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G. L. Greene

Publications and source records attributed to G. L. Greene.

34 records · Page 2Linked to original sources
arXiv2013

Improved Determination of the Neutron Lifetime

The most precise determination of the neutron lifetime using the beam method was completed in 2005 and reported a result of $τ_n = (886.3 \pm 1.2 [\textrm{stat}] \pm 3.2 [\textrm{syst}])$ s. The dominant uncertainties were attributed to the absolute determination of the fluence of the neutron beam (2.7 s). The fluence was measured with a neutron monitor that counted the neutron-induced charged particles from absorption in a thin, well-characterized 6Li deposit. The detection efficiency of the monitor was calculated from the areal density of the deposit, the detector solid angle, and the evaluated nuclear data file, ENDF/B-VI 6Li(n,t)4He thermal neutron cross section. In the current work, we have measured the detection efficiency of the same monitor used in the neutron lifetime measurement with a second, totally-absorbing neutron detector. This direct approach does not rely on the 6Li(n,t)4He cross section or any other nuclear data. The detection efficiency is consistent with the value used in 2005 but was measured with a precision of 0.057 %, which represents a five-fold improvement in the uncertainty. We have verified the temporal stability of the neutron monitor through ancillary measurements, allowing us to apply the measured neutron monitor efficiency to the lifetime result from the 2005 experiment. The updated lifetime is $τ_n = (887.7 \pm 1.2 [\textrm{stat}] \pm 1.9 [\textrm{syst}])$ s.

nucl-ex↗
arXiv2012

Neutron Beta Decay Studies with Nab

Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.

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arXiv2010

Study of the dependence of 198Au half-life on source geometry

We report the results of an experiment to determine whether the half-life of \Au{198} depends on the shape of the source. This study was motivated by recent suggestions that nuclear decay rates may be affected by solar activity, perhaps arising from solar neutrinos. If this were the case then the $β$-decay rates, or half-lives, of a thin foil sample and a spherical sample of gold of the same mass and activity could be different. We find for \Au{198}, $(T_{1/2})_{\rm foil}/(T_{1/2})_{\rm sphere} = 0.999 \pm 0.002$, where $T_{1/2}$ is the mean half-life. The maximum neutrino flux at the sample in our experiments was several times greater than the flux of solar neutrinos at the surface of the Earth. We show that this increase in flux leads to a significant improvement in the limits that can be inferred on a possible solar contribution to nuclear decays.

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arXiv2008

Nab: Measurement Principles, Apparatus and Uncertainties

The Nab collaboration will perform a precise measurement of 'a', the electron-neutrino correlation parameter, and 'b', the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will provide an independent measurement of lambda = G_A/G_V, the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of 'b' in neutron decay, which will provide an independent limit on the tensor weak coupling.

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arXiv2007

High-Efficiency Resonant RF Spin Rotator with Broad Phase Space Acceptance for Pulsed Polarized Cold Neutron Beams

We have developed a radio-frequency resonant spin rotator to reverse the neutron polarization in a 9.5 cm x 9.5 cm pulsed cold neutron beam with high efficiency over a broad cold neutron energy range. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to RF neutron spin flippers based on adiabatic fast passage. The spin rotator does not change the kinetic energy of the neutrons and leaves the neutron beam phase space unchanged to high precision. We discuss the design of the spin rotator and describe two types of transmission-based neutron spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically-polarized 3He neutron spin filters. The efficiency of the spin rotator was measured to be 98.0+/-0.8% on resonance for neutron energies from 3.3 to 18.4 meV over the full phase space of the beam. As an example of the application of this device to an experiment we describe the integration of the RF spin rotator into an apparatus to search for the small parity-violating asymmetry A_gamma in polarized cold neutron capture on para-hydrogen by the NPDGamma collaboration at LANSCE.

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arXiv2006

Upper Bounds on Parity Violating Gamma-Ray Asymmetries in Compound Nuclei from Polarized Cold Neutron Capture

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 $\vec{s_{n}} \cdot \vec{k_γ}$, where $\vec {s_{n}}$ is the neutron spin and $\vec{k_γ}$ 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.

