Spin filters and supermirrors: a comparison study of two methods of high-precision neutron polarisation analysis

The precise knowledge of the neutron polarisation is needed in tests of the electroweak Standard Model using angular correlations in polarised neutron beta decay. We performed an experimental comparison study of two different methods of polarisation analysis of a cold neutron beam which are based on spin-dependent reflection, respectively, transmission. The compared devices are a supermirror analyser used in prior neutron decay studies and an opaque transmission spin filter of either polarised ³He or polarised protons. The results of the neutron polarisation measured with the supermirror analyser and with the spin filter coincided in three different experiments within 0.1–0.2%.     

The resolution function of a pulsed-source neutron chopper spectrometer

We introduce a formulation for the evaluation of the incident neutron intensity distribution for a chopper time-of-flight spectrometer at a pulsed neutron source. This treatment, incorporated with an assumed scattering function of the sample and the detector geometry, enables calculations of the shape of the time-of-flight intensity profiles of the incident and the scattered neutrons sensed by a neutron detector, thus providing direct comparison with experimental results. The resolution function, R(Q, E), is calculated for a nondispersive scatterer at a resonant energy E. The results of the calculations on the basis of this theory are substantiated by measured spectra obtained by the two chopper spectrometers, HRMECS and LRMECS, at the Argonne Intense Pulsed Neutron Source under a variety of experimental conditions. In all cases we find excellent agreement between calculations and experiments. Using these results we present a procedure for the determination of the mean incident neutron energy and the calibration of the energy-transfer scale for pulsed-source chopper spectrometers. These latter do not follow accurately from simple analysis, and are the main objects of this paper.     

Crystal field effects using polarised neutron spectroscopy

Crystal field transitions and quasielastic magnetic scattering were observed in PrAl₃ using a polarised neutron diffractometer/spectrometer. These were positively identified using neutron polarisation analysis. Transitions were observed at 3.5 and 4.5 meV, and both magnetic and nuclear elastic scattering were successfully separated.     

T Violation in the weak scalar and tensor interaction: neutron and nuclei

The motivation and status of a search for time-reversal violation in nuclear and neutron beta decay are discussed. A new experiment for free neutron decay is proposed. A hitherto unmeasured amplitude, R, of the directional correlation J·( p×σ), between the neutron spin J, the electron momentum p and the electron spin σ, will be determined. An accuracy well below 0.01 can be achieved using an intense cold neutron beam and an electron tracking detector, where the spin sensitivity is provided by large angle Mott scattering. This study provides an unique access to the exotic scalar S and tensor T interaction. Finite values, or tight constraints on the time-reversal violating scalar components, can be deduced in a combined analysis of the proposed experiment and a precise determination of the tensor couplings from a recent study of ⁸Li decay. The great interest in weak scalar interaction is stimulated by a multitude of scalar bosons which are introduced in most extensions of the Standard Model.     

Studies of materials using polarised neutrons

The scattering cross-sections of a polarised neutron beam, incident on a magnetic sample, are a function of the polarisation of the beam, and of the nuclear and magnetic structure of the sample. An analysis of the polarisation of the scattered neutrons is valuable in isolating magnetic contributions. The spin distribution, obtained by Fourier inversion of the magnetic structure factors calculated from the cross-sections, has been studied in ferromagnetic metals, intermetallic compounds and alloys. Applications of the method to the study of anti-ferromagnets and canted ferromagnets are outlined. Paramagnetic metals, solid organic radicals and transuranium elements have spin distributions in which orbital and other effects can be compared with theory. The value of polarisation analysis and some of the new technical developments which have made it possible are described. Applications to inelastic phenomena are mentioned.     

Neutron energy spectrum measurements of neutron sources with an NE-213 spectrometer

The neutron energy spectra of the following sources were measured using a fast-neutron spectrometer with NE-213 liquid scintillator: ²⁵²Cf, Am-Be and T(d, n)⁴He from a Van de Graaff accelerator (400 keV). The measured proton recoil pulse-height data were unfolded using the FANTI code to obtain the neutron energy spectrum. The spectrometer gives neutron spectra in the range of 2–16 MeV, with 6% intrinsic efficiency and a resolution between 4% and 11%. The ²⁵²Cf neutron energy spectrum was measured and the results obtained showed good agreement with the spectrum usually published in the literature, which can be fitted by the expression N(E) =√E exp(− E/T) with the constant T = 1.42 MeV.     

Spin-dependent tunnelling in La0.7Sr0.3MnO3/SrTiO3 superlattices

La_0.7Sr_0.3MnO₃ is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La_0.7Sr_0.3MnO₃/SrTiO₃ epitaxial superlattices were grown on LaAlO₃(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La_0.7Sr_0.3MnO₃ and SrTiO₃. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I–V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I–V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.     

