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 provides measurement
services, standards, and fundamental
research in support of NIST's
mission as it relates to neutron technology
and neutron physics. The national
interests served include industrial
research and development, national
defense, homeland security, higher education,
electric power production, and
more specifically, neutron imaging,
scientific instrument calibration and
development, neutron source calibration,
detection of concealed nuclear
materials, radiation protection, and
nuclear and particle physics data.
The Group maintains and disseminates
measurement standards for neutron
dosimeters, neutron survey instruments,
and neutron sources. The Group has
initiated an accelerated program to
develop effective neutron interrogation
and measurement standards for national
defense and homeland security needs,
in collaboration with the Radioactivity Group.
The Group maintains and supports the
Nation's premier fundamental neutron
physics user facility. The Neutron
Interferometry and Optics Facility is
one of the most advanced user facilities
of this kind in the world. The Group's
neutron interferometry program provides
the world's most accurate measurements
of neutron coherent scattering
lengths, important to materials science
research and modeling of the nuclear
potentials. Preparations for new neutron
interferometry experiments for surface
studies of multilayers, for determination
of the charge distribution in the neutron,
and for investigation of quantum
information/coherence phenomena are being carried out.
The Group is at the forefront of basic
research with neutrons in the measurements
of symmetries and parameters
of the weak nuclear interactions,
addressing 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.
We have developed and we maintain the
Nation's only high-resolution neutron
imaging user facility. It is available for
both proprietary and open research at
the NIST Center for Neutron Research
(NCNR). Research at this facility is primarily
dedicated to applying neutron
imaging methods for industrial research
on water transport in fuel cells and on
hydrogen distribution in hydrogen
storage devices. Neutron imaging, vigorously
pursued by the Group, has rapidly
developed to be the technique of choice
for nondestructive, in situ water transport
studies in PEM-type fuel cells.
This facility has provided critical
services to major automotive and fuel
cell companies, and continues to be in high demand.
The Group is also among the world
leaders in developing 3He-based neutron
polarization and analysis techniques.
We are developing and promoting the
applications of neutron spin filters based
on laser-polarized 3He. The technique
has the unique capability to polarize a
polychromatic neutron beam without
introducing additional beam divergence.
This is a very attractive feature for many
materials science and fundamental
physics applications. We are pursuing
applications at the NCNR, the Intense
Pulsed Neutron Source (IPNS) at
Argonne National Laboratory, and the
Los Alamos Neutron Science Center.
Accomplishments
Neutron Technology for Homeland Security
We have participated in the development
of three new ANSI Standards,
N42.32, N42.34, and N42.35, which
provide test and evaluation protocols for
neutron detection in personal electronic
radiation monitors ("pagers"), isotope
identifier instruments, and portal monitors,
respectively. We are managing the
procurement, calibration, and dissemination
of standard, low-intensity neutron
sources to three national laboratories
for testing of commercial instruments
under these standards. We are
also leading the development of new,
additional ANSI standards on neutron
survey meters and active interrogation
devices for detection of fissile material,
explosives, and chemical agents.
The challenge of calibrating low-intensity
sources has motivated the development
of a new source comparator with a
reduced-volume manganese sulfate bath.
For measurement of neutron radiation
protection levels around active interrogation
devices, we have procured and
calibrated state-of-the-art commercial
neutron spectrometry and absorbed-dose measurement systems.
We are developing neutron spectrometers
and epithermal/thermal neutron
detectors with much higher sensitivity
than current state-of-the-art systems.
We are experimenting with muon anticoincidence
techniques for reducing the
cosmic-ray component of neutron
background radiation. Finally, we have
procured and calibrated a small, sealed-tube
source for generating 14 MeV and
2.5 MeV fast neutrons, for testing our
new spectroscopy systems, and for
developing radiation-protection
dosimetry capabilities up to 14 MeV.
