Sapphire Filter Research Articles

A Concept for a Multipurpose Time-of-Flight Neutron Reflectometer at Compact Neutron Sources

The design of a time-of-flight neutron reflectometer proposed for the new generation of compact neutron sources is presented. The reflectometer offers the possibility to use spin-polarized neutrons. The reflectometer design presented here takes advantage of a cold neutron source and uses neutrons with wavelengths in the range of 2–15 Å for the unpolarized mode. In general, due to tight spatial restrictions and the need to avoid moving parts inside the beam channel, a multi-channel collimator guide and reflective neutron guide are used for the first section of the instrument. This enables definition of the desired wavelength band and easy selection of one out of three different Q-resolutions. A low background for the collimator system and the reflectometer is ensured by employing a tangential beam channel and an in-channel sapphire filter. The second section is the time-of-flight (TOF) system, which uses a double-disk neutron chopper followed by polarization elements, the sample environment and the neutron detector system. Monte Carlo simulations and neutron beamline intensity measurements are presented. The design considerations are adoptable for neutron sources where space is limited and sections of the instrument are in a high-radiation environment.

Experimental evaluation of transition rate of sapphire crystal for thermal and fast neutrons using MNSR vertical neutron beam line

Using a perfect single crystal as a neutron filter allows us to have a thermal neutron beam with almost no background of fast neutrons. Single crystals of Al2O3 (sapphire) have proven to be effective filters for fast neutrons and are incorporated into neutron instruments. The present work would experimentally investigate c-axis neutron transmission rate by using different crystal thicknesses. In fact, the optimal thickness for sapphire filter is the one that maximizes the transmission of low energy neutrons and minimizes the transmission of fast neutrons, if there is no significant decrease in thermal neutron flux. In addition, neutron-filtering power of a-axis and c-axis sapphire crystals were compared with each other using different tests on a 2.5 cm slab of the sapphire crystals. The experimental tests were carried out by means of the available neutron flux top of the vertical neutron beam line of the Isfahan Miniature Neutron Source Reactor (MNSR) in two methods of foil activation and flux monitoring. In addition, the thermal and fast neutron dose rate reduction was discussed by using different thicknesses of the c-axis crystal.

Panther — the new thermal neutron time-of-flight spectrometer at the ILL

Panther is a new high-flux medium-resolution direct-geometry thermal-neutron time-of-flight spectrometer at the Institut Laue-Langevin (ILL). It is designed for inelastic neutron-scattering measurements of excitations in condensed matter using single crystals, polycrystalline samples, and liquids. Panther uses double focusing graphite or Cu monochromators, a Fermi chopper, and position-sensitive 3He detectors covering 2 steradians of solid angle. A system of disc choppers and an optional sapphire filter are used to reduce the epithermal neutron background. Thermal neutron background is reduced by a radial oscillating collimator, a beam dump, and an elaborate set of Cd shielding inside the evacuated detector tank. The outside of the tank is covered by a 0.3 m thick layer of borated high-density polyethylene to reduce ambient and cosmic background. The design and performance of the instrument in its current status are described, as well as planned developments.

A comparative study of promising filter materials for neutron imaging facilities

The effect of sapphire and bismuth single-crystal filters and their combinations on the quality of neutron radiographic images and neutron tomography data has been studied. The parameters of the contrast of the neutron image were analyzed depending on the monocrystalline filters. Neutron transmission spectra were obtained for sapphire and bismuth single crystals. Additionally, the effect of filters on the overall intensity of the thermal neutron beam and the background of gamma-rays was investigated. Based on the obtained data, we assume that a single-crystal sapphire filter can be most effectively used for radiographic and tomographic installations using thermal neutrons.

Improvement of moderation efficiency for neutron source system in compact neutron radiography device

In this paper, a neutron source system for compact thermal neutron radiography (TNR) was designed and optimized to improve its moderation efficiency via SuperMC simulations. The DSNH long-life D–T neutron tube with self-target technology independently developed by Northeast Normal University is used as the neutron source. The main optimizations in material and structure include the use of tungsten and lead primary moderators, polyethylene secondary moderators, sapphire filter, and 4 cm magnesium thermal neutron aggregation layer. The improved system provides thermal neutron flux of 3.77×104ncm−2s−1 with thermal neutron content of 85.20% at the exit of the neutron channel, which is suitable for thermal neutron radiography.

