Journal of Neutron Research

The concept of the information content of a scientific measurement is introduced, and a theory is presented which enables the information that may be obtained by a neutron scattering instrument to be calculated. When combined with the time taken to perform the measurement the bandwidth of the instrument is obtained. This bandwidth is effectively a figure of merit which is of use in three respects: in the design of neutron instrumentation, the optimisation of measurements, and in the comparison of one instrument with another.

The need to convert neutron scattering data from site specific data formats to a format understood by data analysis programs has exercised the users of neutron scattering facilities since the invention of neutron scattering and computers. Recent developments in computing, neutron scattering technology and politics have made this problem big enough to justify a proposal for a data exchange format (DEF) for neutron scattering. The proposed data standard is organized in four levels: The first is the physical file format, which will be the Hierarchical Data Format (HDF) [1] developed by the National Center for Super Computer Applications (NCSA). The second level gives guidelines for storing individual data items in the file. A common name convention and a common structure for an exchange file will significantly facilitate the implementation and reuse of software. Therefore the third level defines a recommended structure for arranging and classifying information in the file in a logical manner. The fourth level is a set of naming conventions and a glossary of names for neutron scattering related data items. The standard is extensible. Data items can be added to the file without interfering with the standard. The standard is suggested for voluntary adoption by the neutron scattering community. The authors invite comments from all interested parties.

Polarized neutron diffraction provides information on the Fourier components of magnetization density distributions in crystals. To obtain the densities themselves, one has to solve the inverse Fourier problem. This problem is complicated by the presence of noise and the incompleteness of the data. Several approaches exist and have been widely used, ranging from the most straightforward one, the Fourier synthesis, to the refinement of parametrized models. Recently, the application of the maximum-entropy principle caused a breakthrough in this field, allowing high quality maps to be reconstructed without any a priori knowledge of what the density should look like. This technique can be applied in different ways: all the approaches are discussed and illustrated by an example based on real polarized neutron diffraction data.

The Cambridge Crystallographic Subroutine Library (CCSL) is a library designed to facilitate crystallographic calculations. It defines a data format for describing both nuclear and magnetic structures. Procedures contained in the library use these data to calculate properties which depend on the structure. Subroutines are provided to evaluate quantities such as the structure factor and its differentials with respect to the crystallographic parameters, bond lengths and angles, the indices of magnetic reflections, fourier series etc. The library also includes a selection of main programs for carrying out common tasks; these call the subroutines as required. Examples are given of how the procedures of CCSL can be pieced together to construct powerful programs to analyse different kinds of neutron scattering data. In conclusion some developing ideas for a C++ class library with a similar purpose to CCSL are outlined.

Rapid data inspection and analysis for large arrays is achieved by the program LAMP which was locally written using the IDL® language. Ease of use is ensured by graphical user interfaces, and flexibility is offered by data structure which is optimised for neutron-scattering data and user-macros.

Open-GENIE is the successor to the widely used and highly successful ‘GENIE-V2’ program which is used as the principal front-end neutron data analysis tool at the UK’s ISIS spallation neutron source. In this paper, the background and motivation for the ‘Open GENIE’ project are briefly reviewed as is progress to date. Some of the ideas and concepts making Open GENIE unique are demonstrated. Emphasis is given to how these ideas are expected to develop in the future and how they will impact the field of neutron scattering data analysis.

A description is given of the integrated instrument control and data-acquisition electronics and the software of the neutron reflectometer ROG. For the data acquisition, VME electronics is developed to perform time-of-flight and stroboscopic measurements, using a single ³He detector or a two-dimensional position-sensitive detector. For the latter pixel-dependent discrimination is implemented. The instrument control is performed via a distributed (field bus) system. A flexible integrated software system controls the complete instrument.

A new kind of Charge Coupled Device (CCD) detector, developed at FRM Garching, has been used for computer tomography with thermal neutrons. An object of about 9 cm³ volume was examined in a thermal neutron flux of 3*10⁵ n s⁻¹ cm⁻² with in 12 hours with a spatial resolution of about (200 microns)³.

