-
[show abstract]
[hide abstract]
ABSTRACT: Zheng et al. [ Zheng, Gao, Ye and Zhang Phys. Rev. A 83 061401 (2011)] have measured a strong linear pressure dependence of the nuclear relaxation rate of 3He in glass cells typically used to generate and/or store polarized 3He at room and cryogenic temperatures. Their interpretation is that this linear dependence is a general characteristic of paramagnetic wall relaxation, and they offer a theoretical explanation of the effect based on diffusion theory in the bulk with an incorrect boundary condition. We question the physical basis of the boundary condition and suggest some alternate explanations of the observations. Numerous previous studies support a broadly valid pressure-independent model for wall relaxation.
Phys. Rev. A. 04/2012; 85(4).
-
V Thampy,
J Kang,
J A Rodriguez-Rivera,
W Bao,
A T Savici,
J Hu,
T J Liu,
B Qian,
D Fobes,
Z Q Mao,
C B Fu,
W C Chen,
Q Ye,
R W Erwin, T R Gentile,
Z Tesanovic,
C Broholm
[show abstract]
[hide abstract]
ABSTRACT: Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducting Fe(1+y)Te(1-x)Se(x) induces a magnetic Friedel-like oscillation that diffracts at Q⊥=(1/2 0) and involves >50 neighboring Fe sites. The interstitial >2μ(B) moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in Fe(1+y)Te. A semimetallic five-band model with (1/2 1/2) Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.
Physical Review Letters 03/2012; 108(10):107002. · 7.37 Impact Factor
-
V. Thampy,
J. Kang,
J. A. Rodriguez-Rivera,
W. Bao,
A. T. Savici,
J. Hu,
T. J. Liu,
B. Qian,
D. Fobes,
Z. Q. Mao,
C. B. Fu,
W. C. Chen,
Q. Ye,
R. W. Erwin, T. R. Gentile,
Z. Tesanovic,
C. Broholm
Physical Review Letters 01/2012; 108(10):107002. · 7.37 Impact Factor
-
V. Thampy,
J. Kang,
J. A. Rodriguez-Rivera,
W Bao,
A. T. Savici,
J Hu,
T. J. Liu,
B. Qian,
D. Fobes,
Z. Q. Mao,
C. B. Fu,
W C Chen,
Q Ye,
R. W. Erwin, T R Gentile,
Z. Tesanovic,
C. Broholm
[show abstract]
[hide abstract]
ABSTRACT: Using polarized and unpolarized neutron scattering we show that interstitial
Fe in superconducting Fe_{1+y}Te_{1-x}Se_x induces a magnetic Friedel-like
oscillation that diffracts at Q_(in-plane)=(1/2,0) and involves >50 neighboring
Fe sites. The interstitial >2 mu_B moment is surrounded by compensating
ferromagnetic four spin clusters that may seed double stripe ordering in
Fe_{1+y}Te. A semi-metallic 5-band model with (1/2,1/2) Fermi surface nesting
and four fold symmetric super-exchange between interstitial Fe and two in-plane
nearest neighbors largely accounts for the observed diffraction.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Prior studies revealed neutron beam-induced alkali-metal spin relaxation in 3He cells polarized in-situ by spin-exchange optical pumping. These effects are minor for neutron beams of low to moderate intensity, but become important at the highest neutron flux levels available in the full spectrum beams of high flux neutron sources. It was found that the relaxation consisted of a fast and a slow component, but the origin of neither is understood. This work further explored the mechanisms of this effect by measuring the magnitude and time dependence of alkali-metal spin-relaxation rates as a function of N2 and 3He composition. The experiments were performed on a thermal neutron beam line at the Institut Laue-Langevin with a maximum neutron flux of 2.4 × 109 cm−2 s−1.
Journal of Physics Conference Series 06/2011; 294(1):012011.
-
[show abstract]
[hide abstract]
ABSTRACT: Over a period of well over a decade, a large number and variety of polarized 3He cells for neutron applications have been developed and tested at the U.S. National Institute of Standards and Technology (NIST). These cells have primarily been employed for spin-exchange optical pumping (SEOP), and applied to neutron scattering and fundamental neutron physics. We describe the procedures we have employed for producing these cells, as well as their characteristics and applications. Whereas our best results are for cells blown from boron-free aluminosilicate glass, we summarize results with a range of other methods. We discuss our recent work on SEOP cells for wide angle neutron polarization analysis.
