This second review of ILL instruments (which follows the one on Single Crystal Diffraction Instruments, chaired in 1997-98 by K. Yvon) covers 5 machines:
Powder diffraction (taken in its broad sense, i.e. including disordered systems) covers an extremely wide range of scientific domains, from very fundamental "academic" physics to industry related problems: magnetic and crystal structure determination in crystalline compounds, local atomic arrangements in disordered systems (simple or complex fluids, glasses....). nanocrystalline materials, phase transitions, problems in materials science (internal strains and stresses, crystallographic textures, kinetics of phase transformations or chemical reactions), defect structures, structure and phase transitions of adsorbates, molecules inserted in zeolites etc...
To situate the importance of powder diffraction at ILL, one can recall that in 1997 129 publications came from the 3.5 powder instruments operational at that time (D1A, D1B, D2B, 50% D4), which is more than 20% of the total number of ILL publications of that year. Several of the most cited ILL works used these machines (see below).
The aim of our review will be:
A brief review of the situation for the five powder instruments considered here is given below.
D2B - This very high resolution powder diffractometer is sheduled for many short experiments (more than any other ILL machine last year). D2B produced the ILL's most cited publication (in 1990 on charge transfer in oxide superconductors), and also the most cited ILL publication since the reactor refurbishment (a 1995 Phys. Rev. Letters on Giant Magneto-Resistive materials).
D20 - This extraordinary machine uses the world's first large microstrip detector, and can collect complete diffraction patterns in less than a second, making it faster than synchrotron machines for many chemical kinetics experiments (The neutron machine can use much larger samples, e.g. typical of real batteries and chemical cells). The new possibilities offered by D20 are now only being appreciated, and the machine will be even more powerful with the new monochromators at present under construction. Unfortunately, D20 is still very much a prototype instrument, and the continued deterioration of it's microstrip elements, probably due to insufficient cleaning during assembly, means that a major repair will soon be needed.
D4 - This instrument, installed on the hot source, is a 50 % ILL scheduled liquids diffractometer, rather than a powder machine. It is being modernized with a bank of 9 high pressure He3 microstrip detectors. which will increase its efficiency by more than an order of magnitude, making it a unique instrument for the study of small amorphous samples, such as experiments using isotope contrast. It is generally recognized that, even before this upgrade. D4 is the best machine in its category in the world (This is largely due to its exceptionnally high stability). Although one of the most heavily demanded of the ILL instruments, D4 will however continue to share a beam tube with INI, and remain only 50% scheduled.
D1A - This "half CRG" is at present 50% scheduled for ILL users (and 50% CRG with the Swiss community). It is the only ILL diffractometer available for neutron strain scanning, a field of increasing interest to a number of user groups in all member countries, including industrialists (e.g. British Aerospace, Volkswagen, EDF, etc...). Several of these groups have conducted paid beam time experiments on D1A, and similar paid beam time is planned by other groups. There is currently a proposal by a consortium group of five UK University groups to take over D1A as a 100% CRG: this proposal will be discussed within the present review. D1A has produced some of the ILL's most highly cited publications, but it is supposed that most high resolution Rietveld refinement experiments will be done in future on D2B.
D1B - This machine is now a 100% CRG, run by the Laboratoire de Cristallographie (CNRS Grenoble). D1B has produced more publications than any other ILL machine (according to library records), and has been heavily demanded for work in magnetism and chemical kinetics. However, it is supposed that most D1B experiments will be done in future on D20.
It may appear that with five machines, ILL is well equipped for powder diffraction experiments, but in practice it has fewer ressources in this area than National laboratories such as ISIS and LLB, considering that two of these machines are CRG's, and that they are also used for strain scanning and liquids diffraction. Yet, the three major ILL instruments in this group are world leading, each in its field, working at the limits of resolution or intensity for which a high flux neutron source is needed. The optimization of their use and of their performances is therefore a first priority of the Institute.
23 september 1998
C.H. de NOVION (Review Chairman)
A.W. HEWAT (Head of ILL Diffraction Group)