V. W. Yuan

Neutron polarization with a polarized 3He spin filter

Abstract We report the first use of a polarized 3 He spin filter to polarize epithermal neutrons. The 3 He was polarized to 70% by spin exchange with optically pumped Rb vapor and had a cross sectional area of 0.65 cm 2 and a thickness of 3 × 10 20 atoms cm −2 of 3 He. Neutron polarization up to 20% at 0.734 eV was produced in an epithermal neutron beam at the Los Alamos Neutron Scattering Center and measured by observing the change in neutron transmission produced by the 3 He polarization and also the helicity dependent transmission for a parity-nonconserving resonance in 139 La.

A transverse phase-space measurement technique for high-brightness, H− beams

Abstract A common method of measuring transverse phase-space distributions in charged-particle beams is to intercept the beam with slits, pinhole plates, or wire grids, and to measure the beam distribution after a drift with a parallel-channel collector or a fluorescent screen. Many of the next generation of accelerator applications (e.g., in heavy-ion fusion and transmutation of nuclear waste) will utilize high-beam currents with small phase-space distributions. The power densities of beams will be too large to permit the interception of entire beams. This paper describes a measurement technique that is applicable to H − beams: a small portion of the beam is separated from the full beam by means of photoneutralization with a laser that is upstream from a sweep magnet. Phase-space measurements are made on only the neutralized beam.

Measurement of the parity violating asymmetry Aγ in n→+p→d+γ

The weak pion-nucleon coupling constant H{sub {pi}}{sup 1} remains poorly determined, despite many years of effort. The recent measurement of the {sup 133}Cs anapole moment has been interpreted to give a value of H{sub {pi}}{sup 1} almost an order of magnitude larger than the limit established in the {sup 18}F parity doublet experiments. A measurement of the gamma ray directional asymmetry A{sub {gamma}} for the capture of polarized neutrons by hydrogen has been proposed at Los Alamos National Laboratory. This experiment will determine H{sub {pi}}{sup 1} independent of nuclear structure effects. However, since the predicted asymmetry is small, A{sub {gamma}} {approximately} 5 x 10{sup {minus}8}, systematic effects must be reduced to < 5 x 10{sup {minus}9}. The design of the experiment will is presented, with an emphasis on the techniques used for controlling systematic errors.

A measurement of the absolute neutron beam polarization produced by an optically pumped 3He neutron spin filter

Abstract The capability of performing accurate absolute measurements of neutron beam polarization opens a number of exciting opportunities in fundamental neutron physics and in neutron scattering. At the LANSCE pulsed neutron source we have measured the neutron beam polarization with an absolute accuracy of 0.3% in the neutron energy range from 40 meV to 10 eV using an optically pumped polarized 3 He spin filter and a relative transmission measurement technique. 3 He was polarized using the Rb spin-exchange method. We describe the measurement technique, present our results, and discuss some of the systematic effects associated with the method.

Experience with the ground test accelerator beam‐measurement instrumentation

During the past two years, the Ground Test Accelerator (GTA) has used a variety of off‐ and on‐line beam diagnostic measurements to understand and verify the transverse and longitudinal phase space characteristics of a 35‐mA, low‐energy (2.5‐ to 3.2‐MeV) H−‐beam. For the transverse phase‐space characterization measurements, a slit and collector device samples of the x−x’ and y−y’ phase space, to determine the transverse emittance and Courant–Snyder parameters. The longitudinal phase‐space data are acquired by a laser neutralization technique developed at Los Alamos know as the laser induced neutralization diagnostics approach (LINDA). The transverse and longitudinal phase‐space centroids of the low‐energy, 425‐MHz‐bunched beam are directly measured using the microstrip probe systems. Beam current and transmission are measured by various toroid systems. Beam‐loss‐detection techniques have just been installed and a non‐interceptive beam‐profile measurement has been commissioned. All of these measurement sys...

Current-mode detector for neutron time-of-flight studies

Abstract A system for the detection of high-intensity neutron bursts with instantaneous rates as high as 10 11 Hz is presented. This system uses a transient digitizer to sample the output voltage of a high-current photomultiplier tube as a function of time. The output voltage is proportional to the number of neutrons striking the detector. This detector is used at the Los Alamos Neutron Scattering Center to study fundamental symmetries. Design considerations, construction details and performance benchmarks are presented.

Ion chamber system for neutron flux measurements

Abstract A helium-filled ion chamber detector for intensity measurements of high-intensity epithermal neutron bursts with instantaneous rates as high as 10 11 Hz is presented. This system consists of an ion chamber to detect a portion of the neutron beam, a current-to-frequency converter and CAMAC scalers to readout the chamber. The chambers and readout electronics have a small temperature sensitivity and have high noise immunity. The statistical precision of the system is measured to be 10 −3 for each neutron beam pulse.

Measurement of longitudinal phase space in an accelerated H− beam using a laser-induced neutralization method

Abstract Laser-induced neutralization of H − ions is a process that can be used to measure the longitudinal phase space of accelerated H − beams. The laser-induced neutralization diagnostic approach (LINDA) measures the longitudinal emittance of an H − beam by photoneutralizing different phase slices of beam microbunches and analyzing the energy distribution of the neutral slices. A LINDA system utilizing a pulsed laser and time-of-flight analysis has successfully measured longitudinal emittance of the 5 MeV H − beam exiting the drift-tube linac of the Los Alamos Accelerator Test Stand. Design considerations associated with the LINDA laser-based emittance measuring system are given. The present LINDA system is described and its limitations are discussed. Experimental results are given from an application of the LINDA system to the measurement of longitudinal emittance growth in a drift space and following insertion into the beamline of beam transport elements comprising a single-arm funnel. A new system is proposed which uses a mode-locked laser and spectrometer to improve resolution and shorten measurement time.

An apparatus and techniques of tests for fundamental symmetries in compound-nucleus scattering with epithermal polarized neutron beams

Abstract The epithermal polarized-neutron beam facility used for tests of fundamental symmetries is described. The initial unpolarized beam is obtained from the spallation source at the Los Alamos Neutron Scattering Center. Characteristics of the polarized and unpolarized beams are described, as well as design and performance of a fast-spin reversal system, neutron flux monitor and target cooler. Experimental results for polarized-neutron transmission experiments are given to illustrate the overall quality of the system.

Explanation for Anomalous Shock Temperatures Measured by Neutron Resonance Spectroscopy

Neutron resonance spectrometry (NRS) has been used to measure the temperature inside Mo samples during shock loading. The temperatures obtained were significantly higher than predicted assuming ideal hydrodynamic loading, a discrepancy which we now explain. The effects of plastic flow and nonideal projectile behavior were assessed. Plastic flow was calculated self-consistently with the shock jump conditions: this is necessary for a rigorous estimate of the locus of shock states accessible. Plastic flow was estimated to contribute a temperature rise of 53 K compared with hydrodynamic flow. Simulations were performed of the operation of the explosively driven projectile system used to induce the shock in the Mo sample. The simulations, and related experiments, indicated that the projectile was significantly curved on impact, and still accelerating. The resulting spatial variations in load, including radial components of velocity, should increase the apparent temperature that would be deduced from the width of the neutron resonance by 160 K. These corrections are sufficient to reconcile the apparent temperatures deduced using NRS with the accepted properties of Mo, in particular, its equation of state.