Development of a Neutron Radiography System based on a 10 MeV Electron Linac

A thermal neutron radiography unit using the neutrons which emits a 10 MeV electron linac compact has been designed and simulated via MCNPX Monte Carlo code. The facility was carried out for an extensive range of values for the collimator ratio L/D, the main parameter which describes the quality of the produced radiographic images. The results show that the presented facility provides high thermal neutron flux; while with the use of single sapphire filter fulfills all the suggested values which characterize a high quality thermal neutron radiography system. A comparison with other similar facilities indicates that the use of a photoneutron source using a 10 MeV electrons beam is a useful substitutional for radiographic purposes.

New neutron radiography and tomography facility TITAN at the WWR-K reactor

The TITAN is a new experimental facility for neutron radiography and tomography at the research nuclear reactor WWR-K at the Institute of Nuclear Physics (Almaty, Republic of Kazakhstan). The TITAN facility is located on the radial beamline No1 of the nuclear reactor WWR-K. The neutron beam with a maximal field of view of 200 × 200 mm is formed by a collimator system, and the characteristic parameter L/D ratio can be varied in the range 75–1400. The sapphire filter is used to suppress fast neutron radiation. The detector system is based on a scintillator screen with the high-resolution high-sensitive CCD-camera. The design, schematic layout and main technical parameters of the neutron imaging station are presented. The first obtained results of the neutron radiography and tomography experiments with industrial materials and several cultural heritage objects are presented.

Collimator and sapphire filter using for the prompt gamma neutron activation analysis facility: modelling and simulation

Safety measures play an important role in ensuring any system, although nuclear systems always require more safeguards than necessary. From this principle comes the importance of this research, which aims to conduct a study of neutron simulations and modelling to ensure safe installation of PGAA devices in the NB1 channel from the TRIGA Research Reactor at the Maamoura Center for Nuclear Studies. During this study, we used the Monte Carlo method using the MCNP6/X code. This study aims to determine the neutron profile, neutron distribution, and secondary gamma rays produced during the passage of the neutron flux in the collimator channel, sapphire and the vacuum channel that is maintained for the immediate neutron activation analysis (PGAA).

Simulation of a collimator and sapphire filter for PGAA facility of the Moroccan TRIGA MARK II research reactor

The Prompt Gamma Neutron Activation Analysis (PGAA) facility will be commissioned at the NB1 beam tube of the Moroccan TRIGA MARK II research reactor. PGAA is a non-destructive analytical technique used for multi-elemental analysis and especially for light element analysis. For better performances, PGAA requires small and focused beam, and high Фth/Фep and Фth/Фfast ratios, and low background gamma radiation. For this purpose, fast neutron filters are commonly used to reduce fast and epithermal components of neutron flux. In this paper, the impact of sapphire filter, introduced, within the collimator hole, at the beam port on neutron spectra was evaluated. Monte Carlo, MCNP6.2, code has been used with ENDF VII.1 neutron cross section library to perform this study. An approximate TRIGA MARK II reactor MCNP model has been used to generate neutron spectra at the entrance of the collimator. Then an independent model for both collimator and sapphire filter has been brought into calculation to fulfil our purpose. Results showed an increase of about 8 times for both Фth/Фep and Фth/Фfast ratios using a 10 cm length of sapphire.

PITSI: The neutron powder diffractometer for transition in structure investigations at the SAFARI-1 research reactor

The high-performance neutron powder diffraction instrument, PITSI, is located at the SAFARI-1 research reactor in South Africa. It supports both low and high temperature sample environments to facilitate in-situ parametric studies of crystallographic and magnetic phenomena covering the temperature range 1.5–1800 K. This facility has recently undergone a substantial modernization program in which all aspects of the beam delivery, conditioning and instrumentation have been upgraded. The primary neutron beam delivery system was optimised using Monte Carlo methods. Fast neutron and gamma radiation levels emanating from the radial beamline are suppressed within the biological shielding through the use of a vertical input-aperture and a 160 mm length sapphire filter located in the primary beam channel. Their use in conjunction with a vertically focussed monochromator has been found to minimally influence the useful thermal neutron flux component. Resolution and intensity optimization are achieved by having both the horizontal and vertical focuses of the monochromator variable. A resolution of Δd/d = ∼3 × 10−3 is achieved at a sample-detector distance of 1600 mm. The data acquisition and control system is standardised on the SINQ Instrument Control Software. In-house data reduction software enables flat field correction, geometric correction, vertical integration, normalization, data cropping and frame stitching.

