The Fundamental Neutron Physics Beamline at the Spallation Neutron Source

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory with an anticipated start-up in early 2006, will provide the most intense pulsed beams of cold neutrons in the world. At a projected power of 1.4 MW, the time averaged fluxes and fluences of the SNS will approach those of high flux reactors. One of the flight paths on the cold, coupled moderator will be devoted to fundamental neutron physics. The fundamental neutron physics beamline is anticipated to include two beam-lines; a broad band cold beam, and a monochromatic beam of 0.89 nm neutrons for ultracold neutron (UCN) experiments. The fundamental neutron physics beamline will be operated as a user facility with experiment selection based on a peer reviewed proposal process. An initial program of five experiments in neutron decay, hadronic weak interaction and time reversal symmetry violation have been proposed.     

LENS: A New Pulsed Neutron Source for Research and Education

A new pulsed neutron source is under construction at the Indiana University Cyclotron Facility (IUCF). Neutrons are produced via (p,n) reactions by a low-energy proton beam incident on a thin beryllium target. The source is tightly coupled to a cold methane moderator held at a temperature of 20 K or below. The resulting time-averaged cold neutron flux is expected to be comparable to that of the Intense Pulsed Neutron Source (IPNS) facility at Argonne National Laboratory. The initial experimental suite will include instrumentation for small angle neutron scattering (SANS), moderator studies, radiography, and zero-field spin-echo SANS.     

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.     

New aspects for high-intensity neutron beam production

Neutron scattering is an important tool for the investigation of static and dynamic structures of matter. As it is an intensity limited technique, many attempts have been made to increase the effective beam intensity. High neutron intensities or, more precisely, high phase space densities of neutrons can be obtained at low energies only. Such ultra-cold neutrons can be trapped inside material and magnetic bottles. When neutrons of such densities become up-scattered, highly intense, monochromatic and pulsed beams can be produced, whose intensities can overcome limitations imposed by the classical neutron source strength. We report a recent experiment that demonstrated this alternative, to our knowledge, for the first time ever. Perspectives resulting from this development of highly intense neutron beam production will be discussed. A stationary ultra-cold neutron gas produced becomes transformed into a pulsed and monochromatic cold neutron beam.     

Facilities for Fundamental Neutron Physics Research at the NIST Cold Neutron Research Facility

The features of two fundamental neutron physics research stations at the NIST cold neutron research facility are described in some detail. A list of proposed initial experimental programs for these two stations is also given.     

Investigation into the Mechanical Cause of Failure of Neutron Guide NG-2 at the NIST Research Reactor

This study investigates the fractures that occurred in a glass structure used to carry neutrons from a cold source in the NIST reactor into an experimental hall that contains neutron-scattering instrumentation used to perform experiments in chemistry, materials science, physics, and biology. These guides are typically made of rectangular borosilicate glass tubes, coated on the inside with a neutron-reflecting coating. The guide tubes used at the NIST Center for Neutron Research (NCNR) have internal cross sections of dimensions 150?×?60?mm, with lengths extending over as great as 60?m, with gaps for insertion of the instruments used to evaluate materials. On August 23, 2011, a 5.8 magnitude earthquake occurred in Mineral, Virginia, which resulted in significant ground motion over 150?km away at the NCNR. An initial inspection and vacuum test revealed no significant damage to the seven neutron beam lines. After a few weeks, neutron guide 2 (NG-2) that was located in the Guide Hall near the reactor building wall cracked and broke while being evacuated. The cause of fracture was identified by observation of the glass fragments and analysis of the stress distributions in the guide. The delayed fracture was caused by damage introduced during the earthquake.     

The NIST Cold Neutron Research Facility

The Cold Neutron Research Facility (CNRF) at the National Institute of Standards and Technology (NIST) Research Reactor (NBSR) is now coming on line, with the first seven experimental stations operational, and more stations scheduled to be installed during 1992. The present article provides an introduction to the facility, and to other articles in the current issue that give more details on some of the research opportunities that the facility will bring to NIST.     

