INDEPENDENT PROGRAMS

There are a number of programs of long standing located at the NCNR that involve other parts of NIST, universities, industrial laboratories, or other government agencies.

The Polymers Division of the Materials Science and Engineering Laboratory has two major program elements at the NCNR. In the first, the purpose is to help the U.S. microelectronics industry in addressing their most pressing materials measurement and standards issues. In today's ICs and packages the feature size on a chip is ever shrinking, approaching 250 nm, while the size of a polymer molecule is typically 5 nm to 10 nm. As feature size shrinks, the structure and properties of interfaces play an increasingly important role in controlling the properties of the polymer layers used in interconnects and packages. NIST scientists use both neutron reflectivity and other neutron scattering methods to characterize polymer/metal interfaces with regard to local chain mobility, moisture absorption, glass transition temperature, and crystalline

structure.

In the second program element, the objective is to understand underlying principles of phase behavior and phase separation kinetics of polymer blends, both in the bulk and on surfaces, in order to help control morphology and structure during processing. SANS and reflectivity measurements in equilibrium, in transient conditions, and under external fields, provide essential information for general understanding as well as for specific application of polymer blend/ alloy systems. Customers include material producers and users, ranging from chemical, rubber, tire, and automotive companies, to small molding and compounding companies. The focus of research on polymeric materials includes commodity, engineering and specialty plastic resins, elastomers, coatings, adhesives, films, foams, and fibers.

The ExxonMobil Research and Engineering Company is a member of the Participating Research Team (PRT) that operates, maintains, and conducts research at the NG-7 30 m SANS instrument and the recently commissioned NG-5 Neutron Spin Echo Spectrometer. The mission is to use those instruments, as well as other neutron scattering techniques, in activities that complement research at ExxonMobil's main laboratories as well as at its affiliates' laboratories around the world. The aim of these activities is to deepen understanding of the nature of ExxonMobil's products and processes, so as to improve customer service and to improve

FIGURE 3: PAC substitute Bill Hamilton (ORNL), members Larry Passell (BNL) and Dieter Schneider (BNL), and PAC Chair, Sanat Kumar (Penn State U.) share a lighter moment while considering proposals for beam time at the NCNR.

the return on shareholders' investment. Accordingly, and taking full advantage of the unique properties of neutrons, most of the experiments use SANS or other neutron techniques to study the structure and dynamics of hydrocarbon materials, especially in the fields of polymers, complex fluids, and petroleum mixtures. ExxonMobil regards its participation in the NCNR and collaborations with NIST and other PRT members not only as an excellent investment for the company, but also as a good way to contribute to the scientific health of the Nation.

The Nuclear Methods Group (Analytical Chemistry Division, Chemical Science and Technology Laboratory) has as its principal goals the development and application of nuclear analytical techniques for the determination of elemental compositions with greater accuracy, higher sensitivity and better selectivity. A high level of competence has been developed in both instrumental and radiochemical neutron activation analysis (INAA and RNAA). In addition, the group has pioneered the use of cold neutron beams as analytical probes with both prompt gamma activation analysis (PGAA) and neutron depth profiling (NDP). PGAA measures the total amount of a particular analyte present throughout a sample by the analysis of the prompt gamma-rays emitted during neutron capture. NDP, on the other hand, determines concentrations of several important elements (isotopes) as a function of depth within the

first few micrometers of a surface by energy analysis of the prompt charged-particles emitted during neutron bombardment. These techniques (INAA, RNAA, PGAA, and NDP) provide a powerful combination of complementary tools to address a wide variety of analytical problems of great importance in science and technology, and are used to help certify a large number of NIST Standard Reference Materials.

During the past several years, a large part of the Group's efforts has been directed towards the exploitation of the analytical applications of the guided cold-neutron beams available at the NIST Center for Neutron Research. The Group's involvement has been to design and construct state-of-the-art cold neutron instruments for both PGAA and NDP and provide facilities and measurements for outside users, while retaining and utilizing our existing expertise in INAA and RNAA.

The Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration (FDA), directs and maintains a neutron activation analysis (NAA) facility at the NCNR. This facility provides agency-wide analytical support for special investigations and applications research, complementing other analytical techniques used at FDA with instrumental, neutron-capture prompt-gamma, and radiochemical NAA procedures, radioisotope x-ray fluorescence spectrometry (RXRFS), and low-level gamma-ray detection. This combination of analytical techniques enables diverse multielement and radiological information to be obtained for foods and related materials. The NAA facility supports agency quality assurance programs by developing in-house reference materials, by characterizing food-related reference materials with NIST and other agencies, and by verifying analyses for FDA's Total Diet Study Program. Other studies include the development of RXRFS methods for screening foodware for the presence of Pb, Cd, and other potentially toxic elements, use of instrumental NAA to investigate bromate residues in bread products, and use of prompt-gamma NAA to investigate boron nutrition and its relation to bone strength.