Geant4 Analysis of a Thermal Neutron Real-Time Imaging System

Thermal neutron imaging is a technique for nondestructive testing providing complementary information to X-ray imaging for a wide range of applications in science and engineering. Advancement of electronic imaging systems makes it possible to obtain neutron radiographs in real time. This method requires a scintillator to convert neutrons to optical photons and a charge-coupled device (CCD) camera to detect those photons. Alongside, a well collimated beam which reduces geometrical blurriness, the use of a thin scintillator can improve the spatial resolution significantly. A representative scintillator that has been applied widely for thermal neutron imaging is 6LiF:ZnS (Ag). In this paper, a multiphysics simulation approach for designing thermal neutron imaging system is investigated. The Geant4 code is used to investigate the performance of a thermal neutron imaging system starting with a neutron source and including the production of charged particles and optical photons in the scintillator and their transport for image formation in the detector. The simulation geometry includes the neutron beam collimator and sapphire filter. The 6LiF:ZnS (Ag) scintillator is modeled along with a pixelated detector for image recording. The spatial resolution of the system was obtained as the thickness of the scintillator screen was varied between 50 and $400~\mu \text{m}$ . The results of the simulation were compared to experimental results, including measurements performed using the PULSTAR nuclear reactor imaging beam, showing good agreement. Using the established model, further examination showed that the resolution contribution of the scintillator screen is correlated with its thickness and the range of the neutron absorption reaction products (i.e., the alpha and triton particles). Consequently, thinner screens exhibit improved spatial resolution. However, this will compromise detection efficiency due to the reduced probability of neutron absorption.

A study of a transportable thermal neutron radiography unit based on a compact RFI linac

A transportable thermal neutron radiography system, incorporating a compact proton accelerator as neutron source has been simulated using the MCNP4B code. The neutron source will be produced via the 7Li(p,n)7Be reactions by a 2.5 MeV, 10 mA proton beam into a thick lithium target. Variable values for the collimator ratio were calculated. Thermal neutron radiography parameters are comparable to the research nuclear reactors. Sapphire filter was treated in order to improve the results. Simple and advanced neutron shielding materials considered which was further enhanced with layers of bismuth. The system was compatible with the European Union Directive on ‘Restriction of Hazardous Substances’ (RoHS) 2002/95/EC, hence excluding the use of cadmium and lead.

Design and Performance of a Thermal Neutron Imaging Facility at the North Carolina State University PULSTAR Reactor

A thermal neutron imaging facility has been set up at the North Carolina State University PULSTAR reactor. The PULSTAR is an open pool light water moderated 1 MWth research reactor with six beam tubes. The present facility is set up on beam tube # 5 of the reactor. The facility is intended to have radiographic and tomographic capabilities. The design of the neutron collimator was performed using MCNP5. The collimator includes a 4-in bismuth filter followed by a 6-in single-crystal sapphire filter. Thermal neutron scattering cross-section libraries for sapphire and bismuth were generated and used in the MCNP simulation of the system. Based on the current design, the L/D of the facility ranges between 100 and 150. The neutron flux at the image plane can be varied from 1.8times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> to 7times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> middots with a Cd-ratio of ~450. The resolution of the system for different imaging media was also estimated and found to be ~33 mum for conventional radiography film and ~110 mum for digital image plates. Initial measurements, using ASTM standards, show that the imaging facility achieves a beam quality classification of I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sup>

New thermal neutron prompt γ-ray activation analysis instrument at the National Institute of Standards and Technology Center for Neutron Research

A new thermal neutron prompt γ-ray activation analysis (PGAA) instrument was designed and built to replace the original PGAA system at the NIST Center for Neutron Research. By placing a sapphire filter in the neutron beam shutter assembly, the fast neutron fluence rate was reduced by a factor of 5 and low-energy (50–200 keV) γ-ray intensities were reduced by factors of 5–10. The thermal neutron fluence rate was reduced by only a factor of 1.13. A new external beam tube, sample chamber, beam stop, and support structure were built and a new detection system installed. The new beam tube is made of two cylindrical aluminum sections lined with a lithiated polymer. Both sections are kept under vacuum to reduce the number of neutrons scattered by air into the beam tube walls. The sample chamber is also fabricated from aluminum and lined with lithiated polymer, and may be evacuated to minimize the number of neutrons scattered and absorbed by air. The beam tube and sample chamber assembly is suspended from the aluminum support structure. The detection system consists of a 40% efficient (relative) germanium detector (resolution 2.0 at 1332.5 keV) and a bismuth germanate Compton suppressor. The detection system is shielded by lead, surrounded by borated and lithiated polyethylene, and placed on a table attached to the support structure. The new, more compact beam stop is welded to the support structure. Capture γ-ray photopeaks from H, B, C, N, Na, Al, Fe, Ge, I and Pb in the background spectrum were either of lower intensity or eliminated with the new PGAA instrument. The more efficient detection system, positioned closer to the sample, yielded element sensitivity increases of 5–50%. Limits of detection have been greatly reduced compared with those of the original instrument due to reduced Compton and scattered γ-ray backgrounds (especially in the low-energy region), increased sensitivities, and reduction of background γ-ray photopeak intensities.

