Radiation Hardness of a Fast Neutron Scintillation Spectrometer with a Stilbene Crystal

Neutron detectors with organic oscillators such as stilbene crystals are suggested for use in measuring the characteristics of neutron radiation in the International Thermonuclear Experimental Reactor (ITER). These detectors will be used as neutron-flux monitors in a multichannel neutron collimator for obtaining the spatial characteristics of a plasma source of thermonuclear neutrons with a 1-ms temporal resolution. In addition, operation in the spectrometric mode will ensure the measurement of the thermonuclear-neutron energy spectrum, which yields data on the fusion-plasma temperature and the ratio of the deuterium and tritium concentrations. During the operation of the facility, the detectors will be exposed to irradiation with a large fluence of fast neutrons. The first results obtained in studies of the characteristics of a stilbene crystal irradiated by fast neutrons with a fluence of up to 10¹⁴ neutrons/cm² are presented.     

Irradiation tests of ITER candidate Hall sensors using two types of neutron spectra

We report on irradiation tests of InSb based Hall sensors at two irradiation facilities with two distinct types of neutron spectra. One was a fission reactor neutron spectrum with a significant presence of thermal neutrons, while another one was purely fast neutron field. Total neutron fluence of the order of 10(16)?cm(-2) was accumulated in both cases, leading to significant drop of Hall sensor sensitivity in case of fission reactor spectrum, while stable performance was observed at purely fast neutron spectrum. This finding suggests that performance of this particular type of Hall sensors is governed dominantly by transmutation. Additionally, it further stresses the need to test ITER candidate Hall sensors under neutron flux with ITER relevant spectrum.     

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.     

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.     

A study on the neutron monitoring system of LHD based on Monte Carlo simulations

Neutron monitoring is quite important because fusion neutrons are a direct evidence of fusion reactions. In calibration experiments of a neutron monitoring system, Monte Carlo calculations play an important role to correct various effects. To perform Monte Carlo calculations for a helical type fusion device, we make a program that can automatically generate an input file of a helical coil geometry for the MCNP code. The neutron spatial distributions and spectra for the helical devices are calculated in the geometries automatically generated by this program. We also discuss in calibration experiments.     

The Feasibility of Neutron Moderation Imaging for Land Mine Detection

Neutron moderation land mine detection involves irradiating the ground with fast neutrons and subsequently detecting the thermalized neutrons which return. This technique has been studied since the 1950s, but only using non-imaging detectors. Without imaging, natural variations in moisture content, surface irregularities, and sensor height variations produce sufficient false alarms to render the method impractical in all but the driest conditions. This paper describes research to design and build a prototype land mine detector based on neutron moderation imaging. After reviewing various neutron detector technologies, a design concept was developed. It consists of a novel thermal neutron imaging system, a unique neutron source to uniformly irradiate the underlying ground, and hardware and software for image generation and enhancement. A proof-of-principle imager has been built, but with a point source offset from the detector to roughly approximate a very weak uniform source at the detector plane. Imagery from the detector of mine surrogates is presented. Realistic Monte Carlo simulations were performed using the same two dimensional neutron imaging geometry as the detector in order to assess its performance. The target-to-background contrast was calculated for various soil types and moisture contents, explosive types and sizes, burial depths, detector standoffs, and ground height variations. The simulations showed that the neutron moderation imager is feasible as a land mine detector in a slow scanning or confirmation role and that image quality should be sufficient to significantly improve detector performance and reduce false alarm rates compared to non-imaging albedo detection, particularly in moist soils, where surface irregularities exist and when the sensor height is uncertain. Performance capability, including spatial resolution and detection times, was estimated.     

Modeling the National Ignition Facility neutron imaging system

Numerical modeling of the neutron imaging system for the National Ignition Facility (NIF), forward from calculated target neutron emission to a camera image, will guide both the reduction of data and the future development of the system. Located 28 m from target chamber center, the system can produce two images at different neutron energies by gating on neutron arrival time. The brighter image, using neutrons near 14 MeV, reflects the size and symmetry of the implosion "hot spot." A second image in scattered neutrons, 10-12 MeV, reflects the size and symmetry of colder, denser fuel, but with only ～1%-7% of the neutrons. A misalignment of the pinhole assembly up to ±175?μm is covered by a set of 37 subapertures with different pointings. The model includes the variability of the pinhole point spread function across the field of view. Omega experiments provided absolute calibration, scintillator spatial broadening, and the level of residual light in the down-scattered image from the primary neutrons. Application of the model to light decay measurements of EJ399, BC422, BCF99-55, Xylene, DPAC-30, and Liquid A suggests that DPAC-30 and Liquid A would be preferred over the BCF99-55 scintillator chosen for the first NIF system, if they could be fabricated into detectors with sufficient resolution.     