Spectator detection for the measurement of proton–neutron interactions at ANKE

A telescope of three silicon detectors has been installed close to the internal target position of the ANKE spectrometer, which is situated inside the ultra-high vacuum of the COSY-Jülich light-ion storage ring. The detection and identification of slow protons and deuterons emerging from a deuterium cluster-jet target thus becomes feasible. A good measurement of the energy and angle of such a spectator proton (p_sp) allows one to identify a reaction as having taken place on the neutron in the target and then to determine the kinematical variables of the ion–neutron system on an event-by-event basis over a range of c.m. energies.

The system has been successfully tested under laboratory conditions. By measuring the spectator proton in the pd→p_spdπ⁰ reaction in coincidence with a fast deuteron in the ANKE Forward Detector, values of the pn→dπ⁰ total cross-section have been deduced. Further applications of the telescope include the determination of the luminosity and beam polarisation which are required for several experiments.     

A low momentum tagged antineutron beam

The operating performances of a low-momentum (< 270 MeV/c) tagged beam are reported. The beam is obtained by means of the charge exchange reaction n on a liquid hydrogen target. The neutron associated to the in the two-body reaction is used for the determination of the energy and direction. The measured total rate of tagged is 8.02 ± 0.03 × 10⁻⁵ per incident at 300 MeV/c.     

Shielding design for the PANDA spectrometer at the Munich high-flux reactor FRM-II

The start-up of the Munich high-flux reactor FRM-II is in progress on. At the beam tube SR-2 the spectrometer PANDA has been installed. It is at three-axis neutron spectrometer looking onto a slightly under-moderated cold neutron source. For polarisation analysis, PANDA is equipped with a vertical cryomagnet producing fields up to 14.5 T for the sample. To get an appropriate shielding of the high-intensity instrument, one has to take into account the large cross section of the primary beam, several restrictions using magnetic materials, limitations in loading the site and finally, has to keep the lateral extent of the shielding small to allow for high-scattering angles. The shielding has been designed on the basis of the results, which were achieved by the combined use of both the Monte Carlo code MCNP-4B2 and an analytical method based on one-dimensional dose transmission functions.     

Ultra-High Resolution Inelastic Neutron Scattering

Two types of ultra high energy resolution neutron scattering instruments, the backscattering spectrometer and the spin echo spectrometer, are described. Examples of the types of research which can be done with these instruments are given and plans for a cold neutron backscattering spectrometer which will be built in the NIST Cold Neutron Research Facility (CNRF) are discussed. It is hoped that this information will be of use to researchers considering neutron scattering experiments at NIST.     

Calibration of the electron volt spectrometer, a deep inelastic neutron scattering spectrometer at the ISIS pulsed neutron spallation source

The electron Volt Spectrometer (eVS) is an inverse geometry filter difference spectrometer that has been optimised to measure the single atom properties of condensed matter systems using a technique known as Neutron Compton Scattering (NCS) or Deep Inelastic Neutron Scattering (DINS). The spectrometer utilises the high flux of epithermal neutrons that are produced by the ISIS neutron spallation source enabling the direct measurement of atomic momentum distributions and ground state kinetic energies. In this paper the procedure that is used to calibrate the spectrometer is described. This includes details of the method used to determine detector positions and neutron flight path lengths as well as the determination of the instrument resolution. Examples of measurements on 3 different samples are shown, ZrH₂, ⁴He and Sn which show the self-consistency of the calibration procedure.     

Low-energy neutron spectrometer using position sensitive proportional counter—Feasibility study based on numerical analysis

There is no direct technique to measure a neutron energy spectrum, particularly in the lower energy region, because the reaction Q value for detection is much larger than the neutron energy to be measured. However, such techniques are becoming a necessity, for example, in medical applications such as boron neutron capture therapy. In this study, a new spectrometer to measure low-energy neutrons (from thermal to 100 eV) is investigated numerically. We propose a unique approach of estimating the neutron energy spectrum by analyzing the distribution of neutron detection depths in the detector using an exact relation between the neutron energy and nuclear reaction cross-section. The proposed spectrometer has been established to be feasible to manufacture. The conversion performance of the neutron detection depth distribution to the neutron energy spectrum has also been proven to be acceptable, with the unfolding process based on Bayes’ theorem, even though the detector response function is non-distinctive (without peaks or edges). The present spectrometer is now under development, and its practical performance will be reported as soon as the prototype detector is completed.     

Test of He-based neutron polarizers at NIST

Neutron spin filters based on polarized ³He are useful over a wide neutron energy range and have a large angular acceptance among other advantages. Two optical pumping methods, spin-exchange and metastability-exchange, can produce the volume of highly polarized ³He gas required for such neutron spin filters. We report a test of polarizers based on each of these two methods on a new cold, monochromatic neutron beam line at the NIST Center for Neutron Research.     