Sapphire filter thickness optimization in neutron scattering instruments

The present work is concerned with the optimization of the sapphire fast neutron filter thickness used in neutron diffraction instruments. The optimization is based on maximization of the slow neutron transmission, minimization of the fast neutron transmission, and also taking into consideration the neutron background at the vicinity of an instrument. Scattering properties of the sapphire in the fast and slow neutron regions are discussed.

Neutron Depth Profiling with the New NIST Cold Neutron Source

Neutron depth profiling (NDP) is a method of near-surface analysis for isotopes that undergo neutron-induced positive Q-value charged particle reactions, e.g. (n, α) and (n, p). The method is based on measuring the energy loss of the charged particles as they exit the specimen. Depending on the material under study, depths of up to 10 μm can be profiled and depth resolutions of the order of 15 nm can be obtained. The most studied analytes via NDP at the National Institute of Standards and Technology (NIST) are boron, lithium and nitrogen. These analytes have been determined in a variety of matrices. The NIST research reactor has recently undergone a major upgrade, including the addition of a new liquid-hydrogen cold source, which reaches a lower effective temperature and more fully illuminates the neutron guides. The cold neutron depth profiling (CNDP) facility has been placed on a curved super-mirror guide. Because of the guide curvature, the NDP chamber does not directly view the reactor and its associated fast neutrons and gamma rays, and therefore a 13.5 cm thick sapphire filter used previously in front of the beam port is no longer necessary. The increase in the neutron fluence rate at the end of the guide leads to an improvement in detection limit of more than a factor of four over the previous depth profiling instrument. The reduction in the gamma-ray background in the chamber will improve our ability to measure elements such as nitrogen. © 1997 by John Wiley & Sons, Ltd.

Neutron Depth Profiling with the New NIST Cold Neutron Source

Neutron depth profiling (NDP) is a method of near-surface analysis for isotopes that undergo neutron-induced positive Q-value charged particle reactions, e.g. (n, α) and (n, p). The method is based on measuring the energy loss of the charged particles as they exit the specimen. Depending on the material under study, depths of up to 10 μm can be profiled and depth resolutions of the order of 15 nm can be obtained. The most studied analytes via NDP at the National Institute of Standards and Technology (NIST) are boron, lithium and nitrogen. These analytes have been determined in a variety of matrices. The NIST research reactor has recently undergone a major upgrade, including the addition of a new liquid-hydrogen cold source, which reaches a lower effective temperature and more fully illuminates the neutron guides. The cold neutron depth profiling (CNDP) facility has been placed on a curved super-mirror guide. Because of the guide curvature, the NDP chamber does not directly view the reactor and its associated fast neutrons and gamma rays, and therefore a 13.5 cm thick sapphire filter used previously in front of the beam port is no longer necessary. The increase in the neutron fluence rate at the end of the guide leads to an improvement in detection limit of more than a factor of four over the previous depth profiling instrument. The reduction in the gamma-ray background in the chamber will improve our ability to measure elements such as nitrogen. © 1997 by John Wiley & Sons, Ltd.

EFFECTS OF SIMULATED SPACE ULTRAVIOLET AND VISIBLE RADIATION UPON DRY CYSTS OF COLPODA INFLATA

Abstract— Dry cysts of Colpoda inflata were irradiated with an argon plasma arc lamp, the output of which simulates space radiation in its wavelength distribution although it is of higher intensity. Rapid evacuation reduced the rate of excystment as well as the percentage excysting; low temperature (near that of liquid nitrogen) had little effect on either measurement. All cysts were therefore irradiated at room temperature in a cell flushed with dry nitrogen. Radiation passed for 1 min through a sapphire filter, with a cutoff at 145 nm, prevented excystment. At smaller doses progressively less damaging effects were observed as judged by both the increase in time required for excystment and the percentage excysted. No dark recovery was observed in dry cysts. Exposure of cysts to radiation passed through a series of filters which progressively remove short ultraviolet wavelengths indicate that it is mainly the quartz‐transmitted wavelengths (cutoff 185 nm) which damage the cysts. The presumption is that most of the vacuum‐ultraviolet wavelengths are superficially absorbed; their lesser effect is also partially attributable to their lower intensity. Inasmuch as 1 min of argon lamp radiation is equivalent to 24 min in space, it is evident that Colpoda cysts would not long survive a journey in space, had they reached there in viable condition.