Erik B. Iverson

The new cold neutron chopper spectrometer at the Spallation Neutron Source: Design and performance

The design and performance of the new cold neutron chopper spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge are described. CNCS is a direct-geometry inelastic time-of-flight spectrometer, designed essentially to cover the same energy and momentum transfer ranges as IN5 at ILL, LET at ISIS, DCS at NIST, TOFTOF at FRM-II, AMATERAS at J-PARC, PHAROS at LANSCE, and NEAT at HZB, at similar energy resolution. Measured values of key figures such as neutron flux at sample position and energy resolution are compared between measurements and ray tracing Monte Carlo simulations, and good agreement (better than 20% of absolute numbers) has been achieved. The instrument performs very well in the cold and thermal neutron energy ranges, and promises to become a workhorse for the neutron scattering community for quasielastic and inelastic scattering experiments.

The Spallation Neutron Source High Power Target Station Moderator Performance: Calculations and Studies

We address recent progress in the continued neutronic design of the Spallation Neutron Source Target Station as regards moderator performance. The Spallation Neutron Source Target Station will receive 2 MW of 1 GeV protons at 60 Hz. This level of proton power offers unprecedented neutronic performance for pulsed neutron production for time-of-flight neutron scattering, as well as unprecedented challenges in providing such a performance. We report on the results of recent design optimization studies and the importance of carefully matching moderator performance characteristics to the needs of the neutron scattering instruments.

Time focusing of pulsed-source crystal analyzer spectrometers. Part II: practical expressions

We develop practical expressions for the focusing conditions of fully general crystal analyzer inelastic neutron scattering instruments having two-dimensionally extended sources (moderators), samples, analyzer crystals, and detectors. We show their relationships to focusing conditions for powder diffraction and single-crystal diffraction instruments. We demonstrate that geometries other than the traditional backscattering arrangement, using more general sample, analyzer, and detector orientation angles, can provide high resolution and offer potential advantages in design. The results apply to the use of mosaic as well as perfect single-crystal analyzers.

Fundamental neutron physics beamline at the spallation neutron source at ORNL

Abstract We describe the Fundamental Neutron Physics Beamline (FnPB) facility located at the Spallation Neutron Source at Oak Ridge National Laboratory. The FnPB was designed for the conduct of experiments that investigate scientific issues in nuclear physics, particle physics, astrophysics and cosmology using a pulsed slow neutron beam. We present a detailed description of the design philosophy, beamline components, and measured fluxes of the polychromatic and monochromatic beams.

Hydrogen adsorption on two catalysts for the ortho- to parahydrogen conversion: Cr-doped silica and ferric oxide gel

Molecular hydrogen exists in two spin-rotation coupled states: parahydrogen and orthohydrogen. Due to the variation of energy with rotational level, the occupation of ortho- and parahydrogen states is temperature dependent, with parahydrogen being the dominant species at low temperatures. The equilibrium at 20 K (99.8% parahydrogen) can be reached by natural conversion only after a lengthy process. With the use of a suitable catalyst, this process can be shortened significantly. Two types of commercial catalysts currently being used for ortho- to parahydrogen conversion are: iron(iii) oxide (Fe2O3, IONEX®), and chromium(ii) oxide doped silica catalyst (CrO·SiO2, OXISORB®). We investigate the interaction of ortho- and parahydrogen with the surfaces of these ortho-para conversion catalysts using neutron vibrational spectroscopy. The catalytic surfaces have been characterized using X-ray absorption fine structure (XAFS) and X-ray/neutron pair distribution function measurements.

Characterization of the radiation background at the Spallation Neutron Source

We present a survey of the radiation background at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, TN, USA during routine daily operation. A broad range of detectors was used to characterize primarily the neutron and photon fields throughout the facility. These include a WENDI-2 extended range dosimeter, a thermoscientific NRD, an Arktis 4He detector, and a standard NaI photon detector. The information gathered from the detectors was used to map out the neutron dose rates throughout the facility and also the neutron dose rate and flux profiles of several different beamlines. The survey provides detailed information useful for developing future shielding concepts at spallation neutron sources, such as the European Spallation Source (ESS), currently under construction in Lund, Sweden.

A neutron sensitive microchannel plate detector with cross delay line readout

Microchannel plates containing neutron absorbing elements such as boron and gadolinium in the bulk glass are used as the sensing element in high spatial resolution, high rate neutron imaging systems. In this paper we describe one such device, using both 10B and natural Gd, which employs cross delay line signal readout, with time-of-flight capability. This detector has a measured spatial resolution under 40 μm FWHM, thermal neutron efficiency of 19%, and has recorded rates in excess of 500 kHz. A physical and functional description is presented, followed by a discussion of measurements of detector performance and a brief survey of some practical applications.

The Monte Carlo simulation of neutron transmitted and scattered by disk choppers of various compositions.

Abstract We consider the transmission of neutrons through disk-type neutron choppers, considering both the uncollided neutron fraction (φu) and scattered neutron fraction (φs). We have computed φu, φs, and the ratio φs/φu through plates of five different absorber materials of various thicknesses to give information for selecting optimum materials and thicknesses.

The upgrade of intense pulsed neutron source (IPNS) through the change of coolant and reflector

Abstract The current intense pulsed neutron source (IPNS) depleted uranium target is cooled by light water. The inner reflector material is graphite and the outer reflector material is beryllium. The presence of H 2 O in the target moderates neutrons and leads to a higher absorption loss in the target than is necessary. D 2 O coolant in the small quantities required minimizes this effect. We have studied the possible improvement in IPNS beam fluxes that would result from changing the coolant from H 2 O to D 2 O and the inner reflector from graphite to beryllium. Neutron intensities were calculated for directions normal to the viewed surface of each moderator for four different cases of combinations of target coolant and reflector materials. The simulations reported here were performed using the MCNPX (version 2.1.5) computer program. Our results show that substantial gains in neutron beam intensities can be achieved by appropriate combination of target coolant and reflector materials. The combination of D 2 O coolant and beryllium inner and outer reflectors improves facility performance about 1.3 times. The purpose of this summary is to report our simulation and to recommend to change target coolant and inner reflector materials based on our simulation results.

A sample holder for simultaneous Raman and neutron vibrational spectroscopy

We have built a sample holder (called a center stick or sample stick) for performing simultaneous Raman and neutron vibrational spectroscopy on samples of material at the VISION neutron vibrational spectrometer of the Spallation Neutron Source at Oak Ridge National Laboratory. This equipment holds material samples in the neutron beam within the cryogenic environment of the VISION spectrometer, allowing for samples to be studied at temperatures as low as 5 K. It also provides the capability for gas to be loaded to or evacuated from the sample while it is loaded at VISION. The optical components for directing and filtering light are located within the cryogenic volume, in physical proximity to the sample. We describe the construction of this sample holder and discuss our first measurements of simultaneous Raman and neutron vibrational spectra. The samples that we report on were of 4-nitrophenol at a temperature of 20 K and of cryogenic hydrogen of a number of different orthohydrogen fractions.