Monochromatic Neutron Research Articles

Dosimetry for Neutrons from 0.25 to 15 MeV by the Measurement of Linear Energy Transfer Distributions for Secondary Charged Particles in CR-39 Plastic

In the radiation fields of high energy accelerator facilities, high-altitude aircraft and space flights, high-energy neutron dosimetry of ∼20 MeV or more is a significant issue for radiological protection. We studied the feasibility of experimental measurements of linear energy transfer (LET) distributions for secondary charged particles induced by fast neutrons using CR-39 plastic nuclear track detectors. In order to investigate a method of analyzing the CR-39 detectors that is appropriate for fast neutron dosimetry, two-layer CR-39 stacks were exposed to monochromatic neutrons (0.25, 0.55, 5, and 15 MeV) at the Fast Neutron Laboratory of Tohoku University in Japan. We also conducted Monte Carlo calculations to estimate the detection efficiency of the CR-39 detector for recoil protons. The CR-39 detectors treated by single-step chemical etching were used to obtain LET distributions for LET > 10 keV/µm-water. The results indicated that measurements of short-range particles are very important for obtaining the correct LET distributions. Using the measured LET distributions, we calculated neutron sensitivities, absorbed doses and dose equivalents based on the ICRP 60 Q–L relation and averaged quality factors. The dose equivalents were compared with the neutron fluence-to-dose equivalent conversion factors given by ICRP 74 and the averaged quality factors were compared with weighting factors given by ICRP 60 and ICRP 92.

A new thermal triple-axis spectrometer at the research reactor FRJ-2

A high intensity triple-axis spectrometer is described, which has been commissioned only a year before the final shut-down of the FRJ-2 reactor in May 2006. It was designed both as an instrument for quasi-elastic scattering with polarisation analysis on an area detector, and as a triple-axis spectrometer with polarization analysis for measurements of magnetic and nuclear inelastic scattering. The instrument features two exchangeable monochromators (Cu [200] and PG [002]), which enable both horizontal and vertical focusing. High monochromatic neutron current densities at the sample position of up to 4 × 107cm− 2 s− 1 (for pyrolytic graphite at 34 meV) and energies of up to 117 meV (with copper) have been achieved at the FRJ-2 (thermal flux density: 3 × 1014cm− 2 s− 1). The utilization of 3He filters both as polarizer and analyzer will enable polarization analysis for high-energy neutrons. The spectrometer is presently being prepared for installation at the new Chinese research reactor CARR at the China Ins...

Neutron Laue Diffraction in Perfect and Deformed Silicon Single Crystals Under Ultrasound Excitation

Neutron Laue Diffraction in Perfect and Deformed Silicon Single Crystals Under Ultrasound ExcitationThe effect of ultrasound (US) on neutron Laue's diffraction is studied for perfect and deformed crystals. In a perfect crystal the oscillations of relative diffraction intensity were observed, which depend on the ultrasound wave amplitude. In a deformed crystal the ultrasound distorts the adiabatic motion of the image points along the sheets of dispersive surface. This gives rise to the diffraction intensity behavior which sharply differs from that in the perfect crystal case. The results obtained agree well with theoretical concepts. The described effects can be used for creation of US-controlled new type neutron monochromators or choppers.

Neutron Bragg Diffraction on a Bent Silicon Single Crystal Excited by Ultrasound

Neutron Bragg Diffraction on a Bent Silicon Single Crystal Excited by UltrasoundThe neutron Bragg diffraction on a bent silicon monocrystal excited by ultrasound was investigated. It is shown that for perfect crystal the relative diffraction intensity is proportional to the acoustic wave amplitudew.The calibration parameters between the generator voltage and acoustic wave amplitude were derived assumingw= (2.3±0.3)·10-2Å/V. To explain the results, a modified Penning—Polder—Kato model was applied. In a bent crystal, owing to ultrasound, transitions between the sheets of a dispersion surface take place. This leads to various manifestations of the behaviour of the integral scattering intensity, which drastically differs from the case of a perfect crystal. The observed effects may be used for creating new types of neutron monochromators and choppers governed by the ultrasound wave amplitude as well as by the length and bending radius of the crystal.

