energy range of≈ 5 meV to 250 meV with options for precollimations of 60', 40', or 20' and postcollimations of 40', 20', or 10'. The intensity improvement of the phase-I instrument is due to the twenty-fold increase in detector solid angle provided by a much larger detector bank (see Fig. 2). The accompanying analyzer filter consists of a Be-PG-Be layered arrangement cooled with liquid nitrogen to enhance transmission of the low energy neutrons. Such a low-bandpass filter provides a best resolution at the lowest energy transfers of≈ 1.1 meV FWHM.

The first measurements during the spring of 2000 confirmed the magnitude of the expected gains. For example, Fig. 3 displays the low-temperature neutron vibrational spectrum for triethylene diamine measured using the new FANS-I configuration compared with that measured using the original configuration under identical beam collimations and measurement times. This gain in intensity is accompanied by a somewhat improved signal/noise ratio.

The enhanced capabilities of the FANS-I instrument have already been demonstrated for a variety of materials including protonic conductors, organic solids, metal hydrides, carbon nanotubes, and metal oxides. These experiments confirmed substantial reductions in required sample size and/or measurement time. For exam

University of Pennsylvania Philadelphia, PA 19104

A. K. Cheetham
Materials Department

University of California at Santa Barbara
Santa Barbara, CA 93106

ple, Fig. 4 displays the temperature dependence of the NV spectrum for RbH(SO4)(SeO)0.19, a protonic conductor oxide with lattice protons that become mobile at temperatures less than 473 K. Vibrational spectra were collected with FANS-I at nine different temperatures in less than one day, a feat not possible using the original instrument. It proved particularly interesting that the energy of the mode near 100 meV decreases while that of the mode near 83 meV increases with increases in temperature concomitant with rapid decreases in both peak intensities. Indeed, these proton-related features largely disappear at temperatures as low as 200 K, indicating that there is significant proton motion even at this low temperature.

Figure 5 shows the FANS-I spectrum for solid 3-nitrophenol [HO(CH)NO] compared with a GAUSSIAN calculation for the isolated molecule. This spectrum exemplifies the overall quality and high-resolution capabilities enabled by the marked improvement in sensitivity. There is fair agreement between calculation and experiment for many features in the vibrational density of states, although it is clear that significant intermolecular hydrogen bonding interactions in the solid cause strong perturbations in the OH wagging mode predicted near 45 meV for the isolated molecule.

T

The recently commissioned Disk Chopper Spectrometer (DCS) is a versatile state-of-the-art instrument that is primarily intended for studies of diffusional processes and low energy excitations in materials. It has no equal in North America and is fully competitive with comparable instruments in Europe.

The spectrometer is shown schematically in Fig. 1. Following a tapered offset guide assembly ("neutron optical filter") that removes almost all of the y-rays and high energy neutrons from the reactor beam [1], seven phased disk choppers supply monochromatic bursts of neutrons at the sample position. Three parallel banks of 6 atmosphere 'He detectors, of 400 mm active length, are placed 4010 mm from the sample position, and≈ 90% of the space between the sample and the detectors is argon-filled. Each of the 913 rectangular cross section detectors subtends≈ 0.5° in the scattering plane. The central bank provides continuous angular coverage from -30° to -5° and from 5° to 140°. The overall detector coverage is ≈ 0.65 sr, double that of the IN5 spectrometer at the Institut Laue Langevin, Grenoble. Presently fitted with a≈ 50 mm long room temperature beryllium filter, the instrument operates at wavelengths greater than≈ 4.1 Å. The beryllium will shortly be replaced with an assembly of cold oriented graphite, 100 mm in length, permitting measurements down to≈ 2.3 Å. (Wavelengths near 3.33 Å and 6.67 Å will be unavailable.) The first two and last two choppers are fitted with three slots of different widths, enabling a choice among three "resolution modes" at a given wavelength and master chopper speed.

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The neutron current density at the sample and the energy resolution width at the detectors are shown in Figs. 2 and 3 respectively. In designing and building the DCS, great care has been taken to ensure that distances and detector locations are accurately known. The stability of the chopper phasing results in a resolution lineshape with a sharp leading edge. The sample area is easily accessed at beam level and from above. The data acquisition system has been carefully designed and is extremely reliable. We plan to modify it so that crude pulse height spectra can be extracted. The software is user-friendly and will be improved as time permits. Planned improvements, apart from the crystal filter replacement, include removal of the two innermost reflecting plates within the guides after the first chopper; this will increase the flux with little reduction in versatility. With the new cold source and these optics improvements we anticipate a threefold improvement in flux within the next year.