The design and development of crystalline silicon neutron detectors are discussed. Experimental results obtained on prototype systems, namely a diode and a microstrip device, are reported. The detector performances are discussed with special emphasis on detection efficiency, dead-time and positional resolution. Performances in signal-to-noise ratio when switching from medium-speed to fast VLSI readout electronics are reported for the prototype systems. Progress through various detection schemes is outlined and the expected performances of a position sensitive area detector for neutron scattering application are presented.

The use of medium to long wavelength neutron diffraction for the study of large unit cells to near atomic resolution using quasi-Laue techniques is a good way of increasing measurement capacity in neutron protein crystallography, but requires a large detector area. A cylindrical detector based on image plate technology has been constructed for this purpose, and basic design criteria together with some first results are reported.

The readout of neutron scintillators via CCD cameras has proved to be an efficient and cost effective means of position sensitive neutron detection. Slow-scan CCDs work as integrating detectors, offering high count rate capability and high linearity, whereas intensified CCDs allow the analysis of single neutron events, thus providing good background rejection at lower count rates. This paper describes the workings of a slow-scan CCD camera for neutron detection and reports about properties of various types of scintillators, fiber optic devices, lenses and CCDs that are used for these detectors, and their influence on the detective quantum efficiency.

The principles involved in gas detectors for neutrons and the techniques for position encoding in such detectors are reviewed. This forms the basis for a discussion of the limitations on gas detector position resolution and maximum data rates, and of the aging effects in such detectors. Current capabilities and possible areas for improvement are noted.

A review is presented about the use of scintillating glass optical fibers for neutron imaging. The fibers are assembled in bundles to form plates coupled to integrating image detectors (CCDs). The scintillating glass core of the single fibers, which are oriented in the direction of impinging neutrons, is sensitive to neutrons. Scintillating glasses containing lithium and doped with terbium or europium are the most convenient for detection of slow neutrons. Experimental results are reported.

We report on the scintillation properties of a LiBaF₃ crystal when irradiated with thermal neutrons or gamma rays. This scintillator exhibits cross luminescence and self-trapped-exciton luminescence. The former does not show up under thermal-neutron irradiation, whereas both give a response under gamma irradiation. This offers a unique opportunity for thermal-neutron - gamma discrimination. The effectiveness of the method is demonstrated.

A proposed design for an optically-based, one-dimension scintillation detector to replace the gas-filled position-sensitive proportional counter currently used for a wide-angle neutron detector (WAND) at the High-Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL) is presented. The scintillator, consisting of a mixture of ⁶LiF and ZnS(Ag) powders in an epoxy binder, is coupled to an array of wavelength shifting optical fibers which provide position resolution. The wide-angle neutron detector is designed to cover a 120 degree arc with a 75 cm radius of curvature. The final detector design provides for 600 optical fibers coupled to the scintillator screen with an angular resolution of 0.2 degrees. Each individual pixel of the detector will be capable of operating at count rates exceeding 1 MHz. Results are presented from the measurement of neutron conversion efficiencies for several screen compositions, gamma-ray sensitivity and spatial resolution of a 16 element one-dimensional array prototype.

This paper describes the scintillators in common use for the detection of thermal neutrons and the advantages of using scintillator technology in the design of neutron detectors. Characteristics obtained in recent detectors incorporating these scintillators are given together with proposals for new scintillator detectors under development at ISIS. Desired characteristics of new scintillators for use in neutron detectors at both ISIS and new accelerator sources are defined.

The paper presents the spectrum of solid state neutron scintillation detectors, which have been developed and manufactured at Jiilich with respect to use at neutron scattering instruments at steady state reactors and pulsed spallation sources. Design and physical characteristics of three types of linear and three types of area detectors are described. The detectors are designed for operation as standalone PC-based systems with integrated timing interface based on programmable logic cell arrays. The data acquisiton system within the EISA-PC is suitable for applications requiring huge amounts of memory, e.g. in position-sensitive time-of-flight experiments.