Design, Fabrication and Measurement of Ni/Ti Multilayer Used for Neutron Monochromator

Ni/Ti multilayers, which can be used for neutron monochromators, are designed, fabricated and measured. Firstly, their reflectivities are simulated based on the Nevot–Croce model. Reflectivities of two Ni/Ti multilayer mirrors with periods d = 10.3 nm (M1) and d = 7.8 nm (M2) are calculated. In the calculation, the reflectivity of the Ni/Ti multilayer is taken as a function of the grazing angle with different roughness factors δ = 1.0 nm and δ = 1.5 nm. Secondly, these two multilayers are fabricated by the direct current magnetron sputtering technology. Thirdly their structures are characterized by small-angle x-ray diffraction. The roughness factors are fitted to be 0.68 nm and 1.16 nm for M1 and M2, respectively. Finally their reflective performances are measured on the V14 neutron beam line at the Berlin Neutron Scattering Centre (BENSC), Germany. The experimental data show that the grazing angle of the reflected neutron intensity peak increases, but the reflected neutron intensity decreases, with the decreasing periods of the multilayers.

Synthesis and Characterization of New AA′BWO6 Perovskites Exhibiting Simultaneous Ordering of A-Site and B-Site Cations

Six new AA′BB′O6 perovskites KLaMnWO6, NaLaMnWO6, NaNdMnWO6, NaTbMnWO6, NaNdCoWO6, and NaNdMgWO6 have been prepared. Each possesses the unusual combination of layered ordering of the A-site cations and rock-salt ordering of the B-site cations. The structure and properties of these samples have been characterized using monochromatic X-ray and neutron powder diffraction, UV–vis diffuse reflectance spectroscopy, and SQUID magnetometry. NaLaMnWO6, NaNdMnWO6, and NaTbMnWO6 adopt a structure with monoclinic P21 symmetry arising from the combination of cation ordering and a–a–c+ octahedral tilting. The structures of the other three compounds are similar, but the presence of satellite reflections in the neutron diffraction data suggests a more complicated superstructure. Each of the four AA′MnWO6 samples shows a paramagnetic to antiferromagnetic transition with Néel temperatures ranging from 6 to 15 K. The NaTbMnWO6 compound shows a second magnetic transition at ∼9 K. The origin of two magnetic phase transitions appears to arise from coupling between the Mn2+ sublattice and the Tb3+ sublattice.

Optimization of filtered neutron beams for the calibration of superheated droplet detectors at the RPI

Superheated droplet detectors (SDDs) have been investigated for applications in neutron dosimetry and spectrometry. Varying the detector temperature, it is possible to change the neutron energy detection threshold of SDDs, thus allowing the use of a single detector to measure neutrons of different energy, without any change of the experimental setup. However, the neutron threshold energy versus temperature curves have to be experimentally determined. The determination of the calibration curves requires the use of monochromatic neutron beams. The neutron spectrum from a nuclear reactor covers a wide energy range, from meV to several MeV. Beams of quasi-monochromatic neutrons can be generated by filtering neutrons emerging from the core with suitable materials, such as Fe (for 24 keV neutrons) and Si (144 and 54 keV). These materials have windows in their neutron cross-sections, so that neutrons corresponding to these windows are transmitted, whereas neutrons with other energies are attenuated. We report on the MCNP simulation study of passive monochromators of Si + S and Si + Ti for the production of quasi-monochromatic neutron beams of 54 keV ( Si + S ) and 144 keV ( Si + Ti ) . The simulations allowed the purity versus intensity of the neutron beams to be optimized, within the geometrical constraints of the beam port.

Production of monoenergetic MeV-range neutrons by 3H(p,n)3He reaction

Monochromatic MeV-energy neutron source for secondary reaction was developed utilizing tritium embedded titanium (Ti-3H) thin film via 3H(p,n)3He reaction. We have measured the neutron energies and the energy spread by resonance reactions of 12C(n,tot) and 28Si(n,tot). The available energy was within the range from 0.6 to 2.6 MeV. Energy spread was 1.6% at energy of 2.077 MeV. The flux in the beam direction was determined to be 3.76·107 n/s/sr by irradiating 197Au by about 2 MeV neutrons. This source was shown to be useful for measurements of nuclear data by measuring the total cross sections of neutrons on Fe and comparing these data to the data of ENDF-6.

Effect of hydrogen on Ni∕Ti multilayer neutron monochromator performance

Ni ∕ Ti multilayers with and without hydrogen added to the Ti layers have been prepared by dc-magnetron sputtering to investigate the effect of hydrogen on neutron monochromator performance. The addition of hydrogen further reduces the negative scattering length density of Ti, thereby increasing the contrast with the Ni. Increases in the first order peak reflectivity by factors of 2–3 have been observed in neutron reflectivity measurements. The improved performance is attributed to a larger neutron scattering length density contrast and to a sharpening of the interfaces.