New Imaging Station for Fuel Cell Research
The Neutron Imaging Facility started
full-time operation in 2003 at Beam
Tube 6 (BT-6) at the NCNR. The facility
has an extremely high fluence rate of
1.8 × 107 cm-2 s-1
uniformly spread over a 26 cm diameter area, with an
L/d ratio of 300, which is needed to achieve
optimum geometric sharpness of the
image. This is one of the most advanced
neutron imaging facilities in the world
and the best of its kind in the U.S.
We have pioneered nondestructive
imaging techniques to map critically
important hydrogen and water motion
in operating fuel cells. This facility has
been hosting experiments from industrial
fuel cell developers, interested in looking
at the distribution of water in operational
fuel cell systems using standard
fuel cell hardware. First-generation neutron
experiments with a major industrial
partner show exciting promise of
robust/efficient fuel cell design, substantial
reduction in fuel cell development
time, and establishment of uniform
characterization/performance standards.
This program has been awarded NIST
competence research funding and, in
2003, DOE recognized this program as
one of the top-ten programs relevant to
the "FreedomCAR" program initiated by the President.
Neutron Measurements to
Test "Standard Model"
| |
Figure 3. Pieter Mumm (second from right) and colleagues
setting up a neutron decay detector for the time reversal
asymmetry experiment (emiT).
|
The emiT experiment successfully
completed its data acquisition on
NG-6 in December 2003. (See Fig. 3.)
The experiment is a collaboration
among NIST, the University of Washington, the University of
California (Berkeley), the University
of Michigan, Hamilton College, and Tulane University.
The experiment searches for (or will set
an improved upper bound on) the time-reversal
asymmetry term in neutron beta
decay. It does so by measuring electron-proton
coincidence events from the
decay of polarized neutrons. An asymmetry
in coincidence pairs is formed as
a function of the direction of the neutron
spin. A measurement of a nonzero
asymmetry would be an indication of time-reversal violation.
The measured electron-proton coincidence
rate is a factor-of-ten higher than
in the first run. In addition, the signal-to-background
ratio is two orders of magnitude higher. The analysis of the
data is in progress, and we anticipate a
result that is approximately a factor of
four better than the world current limit.
Work has commenced at NG-6 on an
effort to measure the radiative decay
mode of the free neutron. The usual
beta decay of the neutron into a proton,
electron, and antineutrino is occasionally
accompanied by the emission of a
photon. Despite decades of detailed
experimental studies of neutron beta
decay, this branch of a fundamental
weak decay has never been observed.
Our technique is based on measuring
the triple coincidence of the proton and
electron along with the rare photon.
The experiment should not only
definitively see radiative neutron beta
decay for the first time, but also perform
a 5 % measurement of the photon spectrum.
Polarized 3He Neutron Spin
Filters for Fundamental and Applied Research
| |
Figure 4. Polarization analysis of neutron reflection
from a sample of cobalt "antidots," obtained with a
3He analyzer and a position-sensitive detector.
Qz and Qx are the z and x
components of the wavevector transfer; spin-flip
scattering from magnetic domains is evident in
the off-specular data. |
In collaboration with scientists in the
NIST Materials Science and
Engineering Laboratory and Indiana
University, we are employing 3He-based
polarization analysis for diffuse reflectometry
at the new Advanced Neutron
Diffractometer/Reflectometer at the
NCNR, and the POSY-I reflectometer at IPNS.
Our current focus is on studies of
patterned magnetic materials, which are
relevant to the development of future
magnetic storage media (Fig. 4). In
addition, we are collaborating with
researchers from Hamilton College,
IPNS, and the Spallation Neutron
Source in the application of a continuously
operating 3He polarizer for
single-crystal diffractometry.
In fundamental physics, we are contributing
to the operation and further
development of the 3He polarizer for an
experiment in weak interaction physics
(the "npdγ" experiment) at Los Alamos
National Laboratory. In addition, we are
developing 3He cells for a planned
measurement of the spin dependence of
the 3He scattering length on the NCNR
neutron interferometer. In collaboration
with the University of Utah, we have
documented remarkable magnetic
effects in the polarized 3He storage cells,
which are relevant to suppressing wall relaxation time.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