Journal of Physics Conference Series 06/2011; 294(1):012003.
-
R C Gillis,
R Alarcon,
S Balascuta,
L Barrón-Palos,
J D Bowman,
R D Carlini,
W Chen,
T E Chupp,
C Crawford,
S Covrig, [......],
W D Ramsay,
A Salas-Bacci,
S Santra,
M Sharma,
P-N Seo,
E I Sharapov,
T B Smith,
W M Snow,
W S Wilburn,
V Yuan
[show abstract]
[hide abstract]
ABSTRACT: The NPDGamma Experiment measures the parity-violating correlation Aγ between neutron spin and photon momentum in the reaction + p → d + γ. Knowledge of Aγ and other parity-violating observables in few-body nuclear systems will provide constraints for a parameterized description of ΔS = 0 parity-violating phenomena free from complications of nuclear structure. The NPDGamma experiment uses a polarized cold pulsed neutron beam, a liquid parahydrogen target, and a cylindrical array of 48 CsI(Tl) scintillation detectors operated in current mode to search for the asymmetry. NPDGamma recently completed the first phase of the program to measure Aγ at the Los Alamos Neutron Science Center with the preliminary result Aγ = (−1.2 ± 2.1(stat.) ± 0.1(sys.)) × 10−7, reproducing the previous upper limit from a measurement at a reactor facility. We discuss the theoretical background and experimental method and report on preliminary analysis of the LANSCE data. The second phase of the program to measure Aγ is in progress at the Spallation Neutron Source at Oak Ridge National Laboratory.
Journal of Physics Conference Series 08/2010; 239(1):012012.
-
K L Krycka,
R A Booth,
C R Hogg,
Y Ijiri,
J A Borchers,
W C Chen,
S M Watson,
M Laver, T R Gentile,
L R Dedon,
S Harris,
J J Rhyne,
S A Majetich
[show abstract]
[hide abstract]
ABSTRACT: A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.
Physical Review Letters 05/2010; 104(20):207203. · 7.37 Impact Factor
-
I Cabrera,
M Kenzelmann,
G Lawes,
Y Chen,
W C Chen,
R Erwin, T R Gentile,
J B Leão,
J W Lynn,
N Rogado,
R J Cava,
C Broholm
[show abstract]
[hide abstract]
ABSTRACT: Electric control of multiferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of Ni3V2O8 in an electric field E causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has a handedness that can be reversed by E. We further show a close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that a definite cycloidal handedness is achieved through magnetoelastically induced Dzyaloshinskii-Moriya interactions.
Physical Review Letters 08/2009; 103(8):087201. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have expanded upon the "Magic Box" concept, a coil driven magnetic parallel plate capacitor constructed out of mu-metal, by introducing compensation sections at the ends of the box that are tuned to limit end-effects similar to those of short solenoids. This ability has reduced the length of the magic box design without sacrificing any loss in field homogeneity, making the device far more applicable to the often space limited neutron beam line. The appeal of the design beyond affording longer polarized 3He lifetimes is that it provides a vertical guide field, which facilitates neutron spin transport for typical polarized beam experiments. We have constructed two end-compensated magic boxes of dimensions 28.4 x 40 x 15 cm3 (length x width x height) with measured, normalized volume-averaged transverse field gradients ranging from 3.3 x 10(-4) to 6.3 x 10(-4) cm(-1) for cell sizes ranging from 8.1 x 6.0 to 12.0 x 7.9 cm2 (diameter x length), respectively.
The Review of scientific instruments 07/2009; 80(6):063905. · 1.52 Impact Factor
-
M Sharma,
E Babcock,
K H Andersen,
L Barrón-Palos,
M Becker,
S Boag,
W C Chen,
T E Chupp,
A Danagoulian, T R Gentile,
A Klein,
S Penttila,
A Petoukhov,
T Soldner,
E R Tardiff,
T G Walker,
W S Wilburn
[show abstract]
[hide abstract]
ABSTRACT: We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.