The new V12 ultra-small-angle neutron scattering and tomography instrument at the Hahn–Meitner Institut

The new V12 instrument at the Hahn–Meitner Institute in Berlin is a multiple setup combining several techniques to investigate the internal structure of bulk samples. It consists of two double-crystal diffractometers (DCDs) and an attenuation tomography device operating with monochromatic neutrons. The three instrument parts can be used independently at the same time. The DCDs are mainly used in the ultra-small-angle neutron scattering (USANS) regime, where they overlap the accessible range of small-angle neutron scattering instruments, while tomographic methods collect real-space information on a macroscopic scale. Together they enable structural investigations over six orders of magnitude (50 nm to 5 cm). Scattering and tomographic methods can even be combined by means of diffraction and scattering-enhanced imaging. The sample environment can be varied over a large range of temperatures and pressures for USANS measurements and a polarized USANS option is available.

Polarizing neutron monochromator with reduced higher-order contributions

Polarized neutron reflectivity experiments have been performed to investigate polarizing multilayer monochromators. The neutron monochromators have been sputtered with Ti and Fe–Co–V as layer materials with a nominal period of 11nm. For more than two layers per period, the bulk magnetization of 2.35μB is reduced to 1.6μB. With 22 layers per period and 15 periods the higher-order contributions are decreased to below 0.6% of the first-order Bragg peak intensity (reflectivity of 0.21) at qz=0.06Å−1. The polarization efficiency for the final monochromator is 95.5%.

The SANS facility at the Pitesti 14MW TRIGA reactor

The SANS facility existing at the Pitesti 14MW TRIGA reactor is presented. The main characteristics and the preliminary evaluation of the installation performances are given. A monochromatic neutron beam with 1.5 A ≤ λ ≤ 5 A is produced by a mechanical velocity selector with helical slots. A fruitful partnership was established between INR Pitesti (Romania) and JINR Dubna (Russia). The first step in this cooperation consists in the manufacturing in Dubna of a battery of gas-filled positional detectors devoted to the SANS instrument.

Wavelength tunable device for neutron radiography and tomography

A special double monochromator system was tested for a conventional operating tomography setup in order to use a broad wavelength band of monochromatic neutrons for radiography and tomography. Scanning through the wavelength region of Bragg edges, it is possible to make series of radiographs and tomographs at different wavelengths from 2.0 until 6.5Å. So no beam hardening influences the measurements and is not to be corrected. With this instrument for cold neutron radiography and tomography, energy selecting quantitative radiography, stress and strain mapping, and phase radiography were performed.

In situ deformation apparatus for time-of-flight neutron diffraction:Texture development of polycrystalline ice Ih

This article documents a new in situ deformation apparatus built for neutron diffraction investigations of polycrystalline materials in low-temperature environments and the first experiment in which it was used. We performed texture analysis of fine-grained polycrystalline D2O ice Ih deformed uniaxially between 230 and 240K using time-of-flight neutron diffraction on the high-pressure preferred orientation diffractometer at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory. The new deformation apparatus operates at 1atm of ambient pressure and over temperatures in the range of 77K<T<298K, and accommodates up to 667N of uniaxially applied load. It is suitable for diffraction studies of any bulk polycrystalline material, ideally cylindrical in shape, and is adaptable to multiple neutron spectrometers, including those at other polychromatic and monochromatic neutron facilities. The first experiment on a hexagonal ice sample demonstrates development of fiber texture in the direction of the applied load. The equipment has many applications to earth science, glaciology, and ice engineering.

Status and recent developments in diffractive neutron optics at the ILL

We present the present status and recent advances in diffractive neutron optics at the ILL. Very high-quality copper crystals with intrinsic mosaics of the order of 1′ can now be produced which is of interest for the reproducible production of higher-mosaic anisotropic crystals for neutron monochromators and potentially for a gamma-ray Laue lens for astrophysical applications. Recent progress in the growth of mosaic germanium crystals now allows us to produce high-quality crystals with a controllable and anisotropic mosaic of up to several degrees. Polarising supermirrors of Fe–Si can now be produced with critical angles of up to 4 times the critical angle of natural nickel. Recent applications include a solid-state S-bender, a transmission polariser for small-angle scattering and a polarising monochromator for 8.9 Å neutrons for the production of ultra-cold neutrons.