The first officially approved experiment using the DCS was a comparative study [2] of native bovine -lactalbumin (BLA) and -lactalbumin in a "molten globular" state (MBLA); the latter state is partially folded and compact, with native-like secondary structure but lacking the side-chain packing that characterizes the native state. Molten globules play an important role in understanding protein folding mechanisms, and molten globules also participate in important cellular functions. The DCS measurements (Fig. 4) confirm and extend previous results [3, and see also the article by Z. Bu et al. on

ments have shown that the high frequency dynamics are virtually unchanged.

The moniker "boson peak" has of late been associated with an excess feature that shows up in the vibrational density of states of many materials, generally in the Debye region, between 1 meV and 10 meV. While the molecular origin of the peak is unclear, its characteristic energy suggests that it is a collective excitation between the low energy acoustic modes and localized high energy optic modes (representing local bond vibrations, librations, bendings, etc.). In a recent experiment [4] the thermal softening of the boson peak was studied in detail in a polyester carbonate copolymer that had already been extensively studied using the FCS and the NCNR backscattering spectrometer. The DCS is particularly well suited for such a study because of the large solid angle of detectors and the large beam size. The analysis of the data is in progress.

Other materials recently studied using the DCS include carbon nanotubes and superionic proton conductors.

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Wavelength (Å)

Q2 (Å2)

FIGURE 3. Calculated resolution widths (full widths at half maximum height) for the three "resolution modes" of the instrument. Some experimental widths are also shown.

FIGURE 4. The Q dependence of the full-width at half maximum height of the quasielastic Lorentzian peak for native bovine α-lactalbumin (BLA) and for molten globular bovine α-lactalbumin (MBLA) in 8M urea [2]. Results [3] for BLA from the Fermi chopper spectrometer (FCS) are shown for comparison.

A

perfect crystal diffractometer (PCD), shown in Fig. 1, for ultramea high resolution small-angle neutron scattering (USANS) measurements is now in operation at the thermal neutron beam port, BT-5. The PCD increases the maximum size of features accessible with the NCNR's 30 m long, pinhole collimation SANS instruments by nearly two orders of magnitude, from≈ 102 nm to 104 nm.

The PCD is a Bonse-Hart type instrument with large triplebounce, channel-cut Si (220) crystals as monochromator and analyzer. The perfect crystals provide high angular resolution while the multiple reflections suppress the "wings" of the beam profile, improving the signal-to-noise ratio to values comparable to that obtained with pinhole instruments. This technique, widely utilized for x-rays for many years, has only recently been successfully adapted for neutrons [1] as dynamical diffraction effects arising from the deep penetration of neutrons in thick perfect crystals have become better understood. Neutrons can, in effect, propagate through a thick crystal, and then reflect from the back-face of the crystal. The geometry of this second diffraction path allows part of the beam to bypass the second and third reflections. The design of the NCNR's PCD [2] successfully eliminates the single reflection path by adding shielding along the middle of the long face of each crystal between the first and third reflections (see inset in Fig. 2). The additional shielding reduces the wings in the rocking curve by two orders of magnitude, resulting in a signal-to-noise ratio of 105 at a minimum scattering vector Q = 0.0005 nm. Figure 2 shows typical

rocking curves with and without shielding

of the deleterious back-face reflection. The beam flux obtained

for smaller samples is 3000 cm2 s1, while the maximum intensity obtained is 15 000 s when using the maximum 3 x 5 cm2 beam size. The mainly fast neutron flat background (≈ 0.15 s-1) found at large angles is independent of beam size. The beam intensity will increase somewhat when the present perfect crystals are replaced by ones with a wider channel, and a gap in the middle of the long face, in order to increase the beam width to 4 cm with no contamination from single back-face reflections.

The measurement range of the PCD overlaps that of the NCNR's 30 m SANS instruments. Together they probe structure in materials over four orders of magnitude, from ~1 nm to 10+ nm. Combined measurements on these instruments will enable fuller characterization of hierarchical and highly anisotropic microstructures in materials, for example in fiber or clay impregnated nanocomposites. The PCD is part of the NIST/NSF Center for High Resolution Neutron Scattering (CHRNS) with up to two-thirds of the available beam time to be allocated by the NCNR's Program Advisory Committee to scientists and engineers who submit proposals for peer review.

The PCD USANS instrument can accept any ancilliary sam