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Precision neutron polarimetry for neutron beta decay
No full textThe abBA collaboration is developing a new type of field-expansion spectrometer for a measurement of the three correlation coefficients a, A, and B and the shape parameter b. The measurement of A and B requires precision neutron polarimetry. We will polarize a pulsed cold neutron beam from the SNS using a He-3 neutron spin filter. The well-known polarizing cross section for n-He-3 has a 1/v dependence, where v is the neutron velocity, which is used to determine the absolute beam polarization through a time-of-flight (TOF) measurement. We show that by measuring the TOF dependence of A and B, the coefficients and the neutron polarization can be determined with a small loss of the statistical precision and with negligible systematic error. We conclude that it is possible to determine the neutron polarization averaged over a long run in the neutron beta decay experiment with a statistical error less than 10(-4). We discuss various sources of systematic uncertainty in the measurement of A and B and conclude that the fractional systematic errors are less than 2 x 10(-4)
Hamiltonian paths through two- and three-dimensional grids
No full textThis paper addresses the existence of Hamiltonian paths and cycles in two-dimensional grids consisting of triangles or quadrilaterals, and three-dimensional grids consisting of tetrahedra or hexahedra. The paths and cycles may be constrained to pass from one element to the next through an edge, through a vertex, or be unconstrained and pass through either. It was previously known that an unconstrained Hamiltonian path exists in a triangular grid under very mild conditions, and that there are triangular grids for which there is no through-edge Hamiltonian path. In this paper we prove that a through-vertex Hamiltonian cycle exists in any triangular or tetrahedral grid under very mild conditions, and that there exist quadrilateral and hexahedral grids for which no unconstrained Hamiltonian path exists. The existence proofs are constructive, and lead to an efficient algorithm for finding a through-vertex Hamiltonian cycle
Measurement of the parity-violating neutron spin rotation in He-4
No full textIn the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths of these amplitudes from theoretical calculations are not well known, and experimental measurements of parity-violating (PV) observables in different nuclear systems have not constrained their values. Transversely polarized cold neutrons traveling through liquid helium experience a PV spin rotation due to the weak interaction with an angle proportional to a linear combination of these weak meson exchange amplitudes. A measurement of the PV neutron spin rotation in helium would provide information about the relative strengths of the weak meson exchange amplitudes, and with the longitudinal analyzing power measurement in the p + alpha system, allow the first comparison between isospin mirror systems in weak NN interaction. An earlier experiment performed at NIST obtained a result consistent with zero: phi(PV)(n, alpha) = (8.0 +/- 14( stat) +/- 2.2( syst)) x 10(-7) rad / m[1]. We describe a modified apparatus using a superfluid helium target to increase statistics and reduce systematic effects in an effort to reach a sensitivity goal of 10(-7) rad/m
A new method for precision cold neutron polarimetry using a He-3 spin filter
No full textWe present a new method for precision measurement of the capture flux polarization of a polychromatic ( white), continuous cold neutron beam, polarized by a He-3 spin filter. This method allows an in situ measurement and does not require knowledge of the neutron beam wavelength distribution. We show that a polarimetry precision of 0.1% is possible
First ever storage of ultracold neutrons in a magnetic trap made of permanent magnets
No full textFurther improvement in the accuracy of any neutron lifetime experiment by means of ultracold neutrons (UCN) in material bottles is limited due to unavoidable systematic effects when the UCN are reflected from the walls. However, such effects can be excluded in principle if magnetic trapping of UCN is used. The storage of UCN in a small magnetic trap made of permanent magnets was demonstrated for the first time ever. The measured storage time in this feasibility study was (882 +/- 16) s. At this level of accuracy no depolarization was observed
Detecting the radiative decay mode of the neutron
No full textBeta 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
Bounds on time reversal violation from polarized neutron capture with unpolarized targets
No full textWe have analyzed constraints on parity-odd time-reversal noninvariant interactions derived from measurements of the energy dependence of parity-violating polarized neutron capture on unpolarized targets. As previous authors found, a perturbation in energy dependence due to a parity ( P)-odd time (T)-odd interaction is present. However, the perturbation competes with T-even terms which can obscure the T-odd signature. We estimate the magnitudes of these competing terms and suggest strategies for a practicable experiment
Developing a more rapid test to assess sulfate resistance of hydraulic cements
No full textExternal sulfate attack of concrete is a major problem that can appear in regions where concrete is exposed to soil or water containing sulfates, leading to softening and cracking of the concrete. Therefore, it is important that materials selection and proportioning of concrete in susceptible regions be carefully considered to resist sulfate attack. American Society for Testing Materials ( ASTM) limits the tricalcium aluminate phase in cements when sulfate exposure is of concern. The hydration products of tricalcium aluminate react with the sulfates resulting in expansion and cracking. While ASTM standard tests are available to determine the susceptibility of cements to sulfate attack, these tests require at least 6 months and often up to a year to perform; a delay that hinders development of new cements. This paper presents a new method for testing cement resistance to sulfate attack that is three to five times faster than the current ASTM tests. Development of the procedure was based upon insights on the degradation process by petrographic examination of sulfate-exposed specimens over time. Also key to the development was the use of smaller samples and tighter environmental control
Portrait of Stephen Seltzer
No full textPortrait of Stephen Michael Seltzer. Seltzer's research has included work on dosimetry for medical and space applications. In 1999 Seltzer won the Department of Commerce Silver Medal for leadership in computational dosimetry. The award recognized Seltzer as “the world’s leading authority on radiation transport codes for high-energy ionizing radiation in the region from 1 keV up to 20 GeV, used by thousands world wide for applications in space, medicine and defense. He worked to develop the ETRAN Monte Carlo code in the 1960’s which provides the radiation physics base for the present generation of special purpose codes. All these codes rely on his definitive standard reference data as well as refinements to the theoretical methods used for modeling the charged particle interactions in matter.” In 2012 he was inducted into the NIST Gallery of Distinguished Scientists, Engineers, and Administrators
