Resonance Detector Spectrometer Research Articles

Enhancing the performances of a resonance detector spectrometer for deep inelastic neutron scattering measurements

The possibility is explored to sum up neutron Compton profiles at different scattering angles in deep inelastic neutron scattering measurements within the Resonance Detector (RD) configuration to enhance the statistics for a more reliable extraction of the momentum distribution of the constituents in the target. The RD configuration allows to select the energy of the scattered neutrons up to several tens of electron Volt, thus accessing energy and wave vector transfers well above 1eV and 30Å−1, respectively. In the high-q/ω regime, the final state effects could be considered as negligible, as shown in a series of simulations using a Monte Carlo method with different inverse geometry instrument setups. The simulations show that it could be possible to conceive an instrument set up where the RD configuration allows the proper summation of several spectra at different scattering angles, providing a good separation of the proton recoil signal from that of the heavier atoms, thus avoiding the cell subtraction by fitting procedure.

Cadmium–Zinc–Telluride photon detector for epithermal neutron spectroscopy—pulse height response characterisation

The Resonance Detector Spectrometer was recently revised for neutron spectroscopic studies in the eV energy region. In this technique one makes use of a photon detector to record the gamma emission from analyser foils used as neutron-gamma converters. The pulse-height response of a Cadmium–Zinc–Telluride photon detector to neutron capture emission from 238 U and 197 Au analyser foils was characterised in the neutron energy range 1– 200 eV . The experiment was performed on the VESUVIO spectrometer at the ISIS neutron-pulsed source. A biparametric data acquisition, specifically developed for these measurements, allowed the simultaneous measurements of both the neutron time of flight and γ pulse-height spectra. Through the analysis of the γ pulse-height spectra the main components of the signal associated with resonant and non-resonant neutron absorption were identified. It was also shown that, in principle, energy discrimination can be used to improve the signal to background ratio of the neutron time-of-flight measurement.

Photon detectors for epithermal neutron scattering at high- ω and low- q

Inelastic epithermal neutron scattering at high energy ( ℏω⩾1 eV ) and low wave vector ( q⩽10 A ̊ −1 ) transfers is the unique technique for the investigation of high-energy excitations in a variety of systems, ranging from magnetic materials to semiconductors. The key issue in order to make these measurements feasible on inverse geometry spectrometers, is to develop suitable detection systems for neutrons in the energy range 1–100 eV . The Resonance Detector Spectrometer configuration has to be considered as the most promising approach for electron Volt neutron spectroscopy. This configuration will be employed in the new low angle detector bank, VLAD, planned for VESUVIO spectrometer operating at ISIS source.

Electron-volt spectroscopy at a pulsed neutron source using a resonance detector technique

The effectiveness of the neutron resonance detector spectrometer for deep inelastic neutron scattering measurements has been assessed by measuring the Pb scattering on the eVS spectrometer at ISIS pulsed neutron source and natural U foils as (n,γ) resonance converters. A conventional NaI scintillator with massive shielding has been used as γ detector. A neutron energy window up to 90 eV, including four distinct resonance peaks, has been assessed. A net decrease of the intrinsic width of the 6.6 eV resonance peak has also been demonstrated employing the double difference spectrum technique, with two uranium foils of different thickness.

Detectors for neutron scattering in the eV region

With the advent of high-current high-energy proton accelerators used as pulsed neutron sources for condensed matter studies, interest in extending neutron scattering measurements to the eV region has increased. Among the instruments proposed is the resonance-detector spectrometer (RDS) which uses the sharp low-energy nuclear resonances (usually capture) to define the energy of the scattered neutron. These spectrometers require detectors with high efficiency and good background rejection. Two promising candidates for such detectors, a bismuth germanate (BGO) scintillator and a high-purity germanium (HPGe) detector have been tested at the NBS neutron time-of-flight facility. These tests used simple recoil scattering at high scattering angles and several different resonance foils. Efficiencies and signal to background ratios were measured.

Determination of the momentum distribution of scattering particles from high- Q scattering spectra of a resonance detector neutron spectrometer

We have measured high- Q neutron scattering spectra from various crystaline solids using an eV resonance detector spectrometer, and analyzed the different contributions to the line profile, namely the finite time spread of the neutron pulse, the detector resolution, the finite size of the moderator, sample and detector, the scattering function and multiple scattering processes, by first-principle calculations. The calculated spectra agree well with the experiments. The momentum distribution of scattering particles can be determined from the high- Q limit of the scattering function. However, deviations from this asymptotical form are clearly observed even for momentum transfers around 100 Å −1.

Experimental study of the time-focussing effect in a resonance detector neutron spectrometer

In a real resonance detector spectrometer, the finite size of the neutron beam, the sample and the detector leads to a spread in the neutron time-of-flight. The theory for the time-focussing effect predicts that this spread can be compensated through the recoil shift of the neutron energy, if the moderator, sample and detector are properly aligned. We have studied this effect experimentally with lithium, berylliu, carbon and vanadium. Our data are in good agreement with the theory.

Detector resolution in a resonance detector neutron spectrometer

We have studied the effect of foil temperature and chemical bonding on the energy resolution of an eV neutron resonance detector spectrometer using Sb, Sm, Ho, Ta, and U as resonance atoms. Considerable improvement in the energy resolution can be obtained by cooling the resonance foils. The experimental data for the uranium compound foil show that cooling is effective even below 25 K. Unlike the Mössbauer effect, it is not possible to find a chemical compound of the resonance atoms for which the resonance line width is reduced to its intrinsic value. A difference method is also described to improve the energy resolution.

Neutron compton spectroscopy of pyrolitic graphite

The anisotropic momentum distribution of carbon atoms in pyrolitic graphite has been determined by neutron scattering at high momentum transfer with a resonance detector spectrometer. Our experiments demonstrate the capability of the RDS technique at a spallation neutron source for neutron Compton spectroscopy.

A resonance detector spectrometer at KENS

We have tested a resonance detector spectrometer at the KENS neutron source, using 181Ta, 121Sb and 149Sm resonances and bismuth germanate (BGO) scintillators. In the process we encountered and solved numerous background problems, and discovered a time-focussing principle. We measured the scattering from a number of materials and have so far analyzed results for bismuth, vanadium and graphite, which we present. Tests of cooled absorbers indicate that resolution of 70 meV is possible with 181Ta.

The spin precession resonance spectrometer

We present the concept of a new neutron scattering instrument to be used at a pulsed neutron source. The machine, which starts as a development of the resonance detector spectrometer and incorporates the basic ideas of spin precession energy analysis, is suited for the study of medium-and high-energy excitations in solids and liquids, particularly at small momentum transfers. The most novel characteristic of its performance is a high resolving power, achieved without appreciable loss of neutron flux. In an alternative operation (and sacrificing resolution) the instrument is capable of separating spin-dependent from spin-independent scattering processes.