New Pulsed Cold Neutron Beam Line for Fundamental Nuclear Physics at LANSCE

The NPDGamma collaboration has completed the construction of a pulsed cold neutron beam line on flight path12 at the Los Alamos Neutron Science Center (LANSCE). We describe the new beam line and characteristics of the beam. We report results of the moderator brightness and the guide performance measurements. FP12 has the highest pulsed cold neutron intensity for nuclear physics in the world.     

Detecting the Radiative Decay Mode of the Neutron

Beta decay of the neutron into a proton, electron, and electron antineutrino is occasionally accompanied by the emission of a photon. Despite decades of detailed experimental studies of neutron beta-decay, this rare branch of a fundamental weak decay has never been observed. An experiment to study the radiative beta-decay of the neutron is currently being developed for the NG-6 fundamental physics endstation at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The experiment will make use of the existing apparatus for the NIST proton-trap lifetime experiment, which can provide substantial background reduction by providing an electron-proton coincidence trigger. Tests and design of a detector for gamma-rays in the 10 keV to 200 keV range are under development. The need for a large solid-angle gamma-ray detector that can operate in a strong magnetic field and at low temperature has led us to consider scintillating crystals in conjunction with avalanche photodiodes. The motivation and experimental technique will be discussed.     

Neutron Depth Profiling: Overview and Description of NIST Facilities

The Cold Neutron Depth Profiling (CNDP) instrument at the NIST Cold Neutron Research Facility (CNRF) is now operational. The neutron beam originates from a 16 L D₂O ice cold source and passes through a filter of 135 mm of single crystal sapphire. The neutron energy spectrum may be described by a 65 K Maxwellian distribution. The sample chamber configuration allows for remote controlled scanning of 150 × 150 mm sample areas including the varying of both sample and detector angle. The improved sensitivity over the current thermal depth profiling instrument has permitted the first nondestructive measurements of ¹⁷O profiles. This paper describes the CNDP instrument, illustrates the neutron depth profiling (NDP) technique with examples, and gives a separate bibliography of NDP publications.     

Parity-Violating Neutron Spin Rotation in a Liquid Parahydrogen Target

Our understanding of hadronic parity violation is far from clear despite nearly 50 years of theoretical and experimental progress. Measurements of low-energy parity-violating observables in nuclear systems are the only accessible means to study the flavor-conserving weak hadronic interaction. To reduce the uncertainties from nuclear effects, experiments in the few and two-body system are essential. The parity-violating rotation of the transverse neutron polarization vector about the momentum axis as the neutrons traverse a target material has been measured in heavy nuclei and few nucleon systems using reactor cold neutron sources. We describe here an experiment to measure the neutron spin-rotation in a parahydrogen target (n-p system) using pulsed cold-neutrons from the fundamental symmetries beam line at the Spallation Neutron Source under construction at the Oak Ridge National Laboratory.     

An acceptance diagram analysis of the contaminant pulse removal problem with direct geometry neutron chopper spectrometers

Phased choppers are used to produce pulsed beams of monochromatic neutrons at research reactors and spallation neutron sources. Depending on the design of the instrument, it is very possible that the choppers will transmit neutrons with wavelengths other than those within the desired band of wavelengths. One or more additional choppers are typically needed to remove these contaminant pulses. We describe a method of determining the wavelength- and time-dependent transmission of a system of choppers using acceptance diagrams. The method is illustrated with calculations for the Disk Chopper Spectrometer at the NIST Center for Neutron Research and the proposed Cold Neutron Chopper Spectrometer at the Spallation Neutron Source (Oak Ridge, TN).     

Advantages and limitations of nuclear physics experiments at an ISIS-class spallation neutron source

Nuclear physics experiments have a long history of being conducted on spallation neutron sources. Like other experiments, these measurements take advantage of the identification of the incident neutron energy by the time-of-flight (ToF) technique. However, in some ways these experiments are often in direct conflict with other experiments. Especially in large (ISIS or SNS class) facilities, the design of the source often reflects a compromise between different experimental needs and requirements. It has been a long standing question for nuclear physics experiments how limiting these compromises are and how they can be dealt with. We have therefore calculated the incident neutron energy spectrum, along with the gamma background spectrum, for flight path (FP) 5 at the Los Alamos Neutron Science Center (LANSCE) Manuel Lujan Jr. Neutron Scattering Center (Lujan Center) including a detailed evaluation of the signal shape. We will discuss the advantages and limitations of the nuclear physics experiments at FP-5 in the light of our results.     