The Neutron Interactions and Dosimetry Group (Physics Laboratory) provides measurement services, standards, and fundamental research in support of NIST's mission as it relates to neutron technology and neutron physics. The national and industrial interests served include scientific instrument calibration, electric power production, radiation protection, defense nuclear energy systems, radiation therapy, neutron radiography, and magnetic resonance imaging.

The Group's activities may be represented as three major activities. The first is Fundamental Neutron Physics including mag

netic trapping of ultracold neutrons, operation of a neutron interferometry and optics facility, development of neutron spin filters based on laser polarization of 3He, measurement of the beta decay lifetime of the neutron, and investigations of other coupling constants and symmetries of the weak interaction. This project involves a large number of collaborators from universities and national laboratories. The second is Standard Neutron Fields and Applications utilizing both thermal and fast neutron fields for materials dosimetry in nuclear reactor applications and for personnel dosimetry in radiation protection. These neutron fields include thermal neutron beams, "white" and monochromatic cold neutron beams, a thermal-neutroninduced 235U fission neutron field, and 252Cf fission neutron fields,

both moderated and unmoderated.

The third is Neutron Cross Section Standards including experimental advancement of the accuracy of neutron cross section standards, as well as evaluation, compilation, and dissemination of these standards.

Several universities have also established long term programs at the NCNR. The University of Maryland is heavily involved in the use of the NCNR, and maintains several researchers at the facility. Johns Hopkins University participates in research programs in solid-state physics and in instrument development at the NCNR. The University of Pennsylvania is working to help develop biological applications of neutron scattering. It is also participating in the second stage construction of the filter analyzer neutron spectrometer, along with the University of California at Santa Barbara, DuPont, Hughes, and Allied Signal. The University of Minnesota participates in two PRTS, the NG-7 30 m SANS and the NG-7 reflectometer. The University of Massachusetts also participates in the latter PRT.

46

The reactor operated for 198 full power (20 MW) days during

The

past or foot to

the past year of the maximum available operating time. Routinely, the reactor is scheduled to operate on a seven week cycle, seven times a year. Each operating cycle includes 38 days on-line and 11 days shutdown for refueling, routine maintenance, and surveillance tests. This year, several major tasks became due and required additional shutdown time. Included among these are two shipments of spent fuel; replacement of the shim arm assemblies which required removal of the entire core and took only one month, half of that previously; and major modification to the refueling system projected to take two months which was completed in five weeks. In addition, corrective maintenance of the thermal shield and the thermal column cooling systems, refurbishing of the existing cooling tower to assure uninterrupted operation for at least two more years, and finally the biennial retraining, re-examination and requalification of all licensed operations personnel were conducted.

or 54 % of real time that is equivalent to 77%

The major engineering effort the past year was the design and specification of a completely new plume-abatement cooling tower to be installed adjacent to the existing one. (Figure 1 compares examples of abated and non-abated cooling towers.) As well as being a larger capacity, more effective and more efficient system, the new tower eliminates vapor plumes down to -12 °C ambient. Construction of the new tower basin began in late September 2000 and should be completed before the end of the year. Fabrication of the tower is underway and scheduled for completion in the spring of 2001 to be followed by on-site installation, expected to take approximately four months. No reactor shutdown will be required during this period. The reactor will have to be shut down only for final hookup of electrical, controls, and piping connections and for acceptance and performance testing.

FIGURE 1. No discernible cloud is emitted by the plume-abated cooling tower shown on the left compared to the plumes emerging from the non-abated towers on the right.

Preparations for reactor re-licensing in 2004 for an additional 20 years are proceeding. They include preparation of an updated safety analysis report including seismic evaluation, an environmental report and impact statement, technical specifications and bases, operator requalification program and emergency and security plans. In-service inspections of reactor internals and ultrasonic testing of the primary cooling system plus upgrade of older systems and components will be needed in support of the application for license renewal. Many of the upgrades have already been completed or are in progress. Among the major upgrades planned over the next few years are complete replacement of the nuclear instrumentation panel and associated safety and control systems, complete replacement of the electrical power systems and associated switch gear and replacement and upgrade of the reactor emergency power supply systems.

A LOW BACKGROUND DOUBLE FOCUSING
NEUTRON MONOCHROMATOR

Work continues on the development of a low background double focusing monochromator which was described in the 1999 NCNR report. The actively controlled double focusing monochromator consists of an array of 315 pyrolytic graphite crystals mounted on 21 thin aluminum blades (see Fig. 1). When buckled, each variable thickness blade conforms in shape to an arc of constant radius providing active vertical focus control. Horizontal focus is accomplished by independently controlling the rotation of each blade.

