Institut Max von Laue - Paul Langevin, B.P. 156X, Grenoble Cedex 9, 38042 FRANCE (hewat@ill.fr)
The ILL Grenoble has the world's highest flux neutron source, but compared to synchrotron radiation, neutron intensities are still too weak for some of the more difficult diffraction experiments. Although it would be very expensive to increase the neutron flux, it is most cost-effective to improve the efficiency of its use.
Three areas are being emphasized: focussing/filtering optics, position sensitive detectors, and modern data analysis. Examples are ILL's development of composite monochromators, microstrip detectors, and real-time analysis as used in Rietveld refinement and maximum entropy methods.
Large focussing composite monochromators and neutron guide tubes have long been used at ILL to deliver the maximum flux to the smallest volume. Early examples were used for our first powder diffractometers D1A and D1B. These monochromators were made by hot pressing germanium (or graphite) - with uncertain results. More recently the 'deformed wafer' technique invented at Brookhaven National Laboratory1 has been improved at ILL such that many different neutron wavelengths can be obtained from the one monochromator. D2B, the successor to D1A, now has a 300 mm high vertically focussing monochromator consisting of 15 stacks of 10x1mm wafers. Apart from the increased intensity, the line profile, important for Rietveld refinement, is extremely regular, even at very high resolution. A similar monochromator is being made for D20, the high flux successor to D1B.
Other examples of new developments in optics include the He3 neutron polarising filter, which will be used together with a new high-field magnet on diffractometer D3 to map magnetic electron densities, and the neutron 'band-pass' filter being used on the quasi-Laue diffractometer LADI.
The microstrip technique for neutron detectors, developed at ILL2, saw it's first large scale use on D20. Instead of multiple wires, 1600 individual electrodes are etched onto glass substrates - a kind of 'printed circuit' detector. These glass plates are mounted in a large He3 gas envelope to obtain a detector covering the whole 160o scattering range with a resolution of 0.1o. A medium resolution powder pattern can then be collected on a time scale of microseconds - very fast even compared to synchrotron powder diffractometers. Since neutrons are more penetrating than hard X-rays, this means that in-situ chemical and crystallographic reactions can be studied on this short time scale.
The new microstrip technology is now being applied to our other diffractometers, and will mean more than an order of magnitude gain in efficiency in many cases. For example, 9 individual microstrip detectors operating at very high pressure (15 atmospheres) will greatly improve D4, our diffractometer for the structure of liquids and amorphous materials. The first such high pressure detector has already been tested. At the same time, ILL is developing a 200x200 mm position sensitive detector for use on our single crystal machines. An array of such 2D microstrip detectors will mean a gain of x30 on our 'protein' diffractometer D19, greatly extending its capabilities. The first prototype will be tested this year, but we are already routinely using a smaller 2D microstrip detector on D1A for mapping internal stress in engineering components.
Finally, the third area where gains in efficiency are being made is in data analysis. Because desk-top computers have become so powerful, it has become possible to analyse data as it is collected, to make best use of the available time. In particular, advances in graphics allow us to visualise data and calculations in 3D (for example, the intensity over a 2D detector, the time evolution of a powder diffraction pattern, the spin density in real space etc.)
All three technologies are of course being developed for other applications in crystallography, but are of special interest for neutron diffraction, where the price of efficiency or intensity is particularly high.
1. J.D. Axe, S. Cheung, D. Cox, L. Passell and
T. Vogt J. Neutron Res., 2,3 (1994),
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2. P. Convert, M. Berneron, R. Gandelli, T. Hansen, A. Oed, A.
Rambaud, J. Ratel, J. Torregrossa Physica B, 234-236
(1997) 1082-1083.