When we attach a crystal to the end of the sample-holder stick, we pre-orient it as well as possible at ambient temperature. Once the temperature desired has been reached,the sample often has to be reoriented, to align it with the neutron beam. This is due to the combined effect of thermal expansion of the cryostat and deformation of the crystal itself (changes in the parameters of the crystal lattice).
An unforeseen problem
Surprising as it may seem, the second photo below and report of 22 March 1973  show that ILL had not anticipated the problem of orienting the sample within the sample environment devices (liquid helium cryostats, furnaces, etc.):
M. Gobert (head of the mechanical service) observed that the goniometers being used were not at all designed to support cryostats with very off-centred centres of gravity, especially as some scientists were asking for inclinations of up to ±20°. [...] He suggested designing and building a simpler, more robust, and manually controlled goniometer capable of taking the weight of cryostats positioned with an inclination of ±5°. This proposal has been accepted.
The mechanical service therefore set about developing imposing goniometer devices (combined translation and inclination) allowing up to ±15° of lean, to be manufactured by ACB (Ateliers Chantiers de Bretagne). In around 1978 ILL started developing much more compact models with a range of ±10° to ±20°. Whilst initially manual they progressively became motorised and programmable, produced in-house at the ILL and gradually deployed across the instruments. They were finally imitated by AZ Systèmes.
An idea long-neglected...
Guy Gobert notes in the 1973 report :
There seems to be a need to be able to orient the sample roughly inside the cryostat using a mini-goniometer mounted on the end of a sample-holder stick. It should be possible to manoeuvre the mini-goniometer when the cryostat is cold.
After several fruitless attempts Gobert lost interest. Some of the instrument technicians took up the challenge however, as there was a real problem. The geometry of instruments like D7 or IN5 made any inclination of the cryostat impossible, and things didn't get any better when cryomagnets came onto the scene.
Michel Berneron then came up with an idea and produced (1984) a cunningly-designed eucentric goniometer head, small enough to be fixed to the end of a sample-holder stick and controllable from outside the Orange cryostat . At the time D7 was the only team interested, as the time-of-flight and backscattering instruments, although directly concerned, did not have enough flux to work on single crystals directly.
[Click here to see the gonio head in motion]
The ILL spectrometers have made huge progress since then. In 2013 therefore the gonio head was brought out for a spin, upgraded and made made non-magnetic to broaden its range of application . It provides an inclination of ±7° with 0.02° resolution. The 0.14° of play is managed by the control software.
- Minutes of the cryogenics meeting of 22 March 1973, Dominique Brochier, ILL.
- "A remote control eucentric goniometer head for use within a top-loading helium cryostat", Berneron M. Filhol A. Vernier J.J. Thomas M. (1984) Revue de Physique Appliquée 19, 795-797.
- "In-situ crystal alignment at low temperature", J. Allibon, E. Bourgeat-Lami, L. Chapon, Ph. Decarpentrie, A. Filhol, J.-P. Gonzales, J. Halbwachs, E. Lelièvre-Berna, J. Ollivier, B. Ouladdiaf, G. Pastrello (2014) ILL annual report, pp 88-89.