The design and choice of materials for the system reduces scattering from the supporting structure, a problem common to traditional lead-screw and lever controlled monochromators. Structural material in the beam is limited to the 21 blades and three thin walled aluminum posts. The 315 crystals are accurately suspended with only 630 g of structural material in the beam's direct line of sight.

An engineering mock-up of the focusing system was constructed (see Fig. 2a). This three-blade version of the full-scale 21-blade unit was used to study blade performance, develop control software, quantify horizontal and vertical focus performance, and test mechanical and electrical system components. Figure 2b shows an optical test of vertical focus performance using the mock-up. The

FIGURE 2. Three blade mockup of the double focusing monochromator focusing system. (a) Blades are shown buckled to an arc of a 1 m radius circle. (b) Vertical focusing is optically verified by focusing a white point source onto

a screen.

three blades are covered with reflective mirrors and illuminated with a white point source. The reflected image is focused onto a screen. Imaging tests such as this, as well as mechanical measurements, have verified that errors in blade shape are negligible compared to contributions due to crystal mosaic over the focal range of interest. Similar optical tests have been used to verify the horizontal focus performance.

The full-scale unit is currently under construction. When completed, the 1300 cm2 monochromator will be the heart of the new cold neutron spectrometer under development at the NCNR. It is expected to provide an intense monochromatic neutron flux with 0.1 <AE < 0.5 meV and AQ≈ 0.1 Å1 yielding a peak flux of order 1.0 × 108 n/cm3/s, higher than any currently available worldwide. This new instrument will be ideal for studying materials with excitations having a low characteristic velocity. The enhanced sensitivity will enable inelastic neutron scattering studies of smaller sample size and will provide dynamic information of unprecedented detail when large samples are available.

3

FIGURE 1. Rendered image of the low background doubly focusing monochromator.

THE BT-7 THERMAL TRIPLE AXIS

ANALYZER/DETECTOR SYSTEM

As part of the modernization of the thermal neutron spectrometers a new triple-axis instrument is being designed for the BT-7 thermal beam port. For the analyzer portion of this machine several different types of systems have been proposed. One type is a horizontally focused pyrolytic graphite analyzer system shown in Fig. 3. The analyzer crystal system consists of 13 pyrolytic graphite blades, each 2 cm wide and 15 cm high, with either an individual detector for each of the 13 blades, or a position-sensitive detector using all the blades at once.

This is the modern equivalent of our present analyzer systems, and is expected to be the workhorse for the new thermal triple axis instruments. The blades of the analyzer can be freely rotated by 360 degrees and individually positioned, while the entire unit can be rotated as a whole to achieve the desired focusing condition. Each blade can then be matched with a detector that is capable of being positioned individually by a stepper motor on a circular track around the analyzers. A straight-through beam monitor is incorporated into the shielding behind the analyzer crystals to continuously monitor the flux of neutrons entering the analyzer system. A separate diffraction detector is also provided, which can be moved in front of the analyzer if the energy-integrated signal is to be measured.

Custom gear tracks

The general design philosophy is to make the instrument as user friendly as possible while still meeting all the desired operating criteria. These include a built-in magnetic guide field for polarized beam operation; and various beam defining systems such as collimators, beam apertures, spin flippers, and filters. Ease of exchanging beam collimators before and after the analyzer crystals is an important design feature that presents an engineering challenge. Extracting the wiring from all the moving detectors and motors inside the system will also be a technical challenge.

A second type of analyzer system will consist of a series of up to 30 individual and isolated analyzer/detector systems. Other analyzer options, to be developed in the future, include incorporating a velocity selector into the analyzer system, and developing a "conventional" double-focusing analyzer with a single, well-shielded detector.

THERMAL NEUTRON PROMPT GAMMA-RAY ACTIVATION ANALYSIS (PGAA) FACILITY AT VT-5.

High density polyethylene shielding

The vertical beam tube VT-5 thermal neutron PGAA facility is being upgraded through a collaboration of members of the NCNR, the Nuclear Methods Group, and the U.S. Food and Drug Administration. The current facility consists of an internal neutron-collimating beam tube and shutter assembly, an external beam tube, sample chamber, a beam stop, and a gamma-ray detection system. All components except the internal beam tube and shutter assembly will be replaced and a sapphire filter installed in the shutter assembly. The new components will be designed to reduce background count rates and improve detection limits. The external components will be constructed as a single unit to simplify removal and re-assembly of the instrument to make room for reactor refueling.

Individual detectors which, as a group,

can range over -5°<20analyzer 145°