Physical Review Letters 09/2008; 101(8):083002. · 7.37 Impact Factor
-
M. Sharma,
E Babcock,
K. H. Andersen,
L. Barron-Palos,
M. Becker,
S. Boag,
W C Chen,
T. E. Chupp,
A. Danagoulian, T R Gentile,
A Klein,
S. Penttila,
A. Petoukhov,
T. Soldner,
E. R. Tardiff,
T G Walker,
W. S. Wilburn
[show abstract]
[hide abstract]
ABSTRACT: We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal-spin-exchange polarized He-3 cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable He-3 polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at LANSCE and ILL. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as the square root of the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the recombination-limited ion concentration, but is much larger than expected from earlier work.
03/2008;
-
M. T. Gericke,
J.D. Bowman,
R. D. Carlini,
T. E. Chupp,
K. P. Coulter,
M. Dabaghyan,
M. Dawkins,
D. Desai,
S. J. Freedman, T R Gentile, [......],
S. I. Penttila,
W D Ramsay,
S. Santra,
P. -N. Seo,
E.I. Sharapov,
T.B. Smith,
W. M. Snow,
W. S. Wilburn,
V. Yuan,
H Zhu
[show abstract]
[hide abstract]
ABSTRACT: Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states. In this work we use a statistical approach to estimate the root mean square (RMS) of the distribution of expected parity-odd correlations $\vec{s_{n}} \cdot \vec{k_{\gamma}}$, where $\vec {s_{n}}$ is the neutron spin and $\vec{k_{\gamma}}$ is the momentum of the gamma, in the integrated gamma spectrum from the capture of cold polarized neutrons on Al, Cu, and In and we present measurements of the asymmetries in these and other nuclei. Based on our calculations, large enhancements of asymmetries were not predicted for the studied nuclei and the statistical estimates are consistent with our measured upper bounds on the asymmetries.
09/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: Based on measurements of the temperature dependence of 3He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed 3He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.
Physical Review Letters 04/2006; 96(8):083003. · 7.37 Impact Factor
-
M. T. Gericke,
C. Blessinger,
J. D. Bowman,
R. D. Carlini,
T. E. Chupp,
K. P. Coulter,
M. Dawkins,
M. Dabaghyan,
D. Desai,
S. J. Freedman, [......],
S. I. Penttilä,
W. D. Ramsay,
S. Santra,
P.‐N. Seo,
E. T. Sharapov,
T. B. Smith,
W. M. Snow,
J. Tasson,
W. S. Wilburn,
H. Zhu
[show abstract]
[hide abstract]
ABSTRACT: We have built a CsI(Tl) γ‐ray detector array for the NPDGamma experiment to search for a small parity‐violating directional asymmetry in the angular distribution of 2.2 MeV γ‐rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion‐nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires control of systematic errors at the ppb level, and the use of current‐mode γ‐ray detection with vacuum photo diodes and low‐noise solid‐state preamplifiers. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability, and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of 10−9 in a few hours. We show that the detector array operates at counting statistics and present asymmetry results for B4C , CCl4 , 27Al, Cu, and In. B4C , 27Al, Cu, and In are used throughout the experimental apparatus. © 2005 American Institute of Physics
AIP Conference Proceedings. 05/2005; 769(1):692-695.
-
[show abstract]
[hide abstract]
ABSTRACT: We have employed a polarized <sup>3</sup> He spin filter in conjunction with a position-sensitive detector (PSD) to perform efficient polarization analysis of neutron diffuse reflectivity. This work was carried out on the NG-1 polarized neutron reflectometer at the National Institute of Standards and Technology Center for Neutron Research. We measured the specular and diffuse reflectivity of a patterned magnetic array that has periodic square holes in a Co film. Analysis of the data yielded spin-analyzed two-dimensional Q<sub>x</sub>-Q<sub>z</sub> reciprocal space maps for the sample in magnetized and demagnetized states. We compared the measurements obtained with a <sup>3</sup> He analyzer and a PSD with those obtained using a conventional supermirror analyzer and a <sup>3</sup> He proportional counter. The results are in good agreement. For this experiment, <sup>3</sup> He gas was polarized by the spin-exchange optical pumping method and stored in a uniform magnetic field provided by a shielded solenoid. Improved optical pumping using a spectrally narrowed diode laser array yielded an initial <sup>3</sup> He polarization of 70% in a <sup>3</sup> He cell volume of 280 cm <sup>3</sup> .
