Microtron MT 25 as a source of neutrons

The objective was to describe Microtron MT25 as a source of neutrons generated by bremsstrahlung induced photonuclear reactions in U and Pb targets. Bremsstrahlung photons were produced by electrons accelerated at energy 21.6 MeV. Spectral fluence of the generated neutrons was calculated with MCNPX code and then experimentally determined at two positions by means of a Bonner spheres spectrometer in which the detector of thermal neutrons was replaced by activation Mn tablets or track detectors CR-39 with a (10)B radiator. The measured neutron spectral fluence and the calculated anisotropy served for the estimation of neutron yield from the targets and for the determination of ambient dose equivalent rate at the place of measurement. Microtron MT25 is intended as one of the sources for testing neutron sensitive devices which will be sent into the space.     

New measurement system for on line in core high-energy neutron flux monitoring in materials testing reactor conditions

Flux monitoring is of great interest for experimental studies in material testing reactors. Nowadays, only the thermal neutron flux can be monitored on line, e.g., using fission chambers or self-powered neutron detectors. In the framework of the Joint Instrumentation Laboratory between SCK-CEN and CEA, we have developed a fast neutron detector system (FNDS) capable of measuring on line the local high-energy neutron flux in fission reactor core and reflector locations. FNDS is based on fission chambers measurements in Campbelling mode. The system consists of two detectors, one detector being mainly sensitive to fast neutrons and the other one to thermal neutrons. On line data processing uses the CEA depletion code DARWIN in order to disentangle fast and thermal neutrons components, taking into account the isotopic evolution of the fissile deposit. The first results of FNDS experimental test in the BR2 reactor are presented in this paper. Several fission chambers have been irradiated up to a fluence of about 7 × 10(20) n∕cm(2). A good agreement (less than 10% discrepancy) was observed between FNDS fast flux estimation and reference flux measurement.     

Fast X-ray and Neutron Detectors Based on Natural Diamond

Fast-response detectors based on natural diamond, which has a long charge-carrier lifetime, have been developed and manufactured for performing time-of-flight measurements of the parameters of plasma and other pulsed sources of ultrasoft X-ray and neutron radiations. Detectors of two types—volumetric detectors for detecting neutrons and X rays with quantum energies of >1 keV and surface detectors for detecting ultrasoft X rays with quantum energies of 200–1000 eV—have been manufactured. Their response time is 1–2 ns (the FWHM duration of a current response). The sensitivity of the volumetric detectors to neutrons with an energy of 14 MeV is at a level of 10^–18 C cm²/neutron. The sensitivity of the surface detectors to X rays with an energy of 580 eV is above 5 mA/W.     

High-Intensity Neutron Fields at the Center of the Metallic Core of Fast Reactors as the Primary Standard Neutron Fields

It is proposed to use high-intensity neutron fields with an average neutron energy of 0.7–1.5 MeV at the center of the metallic core of fast reactors as the primary standard neutron fields along with the known standard fields of fission neutrons with average energies of 1.93 and 2.13 MeV, respectively, from ²³⁵U and ²⁵²Cf nuclei, which are used in the world practice of taking neutron measurements. It is also proposed to create standard fields of nuclear fission neutrons with an average energy of 2 MeV by the bombardment of ²³⁵U converters with a beam of fast monoenergetic neutrons with energies of 2.5 and 14 MeV and 50-MeV protons.     

A digital method for pulse-shape discrimination between particles

Results from experimental and theoretical investigations of a digital technique for pulse-shape discrimination are presented. Pulses from a scintillation (stilbene-based) detector are recorded by a digital oscilloscope and transmitted to a computer for carrying out particle identification. For a stilbene scintillation detector irradiated by neutrons and γ rays, the γ-ray suppression factor is ～0.5 × 10³?10⁴ and the neutron identification efficiency is ～0.70-0.90 over an energy range of ～30–500 keV at a counting rate up to ～10⁵ pulse/s. It is shown that this technique can form the basis for developing a detector unit that comprises a fast ADC and a digital signal processor and provides online identification of particles by scintillation pulse shape.     

Effect of neutron irradiation on the electrodes of a vacuum photoemission detector

The effect of neutron irradiation on Ta-Be photocathodes of a vacuum photoemission detector (VPD) has been investigated. The VPD has been designed for tomography of plasmas using thermal X-ray radiation on the ITER international tokamak reactor. The exposure of VPD electrode samples to neutrons with a fluence of 1.6 × 10¹⁹ neutrons/cm² (E > 0.1 MeV) is shown to change the relative concentration of Ta and Be in the surface layer. As a result, the detector sensitivity in the DT operating mode of the ITER is inevitably altered. Some recommendations concerning selection of materials for the VPD electrodes are presented. The conclusion is drawn that sputtering by thermonuclear neutrons in the ITER may lead to deposition of a conductive film on the insulator surface and that a special shape of insulators must be therefore used to avoid formation of such a coating.     

