DRACULA will be the world’s fastest Diffractometer for Rapid ACquisition over Ultra Large Angles, competing favourably with machines being constructed on the new American and Japanese pulsed sources. The project was rated as top priority by the ILL Instrument Committee in 2003, and approved by the ILL Science Council in Spring 2004. The idea is to take advantage of the very high time-averaged flux that a continuous source can deliver with doubly focusing monochromators, and then to match the solid angle of the large detectors that have been used so successfully on pulsed sources. Such monochromators and detectors have already been developed at ILL for IN8 and D19 respectively, and the idea of using big 2D detectors has been demonstrated successfully on D2B. Instruments like Dracula will be essential if Europe is to maintain it’s lead in neutron diffraction until ESS can finally be built.
It was shown that the 200mm thermal beam tube H9 was the best possible position for DRACULA; the beam height is similar to that on D20, extending to 300mm with 200mm at high flux, and the beam width is much greater, allowing horizontal as well as vertical focusing. H9, unlike other beam tubes, is not restricted by in-pile beam rotors, the Lohengrin chariot and target are not very absorbing, and the fast neutron flux of ~104 n.cm-2.sec-1 is not a serious problem. Indeed H9 is best for DRACULA for the same reasons that it is best for TOMOGRAPHY - an intense neutron flux over large areas.
It was further shown that DRACULA could co-exist with TOMOGRAPHY, since no primary collimator would be used and the large germanium monochromator would uniformly absorb only 15% of the white neutron beam. Indeed, it would be an opportunity for TOMOGRAPHY to rebuild a better casemate with money from a joint project. Moving this casemate 4m further from the reactor (18m instead of 14m) would reduce slightly the flux, but also increase resolution and reduce background. TOMOGRAPHY plan in any case to use a collimator and a large filter for these purposes.
A detailed proposal for DRACULA can be found on http://www.ill.fr/dif/AlanHewat/