The Japanese Spallation Neutron Source Facility is based on a 3 GeV, 1 MW, 25 Hz proton accelerator. It is apart of the High Intensity Proton Accelerator Project that is a joint project between the Tokai Establishment of the Japan Atomic Energy Agency and the High Energy Accelerator Research Organization. Neutronic optimization of the target-moderator-reflector assembly has been conducted assuming that the hydrogen is almost 100% para. Target station design is also underway, and five-year contracts will be made in November this year. Various device development programs, such as neutron-optical devices and detector systems, are also actively underway.
The Spallation Neutron Source (SNS) is an accelerator-based short-pulse neutron scattering facility designed to provide an order of magnitude more power than the most powerful existing facility of this type. The SNS is being constructed at Oak Ridge National Laboratory and is on schedule for completion in 2006. The unprecedented power of this facility brings many new opportunities and challenges for neutron scattering instrumentation. This instrumentation will cover a broad spectrum of science, with every instrument designed to be best-in-class. The SNS has provisions to accommodate up to 24 neutron beam instruments, and design and construction of a number of these instruments are already underway. Some of these instruments are funded within the SNS construction project and some from external sources. This paper will discuss the status of these funded instrumentation activities and of some other instrumentation activities in the planning stage, and will also discuss the process for providing additional instruments. The paper will also indicate the performance expected from many of these instruments and will address some of the challenges and opportunities faced in instrumenting a new spallation source of this unprecedented intensity.
The choice of pulse repetition rates and pulse lengths provided by a pulsed neutron source cannot ideally fit the requirements of all of the various neutron scattering experiments. Novel multiplexing chopper systems we propose can help to enhance instrument performance by tailoring the effective pulse parameters to better meet experimental needs. In particular schemes to multiply pulse repetition rates and wavelength bands and to enhance wavelength resolution will be discussed, together with an efficient chopper system design tool based on the new concept of wavelength filtering.
Many topics of interest for neutron scattering demand small sampling volumes. Then the scattering instruments should include focusing devices in order to deliver a sharp spatially shaped neutron beam at the sample position. Moreover the wavelength bandwidth should be sufficiently large if time-of-flight method is used. In this contribution a new compact focusing device is proposed. The device is made of a stack of bent silicon wafers, each having a glancing reflective layer deposited on one side and a neutron absorbing layer on the other side. This device acts as a lens by superposing the images delivered by individual mirrors. The aberrations are minimized due to the short length of the device. From this point of view this type of superposition lens is equivalent to a long elliptic or parabolic mirror. Consequently a two dimensional focusing could be obtained by combining two devices in a Kirkpatrik-Baez set-up. Basic design principles are described and Monte-Carlo simulation results are presented. Possible applications in neutron instrumentation are reviewed.
The relatively low flux from neutron sources means that structural analysis using neutron diffraction requires large crystals that are often not available.We are exploring the possibilities of a polycapillary focusing optic to produce a small intense beam spot of size ⪅0.5 mm for small crystals.We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm,suitable for time-of-flight diffraction.Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength.We have performed similar measurements on a polychromatic beam on a pulsed neutron source,where the results are subject to background from short wavelength neutrons.The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of two.In a diffraction measurement from an alpha quartz crystal using a 2.1° convergent beam from a pulsed neutron source,we observed six diffraction peaks in the 1.5 Å -4 Å wavelength bandwidth transmitted by the optic.These diffraction spots show an intensity gain of 5.8 ±0.9 compared to a direct beam diffracting from the same sample volume as that illuminated by the convergent beam.
Barring a monumental failure of design execution or of performance estimation, the liquids reflectometer at the SNS will provide unprecedented capabilities for the study of liquid and solid surfaces. Design of the instrument is well underway and procurement of the guide components has begun. Nuetrons from a coupled 20-K supercritical hydrogen moderator will be delivered via a multi-channel supermirror bender and tapered guide onto either a horizontal or tilted surface. Collimating slits select the beam incident angle from a 0-5° vertical intensity distribution provided by optics. With the SNS running at 2 MW, the instrument will be able to accumulate a complete specular reflectivity scan from D2O (R < 10-7, Q > 0.5 Å-1) in less than 10 minutes. We will describe the optical design of the SNS liquids reflectometer, compare it with a conventional instrument, and estimate its time resolution for a model kinetic system.
Neutron diffraction is now established as a very reliable bulk and surface technique, with draft industrial standards in place and industrial take up increasing. The Strain Imager will be the new instrument for residual stress analysis of engineering components at the ILL, the worldTs most powerful steady state neutron source.
Thanks to the improved flux of the new m = 2 supermirror guide (gaining a factor 3 to 5 on the existing guide), and improved optics the Strain Imager will increase the range of potential engineering applications by improving the resolution and shortening the measurement times. It is designed to allow measurements in components up to 2 metres in length and more than 500 kilogram weight (at a depth up to several centimetres), yet having a spatial resolution better than 0.5x0.5x0.5 mm3 . On the other hand, precise surface measurements will be standard, owing to the presence of collimators and to a positioning precision better than 50 μm.
Special features include: a) the sample stage O the first to use a Stewart platform allowing 6 axis sample movement working in sample centric co-ordinates; b) a double- focusing bent Si monochromator, with very good resolution, very high intensity and a variable take-off angle from 55° to 125°; and c) a flexible optical system for the incident and diffracted beams. It will allow the use of combinations of different collimators and variable size slits for automated definition of the gauge volume.
The feasibility of Guinier cameras for small angle neutron scattering (SANS) is analyzed theoretically and experimentally. Small angle X-ray scattering (SAXS) is commonly measured with Guinier cameras1 that use bent perfect crystals to focus to detector beams from point sources of characteristic X-rays. Neutron Guinier cameras do not exist yet, although focusing to detector has occasionally been tried. The philosophy of current SANS pinhole instruments is to gain intensity from broad wavelength bands at tight collimation. With characteristic X-rays, intensity gains can only come from broad angular divergences. Neutron focusing instruments represent a return, at a higher level, to the philosophy of characteristic X-rays. Such a return is advocated in this paper for SANS.
The resolution of Guinier cameras is defined not by the collimation (which is relaxed), but by the beam size at focus and the spatial resolution of the position sensitive detector (which should match each other). Within the recent concept of neutron imaging2 multi-wafer monochromators can provide image sizes comparable to the thickness of one wafer in the bent packet. The imaging may be non-dispersive, at broad wavelength bands, like with mirrors in conventional optics. These are the right ingredients for convergent neutron beams in Guinier cameras. The paper addresses the question whether the increased angular divergence can compensate for the reduced size of the source that is imaged into a sharp spot at detector.
A neutron Guinier camera at thermal neutron energies is evaluated. It turns to be quite feasible, providing moderate resolution at high intensity with detection systems in current use for high-resolution neutron diffraction. High-resolution SANS is also possible with detection by image plates or microchannel plate systems.
Tests were performed using a single wafer and a packet of bent silicon wafers in both Bragg and Laue (transmission) geometry in non-dispersive imaging arrangements. Experiments have confirmed expectations. SANS data obtained in neutron Guinier camera conditions on samples of collagen and lipids are presented.
Vertically "focussed" monochromators are widely used on neutron scattering instruments. As scientists search to improve instrument performance, there has been a trend to increasing the vertical height of monochromators. Vertical focussing shifts the centre of intensity of the beam horizontally in a systematic way. With large monochromators the shift becomes large. While the effect is simple to explain, it is less widely appreciated than it should be. This work presents equations describing the shifts.
