Skip to main content

Alan Thompson

Followers

4

Following

5

Co-authors

4

Public Views

Interests

Uploads

Papers by Alan Thompson

Metastability exchange optical pumping of 3He gas up to hundreds of millibars at 4.7 Tesla

The European Physical Journal D, 2013

Metastability exchange optical pumping (MEOP) is experimentally investigated in 3 He at 4.7 T, at... more Metastability exchange optical pumping (MEOP) is experimentally investigated in 3 He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 2 3 S-2 3 P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of 3 He atoms in the rf discharge plasma. The collisional broadening rate is measured (12.0 ± 0.4 MHz mbar -1 FHWM) and taken into account for accurate absorption-based measurements of both nuclear polarization in the ground state and atom number density in the metastable 2 3 S state. The results lay the ground for a comprehensive assessment of the efficiency of MEOP, by comparison with achievements at lower field (1 mT-2 T) over an extended range of operating conditions. Stronger hyperfine decoupling in the optically pumped 2 3 S state is observed to systematically lead to slower build-up of 3 He orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high pressure but in a less dramatic way than observed at 2 T in the same sealed glass cells. To date, thanks to the linear increase in gas density, they correspond to the highest nuclear magnetizations achieved by MEOP in pure 3 He gas. The improved efficiency puts less demanding requirements for compression stages in polarized gas production systems and makes high-field MEOP particularly attractive for magnetic resonance imaging of the lungs, for instance.

Demonstration of a Backscatter-Suppressed Beta Spectrometer for Neutron Decay Experiments

The proposed new measurement of the electron-antineutrino correlation coefficient ``a'' i... more The proposed new measurement of the electron-antineutrino correlation coefficient ``a'' in neutron decay (previous talk) requires an electron spectrometer with a high degree of backscatter suppression to minimize the low-energy tail in the response function. We have designed, constructed, and tested a prototype of a spectrometer which consists of a conical array of plastic scintillator detectors coupled to photomultipliers. Results

Observation of the radiative decay mode of the neutron

We report observation of the radiative decay mode of the neutron, in which the usual decay produc... more We report observation of the radiative decay mode of the neutron, in which the usual decay products are accompanied by an inner-bremsstrahlung photon. The experiment was carried out at the fundamental physics beam line at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) Photons with energies between 15 keV and 340 keV were detected by

Production of highly polarized 3He using spectrally narrowed diode laser array bars

Journal of Applied Physics, 2003

We have produced 70%–75% 3He polarization by spin-exchange optical pumping in cells ≈100 cm3 in v... more We have produced 70%–75% 3He polarization by spin-exchange optical pumping in cells ≈100 cm3 in volume. The polarization achieved is consistent with known spin-exchange and spin-relaxation rates, but only when the recently discovered temperature dependence of 3He relaxation is included. Absolute 3He polarization measurements were performed using two different methods in two different laboratories. The results were obtained with either a spectrally narrowed laser or one type of broadband laser. Based on tests of several larger cells at pressures near 1 bar, we find that the power required to reach the same polarization is typically three times lower for the spectrally narrowed laser. This last result indicates that spectrally narrowed lasers will be important for obtaining the highest polarization in large volume neutron spin filters. Polarization in excess of 55% as obtained in cells up to 640 cm3 in volume and 70% polarization is anticipated with available increases in spectrally n...

Toward precision polarimetry of dense polarized 3He targets

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1998

We describe several new measurement and analysis techniques used to determine the polarization of... more We describe several new measurement and analysis techniques used to determine the polarization of the 3He target in a recently completed measurement of the neutron spin structure function g; at SLAC (E-154). The polarization was determined using two independent methods. The first method used a standard technique of Adiabatic Fast Passage, calibrated by a measurement of Boltzmann polarization in a sample of water. We describe several systematic effects affecting this calibration procedure. The second method used a shift of the Rb Zeeman resonance frequency due to the polarization of 3He. Implementation and calibration of this technique are discussed in detail. Finally, the density of 3He in the cell was measured using two independent methods, one of them based on the pressure broadening of Rb Dl and D2 lines due to 3He.

Neutron Test Results of Preliminary Measurements and Calculations Using a Handheld Instrument

Initial neutron measurements and calculations were performed to investigate the response of handh... more

format_quoteRID response to unmoderated 252Cf source in low scatter is 5-9 times lower than in vehicle environment.format_quote

aCORN: An experiment to measure the electron–antineutrino correlation in neutron decay

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficien... more We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

Extreme ultraviolet photon conversion efficiency of tetraphenyl butadiene

Applied Optics, 2019

Extreme ultraviolet (EUV) radiation can be converted to visible light using tetraphenyl butadiene... more Extreme ultraviolet (EUV) radiation can be converted to visible light using tetraphenyl butadiene (TPB) as a phosphor. 1 µm films of TPB were prepared using thermal vapor deposition of the pure material and by spin coating suspensions of TPB in high-molecular-weight polystyrene/toluene solutions. Calibrated sources and detectors were used to determine the effective photon conversion efficiency of the films for incident EUV radiation in the wavelength range of 125 n m ≤ λ ≤ 200 n m . After exposure to atmosphere, the efficiency of the vapor-deposited films decreased significantly, while the efficiency of the spin-coated films remained unchanged. The production of TPB films by spin coating offers the advantages of simplicity and long-term stability.