Review of Scientific Instruments 11/2004; · 1.37 Impact Factor
-
W C Chen,
C Bailey,
J A Borchers,
R F C Farrow, T R Gentile,
D Hussey,
C F Majkrzak,
K V O Donovan,
N Remmes,
W M Snow,
A K Thompson
[show abstract]
[hide abstract]
ABSTRACT: At the National Institute of Standards and Technology (NIST) and Indiana University, we are developing polarized 3 He analyzers for neutron reflectometry. We have employed 3 He analyzers at two polarized neutron reflectometers, NG1 at the NIST Center for Neutron Research (NCNR) and POSY I at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The long-term goal for both efforts is to perform efficient studies of magnetic diffuse scattering. At the NCNR, we tested a 3 He analyzer by comparing measurements of specular scattering obtained with a supermirror analyzer to those obtained with a 3 He analyzer. For this test, we measured the spin-flip and nonspin-flip scattering from a Mn 0:52 Pd 0:48 =Fe thin film. The results with the 3 He analyzer show very good agreement with those obtained with the supermirror analyzer. We have also carried out tests of the 3 He analyzer for application to magnetic diffuse scattering experiments. We discuss the development of a 3 He analyzer for IPNS that will be employed for studies of patterned magnetic arrays.
01/2003; 1280(32).
-
[show abstract]
[hide abstract]
ABSTRACT: A polarized 3He neutron-spin filter was used to analyze polarized neutrons reflected from a 1000--thick Fe film immersed in a magnetic field. The spin filter analyzed both the specularly reflected component and the off-specular 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 3He was polarized by metastability-exchange 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 3He 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 3He neutron-spin filter in polarized neutron reflectometry.
Applied Physics A 11/2002; 74:s234-s236. · 1.63 Impact Factor
-
S. I. Penttila,
J. D. Bowman,
R. D. Carlini,
T. Case,
T. E. Chupp,
K. P. Coulter,
S. J. Freedman, T. R. Gentile,
M. T. Gericke,
G. L. Greene, [......],
G. S. Mitchell,
K. Morimoto,
H. Nann,
S. A. Page,
W. D. Ramsay,
Ė. I. Sharapov,
T. B. Smith,
W. M. Snow,
S. W. Wilburn,
V. W. Yuan
[show abstract]
[hide abstract]
ABSTRACT: The {rvec n} + p {yields} d + {gamma} experiment under construction at LANSCE studies the weak interaction between neutrons and protons. The experiments will measure the directional dependence of the parity-violating {gamma}-ray asymmetry, A{sub {gamma}}, in the polarized cold neutron capture by para-hydrogen. The goal is to measure A{sub {gamma}} with uncertainty of 0.5 x 10{sup -8}, 10% of its predicted value. A{sub {gamma}} primarily isolates the {Delta}I = 1 component of the hadronic weak interaction and thus will determine the long-range weak pion-nucleon coupling constant H{sub {pi}}{sup 1}. The experiment is carefully designed for the LANSCE pulsed spallation neutron source to achieve the proposed statistical precision and to control systematic errors. We discuss the experiment and its status.
12/2001
-
[show abstract]
[hide abstract]
ABSTRACT: Neutron spin filters based on polarized 3 He promise to significantly expand the range of applications of polarized neutron scattering, particularly for experiments with divergent beams. At NIST and at Indiana University, we are pursuing both methods of polarizing 3 He by optical pumping: the metastability exchange method, and spin exchange with optically pumped Rb. The status of the respective apparatus and application to small angle neutron scattering (SANS) at the National Institute of Standards and Technology (NIST) and neutron reflectometry at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory are discussed. r 2001 Elsevier Science B.V. All rights reserved.
Physica B. 01/2001; 305:203-207.