We have carried out simulations of a time-focused pulsed-source crystal analyzer (inverse geometry) spectrometer using the VITESS Monte Carlo neutron scattering instrument simulation code. The configuration of the instrument is one suggested by the recently reported general theory of this class of instrument. That theory provides the basis for design to accomplish high resolution while allowing other than backscattering geometry and more flexibility in choices of the type of analyzer crystal and the detector location. The VITESS code has all the capabilities needed to treat this type of spectrometer: three-dimensional generality, time-of-flight, off-cut mosaic crystal reflection, and high computational efficiency, all of which we exercised. We analyzed a configuration with a 50.-m incident flight path, 2.-m distance from sample to analyzer, and 1.8-m distance from analyzer to detector, assuming elements 1.-mm thick and considering reflectivity widths up to 0.5°. The Bragg angle at the analyzer was 80.° and the assumed d-spacing was 3.13 Å. We report results concerning the orientations of the moderator (neutron source), the sample, the analyzer crystal, and the detector that prove out the focusing conditions resulting from the theory. Calculations for realistic sizes of elements and 90° scattering angle indicate an elastic-scattering time-of-flight resolution Δt/t approximately 6. x 10-5 (far less than a conventional estimate cot ΘΔΘ) for the instrument geometry alone, absent the contributions from finite moderator emission-time width and finite-width monochromator d-spacing distribution. Simulations of a second analyzer arm at 60° also show the focusing effect, although we have so far been unable to carry this out for the same sample orientation as for 90°, as theory assures should be possible. The calculations also provide indications of the limits of the linear focusing theory.
3He neutron spin filters and zero-field polarimeters are being intensively developed in order to better investigate exotic magnetic systems and prepare the construction of polarized neutron instruments at spallation neutron sources. There are several incentives for using 3He spin filters. They are simple transmission devices having minimal impact on the optics of the neutron beam and they are broadband with properties that vary only weakly with neutron energy. However, a very homogeneous magnetic field is required for long relaxation time of the 3He polarization. We will present devices which remove this difficulty, even in front of a cryomagnet producing a large inhomogeneous stray field.
With a zero-field polarimeter Cryopad, the three components of the scattered spin-polarization vector are measured, which is not possible by standard uniaxial polarization analysis. In the case of magnetic structures, Cryopad allows the direct determination of the direction and phase of the magnetic interaction vector. Hence, it is the most powerful tool for solving non-trivial antiferromagnetic structures. In recent years, the technique has been used to correct major errors in magnetic structures previously determined with other techniques. Efforts are being made to adapt this technique to the case of a spallation neutron source.
Our goal is to develop magnetically remanent neutron supermirrors using the material combinations Fe/Si and FeCo/Si. With these we plan to build compact neutron transmission polarizers and neutron polarizers which can be operated with their magnetization oriented antiparallel to the guide field. In the latter case no spin flipper is necessary to switch to the other spin state.
The supermirrors are produced by magnetron sputtering. The preparation conditions were optimized by producing series of multilayers where the sputtering parameters gas pressures, plasma power and the speed of substrate translation were varied. Neutron reflectivity and transmission were measured with the polarized neutron diffractometer TOPSI at SINQ. Stress and magnetic behavior were determined using a profilometer and a vibrating sample magnetometer, respectively.
The existence of an easy axis of magnetization is caused by anisotropic tensile stress in the Fe layers. Since the stress in Si is compressive it is possible to reduce the total stress while keeping the remanence by adding the reactive gases O2 and N2 to Si. In this way it was possible to produce Fe/Si supermirrors which show polarizing efficiencies of 96% to 99%. These supermirrors having 299 layers in total, reflect spin up neutrons up to q = 0.55/nm (m=2.5) which allows for their use as transmission polarizers in Ni coated beam guides.