The SLAC E-154 [sup 3]He polarimeter

We describe the NMR and Rb Zeeman frequency shift polarimeters used for determining the 3He polar... more We describe the NMR and Rb Zeeman frequency shift polarimeters used for determining the 3He polarization in a recent precision measurement of the neutron spin structure function gl at SLAC (E-154). We performed a detailed study of the systematic errors associated with the calibration of the NMR polarimeter. A new technique was used for determining the 3He polarization from the frequency shift of the Rb Zeeman resonance.

Deep inelastic scattering of polarized electrons by polarizedHe3and the study of the neutron spin structure

Physical Review D, 1996

Work supported by Department of Energy contract DE{AC03{76SF00515.

format_quoteIntegrating g_n1 over x results in n_1 = 0.0284 ± 0.0061(stat) ± 0.0059(sys), revealing contributions to scattered states.format_quote

Search for aT-odd,P-even triple correlation in neutron decay

Physical Review C, 2012

Background Time-reversal-invariance violation, or equivalently CP violation, may explain the obse... more Background Time-reversal-invariance violation, or equivalently CP violation, may explain the observed cosmological baryon asymmetry as well as indicate physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation D Jn • (pe × pν) is a parity-even, time-reversal-odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects contribute at the level of 10 −5 and can be calculated with a precision of 1% or better. Purpose We have improved the sensitivity to T-odd, P-even interactions in nuclear beta decay. Methods We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract D and study systematic effects in a blind analysis. Results We find D = [−0.94 ± 1.89(stat) ± 0.97(sys)] × 10 −4. This differs from our recent paper (Phys. Rev. Lett. 107 102301 (2011)) due to refinement of corrections for background and backscattering. Conclusions This is the most sensitive measurement of D in nuclear beta decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants (CA/CV = |λ|e iφ AV) with φAV = 180.012 • ± 0.028 • (68% confidence level). This result can also be used to constrain time-reversal violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks.

$TABLE III: Averages over all detectors of background frac- tions for each background component.$

Test of 3He-based neutron polarizers at NIST

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2000

Neutron spin "lters based on polarized He are useful over a wide neutron energy range and have a ... more Neutron spin "lters based on polarized He 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 He gas required for such neutron spin "lters. 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.

Simultaneous polarization analysis of Zeeman splitting in polarized neutron reflectometry using a polarized 3 He neutron-spin filter

Applied Physics A: Materials Science & Processing, 2002

A polarized 3 He neutron-spin filter was used to analyze polarized neutrons reflected from a 1000... more A polarized 3 He neutron-spin filter was used to analyze polarized neutrons reflected from a 1000-Å-thick Fe film immersed in a magnetic field. The spin filter analyzed both the specularly reflected component and the offspecular component caused by the phenomenon of Zeeman splitting of surface-scattered neutrons observed by Felcher et al. and investigated in more detail in subsequent experiments. The sample of polarized 3 He was polarized by metastabilityexchange optical pumping and compressed into a glass cell at the Indiana University Cyclotron Facility. The polarized gas was transported by car in a battery-powered solenoid holding field to the neutron reflectometry instrument POSY I at the Intense Pulsed Neutron Source at Argonne National Laboratory. Using the large solid angle of the polarized 3 He spin filter, we were able to simultaneously analyze both components to the scattering and verify (as expected) that the polarization of the specular component was unchanged upon reflection and that the polarization of the off-specular component was reversed. To our knowledge this work represents the second experiment to employ a polarized 3 He neutron-spin filter in polarized neutron reflectometry.

