Our main research activity is the investigation of the weak interactions of low energy neutrons. Measurements in this area can address issues of fundamental importance in nuclear physics, particle physics, and astrophysics. [W. M. Snow, Fundamental Physics with Slow Neutrons, Proceedings of the International Conference on Nuclear Data for Science and Technology, to be published by the American Institute of Physics (2004)] All of these experiments use intense beams of low energy neutrons produced either at a nuclear reactor or at a "spallation" neutron source, where neutrons are produced by collision of intense ~Gev proton beams on heavy nuclei. The $1.4B Spallation Neutron Source, under construction at ORNL, will be the most intense source of this type in the world when it starts operation in 2007. The list below describes some of the measurements that we have performed and some that are in progress:
(1) We have performed measurements of the weak interactions of low energy neutrons with heavy nuclei (xenon in our case). This work was a small contribution to the experimental program carried out by the TRIPLE collaboration to investigate the weak interaction of neutrons with heavy nuclei in an attempt to learn about the weak interactions among neutrons and protons. We discovered a large (2%) parity-odd analyzing power at a newly discovered p-wave resonance in 131Xe at 3.2 eV. [J. J. Szymanski et al, Observation of a Large Parity Non-conserving Analyzing Power in Xe, Phys. Rev. C 53, R2576 (1996)] If techniques to polarize large amounts of xenon for medical imaging make further progress, it may be possible to use this resonance to search for time-reversal invariance using this resonance as an amplifier.
(2) We have finished an experiment to measure the decay rate of the neutron using a Penning trap to hold and later count one-by-one the protons from neutron decay. [M. S. Dewey et al, Measurement of the Neutron Lifetime Using a Proton Trap, Phys. Rev. Lett. 91, 152301 (2003)] A precision measurement of this quantity is useful in determining one of the parameters of the Standard Model and testing the "universality" of the weak interaction. In addition, it essentially determines the amount of 4He in the universe after the early part of the Big Bang is over (but before stars formed and burned hydrogen into helium). Our result, 886.8 +/- 1.2(stat) +/- 3.2(sys) seconds, is the most precise measurement using a cold neutron beam and is in agreement with other measurements using “ultracold" neutrons stored in material bottles. We are attempting to improve on the precision of this result by an absolute calibration of the efficiency of the neutron fluence monitor in the lifetime experiment. This calibration has required the construction of a monochromatic neutron beam facility NG6M at NIST, an absolute measurement of the mean energy of the beam using diffraction from perfect silicon crystals, and the development of a neutron radiometer to measure absolute neutron fluence. [J. Richardson et al, Accurate Determination of Thermal Neutron Flux via Cryogenic Calorimetry, IEEE Trans. Nucl. Sci. 45, 550 (1998)] [Z. Chowdhuri et al, A Cryogenic Neutron Radiometer for Absolute Neutron Fluence Measurement, Rev. Sci. Inst. 74, 4280-4293 (2003)]
(3) We are preparing for a measurement of the weak interaction between a neutron and a light nucleus (4He in this case). [C. D. Bass et al, Measurement of the parity-violating neutron spin rotation in 4He, submitted to J. Res. NIST (2004)] Again, the idea is to learn about the weak interaction of a neutron with other neutrons and protons, but this time in a system that is simple enough for the results to be interpreted cleanly in terms of the neutron-neutron and neutron-proton weak interaction. The experiment will attempt to measure a ~0.1 microradian rotation of the plane of polarization of a neutron as it moves through a meter of superfluid helium. This experiment, along with the NPDGamma experiment and others, can be used to determine the weak nucleon-nucleon interaction.[W. M. Snow, The Nucleon-Nucleon Weak Interaction and Low Energy Neutrons, Proceedings of the International Conference on Electron-Nucleus Scattering VIII, Elba, Italy, to be published in European Physical Journal A (2004)].
(4) We are playing a leading role in the NPDGamma collaboration that is preparing an experiment to search for the weak interaction of a neutron with a proton. [W. M. Snow et al, Measurement of the Parity Violating Asymmetry A gamma in n + p —› d + gamma, Nucl. Inst. and Meth. A440, 729 (2000)], [W. M. Snow et al, Progress Toward a New Measurement of the Parity Violating Asymmetry in n + p —› d + gamma, Nucl. Inst. and Meth. A515, 563 (2003).] This interaction, which has never been measured, is the only remaining interaction of the known forces among the particles of "normal" matter (protons, neutrons, and electrons), which has yet to be determined. The collaboration has recently commissioned a new cold neutron beam line at the LANSCE facility at Los Alamos for this experiment, and has observed parity-odd gamma asymmetries in 35Cl and 139La [G. S. Mitchell et al, A measurement of parity-violating gamma-ray asymmetries in polarized cold neutron capture on 35Cl, 113Cd, and 139La, Nucl. Inst. Meth. A521, 468-479 (2004).]