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.     

Beamline Performance Simulations for the Fundamental Neutron Physics Beamline at the Spallation Neutron Source

Monte Carlo simulations are being performed to design and characterize the neutron optics components for the two fundamental neutron physics beamlines at the Spallation Neutron Source. Optimization of the cold beamline includes characterization of the guides and benders, the neutron transmission through the 0.89 nm monochromator, and the expected performance of the four time-of-flight choppers. The locations and opening angles of the choppers have been studied using a simple spreadsheet-based analysis that was developed for other SNS chopper instruments. The spreadsheet parameters are then optimized using Monte Carlo techniques to obtain the results presented in this paper. Optimization of the 0.89 nm beamline includes characterizing the double crystal monochromator and the downstream guides. The simulations continue to be refined as components are ordered and their exact size and performance specifications are determined.     

Neutron science and technology on J-PARC

This paper has briefly reviewed a history of neutron sources in Japan and highlighted the 1 MW JSNS (Japan Spallation Neutron Source), which is a central facility of the multi-purposed J-PARC (Japan Proton Accelerator Research Complex) to be completed in 2008. JSNS will provide worldwide users with most intense pulsed neutron beams available for a wide variety of fields ranging from fundamental research of material/life sciences to industrial/medical applications to open up a new era of science and technology.     

The Spallation Neutron Source (SNS) project: a fertile ground for radiation protection and shielding challenges

The Spallation Neutron Source facility presently under construction in the USA consists of a front end, a linac, an accumulator ring, a target station and a neutron instrument hall, producing pulsed neutron beams driven by a proton beam of 1 GeV energy and 1.4 MW power with a repetition rate of 60 Hz. The layout of the facility and the radiation protection and shielding concept of the facility is laid out in numerous examples in a walk from the proton beam generation to the neutron utilisation.     

Analytical shielding calculations for a proton therapy facility

The University of Pennsylvania is building a proton therapy facility in collaboration with Walter Reed Army Medical Center. The proposed facility has four gantry rooms, a fixed beam room and a research room, and will use a cyclotron as the source of protons. In this study, neutron shielding considerations for the proposed proton therapy facility were investigated using analytical techniques and Monte Carlo simulated neutron spectra. Neutron spectra calculations were done using the GEANT4 (v6.2) simulation code for various materials: water, carbon, iron, nickel and tantalum to estimate the neutron production at proton beam energies of 100, 175 and 250 MeV. Dose equivalent calculations were performed using analytical methods at various critical points within the facility, by folding the GEANT4 produced neutron spectra with dose equivalent rate data from the National Council on Radiation Protection and Measurements (NCRP) Report #144.     

Experimental and numerical characterization of the neutron field produced in the n@BTF Frascati photo-neutron source

A photo-neutron irradiation facility is going to be established at the Frascati National Laboratories of INFN on the base of the successful results of the n@BTF experiment. The photo-neutron source is obtained by an electron or positron pulsed beam, tuneable in energy, current and in time structure, impinging on an optimized tungsten target located in a polyethylene-lead shielding assembly. The resulting neutron field, through selectable collimated apertures at different angles, is released into a 100 m² irradiation room. The neutron beam, characterized by an evaporation spectrum peaked at about 1 MeV, can be used in nuclear physics, material science, calibration of neutron detectors, studies of neutron hardness, ageing and study of single event effect. The intensity of the neutron beam obtainable with 510 MeV electrons and its fluence energy distribution at a point of reference in the irradiation room were predicted by Monte Carlo simulations and experimentally determined with a Bonner Sphere Spectrometer (BSS). Due to the large photon contribution and the pulsed time structure of the beam, passive photon-insensitive thermal neutron detectors were used as sensitive elements of the BSS. For this purpose, a set of Dy activation foils was used. This paper presents the numerical simulations and the measurements, and compares their results in terms of both neutron spectrum and total neutron fluence.