to develop and provide neutron standards and measurements needed for fundamental physics, homeland security, the hydrogen economy, worker protection, and nuclear power.
INTENDED OUTCOME AND BACKGROUND
The Neutron Interactions and Dosimetry Group maintains and supports the nation’s premier fundamental neutron physics user facilities, including a weak
interactions neutron physics station, the Neutron Interferometry and Optics Facility (NIOF), the Ultra Cold Neutron Facility (UCNF), an Advanced
Monochromatic Neutron Test Facility, and the nation’s only high-resolution Neutron Imaging Facility (NIF) for fuel-cell research. We maintain and
disseminate measurement standards for neutron dosimeters, neutron survey instruments, and neutron sources, and improve neutron cross-section standards
through both evaluation and experimental work.
The Group is at the forefront of basic research with neutrons. Experiments involve precision measurements of symmetries and parameters of the “weak”
nuclear interaction, including measuring the neutron lifetime using both thermal and ultracold techniques, setting a limit on an important
time-reversal-violating asymmetry coefficient, and characterizing the radiative decay mode of the neutron. These data address fundamental issues
that are important in the understanding of theories of evolution of the cosmos. It is an internationally renowned program that maintains an
extensive level of cooperation with premier national and international academic and research institutions.
The neutron interferometry program provides the world’s most accurate measurements of neutron coherent scattering lengths, which are important to
materials science research and modeling of the nuclear potentials. We recently carried out new interferometry experiments to determine the charge
distribution of the neutron and for reciprocal space imaging. We are developing and promoting the applications of efficient neutron spin filters
based on laser-polarized 3He, and are pursuing applications for these filters at NIST as well as at neutron research centers throughout the U.S.
To support neutron standards for national security needs, we are developing key technical infrastructure. Advanced liquid scintillation neutron
spectrometry techniques will lead to characterization of neutron fields and detection of concealed neutron sources with low false-positive rates.
We are planning to organize and lead a Consultative Committee for Ionizing Radiation (CCRI) comparison of thermal neutron fluence rate measurements,
characterizing four different beam qualities at the NCNR, and to carry out comparisons of NIST standard neutron sources. The Group is leading an
effort that will result in a new international evaluation of neutron cross-section standards.
Accomplishments
Active Interrogation Standards
Active interrogation involves directing nuclear radiation into a closed container and measuring secondary radiations to gain information about the
contents of the container. Typically, but not always, neutrons are used as the impinging radiation. Active interrogation has the potential for
detecting smaller quantities of special nuclear materials than is possible by using passive detectors. It also holds the promise of detecting
nonnuclear materials, hazardous chemicals, and explosives.
NIST organized a drafting committee and held four meetings to prepare a new ANSI Standard, N42.41, Minimum Performance Criteria for Active
Interrogation Systems used for Homeland Security, which is being published by the IEEE. A cargo-container testbed with three massive cargo regions,
as needed for testing under ANSI N42.41, has been set up for use at NIST. It is capable of mobility to other locations.
First Observation of the Radiative Decay Mode of the Neutron
Beta decay of the neutron into a proton, electron, and electron antineutrino is occasionally accompanied by the emission of a photon. An experiment to
study the radiative beta-decay of the neutron was completed at the NG-6 fundamental physics end station. The experiment employed the magnet previously
used for the NIST proton trap neutron lifetime apparatus, with the addition of a detector for photons with energies above 15 keV. A photon detector
that operates efficiently in the high magnetic field, low temperature environment of the magnet was developed. This apparatus allowed half of the
available electrons and all of the available protons from neutron beta-decay to be detected, which provided a high-rate, background-rejecting trigger
for the observation of radiative decay photons. In this first-generation experiment, the photon detector consisted of a single 12 mm b y 12 mm b y 200
mm scintillating cr ystal coupled to an avalanche photodiode.
We observed electron-proton-photon coincidences that were unambiguously due to the radiative decay of the neutron.
We reported the first observation of the radiative decay mode, and a manuscript detailing the measurement was published in the journal Nature.
Seeing Inside an Operating PEM Fuel Cell
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Figure 6. An image of water content in a representative section of an operating PEM fuel cell,
in the “through-plane” dimension. Corresponding water content is shown inthe graph.
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We are applying neutron-imaging methods for industrial research on water transport in fuel cells and on hydrogen distribution in hydrogen storage
devices. This facil-ity has provided critical services to major automotive and fuel cell companies during the last few years. This is a high demand
and high profile, nationally recognized program.
Recent advances in neutron imaging detec-tor technology, based on microchannel plates (MCPs) have resulted in an order-of-magnitude improvement in
the achievable neutron image spatial resolution. This enhanced resolution of 25 µm allo ws the fuel cell researcher to directly measure the
through-plane water distribution in the gas diffusion layer (GDL) and distinguish the water in the anode from that in the cathode in the membrane
electrode assembly (MEA). This was not possible with scintillator-based neutron detection. Measuring the through-plane water distribution gives
insight into the water transport mechanisms in the GDL and MEA. Ongoing developments of neutron MCP detectors should reach an ultimate resolution of
about 10 µm, which will enable the study of commercially viable membranes that are from 25 µm to 50 µm thick.
The initial effort at visualizing the in situ through-plane water distribution of a proton exchange membrane (PEM) fuel cell was recently published,
using an MCP detector at the thermal neutron imaging facility at the NIST Center for Neutron Research, as shown in Fig. 6. The visualization of the
through-plane water distribution with a spatial resolution of 30 µm with neutron imaging was demonstrated.
The analysis suffered from the inability to resolve the membrane swelling. Future work will focus on mitigating this problem.
For instance, thinner membranes will have a smaller total volume change, reducing the problem. Additionally, rather than being directly coupled to
the detector, the cell will be mounted from the bottom, providing a reference surface for determining the location of cell components.
Future work with the enhanced resolution will focus on measuring the GDL porous media properties and their interplay with the dynamic fuel cell
operation. Also, using thick membranes, we will measure the proton conductivity as a function of membrane hydration in an operating fuel cell.
Measurements of the Neutron
Vertical Coherence
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Figure 7. Schematic diagram of the coherence length experiment.
The neutron is coherently split and recombined after traveling two paths.
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We have measured the vertical coherence function of a single crystal neutron interferometer for different vertical beam distributions.
We carried out the measurements by introducing a vertical path separation via a pair of prisms placed in thetwo beam paths of a single crystal
neutron interferometer and measured the loss in fringe visibility (contrast) as this separationis increased. This loss of contrast is directly
related to the vertical coherence of the neutron beam. We showed that the measured coherence length is consistent with the experimental distribution
of the incomingneutron beam momentums in the vertical direction. We also demonstrated that the loss in contrast with beam displacementin one leg of
the interferometer could be recovered by introducing a corresponding displacement in the second leg of theinterferometer.
Results from this experiment raise intriguing possibilities for the development of Fourier Spectroscopy to enable very precise direct measurement of
vertical momentum distribution of beams reflected from surfaces and hence allow characterization of various surface properties (as done in normal
reflectivity measurements) of a sample. Having a single crystal neutron interferometer with a long coherence length also provides studies coherence
scattering over scales that are not easily accessible by other approaches.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
