Appl. Phys. A 74 [Suppl.], S329–S331 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201611 Applied Physics A Materials Science & Processing D17: the new reflectometer at the ILL R. Cubitt ∗ , G. Fragneto Institut Laue Langevin (ILL), 6 Jules Horowitz, 38042 Grenoble, France Received: 18 July 2001/Accepted: 11 December 2001 –  Springer-Verlag 2002 Abstract. The reflectometer D17 has been operational for a year and has already proved to be an excellent tool for in- vestigating surfaces and interfaces in the realms of physics, biology and chemistry. The instrument has two modes of op- eration, time-of-flight and monochromatic, the latter incorpo- rating the polarised-neutron option. Both modes are flexible in the wave-vector-transfer (q) resolution. The loosest reso- lution required to resolve the sample structure can be chosen (and hence the highest flux), enabling the lowest reflectivities and hence the widest q-range to be measured. PACS: 61.12.h Surface and interface science has been a growing area for many years and particularly in the areas of magnetism, surfac- tant chemistry and model biological membranes [1], the de- mand for neutron reflectometry time has exceeded the supply. The ILL has had two reflectometers, ADAM [2] and EVA [3] but these, being funded by external institutes, can give only 30% of their time to ILL users. It was thus decided to rebuild D17 to become a dedicated ILL reflectometer. The instrument was to be as flexible as possible without the complication of non-optimised options. Starting with a vertical guide implied that, with the aid of focusing, the instrument would only be optimised for a horizontal reflection plane (vertical samples). This ruled out free liquid surface experiments which were re- served for another new reflectometer which is presently under consideration [4]. Due to the focusing the white beam flux at the sample position is 9.6 × 10 9 n/s/cm 2 . The complication of having both monochromatic and TOF modes was justified by the specific advantages of each case and that neither option would adversely affect the other. TOF allows continuously variable resolution and as an entire order of magnitude in q can be measured simultaneously in less than a minuite, ki- netic studies are possible. On the other hand a monochromatic beam is far easier for magnetic experiments (no q-dependent polarisation efficiency) and is the most efficient in that the ∗ Corresponding author. (Fax: +33-476/207120, E-mail: cubitt@ill.fr) DOUBL E CHOPPERS FOCUSING SUPERMIRROR GUIDE COLLIMATION SLITS SAMPLE EVACUATED DETECTOR TUBE MULTI - DETECTOR MONOCHROMATOR (removed in TOF mode) D17 SIDE VIEW IN TOF MODE D17 TOP VIEW IN MONOCHROMATIC MODE MONITOR BEAMSTOP CHOPPERS ( stopped in monochromatic mode) ATTENUATORS CHOICE OF MONOCHROMATORS 1.1 - 3.4m MAXIMUM BEAM HEIGHT AT SAMPLE POSITION =70MM Fig. 1. Layout of D17 showing the two modes of operation peak flux is used for each point in q. Figure 1 shows a sim- plified layout of the instrument. 1 Time-of-flight mode The pulsing of the beam is realised by a double chopper sys- tem with variable separation (1–11 cm) and projected sector opening (0–45 ◦ ). Both choppers spin in the same direction