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nce again, it is a pleasure to be able to reflect on the accomplishments of the NIST Center for Neutron Research over the past year. The reactor shim control arms were replaced during a planned shutdown early in 2000. As a result, the reactor was scheduled to operate for 212 days during the reporting period, and did operate 198 days, or 93 % of the scheduled time, as a consequence of one unplanned maintenance shutdown. Construction was begun on the new cooling tower which will not only provide needed capability for the next 25 years, but will also reduce the plume visibility during cold weather. The cold source availability for the period was 98 %; i.e., the cold source held the reactor from operation 4 days during the year. The second-generation liquid hydrogen cold source passed all required pressure tests, and the final assembly is now being prepared for insertion into the reactor in 2001. Finally, steady progress has been made in preparing for a license renewal application to the Nuclear Regulatory Commission, in order to extend the period of operation beyond 2004.
Three high-resolution inelastic scattering instruments, the High Flux Backscattering Spectrometer, the Disk Chopper time-offlight Spectrometer, and the Neutron Spin Echo spectrometer (highlighted in the 1999 report), are now being offered to users who can tolerate the quirks inherent in getting a new instrument on-line. USANS, the perfect crystal small angle scattering spectrometer (part of the NSF/NIST CHRNS), is installed at the reactor and available for proposals; the first phase of the high intensity Filter Analyzer Neutron Spectrometer is operating with high intensity and good backgrounds; and the design and manufacture of new thermal neutron spectrometers is underway. (USANS, DCS, and FANS are highlighted in this report.) This simultaneous development program has put severe strains on our resources, but we can now look forward to many years of benefit from the results. During the past year, a
proposal was made to the National Science Foundation (NSF) to allow joint NIST/NSF operation of the three high-resolution instruments, and to construct a new cold neutron triple axis spectrometer. The final disposition of this application is not yet available, but the external reviews are complete, a site visit has taken place, and the NSF is now considering the appropriate action. If this proposal is funded, then we will be able to operate these new inelastic scattering instruments properly in a full user mode.
Finally, as always, the results are seen in the output of the researchers who use the facility. As has become our practice, we are presenting highlights of this work in the following chapters of this report. I think that all can agree that the results truly speak for themselves.
THE NIST CENTER FOR NEUTRON RESEARCH (NCNR)
odern technological society is dependent upon increasingly
sophisticated use of materials, many of whose attributes are dictated by their sub-microscopic structural and dynamical properties. A wide range of scientific techniques, of which the many types of scattering (for example, x-rays, light, electrons, neutrons) are arguably the most important, provide knowledge of these properties. Of these probes, neutrons are perhaps least familiar, but they provide important advantages for many types of measurements.
Neutrons, as prepared for use at modern sources, are moving at speeds comparable to those of atoms moving at room temperature, thus providing the ability to probe dynamical behavior. At the same time, neutrons are well matched to measurements at length scales ranging from the distances between atoms to the size of biological or polymer macromolecules. Neutrons are sensitive to the magnetic properties of atoms and molecules, allowing study of the underlying magnetic properties of materials. They also scatter quite differently from normal hydrogen atoms than they do from heavy hydrogen (deuterium), allowing selective study of individual regions of molecular systems. Finally, neutrons interact only weakly with materials, providing the opportunity to study samples in different environments more easily (at high pressures, in shear, in reaction vessels, etc.), and making them a non-destructive probe. These favorable properties are offset by the relative weakness of the best neutron sources compared to x-ray or electron sources, and by the relatively large facilities required to produce neutrons. As a result, major neutron sources are operated as national user facilities to which researchers come from all over the United States (and abroad) to perform small-scale science using the special measurement capabilities provided.
In addition to scattering measurements, neutrons can be used to probe the atomic composition of materials by means of capture and resultant radioactive decay. The characteristics of the decay act as "fingerprints" for particular atomic nuclei, allowing studies of environmental samples for pollutants (e.g., heavy metals), characterization of Standard Reference Materials, and many other essential measurements. While the scattering and capture users of neutrons are little concerned with understanding the inherent properties of the neutron, there are important areas in physics that can be explored by carefully measuring fundamental neutron behavior. Examples include the lifetime of the free neutron, an important quantity in the theory of astrophysics; the beta decay process of the neutron, the
details of which are stringent tests of nuclear theory; and the effects of various external influences such as gravity or magnetic fields on
The NCNR utilizes neutrons produced by the 20 MW NIST Research Reactor to provide facilities, including the Nation's only internationally competitive cold neutron facility, for all of the above types of measurements to a national user community. There are approximately 35 stations in the reactor and its associated beams that can provide neutrons for experiments. At the present time 27 of these are in active use, of which 6 provide high neutron flux positions in the reactor for irradiation, and 21 are beam facilities. A schematic layout of the beam facilities and brief descriptions of available instrumentation are given below. More complete descriptions can be found at http://www.ncnr.nist.gov.
These facilities are operated both to serve NIST mission needs and as a national facility, with many different modes of access. Some instrumentation was built years ago, and is not suited to general user access; however, time is available for collaborative research. NIST has recently built new instrumentation (see the highlights in this report on FANS, DCS, and USANS), and reserves 1/3 of available time for mission needs with the balance available to general users. In other cases, instrumentation was built and is operated by Participating Research Teams (PRT); PRT members have access to 75% of available time, with the balance available to general users. In a special case, NIST and the National Science Foundation established the Center for High Resolution Neutron Scattering at the NCNR, with a 30 m Small Angle Scattering (SANS) instrument, a cold neutron triple axis spectrometer, and the thermal neutron perfect crystal SANS commissioned this year. For these facilities, most time is available for general users. While most access is for research, whose results are freely available to the general public, proprietary research can be performed under full cost recovery. Each year, approximately 1600 researchers (persons who participated in experiments at the facility, but did not necessarily come here) from all areas of the country, from industry, academe, and government use the facility for measurements not otherwise possible. The research covers a broad spectrum of disciplines, including chemistry, physics, biology, materials science, and engineering.