We discuss a method, based on the neutron spin echo technique, which can be used to enhance a variety of neutron scattering experiments. In the method, precession of the neutron's spin in a magnetic field is used to code a particular component of the neutron's incident or scattered wavevector. The method allows good resolution to be obtained along any chosen direction in wavevector-and-energy-transfer (Q,E) space and is independent of other resolution elements such as collimators or monochromators. Such components can thus be chosen to maximize signal intensity. The equipment we describe uses thin, magnetic films deposited on silicon substrates to manipulate neutron spins in the manner required to implement the spin echo method. These films and their mounts are inexpensive, easy to build and adjust, and can be added as a "bolt-on" option to any constant-wavelength neutron spectrometer that already provides polarized neutrons. Resolutions comparable with the best achievable with tight collimation or monochromatization should be easily attainable. The gains in intensity achievable for reflectometry and SANS are discussed.
We present concepts for two new instruments for steady state or pulsed neutron sources:
• An ultra-small small angle scattering option for small angle scattering instruments - the principle might be applied for high resolution reflectometry as well;
• A high resolution spin echo option for a long baseline SANS instrument.
Further we present first data from a new resonance spin echo option, dedicated to enhance the resolution of thermal triple axis spectrometers by 2 orders of magnitude.
The conceptual design of a new instrument is presented, which makes use of the spin echo technique to analyze the scattering angle of neutrons impinging upon a rough or corrugated surface at grazing incidence. In a grazing incidence geometry the roughness of the surface and of submersed interfaces give rise to a neutron scattering pattern at angles well resolved from that of specular reflection for corrugation lengths up to several microns, but only in the plane of specular reflection. In contrast, scattering caused by corrugations of comparable length perpendicular to the reflection plane is limited to angles that can be separated only with an extreme tightening of the instrumental resolution. However this scattering can be well resolved by spin-echo methods for polarized neutrons. Results from scattering in and out of the reflection plane provide complementary information on the structure of the roughness and its location in a system with complex layering. This spin-echo technique may also distinguish static from time-dependent roughness.
Spin echo small-angle neutron scattering (SESANS) is a novel technique that measures correlation functions in real space. Recent theoretical study on SESANS has enabled the interpretation of this correlation function. It has also revealed the range of applications and limitations of the SESANS technique. On a two-dimensional SESANS instrument, the experimental correlation function is the pair-distance distribution function of the scattering particle. On a one-dimensional instrument, the correlation function is an integral function of the pair-distance distribution function. SESANS is suitable for studying particles from a nanometer to a few tens of micrometers in size, a range that is similar to that covered by the traditional Bonse-Hart ultra-small-angle neutron scattering instrument. The greatest advantage of SESANS lies in the fact that it can use divergent neutron beams, thus drastically increasing the counting rate. The resolution of a SESANS instrument is limited by the integrated Larmor precession field and by the neutron wavelength. Because any SESANS instrument will have a limited momentum transfer coverage, truncation errors can result in the measured correlation functions. The effect of the inhomogeneity of the Larmor field can be handled as smearing. On a one-dimensional SESANS instrument, the off-plane scatterings also result into smearing effects.
For probing the structure and dynamics of matter at an atomic, molecular and mesoscopic level neutron scattering and diffraction methods are of utmost importance. Suffering presently from relatively low source strengths, compared e.g. to X-ray investigations, neutron scattering methods will greatly benefit from the increase of the instantaneous flux, which will be attained at the next generation of pulsed spallation neutron sources with several MW average beam power in the proton beam hitting the spallation target. In particular at ESS, the strongest of these sources, the thermal neutron flux will exceed the highest available today by about two orders of magnitude in comparison with generic instruments at ILL or ISIS. In addition, improved neutron optics can further enhance the flux on the sample by another factor 5-10. Hence, the count rate loads on the neutron detectors could rise by up to three orders of magnitude and typically by factors of 50-150; however, in more and more experiments intensity will be sacrificed by using small and even sub-millimeter samples. Consequently, the development of neutron detectors with higher counting rate capability, better time-of-flight and position resolutions and improved background is prerequisite for taking full advantage of the improved source strength. In this paper the detector requirements for ESS will be reviewed in comparison with the current state-of-the-art and with promising novel solutions.