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arXiv2006

General classification and analysis of neutron beta-decay experiments

A method for the general analysis of the sensitivities of neutron beta-decay experiments to manifestations of possible deviations from the Standard model is proposed. In a consistent fashion, we take into account all known (radiative and recoil) corrections which are incorporated within the Standard Model to provide a description of angular correlations in neutron decay in the first order of approximation, or down to the level of $\sim 10^{-5}$. The contributions from models beyond the Standard model are, for low energy neutron decay, parameterized in terms of vector, axial-vector, scalar and tensor coupling constants and in terms of parameters related to specific models. For the present analysis we derive the exact expressions for the neutron beta decay probability which includes all possible manifestations models beyond the Standard Model down to level of $\sim 10^{-5}$ without time-reversal violation. Based on the general expressions for manifestation of the deviations from the standard model, we present analysis of the sensitivities for selected neutron decay experiments.

nucl-th↗
arXiv2004

Measurement of the Neutron Lifetime by Counting Trapped Protons in a Cold Neutron Beam

A measurement of the neutron lifetime $τ_{n}$ performed by the absolute counting of in-beam neutrons and their decay protons has been completed. Protons confined in a quasi-Penning trap were accelerated onto a silicon detector held at a high potential and counted with nearly unit efficiency. The neutrons were counted by a device with an efficiency inversely proportional to neutron velocity, which cancels the dwell time of the neutron beam in the trap. The result is $τ_{n} = (886.6\pm1.2{\rm [stat]}\pm3.2{\rm [sys]})$ s, which is the most precise measurement of the lifetime using an in-beam method. The systematic uncertainty is dominated by neutron counting, in particular the mass of the deposit and the $^{6}$Li({\it{n,t}}) cross section. The measurement technique and apparatus, data analysis, and investigation of systematic uncertainties are discussed in detail.

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arXiv2003

Measurement of the Neutron Lifetime Using a Proton Trap

We report a new measurement of the neutron decay lifetime by the absolute counting of in-beam neutrons and their decay protons. Protons were confined in a quasi-Penning trap and counted with a silicon detector. The neutron beam fluence was measured by capture in a thin 6LiF foil detector with known absolute efficiency. The combination of these simultaneous measurements gives the neutron lifetime: tau_n = 886.8 +/- 1.2[stat] +/- 3.2[sys] seconds. The systematic uncertainty is dominated by uncertainties in the mass of the 6LiF deposit and the 6Li(n,t) cross section. This is the most precise measurement of the neutron lifetime to date using an in-beam method.

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arXiv2001

Measurement of the 3He mass diffusion coefficient in superfluid 4He over the 0.45-0.95 K temperature range

We have measured the mass diffusion coefficient D of 3He in superfluid 4He at temperatures lower than were previously possible. The experimental technique utilizes scintillation light produced when neutron react with 3He nuclei, and allows measurement of the 3He density integrated along the trajectory of a well-defined neutron beam. By measuring the change in 3He density near a heater as a function of applied heat current, we are able to infer values of D with 20% accuracy. At temperatures below 0.7 K and for concentrations of order 10^{-4} we find D=(2.0+2.4-1.2)T^-(6.5 -/+ 1.2) cm^2/s, in agreement with a theoretical approximation.

cond-mat↗
arXiv2001

Magnetic trapping of ultracold neutrons

Three-dimensional magnetic confinement of neutrons is reported. Neutrons are loaded into an Ioffe-type superconducting magnetic trap through inelastic scattering of cold neutrons with 4He. Scattered neutrons with sufficiently low energy and in the appropriate spin state are confined by the magnetic field until they decay. The electron resulting from neutron decay produces scintillations in the liquid helium bath that results in a pulse of extreme ultraviolet light. This light is frequency downconverted to the visible and detected. Results are presented in which 500 +/- 155 neutrons are magnetically trapped in each loading cycle, consistent with theoretical predictions. The lifetime of the observed signal, 660 s +290/-170 s, is consistent with the neutron beta-decay lifetime.

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arXiv2000

New Limit on the D Coefficient in Polarized Neutron Decay

We describe an experiment that has set new limits on the time reversal invariance violating D coefficient in neutron beta-decay. The emiT experiment measured the angular correlation J . p_e x p_p using an octagonal symmetry that optimizes electron-proton coincidence rates. The result is D=[-0.6+/-1.2(stat)+/-0.5(syst)]x10^(-3). This improves constraints on the phase of g_A/g_V and limits contributions to T violation due to leptoquarks. This paper presents details of the experiment, data analysis, and the investigation of systematic effects.

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arXiv2000

Magnetic Trapping of Neutrons

Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force and the creation of matter during the Big Bang. Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral and charged particle traps are widely used tool for studying both composite and elementary particles, because they allow long interaction times and isolation from perturbing environments. Here we report the magnetic trapping of neutrons. The trapping region is filled with superfluid 4-He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons have a lifetime in the trap of 750 +330/-200 seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement.

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