Studying the dependence of the ratio of the prompt neutron yields in the symmetric and asymmetric fission of <Superscript>235</Superscript>U and <Superscript>238</Superscript>U nuclei on the energy of neutrons inducing this fission

The ratios have been measured of the prompt neutron yields in the symmetric and asymmetric fission of ²³⁵U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors, as well as in the ²³⁸U fission induced by 14.7-MeV neutrons. The well-known measured integrated cross sections of the system of dosimetric nuclear reactions are compared to the cross sections calculated using the differential cross sections from the data libraries known worldwide, the three-component representation of fission neutron spectra from ²³⁵U and ²⁵²Cf nuclei, and the KASKAD spectrum reconstruction program.     

A Prototype of a Neutron and γ-Ray Spectrometer for Studying Solar Activity at Distances of 0.5 Astronomical Units to 25 Solar Radii

The InterSONG instrument now being developed at the Skobel’tsyn Institute of Nuclear Physics will allow the sensitivity of experiments on solar neutrons to be significantly raised by performing measurements in the immediate neighborhood of the Sun. It is expected that this instrument will be used on the Inter-HELIOS automatic interplanetary station operating at distances of as many as 25 solar radii. The instrument is based on a LiI scintillating crystal enriched with ⁶Li, and a boron-containing plastic scintillator that also acts as a fast neutron moderator. The instrument is designed to detect neutrons with energies of 0.1–100 MeV and electromagnetic radiation over the range 0.03–10 MeV.__________Translated from Pribory i Tekhnika Eksperimenta, No. 3, 2005, pp. 24–36.Original Russian Text Copyright © 2005 by Bogomolov, Kuznetsov, Lishnevskii, Rubinshtein, Ryumin, Nemchenok, Sobolevskii, Ufimtsev.     

Asymmetric fission of <Superscript>235</Superscript>U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors

The contributions that neutrons emitted from the neck of excited dumbbell-shaped fissioning ²³⁵U nuclei at the instant of their fission make to the prompt fission neutron spectrum, as well as the ratios of these contributions, have been measured for nuclear fission induced by thermal neutrons and fast neutrons of pulsed nuclear reactors.     

Angular correlations in detection of α-γ coincidences in the nanosecond tagged neutron technology

The characteristics of the setup for analyzing the elemental composition of organic substances using spectroscopy of γ rays induced by tagged neutrons with an energy of ～14 MeV in an inelastic neutron scattering reaction are discussed. An ING-27 D-T neutron generator with a built-in position-sensitive α detector, fast neutron and γ detectors, and a multichannel detecting system providing a subnanosecond time resolution are parts of the experimental setup. The correlation between the exit angle of a neutron and the number of the α pixel, as well as the anisotropy in emission of γ rays in the inelastic neutron scattering reaction, are measured, and the errors of these measurements are estimated.     

A method for determining the intensity of concomitant neutron source D(d, n)3He when studying the characteristics of delayed neutrons from nuclear fission induced by neutrons from reaction T(d, n)4He

A factor that may be responsible for the discrepancies in the total delay neutron yields, the relative yields of separate groups of delayed neutrons, and half-lives of their precursors has been investigated. These discrepancies are shown to be attributable to the effect of a concomitant neutron source—reaction D(d, n)³He. Such a source is unavoidably present in an experiment where reaction T(d, n)⁴He on a solid-state target is used as a neutron source. A method has been developed to calculate the contribution of neutrons from the reaction D(d, n)³He to the measured total and relative yields of delayed neutrons and the half-lives of their precursors in heavy nuclei fission induced by neutrons with energies of 14–18 MeV. The energy dependence of the parameters of delayed neutron groups from the neutron-induced fission of ²³⁸U nuclei in the energy range of 14.2–17.9 MeV is presented.     

Characteristics of the neutron field at the core center in the ИBB-2M water-moderated water-cooled reactor

The neutron spectrum has been measured at the center of the core in the ИBB-2M research water-moderated water-cooled reactor. A technique has been developed to measure the energy spectrum of neutrons in high-intensity fields (with a flux density of >10¹² cm^?2 s^?1) without recourse to fission detectors enclosed in boron shields.     

High-Intensity Neutron Fields for Calibrating the Detectors of a Standard Control System of High-Power Nuclear Reactors

The characteristics of ten high-intensity modelling neutron fields in research nuclear reactors with various ratios of the contributions of thermal and epithermal neutrons have been measured in order to be used for calibrating the internal detectors of the standard control system of high-power nuclear reactors. The neutron fields in the F-1, Gidra, and KVANT reactors are certified as working standards of the first and second kind.     

