effect was simulated with a high-energy Si atom and subsequent cascade collision, a multiple-phase simulation procedure was employed and the number of four kinds of point defects was counted. Finally, a relatively smaller simulation cell was used to analyze the impact of the incident atom with various velocities on the thermal conductivity of 3C-SiC. The computed results showed the cascade collision, caused by a higher-energy atom, would induce the lower thermal conductivity of 3C-SiC.

Simulation of a new neutron beam guide design for generating high neutron flux

In a neutron scattering instrument, beam flux is important for reducing the measurement time and increasing the statistics of the acquired data. Neutron focusing optical devices, such as parabolic or elliptical supermirrors, have been used to improve the flux at the sample. The aim of this study is to design a focusing guide for gaining high neutron flux with appropriate beam divergence. In this study, a method of arranging multiple layers of parabolic and elliptical mirrors, generating several focal points, is proposed to minimize the beam loss due to absorption during neutron beam focusing. The proposed focusing guide design is simulated via a Monte Carlo neutron ray-tracing simulation package. The practical realization of this proposed design will aid in engineering advanced neutron beam guiding devices and systems capable of producing and delivering neutron beams with high flux, improved intensity, and minimum divergence.

Development of a CCD based thermal neutron imaging detector for the Israeli Research Reactor IRR-1 at Soreq NRC

Among the imaging systems used for thermal neutron imaging worldwide, the most prevalent configuration is CCD camera based. At facilities with sufficiently high neutron flux (>1 × 106 n/cm2/sec), the basic design does not necessitate amplification of scintillation photons prior to their registration by the CCD camera. However, for high resolution imaging at low flux reactors, insufficient amount of scintillation photons may reach each CCD pixel, thus, photon statistics becomes the limiting factor over neutron statistics.As described in this work, introduction of an image-intensifier into the system’s optical chain removed photon statistics limitation, allowed fine focus adjustments and improved image quality. This led to the possibility of performing good quality tomographic scans.

MBGEM: a stack of borated GEM detector for high efficiency thermal neutron detection

A new position-sensitive thermal neutron detector based on boron-coated converters has been developed as an alternative to today’s standard ^3mathrm{He}-based technology for application to thermal neutron scattering. The key elements of the development are the boron-coated GEM foils (Sauli in Nucl Instrum Methods Phys Res Sect A Accel Spectrom Detect Assoc Equip 386:531, 1997) that are used as a multi-layer neutron converter via the ^{10}mathrm{B}(n,alpha )^7mathrm{Li} reaction together with an efficient collection of the produced secondary electrons. This paper reports the test performed on a 3 layers converter prototype coupled to a GEMPix detector (Murtas in Radiat Meas 138:106421, 2020), carried out in order to study the possibility to produce a large-scale multi-layer neutron detector capable to reach high detection efficiency with high spatial resolution and able to sustain the high neutron flux expected in the new neutron spallation source under development like the ESS.

Neutronics optimization study on the first wall design of CFETR for TBR enhancement

Tritium breeding capability is one of the most important objectives for fusion reactor design, and the tritium self-sufficiency has been set as a crucial target for the China Fusion Engineering Test Reactor (CFETR). The first wall (FW) is the first component of the blanket facing plasma, which bears high heat flux and high neutron current. Due to the neutrons entering into the blanket pass through the FW and then absorbed by the breeder materials, the FW design directly influences the blanket neutronics and tritium breeding performance. In this study, the helium-cooled solid breeder (HCSB) blanket of CFETR is selected as the reference design, and the impacts of the FW design parameters on tritium breeding performance have been studied comprehensively. A new conceptual mixed armor design and FW neutronics optimization guidelines have been proposed to enhance the tritium breeding ratio (TBR). Based on the mixed armor design and the optimization guidelines, the preliminarily optimized helium-cooled FW design shows about 9% TBR enhancement.

Study of p-type silicon MOS capacitors at HL-LHC radiation levels through irradiation with a cobalt-60 gamma source and a TCAD simulation

During the era of the High Luminosity LHC (HL-LHC) the devices in its experiments will be subjected to increased radiation levels with high fluxes of neutrons and charged hadrons, especially in the inner detectors. A systematic program of radiation tests with neutrons and charged hadrons is being carried out by the CMS and ATLAS Collaborations in view of the upgrade of the experiments, in order to cope with the higher luminosity at HL-LHC and the associated increase in the pile-up events and radiation fluxes. In this work, results from a complementary radiation study with 60Co-γ photons are presented. The doses are equivalent to those that the outer layers of the silicon tracker systems of the two big LHC experiments will be subjected to. The devices in this study are MOS capacitors fabricated on the float-zone oxygenated p-type silicon wafer. The results of CV measurements on these devices are presented as a function of the total absorbed radiation dose following a specific annealing protocol. The measurements are compared with the results of a TCAD simulation.

Status and future developments of the Linear IFMIF Prototype Accelerator (LIPAc)

LIPAc is the Linear IFMIF Prototype Accelerator developed within the framework of the IFMIF project under the Broader Approach (BA) agreement signed between EURATOM and the Japanese Government in 2007. The IFMIF accelerator aims to provide an accelerator-based D-Li neutron source to produce high intensity neutron fluxes with appropriate energy spectrum in order to characterize materials envisioned for future fusion reactors. Because the IFMIF accelerator has to reach unprecedented performances, the feasibility is being tested through the design, manufacturing, installation, commissioning and testing activities of a 1:1-scale prototype accelerator, namely LIPAc, from the injector to the first cryomodule together with the High Energy Beam Transport line and the High Power Beam Dump. After outstanding results obtained in 2019, the LIPAc project has entered 2020 in the preparation of the third commissioning stage, i.e., validation in continuous-wave mode of the complete accelerator up to 5 MeV with its final beam dump. The validation until the nominal energy of 9 MeV will be made after the completion of cryomodule assembly. After a brief overview of the goals already achieved in the framework of the IFMIF/EVEDA program, this paper will present a synthesis of the results that have been obtained so far with the LIPAc accelerator as well as the future developments planned beyond 2020.

0.1–10 MeV Neutron Soft Error Rate in Accelerator and Atmospheric Environments

Neutrons with energies between 0.1 and 10 MeV can significantly impact the soft error rate (SER) in SRAMs manufactured in scaled technologies, with respect to high-energy neutrons. Their contribution is evaluated in accelerator, ground-level, and avionic (12 km of altitude) environments. Experimental cross sections were measured with monoenergetic neutrons from 144 keV to 17 MeV, and the results benchmarked with Monte Carlo simulations. It was found that even 144 keV neutrons can induce upsets due to elastic scattering. Moreover, neutrons in the 0.1-10 MeV energy range can induce more than 60% of the overall upset rate in accelerator applications, while their contribution can exceed 18% in avionics. The SER due to neutrons below 3 MeV, whose contribution has always been considered negligible, is found to be up to 44% of the total upsets in accelerator environments. These results have strong radiation hardness assurance (RHA) implications for those environments with high fluxes of neutrons in the 0.1-10 MeV energy range.

Measurement of the neutron decay electron-antineutrino angular correlation by the aCORN experiment

The aCORN experiment measures the neutron decay electron-antineutrino correlation ($a$-coefficient) using a novel method based on an asymmetry in proton time-of-flight for events where the beta electron and recoil proton are detected in delayed coincidence. We report the data analysis and result from the second run at the NIST Center for Neutron Research, using the high-flux cold neutron beam on the new NG-C neutron guide end position: $a = -0.10758 \pm 0.00136 (\mbox{stat}) \pm 0.00148 (\mbox{sys})$. This is consistent within uncertainties with the result from the first aCORN run on the NG-6 cold neutron beam. Combining the two aCORN runs we obtain $a = -0.10782 \pm 0.00124 (\mbox{stat}) \pm 0.00133 (\mbox{sys})$, which has an overall relative standard uncertainty of 1.7 \%. The corresponding result for the ratio of weak coupling constants $\lambda = G_A/G_V$ is $\lambda = -1.2796\pm 0.0062$.