Neutron Standards and Measurements:
to develop and provide neutron standards and
measurements needed for worker protection, nuclear power, homeland security,
and fundamental applications.
INTENDED OUTCOME AND
BACKGROUND
The Neutron Interactions and Dosimetry Group has initiated a
homeland-security program to improve the ability to detect fissile materials in
transit. Working with the Radioactivity Group, we are creating a
cargo-container inspection testbed at NIST in order to improve our ability to
test commercially developed, homeland-security neutron detectors. In addition
to testing detectors, the group is developing a high-sensitivity neutron
spectrometer for detecting fissile materials in transit.
Another project with homeland-security implications, as well as energy and
environmental applications, involves imaging chemical processes in fuel cells.
Compared to other forms of radiation, neutrons are highly efficient in probing
complex structures. This is because of their tremendous penetration capability
in almost all known materials and their unique ability to distinguish different
materials with very similar physical properties. They are particularly
effective in detecting hydrogenous materials and light elements.
A new, neutron-imaging facility has been constructed at the high-intensity,
thermal neutron beamline, BT6, at the NIST National Center for Neutron Research
(NCNR) nuclear reactor. The primary use of this facility will be to
non-destructively characterize water-transport mechanisms using real-time
imaging of fuel cells.
Neutron lifetime and decay-asymmetry experiments improve our knowledge of the
fundamental nucleon weak couplings and may ultimately lead to a significant
test of the unitarity of the CKM matrix for three generations of quarks. If
these measurements establish a violation of unitarity, then it will provide an
important clue to the physics beyond the Standard Model of particle physics.
The search for a time-reversal-violating asymmetry also probes physics beyond
the Standard Model. We operate three beams at the NCNR to carry out such
measurements. These include a 0.5 nm monochromatic beam, a 0.9 nm
monochromatic beam dedicated to ultracold neutron production, and a
high-intensity polychromatic beam.
The primary focus of our polarized 3He program is the development of
neutron spin filters and application of these devices to both neutron
scattering and fundamental neutron physics. We are developing two
optical-pumping methods, spin-exchange and metastability-exchange, for producing
the polarized 3He gas. We are pursuing applications at the NCNR, the
Intense Pulsed Neutron Source at Argonne National Laboratory, and the Los
Alamos Neutron Science Center.
The Neutron Interferometry and Optics Facility is a national user facility
operated by the group. Radiography and tomography services and research involve
academic and industrial customers and collaborators. During the past year,
analysis of the n-H, n-D, and n-3He scattering-length data was
completed and the time-dependent water distribution in an operating fuel cell
was non-destructively imaged and quantified. The success of the fuel-cell
imaging drove the development of the neutron-imaging facility described above.
Neutron cross-section standards for key, fundamental reactions are important
since almost all neutron cross sections are measured relative to them. Any
improvement in a cross-section standard leads to improvement in all
measurements that have been or will be made relative to that standard. We have
improved neutron cross-section standards through both data evaluation and
experimental work, and are leading an effort that will result in a new
international evaluation of neutron cross-section standards.
Accomplishments
Detection of Nuclear Materials in Transport
Spent fuel from nuclear reactors, weapons-grade plutonium, and certain
weapons-initiator materials emit large numbers of neutrons from spontaneous
fission or ( ,n) reactions. These are
materials which terrorist groups could obtain fairly easily for constructing
radiation-dispersal weapons or fission weapons. Because U.S. borders are easily
crossed in many wilderness areas, and because these materials could be obtained
from sites within the U.S., it is not sufficient to inspect baggage and cargo
at points of entry.
Very sensitive neutron detectors coupled with surveillance cameras could be
positioned at highway choke points on the outskirts of major cities or near
other sensitive locations to detect neutrons from contraband nuclear components,
and provide warnings to security personnel to try to prevent an attack. The
major advantage of neutron detectors over beta and gamma detectors is that the
background rates and incidences of false positives would be much lower. There
is only a very low level of neutron background due to cosmic rays, much lower
than the natural beta/gamma background.
We are developing a very-high-sensitivity neutron spectrometer that could be
configured for unattended, remote-sensing duty. This spectrometer is based on
photon emission from recoil protons in a liquid scintillator followed by
thermal neutron capture. It uses a segmented geometry to improve energy
resolution. This spectrometer is based on a technique developed for detecting
background neutrons in a neutrino-detection experiment and is expected to have
sensitivity of 10 % or higher. The energy resolution will be optimized for
detecting secondary fission neutrons from 235U triggered by active
portal monitors and released through the ( ,n) reaction or fast-neutron activation.