Advances in neutron scattering studies will be given a large boost with the advent of new spallation and reactor sources at present under consideration or construction. An important element for future experiments is a commensurate improvement in neutron detection techniques. At Brookhaven, a development program is under way for greatly increasing the angular coverage, rate capability and resolution of detectors for scattering studies. For example, a curved detector with angular coverage of 120° by 15° has recently been developed for protein crystallography at a spallation source. Based on neutron detection using 3He, the detector has the following major, new attributes: eight identical proportional wire segments operating in parallel, a single gas volume with seamless readout at segment boundaries, parallax errors eliminated in the horizontal plane by the detector's appropriate radius of curvature, high-throughput front-end electronics, position decoding based on high performance digital signal processing. The detector has a global rate capability greater than 1 million per second, position resolution less than 1.5 mm FWHM, timing resolution about 1 μs, efficiency of 50% and 90% at 1Å and 4 Å respectively, and an active area 1.5 m x 20 cm.
For thermal neutron experiments at ESS very high resolution and fast hybrid microstrip gas chamber (MSGC) detectors are being developed at HMI, which lend themselves for setting up large-area detector arrays comprising detector cells of 285 mm x 285 mm size. These detectors utilize a composite 157Gd/CsI foil converter and novel robust multilayer microstrip glass plates, which are optimized for a low-pressure two-stage gas amplification mode. Using a delay-line based, interpolating readout mode with sub-segmentation on the plates, the detectors have the potential to combine position resolutions of 0.1-0.3 mm FWHM, time-of-flight (TOF) resolutions of less than 10 ns and a counting rate capability in the range of Mevents/s per cell, which is limited by the present version of the data acquisition system. To handle these high data rates a PCI-bus board has been developed comprising four 8-channel multihit TDC chips (~150 ps LSB) of the F1 type, one digital signal processor (1 GFLOP DSP) for online data processing and a histogram memory (HM) of 256 MB. By means of the DSP the data of the registered coordinates X, Y, TOF and pulse-height, measured by means of time-over-threshold discriminators in the TDCs, can either be calibrated and transformed online into 2D spectra accumulated in the HM or read out in list-mode.
The topic of this study is to build a 2D or 3D thermal neutron tomography system with a good spatial resolution (better than σ = 100 μm) for imaging of objects of some centimeters diameter. Two ways of detection have been investigated: Image Plate technology and Micromegas, a detector developed for particle physics, with a 50 μm thick foil of Lithium as neutron converter. With the classical Image Plate technology, a spatial resolution around 50 μm has been obtained but it would be very difficult to develop an industrial tomography system based on this kind of detector. In the case of a Micromegas detector, with one-dimensional readout, a 60 μm spatial resolution has been reached. This result, together with an easier industrialization make the Micromegas technology very promising for thermal neutron tomography.
Semiconducting boron-rich boron-carbon alloys have been deposited by plasma-enhanced chemical vapor deposition. Heterojunction diodes made with 276nm thick nanocrystalline layers of these alloys have been used as real-time solid-state neutron detectors. Individual neutrons were detected and signals induced by gamma rays were determined to be insignificant. Linearity of detection was demonstrated over more than two orders of magnitude in flux. The neutron detection performance was unaffected by > 1 x 1015 neutrons / cm2. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron carbon alloy layers enriched in 10B could lead to increased detection efficiency.
Solid-state devices for the detection of thermal neutrons may possess many interesting features compared to gas-filled detectors. The AlB12, a wide band gap semiconductor, is an interesting candidate for such a detector, which can be used without cooling, e.g. at room temperature. Monocrystalline AlB12 was grown by crucible free electron beam heating method and the obtained samples have been characterized crystallographically and electrically.