Free neutron decay: a review of the contemporary situation

Experimental data on neutron decay parameters are reviewed. These are the neutron lifetime and various angular correlations between spin and momenta of the particles involved in neutron decay. The restrictions on scalar and tensor terms in the weak Hamiltonian, as well as possible admixtures by V+A currents, resulting from neutron decay experiments, are discussed. Reasonable agreement with the predictions of the V−A theory can be certified.     

The calibration of slow neutron polarised beam instruments

A new procedure for calibrating the efficiencies of the polarisation sensitive components in slow neutron polarised beam instruments is described. The measurement of one “flipping ratio” and two “shim ratios” allows the effective polarisation, the spin flipping efficiency, and a depolarising shim efficiency to be determined unambiguously. Results of its application to the calibration of a neutron time-of-flight polarised beam reflectometer are presented.     

Performance test of neutron resonance spin echo at a pulsed source

Neutron resonance spin echo (NRSE) spectroscopy enables us to measure neutron quasielastic scattering with high-energy resolution. It is desirable to apply NRSE spectroscopy to pulsed neutron sources because this application allows a very wide range of the spin echo time. We have already developed the neutron resonance spin flipper applicable to a polychromatic pulsed neutron beam, which is necessary for the TOF–NRSE method. Using this flipper, we have succeeded in observing spin echo signals with visibility higher than 0.65 for the pulsed neutron beam with wavelength from 0.3 to 0.9 nm. We discuss the prospect of the NRSE spectrometer with high-energy resolution on the basis of the present result.     

Observation of a triple correlation in ternary fission: is time reversal invariance violated?

In ternary fission, besides the two main fission fragments, a third (usually light) charged particle is emitted. A triple correlation has been studied involving the momenta for a specific fission fragment , the momenta of the ternary particle and the spin of the polarized cold neutron inducing fission . The correlation observable reverses sign upon time reversal and thus a non-vanishing value for the expectation value B could possibly be due to TRI being violated. However, final-state interactions or specific properties of the emission mechanism for ternary particles could equally well lead to a non-zero B with TRI being perfectly conserved. The reaction chosen was ²³³U(n,f). An unexpectedly large correlation was observed. From the raw data the value for B is B=−(0.78±0.02)×10⁻³ with the sign corresponding to light fragments. Corrections for neutron polarization, geometric efficiency, resolution of detectors and background increase this figure by a factor of (1.5±0.3).     

The high-resolution time-of-flight spectrometer TOFTOF

The TOFTOF spectrometer is a multi-disc chopper time-of-flight spectrometer for cold neutrons at the research neutron source Heinz Maier-Leibnitz (FRM II). After five reactor cycles of routine operation the characteristics of the instrument are reported in this article. The spectrometer features an excellent signal to background ratio due to its remote position in the neutron guide hall, an elaborated shielding concept and an s-shaped curved primary neutron guide which acts i.a. as a neutron velocity filter. The spectrometer is fed with neutrons from the undermoderated cold neutron source of the FRM II leading to a total neutron flux of 10¹⁰n/cm²/s in the continuous white beam at the sample position distributed over a continuous and particularly broad wavelength spectrum. A high energy resolution is achieved by the use of high speed chopper discs made of carbon-fiber-reinforced plastic. In the combination of intensity, resolution and signal to background ratio the spectrometer offers new scientific prospects in the fields of inelastic and quasielastic neutron scattering.     

Growth and properties of the CuInS2 thin films produced by glancing angle deposition

We use the glancing angle deposition technique (GLAD) to grow CuInS₂ thin films by a vacuum thermal method onto glass substrates. During deposition, the substrate temperature was maintained at 200 °C. Due to shadowing effect the oblique angle deposition technique can produce nanorods tilted toward the incident deposition flux. The evaporated atoms arrive at the growing interface at a fixed angle θ measured from the substrate normal. The substrate is rotated with rotational speed ω fixed at 0.033 rev s^− 1. We show that the use of this growth technique leads to an improvement in the optical properties of the films. Indeed high absorption coefficients (10⁵–3.10⁵ cm^− 1) in the visible range and near-IR spectral range are reached. In the case of the absence of the substrate rotation, scanning electron microscopy pictures show that the structure of the resulting film consists of nanocolumns that are progressively inclined towards the evaporation source as the incident angle was increased. If a rapid azimuthal rotation accompanies the substrate tilt, the resulting nanostructure is composed of an array of pillars normal to the substrate. The surface morphology show an improvement without presence of secondary phases for higher incident angles (θ > 60°).