Advances in Neutron Imaging of Fuel Cells
Neutron imaging is ideally suited for non-destructive, in situ
visualization and quantification of water-transport phenomena in operating,
polymer-electrolyte-membrane (PEM) fuel cells.
| |
Figure 3. Image of water transport in an operating fuel cell obtained by
neutron transmission radiography. |
With a prototype facility we have demonstrated that the time-dependent water
distribution in an operating fuel cell could be non-destructively imaged and
quantified. A one-second time resolution was achieved. The total water content
inside the fuel cell as a function of time was also quantified. The water
content showed periodic behavior with time. From time-lapse images it appeared
that the water drains only when it reaches a certain volume. The periodic
drainage also appeared to be uncorrelated with the temperature fluctuations and
hydrogen-flow variations (which were small and random, inconsistent with
observed periodicity).
Because the quantification of time-dependent water distribution is a critical
tool in theoretical modeling of fuel-cell water-transport characteristics, and
because our capabilities are unique in the world, we have recently designed a
new, state-of-the-art neutron-imaging facility for fuel-cell research. This
facility will be used for critical water-transport studies in the membrane
electrode assembly (MEA) and the flow channels, for the measurements of the
hydrogen-diffusion coefficient across the MEA and water/vapor phase, and for
evaluation of the integrity of various interfaces.
Prospects for an Improved Measurement of the Neutron Lifetime: Magnetic
Trapping of Ultracold Neutrons
Recent success in magnetic confinement of ultracold neutrons in an Ioffe-type
superconducting magnetic trap should lead to an improved measurement of the
neutron lifetime  n.
| |
Figure 4. Students in Guide Hall at experiment station for magnetic
trapping of ultracold neutrons. |
The trap is loaded through inelastic scattering of 0.89 nm neutrons with
phonons in superfluid 4He. Trapped neutrons are detected when they
beta decay. Energetic decay electrons ionize helium atoms in the superfluid,
resulting in efficient conversion of electron kinetic energy into light
(scintillation). The changing rate of neutron decays in the trap is
proportional to the number of neutrons in the trap. To the extent that this
population is changing only by the beta-decay of the trapped neutrons, the rate
of detected neutron decays will decrease exponentially with a lifetime of
n.
The advantages of this technique over previous experiments are: continuous
detection of scintillations from decay electrons, and the elimination of wall
losses and betatron oscillations. Analysis indicates that systematic errors due
to neutron losses should be controllable to 10-5 n. A measurement of
n at the 10-3 level
of statistical accuracy should be possible, given the flux available from the
NG6 beamline at the NCNR.
Neutron Spin Filters Based on Polarized 3He
Notable progress has been made in developing and applying neutron spin filters
based on polarized 3He gas. We continue to employ two
optical-pumping methods, spin-exchange and metastability-exchange, to produce
the polarized gas for applications in both neutron scattering and fundamental
neutron physics.
Specular reflection tests using a polarized-3He spin filter on the
NCNR neutron guide, NG1, reflectometer were performed to compare the use of a
3He analyzer to that of a conventional supermirror analyzer. The
3He cell was 6 cm in diameter, and the gas was polarized to
57 % using spin-exchange techniques. The cell was polarized off-line and
used on the beamline in the absence of optical pumping. The polarization
lifetime of the cell is 550 h, which is close to the theoretical limit set
by dipole-dipole relaxation. A compact, magnetically-shielded solenoid
permitted a relaxation time of 350 h on the NG1 instrument in the presence
of stray fields from a 0.6 T magnet at the sample position and guide field
magnets.
The efficiency of 3He spin filters depends strongly on the
achievable 3He polarization. We have reproduced a value of 70 %
polarization that the University of Wisconsin had obtained with a unique,
optical quality, long-lifetime cell provided by us. We have also duplicated the
spectrally-narrowed, high-power diode laser developed at Wisconsin, which has
particular relevance to the size and pressure ranges of neutron-spin-filter
cells. In large spin-filter cells, we have obtained 55 % to 65 %
3He polarization with both broadband lasers and the spectrally
narrowed system, but the much higher efficiency of the narrowed system will
permit further improvement with modest laser-power levels.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