(5) We have finished a series of precision measurements of the coherent neutron scattering amplitudes from hydrogen, deuterium, and 3He using the Neutron Interferometer and Optics Facility at NIST. [T. Black et al, Precision Neutron Interferometric Measurement of the n-D Coherent Neutron Scattering Length and Consequences for the Nuclear Three-Body Force, Phys. Rev. Lett. 90, 192502 (2003)]; [K. Schoen et al, Precision Neutron Interferometric Measurement and Updated Evaluations of the n-D Coherent Neutron Scattering Lengths, Phys. Rev. C67, 044005 (2003)]; [P. R. Huffman et al, A Precision Neutron Interferometric Measurement of the n-3He Coherent Neutron Scattering Length, Phys. Rev. C70, 014004 (2004)] These measurements disagreed with almost all of the predictions using existing NN interaction potentials and will be used in conjunction with other measurements in nuclear few body systems to constrain models of nuclear 3-body forces. Other measurements in the n-3H and n-4He systems are possible.
Another focus of our efforts is to develop a new type of low energy neutron polarizer and analyzer based on polarized 3He gas. The 3He gas is polarized by optical pumping, a process by which the angular momentum in a circularly-polarized laser beam is efficiently transferred first to an electron in the 3He atom and then, by the hyperfine interaction, to the nucleus of 3He, which has spin-1/2. The interaction of a low energy neutron beam with 3He turns out to be strongly spin-dependent. So, if an unpolarized neutron beam enters a polarized 3He target, one spin state interacts more strongly with the gas than the other and the exiting neutron beam is highly polarized. We have constructed and characterized a polarized 3He compression system using metastable optical pumping of 3He [D. Hussey et al, A Polarized 3He Gas Compression System Using Metastability-Exchange Optical Pumping, submitted to Rev. Sci. Inst. (2004)] and conducted measurements using these polarizers to demonstrate their potential for use in absolute neutron polarimetry [D. R. Rich et al, A Measurement of the Absolute Neutron Beam Polarization Produced By an Optically-Pumped 3He Neutron Spin Filter, Nucl. Inst. Meth. A481, 431 (2002)].
In addition to applications of these neutron polarizers to weak interaction experiments, we are pursuing applications in neutron scattering. Neutron scattering is a very useful tool in physics, chemistry, materials science, and biology. Polarized neutron scattering in combination with analysis of the polarization of the scattered neutrons allows one to increase the contrast for certain atomic and molecular structure and dynamics of interest to the experimenter. In particular, neutron scattering is a very important tool in the study of magnetism. We intend to use polarized 3He neutron polarizers and analyzers to make possible new classes of neutron scattering experiments in a wide range of fields. As one example, we used a polarized 3He analyzer to verify the interpretation of the phenomenon of “Zeeman splitting" in polarized neutron reflectometry [D. Hussey et al, Simultaneous Polarization Analysis of Zeeman Splitting of Surface Scattered Neutrons Using a Polarized 3He Neutron Spin Analyzer, Applied Physics A 74, S234-S236 (2003)].
Finally, we are involved in the development of the LENS neutron source at IUCF. LENS is a university-based pulsed cold neutron source for education, research, and instrumentation development [D. V. Baxter et al, LENS- A University-Based Pulsed Neutron Source for Education and Research, proceedings of the 16th meeting of the International Collaboration on Advanced Neutron Sources, Dusseldorf, Germany, May 12-15 (2003)]. We are working on calculations of the expected performance of a phase II solid methane cold neutron moderator, which we expect to operate at about ~7K temperature, to see if we can develop a cold neutron moderator with a lower energy spectrum than achieved at other neutron sources. In addition to its direct application at LENS, such a moderator could also be useful as a premoderator for ultracold neutron sources. We are also designing a neutron spin echo spectrometer for small angle neutron scattering (SESANS) which can access distance scales in the ~10-1000 Angstrom range and can make efficient use of the phase space of a cold neutron beam at a pulsed neutron source.