Lunar Cellular Network Neutron Spectrometer

Detecting the radiative decay mode of the neutron

Journal of Research of the National Institute of Standards and Technology, 2005

Like all elementary particle decays to charged particles in the final state, the beta decay of th... more Like all elementary particle decays to charged particles in the final state, the beta decay of the free neutron has a radiative mode: n → p + e-+ ν − e + γ. Despite decades of detailed experimental studies of neutron beta-decay (including very precise measurements of the neutron lifetime and beta-decay correlation coefficients), this rare decay branch has never been observed, while it has been extensively investigated in more exotic systems [1]. Recently, theoretical investigations of this process have been underway. Gaponov and Khafizov calculated the photon energy spectrum and branching ratio within a quantum electrodynamics framework [2], while Bernard et al. have calculated the photon energy spectrum and photon polarization in a heavy baryon chiral perturbation theory (HBχPT) including explicit ∆ degrees of freedom [3]. Feynman diagrams of the various processes involved in the Bernard et al. calculation are shown in Fig. 1. It should be noted that the Gaponov and Khafizov calculation, performed in a

Noble gas excimer scintillation following neutron capture in boron thin films

by Jacob McComb, Charles Clark, Alan Thompson, and Robert Vest

Far-ultraviolet scintillation signals have been measured in heavy noble gases (argon, krypton, xe... more Far-ultraviolet scintillation signals have been measured in heavy noble gases (argon, krypton, xenon) following boron-neutron capture (10B(n,a)7Li) in 10B thin films. The observed scintillation
yields are comparable to the yields from some liquid and solid neutron scintillators. At noble gas pressures of 107 kPa, the number of photons produced per neutron absorbed following irradiation of a 1200 nm thick 10B film was 14 000 for xenon, 11 000 for krypton, and 6000 for argon. The absolute scintillation yields from the experimental configuration were calculated using data from (1) experimental irradiations, (2) thin-film characterizations, (3) photomultiplier tube calibrations, and (4) photon collection modeling. Both the boron films and the photomultiplier tube were characterized at the National Institute of Standards and Technology. Monte Carlo modeling of the reaction cell provided estimates of the photon collection efficiency and the transport behavior of 10B(n,a)7Li reaction products escaping the thin films. Scintillation yields increased with gas pressure due to increased ionization and excitation densities of the gases from the 10B(n,a)7Li reaction products, increased frequency of three-body, excimer-forming collisions, and reduced photon emission volumes (i.e., larger solid angle) at higher pressures. Yields decreased for thicker 10B thin films due to higher average energy loss of the 10B(n,a)7Li reaction products escaping the films. The relative standard uncertainties in the measurements were determined to lie between 14% and 16%. The observed scintillation signal demonstrates that noble gas excimer scintillation is promising for use in practical neutron detectors.

A sectioned spectrometer of fast neutrons

Instruments and Experimental Techniques, 2009

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped w... more Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched 6 Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ~10-15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10-4 to 10 2 cm-2 s-1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 10 2 s-1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (~1.2 l); for 3-MeV neutrons, its value is 0.2-0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ~600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.

Status of the NIST spin filter project

Journal of Neutron Research, 1996

Abstract Polarized 3He has long been recognized as a promising candidate for producing beams of p... more Abstract Polarized 3He has long been recognized as a promising candidate for producing beams of polarized neutrons by selective absorption of one neutron spin state from an initially unpolarized beam. This technique has advantages over the traditional supermirror technique in that the neutron beams produced have less divergence added by the polarization technique (an advantage for SANS applications), lower backgrounds from gamma production (mainly important for fundamental physics research), and most importantly. 3He can polarize thermal and epi-thermal neutrons as well as cold neutrons. We are pursuing parallel efforts at NIST to produce a practical spin filter either by polarizing 3He using spin-exchange with optically pumped alkali vapors or by direct optical pumping of metastable. 3He followed by compression with a mechanical pump. The status and prospects of both of these projects are reported in this paper.

Observation of far-ultraviolet signatures of the $^ 3$ He ($ n, tp $) reaction

In previous work [1] on the production of Lyman alpha radiation in the the ^3He(n,tp) reaction in... more In previous work [1] on the production of Lyman alpha radiation in the the ^3He(n,tp) reaction in a cell of ^3 He gas at atmospheric pressure, we found that 50 Lyman alpha photons were produced per absorbed neutron. These photons are produced by excited states of T and H created by charge-exchange and excitation collisions of t and p with

Observation of the 3He(n,tp) reaction by detection of far-ultraviolet radiation

Journal of Research of the National Institute of Standards and Technology, 2008

Nuclear physics and atomic, molecular and optical (AMO) physics take place on disparate energy sc... more Nuclear physics and atomic, molecular and optical (AMO) physics take place on disparate energy scales, with nuclear excitations having characteristic energies of 1 MeV (10 6 electron volts) while the energies encountered in atomic electron excitation are from 1 eV to 1000 eV. This is one of the reasons that nuclear and AMO physics have largely developed as independent disciplines, with different experimental and theoretical techniques. There has always been some interplay between them, such as the determination of nuclear moments and charge distributions from high-resolution atomic spectroscopy . The present investigation is at the boundary between these two disciplines. It is concerned with atomic electron excitation during the course of the low-energy 3 He(n,tp) reaction. We have found that this reaction generates a quantity of Lyman alpha