A method for nondestructive assay of nuclear materials in facilities with a pulse neutron generator and a digital technology for discrimination between neutrons and photons

A method for determining the ²³⁵U concentration in fuel assemblies of a high-power channel-type PBMK reactor is described. The measure of ²³⁵U content of an analyzed sample is the number of neutrons from thermal-neutron fission of ²³⁵U, normalized to the number of γ quanta produced in thermal neutron capture by hydrogen nuclei in the scintillator or by ¹⁰B in the glass of a photomultiplier tube. A pulse neutron generator based on DT reaction is the neutron source, and an organic scintillator with the pulse shape discrimination between neutrons and γ rays with the aid of the digital technology is a detector. The scintillator is also used as a neutron moderator. Simulation of the method shows that the ²³⁵U content of the analyzed sample can be determined for 1 min with an accuracy of 1% or better. The efficiency of the method has been confirmed by experimental investigations on a model of the setup.     

A Ring Neutron Detector for a Time-of-flight Diffractometer Based on Linear Scintillation Detectors with Silicon Photomultipliers

A ring neutron detector has been developed for a time-of-flight diffractometer based on linear scintillation detectors. Light is transported over an organic glass light guide with a diffuse reflector. This scheme makes it possible to collect more photons than are collected in detectors based on wavelength-shifting fibers and to use avalanche photodiodes (SiPMs) instead of photomultiplier tubes. Testing confirmed that these detectors could be used as an alternative for widely used proportional neutron counters filled with ³He.     

Calibration of the neutron detectors for the cluster fusion experiment on the Texas Petawatt Laser

Three types of neutron detectors (plastic scintillation detectors, indium activation detectors, and CR-39 track detectors) were calibrated for the measurement of 2.45 MeV DD fusion neutron yields from the deuterium cluster fusion experiment on the Texas Petawatt Laser. A Cf-252 neutron source and 2.45 MeV fusion neutrons generated from laser-cluster interaction were used as neutron sources. The scintillation detectors were calibrated such that they can detect up to 10(8) DD fusion neutrons per shot in current mode under high electromagnetic pulse environments. Indium activation detectors successfully measured neutron yields as low as 10(4) per shot and up to 10(11) neutrons. The use of a Cf-252 neutron source allowed cross calibration of CR-39 and indium activation detectors at high neutron yields (～10(11)). The CR-39 detectors provided consistent measurements of the total neutron yield of Cf-252 when a modified detection efficiency of 4.6×10(-4) was used. The combined use of all three detectors allowed for a detection range of 10(4) to 10(11) neutrons per shot.     

A digital method for pulse-shape discrimination between neutrons and γ rays at a high counting rate and a low energy of detected radiation

A digital method for pulse-shape discrimination between neutrons and γ rays was used in measurements at counting rates of up to ～10⁶ counts/s in the energy range of ～2–800 keV. Pulses produced by neutrons and γ rays in a stilbene-based scintillation detector were digitized by a digital oscilloscope and transmitted to a computer for carrying out particle identification. Identification was performed for radiation of radionuclide sources and a pulsed neutron generator operating in a repeated triggering mode. Amplitude spectra of pulses identified as neutrons and γ rays of radiation from the generator were measured. At a detector counting rate of ～8.5 × 10⁵ counts/s, ～90% of all recorded pulses were recognized as neutrons. In the energy range of ～30–800 keV, the γ-ray suppression factor was ～10⁴–10³ at counting rates of ～1.5×10⁵–5 × 10⁵ counts/s, while the efficiency of identifying neutrons was >0.9. The suppression factor for γ rays with an energy of ～10 keV was ～300, and the neutron identification efficiency was ～0.75.     

A sectioned spectrometer of fast neutrons

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched ⁶Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ～10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10^?4 to 10² cm^?2 s^?1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 10² s^?1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (～1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ～600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.     

A method for measuring the energy spectrum of neutrons with energies of 1–15 MeV

The characteristics of the detector prototype for 1–15 MeV neutrons are described. The prototype is a full absorption detector consisting of interlaced plastic scintillators and ⁶Li doped glasses. A neutron incident on the detector deposits all its energy in the plastic scintillator, is moderated to thermal energies, and detected in the lithium glass. The measured time of complete neutron moderation is ～60 μs. Recording two signals in this time interval from the first event of neutron scattering in the plastic scintillator and from the neutron absorption by a lithium atom in the glass, it is possible to effectively suppress background thermal neutrons and γ rays and, therefore, detect low-intensity neutron sources. Owing to the proposed detector design, the direction toward the neutron source can be determined.     

First results of use of a scintillating-fiber position-sensitive detector for recording the target image in a 14-MeV neuron beam at the ISKRA-5 facility

The design of the scintillating-fiber detector is described, and the first results obtained in recording the target image in a 14-MeV neutron beam at the ISKRA-5 facility are presented. The scintillating-fiber position-sensitive detector has been designed for diagnosing laser fusion processes by recording the spatial distribution of thermonuclear neutrons escaping from the target. Position-sensitive detection is effected by conversion of neutron radiation into light in a scintillating fiber array. Discrimination of neutrons from γ rays by their time of flight and image intensification are performed with the aid of a frame camera. Images are recorded by a CCD camera. A technique for recording penumbra images is used for imaging at low neutron yields (3 × 10⁸−10¹¹).