Neutron monochromator based on dispersive double reflections excited in a cylindrically bent-perfect-crystal (BPC) slab

A high-resolution neutron monochromator based on a multiple-reflection effect realized in a cylindrically BPC element has been recently tested with the aim of its possible employment in the scattering instruments for powder diffractometry. The feasibility of a diffractometer employing such monochromator for high or ultra-high resolution studies has been proved. Feasibility of its employment as a high-resolution monochromator is demonstrated on diffraction by a polycrystalline standard sample of α-Fe of volume of several tens of mm 3.

Inelastic scattering measurement option of TOF-USANS instrument at J-PARC

In a TOF-USANS instrument based on a double crystal diffractometer of Si perfect crystals with (1 1 1) fundamental reflection, monochromatic neutrons of wavelength λ and its harmonics ( λ/ n, n=3,4,5,7) by the first crystal enter a sample, and scattering beams from the sample are analyzed by the second crystal. The neutron beams of these entire wavelengths are used in the normal mode to measure USANS intensity effectively. On the other hand, the TOF-USANS instrument would be considered to be a crystal-monochromator-type direct geometry inelastic-instrument, including the analysis by the second crystal. Therefore, by installing a band definition chopper to choose specific wavelength as option, it would be thought that quasi-elastic and inelastic scattering measurements become possible.

MC ray-tracing optimization of lobster-eye focusing devices with RESTRAX

The enhanced functionalities of the latest version of the RESTRAX software, providing a high-speed Monte Carlo (MC) ray-tracing code to represent a virtual three-axis neutron spectrometer, include representation of parabolic and elliptic guide profiles and facilities for numerical optimization of parameter values, characterizing the instrument components. As examples, we present simulations of a doubly focusing monochromator in combination with cold neutron guides and lobster-eye supermirror devices, concentrating a monochromatic beam to small sample volumes. A Levenberg–Marquardt minimization algorithm is used to optimize simultaneously several parameters of the monochromator and lobster-eye guides. We compare the performance of optimized configurations in terms of monochromatic neutron flux and energy spread and demonstrate the effect of lobster-eye optics on beam transformations in real and momentum subspaces.

Scattering-related contrast signals in neutron computerized tomography and the new V12 instrument at HMI Berlin

Double-crystal diffractometers (DCD) are widely used for structural investigations at the limit between macroscopic and microscopic inner structures of sample materials. Operating in an ultra-small-angle scattering q-range between 10 −4 and 10 −1 nm −1 structures between 50 nm and nearly 100 μm can be resolved. Hence the DCD connects the resolvable ranges of small-angle neutron scattering (SANS) instruments and neutron tomography facilities. However, the DCD does not only link the resolvable size ranges but can also be operated to yield both, q-space information on microscopic structures combined with real space information in the range of macroscopic inner structures. This method was developed in recent years at the V12 DCD at HMI by introducing refraction and ultra-small-angle neutron scattering (USANS) contrast for tomography. The new V12 DCD set-up has now been optimized to exploit all the opportunities of USANS, refraction and USANS contrast tomography and conventional attenuation contrast tomography with an intense monochromatic neutron beam. The new contrast methods will be introduced as well as the final set-up of the V12 instrument. Additionally, several examples and results achieved by the new techniques and instrument will be given.

Monte Carlo simulations for instrumentation at SINQ

The Paul Scherrer Institut (PSI) operates a spallation source SINQ equipped with 11 different neutron scattering instruments. Beside the optimization of the existing instruments, the extension with new instruments and devices are continuously done at PSI. For design and performance studies different Monte Carlo packages are used. Presently two major projects are in an advanced stage of planning. These are the new thermal neutron triple-axis spectrometer Enhanced Intensity and Greater Energy Range (EIGER) and the ultra-cold neutron source (UCN-PSI). The EIGER instrument design is focused on an optimal signal-to-background ratio. A very important design part was to realize a monochromator shielding which covers best shielding characteristic, low background production and high instrument functionality. The Monte Carlo package MCNPX was used to find the best choice. Due to the sharp energy distribution of ultra-cold neutrons (UCN) which can be Doppler-shifted towards cold neutron energies, a UCN phase space transformation (PST) device could produce highly monochromatic cold and very cold neutrons (VCN). The UCN-PST instrumentation project running at PSI is very timely since a new-generation superthermal spallation source of UCN is under construction at PSI with a UCN density of 3000–4000 n cm −3. Detailed numerical simulations have been carried out to optimize the UCN density and flux. Recent results on numerical simulations of an UCN-PST-based source of highly monochromatic cold neutrons and VCN are presented.