As a result of the ever increasing demand for higher neutron flux, future neutron scintillator detectors will require faster scintillators with a high light yield and low gamma sensitivity. Initial measurements with single element prototypes demonstrated that the new lithium gadolinium borate (LiGdBO) scintillator, using 6Li as the neutron absorber, was a promising candidate. As a result of this work, a full size position sensitive detector module has been made consisting of 120 LiGdBO elements. This module was installed on the High Resolution Powder Diffractometer (HRPD) beam line at ISIS for evaluation. The LiGdBO detector has been constructed with the same geometry and theoretically the same neutron absorption efficiency as the existing HRPD ZnS scintillator detector modules. In this way it was possible to make a more exact comparison of the relative detector performances. Measurements have shown that the neutron energy resolution and detection efficiency of both types of detector are identical. Gamma sensitivity and quiet count rate are somewhat higher for the LiGdBO than the ZnS scintillator detector, but still at an acceptable level for many applications. The short decay time of the LiGdBO scintillator has enhanced the count-rate capability of the detector by an order of magnitude. These measurements show that realistic large area position sensitive neutron detectors can be fabricated with LiGdBO scintillator using an optical fibre readout. LiGdBO is thus a promising scintillator for future detectors at the new high flux facilities currently under construction.
Traditional neutron gas counters show limits when a spatial resolution under 1 millimeter in both directions is required. The detction of the light signal induced by a neutron interaction in solid scintillators allow to achieve a better spatial accuracy, but at the cost of a higher sensitivity to γ-rays which represent the major source of noise in most of the neutron instuments.
We have studied at the ILL a detector based on a Position Sensitive Photo-Multiplier Tube directly coupled to a scintillator, either GS20 Lithium glass (0.5mm and 1mm thick) or 6LiF/ZnS:Ag (0.42mm thick).
The simplicity of this device is very well suited when the position resolution is the main parameter of choice.
These two types of scintillators are compared in term of energy spectrum, detection efficiency for thermal neutrons and for 60Co γ-rays, which, even if not representative of γ environment on experimental sites allow this comparison. The counting rate limit is estimated from the light decay time and, position resolution, linearity and edge effects are discussed. We conclude that the GS20 scintillator give the best results, provided its thickness is properly chosen according to the wavelength of neutrons to be detected.
The Spallation Neutron Source (SNS) under construction at the Oak Ridge National Laboratory (ORNL) will be the most important new neutron scattering facility in the United States. Neutron scattering instruments for the SNS will require large area detectors with fast response (< 1 microsecond), high efficiency over a wide range of neutron energies (0.1 to 10 eV), and low gamma ray sensitivity. We are currently developing area neutron detectors based on a combination of a 6LiF/ZnS(Ag) scintillator screen coupled to a wavelength-shifting fiber optic readout array. A 25 x 25 cm prototype detector is currently under development. Initial tests at the Intense Pulsed Neutron Source at the Argonne National Laboratory have demonstrated good imaging properties coupled with very low gamma ray sensitivity. The response time of this detector is approximately 1 microsecond. Details of the design and test results of the detector will be presented.
In order to broaden the applicability of neutron interferometry, a new type of multilayer cold-neutron interferometer based on a pair of etalons has been developed. The range of experimental application of conventional multilayer cold-neutron interferometer was limited due to the small spatial separation between the two coherent beams. Using etalons with an air gap of 9.75μm in spacing we have observed interference fringes with the contrast of 60%. The present results have demonstrated the feasibility of developing a cold neutron interferometer with a large path separation to carry out high precision measurements and new types of experiment.
The present calculations describing the Bonse-Hart Ultra-Small-Angle Neutron Scattering (USANS) Instrument with triple-bounce Si channel-cut crystals show that significant gains in neutron flux and Q-resolution can be achieved using multiple high-order Bragg reflections. These reflections become usable only after combining the Bonse-Hart and Time-of-Flight techniques, thus this variant of the USANS camera needs a pulsed neutron source. We clearly demonstrate that new instruments of that type installed at the SNS water moderator will improve the current state-of-the art USANS camera dramatically increasing the neutron flux and sharpening the Q-resolution by almost one order of magnitude.