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 protective subsurface container for holding and temporary storage of activated targets

An accelerator-driven source of epithermal neutrons has been developed by the Budker Institute of Nuclear Physics for carrying out investigations into neutron capture therapy of malignant tumors. Safe handling of targets with ⁷Be radionuclide accumulated in them is one of the problems encountered in generation of neutrons in the ⁷Li(p, n)⁷Be reaction. It is proposed that targets will be decontaminated in a natural course, being placed in a subsurface container located in the room of the accelerating facility. The maximum activity of the targets enclosed in the container after scheduled generation of neutrons at the facility is estimated. Analytical estimates and Monte Carlo calculation of γ-ray transport are performed to determine the optimum container size, such that the γ-ray flux from its contents is reduced to the acceptable level. The preliminary design of the container and its embodiment are presented.     

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.     

Investigating the Thermal and Epithermal Neutron Spectra in the Moderators of Nuclear Reactors

A method for representing a neutron spectrum in the thermal range as a superposition of a Maxwellian equilibrium neutron spectrum with a temperature equal to that of the moderator and a Maxwellian quasi-thermal nonequilibrium neutron spectrum is proposed. A method for representing the reactor spectra in the energy range of epithermal neutrons as a superposition of five to seven partial Maxwellian equilibrium spectra of elastically scattered neutrons was studied.     

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 thermal neutron detector based on single-crystalline silicon

It is shown that a change in the specific electrical conductivity (SEC) in silicon single crystals after irradiation and radiation defect annealing is directly proportional to the thermal neutron fluence. This fact is used as a basis of a method for measuring the neutron flux. In this case, the proportionality factor is dependent on the neutron spectrum, being independent of the initial SEC, which significantly simplifies detector calibration. In measurements of the thermal neutron fluence in terms of relative and absolute units, the rms was 4 and 8%, respectively.     

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.     

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.     

A method for decreasing the loading on scintillation channels in facilities with pulsed neutron sources

A method is proposed for decreasing the loading on scintillation channels in nuclear material detection and monitoring facilities comprising pulsed neutron sources, neutron moderators, and scintillators with a system for pulse shape discrimination between neutrons and photons. This method is based on the use of composite scintillators containing cylindrical shells of a thermal-neutron absorbing material. Selecting the sizes of zones in a composite scintillator and the absorber type (cadmium and lithium carbonate with different 6 Li content), it is possible to considerably increase the decay time constant for thermal neutrons in a composite scintillator and thereby reduce its load as compared to a homogeneous scintillator over the same time period after a pulse from the neutron source. The sizes of the scintillator component parts and the absorber material are optimized, which provides a means for decreasing the load on the scintillation channel several-fold while maintaining constant detection efficiency for fast fission neutrons. The capabilities of this method for decreasing the load are demonstrated by the example of the operational prototype of the fissile material detection and monitoring facility with a graphite moderator and LS-13 scintillators. The efficiency of the facility used in this method is compared to that of a facility with a deuterium-containing scintillator, in which no radiative capture photons are produced.     

A spectrometer of polarized neutrons

A polarized neutron spectrometer intended for studies of interactions of nuclei and condensed matter with polarized neutrons with energies in the range from thermal energies to several electron volts was designed at the IBR-2 pulsed reactor (JINR, Dubna). Diffraction on magnetized Co(92%)–Fe(8%) single crystals was used to polarize neutrons and analyze the polarization. The attained neutron polarization was ∼95% within the entire energy range.     

Down-scattered neutron imaging detector for areal density measurement of inertial confinement fusion

A custom developed (6)Li glass scintillator (APLF80+3Pr) for down-scattered neutron diagnostics in inertial confinement fusion experiments is presented. (6)Li provides an enhanced sensitivity for down-scattered neutrons in DD fusion and its experimentally observed 5-6 ns response time fulfills the requirement for down-scattered neutron detectors. A time-of-flight detector operating in the current mode using the APLF80+3Pr was designed and its feasibility observing down-scattered neutrons was demonstrated. Furthermore, a prototype design for a down-scattered neutron imaging detector was also demonstrated. This material promises viability as a future down-scattered neutron detector for the National Ignition Facility.     

In situ polarized (3)He system for the Magnetism Reflectometer at the Spallation Neutron Source

We report on the in situ polarized (3)He neutron polarization analyzer developed for the time-of-flight Magnetism Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Using the spin exchange optical pumping method, we achieved a (3)He polarization of 76% ± 1% and maintained it for the entire three-day duration of the test experiment. Based on transmission measurements with unpolarized neutrons, we show that the average analyzing efficiency of the (3)He system is 98% for the neutron wavelength band of 2-5?A. Using a highly polarized incident neutron beam produced by a supermirror bender polarizer, we obtained a flipping ratio of >100 with a transmission of 25% for polarized neutrons, averaged over the wavelength band of 2-5?A. After the cell was depolarized for transmission measurements, it was reproducibly polarized and this performance was maintained for three weeks. A high quality polarization analysis experiment was performed on a reference sample of Fe∕Cr multilayer with strong spin-flip off-specular scattering. Using a combination of the position sensitive detector, time-of-flight method, and the excellent parameters of the (3)He cell, the polarization analysis of the two-dimensional maps of reflected, refracted, and off-specular scattered intensity above and below the horizon were obtained, simultaneously.