J. C. Ramsey

Tomographic Imaging with Cosmic Ray Muons

Over 120 million vehicles enter the United States each year. Many are capable of transporting hidden nuclear weapons or nuclear material. Currently deployed X-ray radiography systems are limited because they cannot be used on occupied vehicles and the energy and dose are too low to penetrate many cargos. We present a new technique that overcomes these limitations by obtaining tomographic images using the multiple scattering of cosmic radiation as it transits each vehicle. When coupled with passive radiation detection, muon interrogation could contribute to safe and robust border protection against nuclear devices or material in occupied vehicles and containers.

Determination of the axial-vector weak coupling constant with ultracold neutrons.

A precise measurement of the neutron decay β asymmetry A₀ has been carried out using polarized ultracold neutrons from the pulsed spallation ultracold neutron source at the Los Alamos Neutron Science Center. Combining data obtained in 2008 and 2009, we report A₀ = -0.119 66±0.000 89{-0.001 40}{+0.001 23}, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon g{A}/g{V}=-1.275 90{-0.004 45}{+0.004 09}.

Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

How long does a neutron live? Unlike the proton, whose lifetime is longer than the age of the universe, a free neutron decays with a lifetime of about 15 minutes. Measuring the exact lifetime of neutrons is surprisingly tricky; putting them in a container and monitoring their decay can lead to errors because some neutrons will be lost owing to interactions with the container walls. To overcome this problem, Pattie et al. measured the lifetime in a trap where ultracold polarized neutrons were levitated by magnetic fields, precluding interactions with the trap walls (see the Perspective by Mumm). This more precise determination of the neutron lifetime will aid our understanding of how the first nuclei formed after the Big Bang. Science, this issue p. 627; see also p. 605 Ultracold polarized neutrons are levitated in a trap to measure their lifetime with reduced systematic uncertainty. The precise value of the mean neutron lifetime, τn, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

A multilayer surface detector for ultracold neutrons

Abstract A multilayer surface detector for ultracold neutrons (UCNs) is described. The top 10 B layer is exposed to vacuum and directly captures UCNs. The ZnS:Ag layer beneath the 10 B layer is a few microns thick, which is sufficient to detect the charged particles from the 10 B(n,α) 7 Li neutron-capture reaction, while thin enough that ample light due to α and 7 Li escapes for detection by photomultiplier tubes. A 100-nm thick 10 B layer gives high UCN detection efficiency, as determined by the mean UCN kinetic energy, detector materials, and other parameters. Low background, including negligible sensitivity to ambient neutrons, has also been verified through pulse-shape analysis and comparison with other existing 3 He and 10 B detectors. This type of detector has been configured in different ways for UCN flux monitoring, development of UCN guides and neutron lifetime research.

New result for the neutron

Background: The neutron β-decay asymmetry parameter A_0 defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A_0 permit an extraction of the ratio of the weak axial-vector to vector coupling constants, λ≡gA/gV, which under assumption of the conserved vector current hypothesis (gV=1) determines gA. Precise values for gA are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron β-decay angular correlation performed with UCN. This article reports the most precise result for A_0 obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A_0. Results: The UCNA experiment reports a new 0.67% precision result for A_0 of A_0=−0.12054(44)_(stat)(68)_(syst), which yields λ=gA/gV=−1.2783(22). Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces A0=−0.12015(34)stat(63)syst and λ=gA/gV=−1.2772(20). Conclusions: This new result for A0 and gA/gV from the UCNA experiment has provided confirmation of the shift in values for gA/gV that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were >10%, whereas those in the more recent ones (published after 2002) have been of the scale of <2%. The impact of these older results on the global average will be minimized should future measurements of A0 reach the 0.1% level of precision with central values near the most recent results.

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A technical demonstration of cosmic-ray muon radiography of a UO2 fuel assembly was performed at Toshiba Nuclear Critical Assembly (NCA). The fuel assembly in the NCA was imaged through obstacles such as steel and concrete. The result suggested that the method can be applicable to assess the damage to the reactors at the Fukushima Daiichi nuclear power plant. Here, both scattering and displacement methods are presented, and the results are shown to agree with Monte Carlo simulations. In addition, detailed Monte Carlo simulations of the Fukushima Daiichi reactor were performed, which showed capability of muon radiography to locate the fuel in the damaged reactors.

-asymmetry parameter

A 1.2 × 1.2 m2 muon tracker was moved from Los Alamos to the Toshiba facility at Kawasaki, Japan, where it was used to take ∼4 weeks of data radiographing the Toshiba Critical Assembly Reactor with cosmic ray muons. In this paper, we describe the analysis procedure, show results of this experiment, and compare the results to Monte Carlo predictions. The results validate the concept of using cosmic rays to image the damaged cores of the Fukushima Daiichi reactors.

A_0

In this paper, we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons (UCN), which provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. An active detector that can be lowered into the trap has been used to measure the neutron distribution as a function of height and measure the influence of marginally trapped UCN on the neutron lifetime measurement. In addition, measurements have demonstrated phase-space evolution and its effect on the lifetime measurement.

from UCNA

A new measurement of the neutron β-decay asymmetry A_0 has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to A_0=−0.11954(55)_(stat)(98)_(syst), corresponding to the ratio of axial-vector to vector coupling λ ≡ g_A/g_V = −1.2756(30).

Cosmic-ray muon radiography of UO2 fuel assembly

The study of neutron cross sections for elements used as efficient “absorbers” of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, “absorption” includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH_2 using simultaneous measurements of the radiative capture cross sections for these elements. We measured σ_(up)=1972±130 b for hydrogen in CH_2, which is below theoretical expectations, and σ_(up)=25±9 b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids.