The Workshop "Review of Neutron Diffraction ILL Instruments for Powders and Disordered Systems" was held at ILL, Grenoble, on 22 and 23 march 1999.  The programme included 5 sessions dedicated to the individual instruments (D2B, D20 D4, and the two CRG : D1A and D2B), and 3 "horizontal" sessions on "Continuous versus TOF neutron sources", "Synchrotron versus neutrons", and " Data acquisition and software" respectively.

The detailed programme is joined in Appendix. There were 50 participants. From all reports, people thought the meeting went well, the talks were interesting and the discussions lively.

The main conclusions, presented at the end of the workshop, are summarized below.

The final report of the Powder Diffraction Review, including Users Questionnaire, Statistics, Highlights, etc.... will be presented at the Scientific Council of Autumn 1999.

1) The ILL Suite of Diffractometers for Powders and Liquids

All the concerned instruments are world-class. D2B, D20 and D4 are the best in their categories, D2B being in competition with HRPD in ISIS.

Nevertheless, the total capacity of access on powder diffractometers at ILL is smaller than what it should be, taking into account the size of the community and the scientific production. This is of course enhanced with the present shut-down of D20.

This leads to the following statements :

- the first priority is to repair D20;

- the second high priority is to improve the performances of ILL instruments;

- some of the performed experiments probably do not require the high flux of ILL, and optimum use of the european medium size sources should be made;

- the 1/2 instrument D1A should be kept in operation for ILL subcommittees at least until the definitive repair of D20;

- a new powder diffraction instrument would be useful. But it has to be discussed in the frame of the overall ILL instrument suite. And no consensus was made on the characteristics of this instrument : a TOF diffractometer (optimal for high pressures) ? a multipurpose instrument, intermediate between D2B and D20, i.e. combining high flux and high resolution ? a large-wavelength diffractometer on cold source (but do not forget D16) ? a short-wavelength diffractometer on hot source ?

2) The problem of D20

The situation and the position of ILL management were presented by P. Leconte during the final Round-Table Discussion. The multidetector was opened, and chromium oxide was found on the anodes. The origin of oxygen is not clear : from the gas (i.e. from leaks) or from the glass (P. Leconte favours the latter explanation).

The microstrip detector will be repaired in the present configuration, with some slight modifications. This should require at least one year.

Therefore, the workshop made the following recommendations :

- detailed chemical and metallurgical analysis should be made, the advice of corrosion and materials scientists should be taken to understand the origin of the formation of Cr oxide;

- a temporary solution should be put in place as soon as possible, in order to satisfy, at least partially, the need of the community of users;

- the best temporary solution would be to loan the CEA/Siloe BF3 wire multidetector, which is presently in Portugal (offer to portuguese colleagues some beam time in ILL in exchange ?);

- the second best (and cheapest) temporary solution is to put in place the "old" small D20 detector;

- this temporary solution, and the partial reopening of D20 will take forces from instrument responsibles and technician; it should not delay the final reparation of D20.
 
 

3) Instruments upgrade

The D4C upgrade was presented by H. Fisher : it should be finished at the end of the year.

The "super-D2B" upgrade project was considered as the first priority, in particular the new (and higher) detector set. This would allow to use routinely the D2B instrument in its highest resolution mode.

4) Strain Scanner

The project is considered as a high priority, whatever the administrative form (CRG or other).

A full-time instrument is required.

Therefore, this diffractometer should be totally disconnected from D1A.

The best location (behind D1B or other) is to be decided by ILL.

5) Other topics

Neutrons versus synchrotron, as well as instruments on continuous versus TOF sources (e.g. ILL versus ISIS), are competitive and complementary in this field.

Very generally speaking, it appears that neutrons are better than X-rays for refinement, and to obtain quantitative and precise data (e.g. atomic positions of oxygen in oxides). This is due to several factors : the weak neutron-matter interaction, the easiness of corrections, the independence of scattering length with q. This is specially true for instruments on continuous sources, owing to the great stability of neutron flux produced by reactors (consider for example D4 versus SANDALS)

Data acquisition and software takes an increasing importance. It raises manpower difficulties, as many users as well as the instrument responsibles have generally not the time to develop satisfactorily the programmes. The solution put in place for FULLPROF and for the CCP14 project (i.e. a network) is probably the good one, and should be generalized.