Si-strip detectors show qualities which are desirable in many neutron experiments, like excellent position– and time resolution and a low sensitivity to background radiation. To demonstrate the feasibility of this detector type for the detection of thermal neutrons, we tested a prototype of a one dimensional position sensitive neutron detector based on a Si-strip detector with a Gd-foil converter. A complete system consisting of the detector with VLSI electronics and an on-line data acquisition system has been constructed and tested. We present the first results of the measured position resolution and detector efficiency for neutrons with 1.1 Å wavelength.

Perfect crystal neutron interferometry has provided the basis for the realization of many basic experiments for testing quantum mechanics with massive particles. The following quantum effects have been verified: 4π-symmetry of spinors, spin-superposition, gravity-induced quantum interference effects, the Aharonov-Casher effect, the magnetic Josephson effects, the scalar Aharonov-Bohm effect etc. More recently, steps towards neutron quantum optics have been undertaken. Measurements have dealt with the coherence properties of the neutron beam, the photon exchange between the neutron and various electromagnetic fields, the particle number-phase uncertainty, squeezing phenomena, and the Wigner-functional behavior of a neutron system. These achievements opened the field of direct Fourier spectroscopy which brings a direct access to the space and time dependent correlation functions which describe condensed matter. Various perspectives for further experiments in the field of fundamental and condensed matter physics are discussed.

Cold neutron interferometry using multilayer mirrors is discussed. The interferometry includes two kinds of multilayer interferometers, which are analogous optically to the Mach-Zehnder and the Jamin interferometer in classical optics. The Mach-Zehnder multilayer interferometer has been installed at JRR-3M reactor of JAERI. We describe the conditions required for the Mach-Zehnder multilayer interferometer and the characteristics of the interferometer. The Jamin multilayer interferometer, called a phase echo interferometer, shows a phase echo effect analogous functionally to the spin echo principle. We describe briefly the first successful performance tests. We propose a phase echo spin interferometer using multilayer spin splitters, based on the phase echo principle and the coherent superposition of the spin eigenstates.

Crystals, counters and computers are the most important new tools for powder diffraction. New crystal monochromators allow a wide band of wavelengths to be focussed to give higher intensity. New position sensitive detectors cover a larger angular range, increasing efficiency and the definition of the powder profile. Finally, new computers refine and display the data in real time, converting measurements into real experiments. These new technical developments will allow neutron powder diffraction on reactors to remain unique tools in chemical crystallography, despite advances made with pulsed sources and synchrotrons. In this paper we describe a new monochromator and filter for D2B, new detectors for DlA and D20 and new computers and software for all diffractometers.

Industrial manufacture of surfaces coated with supermirror of enhanced critical angle and high reflectivity allows new developments in the use of neutron guide. We report some new results on Ni/Ti multilayer: influence of adding Carbon in the Ni layers and time evolution with irradiation of the reflecting properties. Also we present briefly the flux and wavelength distribution available at the end position of a multilayer coated bender and of a classical guide where only the curved section is made of supermirror-type elements.

Recent theoretical and experimental progress on the development of new techniques for measuring the neutron electric dipole moment and neutron lifetime, and for producing ultracold neutrons in general, is reviewed, with emphasis on the electric dipole moment work. The basic idea is to produce and store ultracold neutrons in a superfluid ⁴He bath which has been doped with a low concentration of polarized ³He which serves as an ultracold neutron polarizer, analyzer, and magnetometer.

Ultra Cold Neutrons (UCN) can be produced at spallation sources using a variety of techniques. To date the technique used has been to Bragg scatter and Doppler shift cold neutrons into UCN from a moving crystal. This is particularly applicable to short-pulse spallation sources and is the method planned for a UCN source that is under construction at LANSCE. However, large gains should be possible using cryogenic UCN sources. Spallation sources are particularly well suited for such sources given the lower heat loads and simpler engineering requirements than are possible at a reactor. The most attractive cryogenic source appears to be a frozen deuterium source and research is under way at Gatchina to demonstrate technical feasibility. If successful, a source of this type could be implemented at future spallation sources with a UCN density that may be two orders of magnitude higher than that presently available at reactors.

P-violating neutron-spin rotation was firstly observed in a p-wave neutron-nucleus resonance. A lanthanum target was placed in a superconducting box. The incident neutron spin was set at transverse direction to the neutron beam in the horizontal plane. The neutron spin rotates around its momentum axis in the presence of P-violating interaction. The neutron spin passes through a Meissner sheet without any change in its direction. The rotation angle was converted to a projection angle on a vertical magnetic field in the downstream, which was analyzed by a polarized ³He neutron polarimeter. The experimental result of the P-violating rotation is consistent with the theory of s- and p-wave parity mixing in the compound nucleus state. For the T-violation experiment, measurement of T-odd triple correlation between the neutron spin, the neutron momentum and the nuclear spin, a new method is proposed, which greatly reduces systematic errors. The experiment can be compared with the neutron electric dipole moment. Target-nucleus polarization was developed for the T-violation experiment by using a dynamic nuclear polarization. A large lanthanum nuclear polarization was obtained.

We describe an apparatus for studies of parity violation in neuwn-nucleus scattering. This experiment requires longitudinally polarized neutrons from the Los Alamos Neutron Scattering Center over the energy range from 1 to l000 eV, the ability to reverse the neutron spin without otherwise affecting the apparatus, the ability to detect neutrons at rates up to 500 MHz and an appropriate data acquisition system. We will discuss the neutron polarizer, fast neutron spin reverser, detector for transmitted neutrons and high rate data acquisition system.

Antisymmetric interactions with respect to spin direction give rise to spin rotations in certain perfect crystals. A new perfect crystal technique, termed Dual-Polarised-Beam technique (DPB), can be used to boost the sensitivity of the spin rotation measurements by several orders of magnitude. The typical Zeeman splitting is shown and a future experiment of neutron EDM detection with DPB using new crystals is also discussed.

Polarized neutron scattering is a probe of nuclear spin densities. It finds an important application in nuclear spin contrast variation. As an example the results from studies on ribosome bound tRNAs are presented. Selective nuclear spin polarization can be controlled by the rf bandwidth. The creation of spin label dependent polarization domains is at hand.

To show the rich scale of possibilities of the classical polarization analysis we first give a short overview of measuring techniques. Then we compare some known instruments and give some details for the use of classical polarization analysis in a multidetector instrument. Some examples of measurements already performed are discussed such as partial structure factors with separation of elastic coherent and incoherent scattering, diffraction patterns with elastic antiferromagnetic peaks and finally some examples of quasielastic and inelastic measurements with polarization analysis for separation of coherent, incoherent and magnetic scattering.

Because of the stringent requirements on the magnetic precession fields this technique has taken a long implementation time. The solution of the homogeneity and of the stray-field problem is the Optimal Field Shape (OFS) which allows the use of wide and divergent neutron beams. On the ISSP thermal three-axis spectrometer equipped with OFS coils relative energy resolutions of l0⁻⁵ have now been obtained. At the ILL the implementation of super conducting OFS coils is expected to yield 10⁻⁶. The quality of the spin-echo polarisation and the spin echo phase are independent of the scattering angle. A further possibility of TASSE (Three Axis Spectrometer with Spin Echo) which has been developed and is to be tested, is complete phonon focusing, that is matching of both slope and curvature of the constant spin echo phase surface to those of the dispersion surface of the excitation, the lifetime of which is to be determined.

We summarise the theory of polarised neutron reflectivity and describe the PADA reflectometer at the Orphée reactor at Saclay France. We then discuss some experimental difficulties and highlight how this apparatus can be applied to the study of thin magnetic films. Finally, we give an example of a recent experiment on Au/Co/Au trilayers.

We present the neutron resonance spinecho-spectrometer (NRSE) that was recently installed at the LLB-Saclay. Brief quantum mechanical descriptions of the spectrometer principle and of the main components, the RF-flippers, are given. We also discuss the technological details of these flippers. The spinecho-curves clearly demonstrate the good performance of the instrument. A first quasielastic neutron scattering measurement on the structural relaxations of the glass former deuterated glycerol shows the high flexibility and reliability of the new spectrometer, but also indicates the need to increase the primary intensity, planned in near future.