Project Plan in its initial version (April 2000)
The aim of the project is to develop and to build a real-time neutron detector for small-angle scattering applications which is capable of counting 2 MHz of neutrons on the whole detector with dead-time losses of not more than 10%, rather than the 50 KHz of the current detector. In all other aspects the new detector should be at least as efficient as the current detector of the instrument D22 at the ILL, i.e. it should in particular have a spatial resolution of 7.5 x 7.5 mm, an active area of 96 x96 cm, a sensitivity of 75% for neutrons of 5Å wavelength, and a similar quiet count rate.
We envisage two possible solutions to achieve this goal. The first is that of an "organ-pipe" detector composed of about 120 single linear position sensitive detectors arranged in a vertical plane or (more probably) in a zig-zag. The second is an array of linear position sensitive wires situated in a housing similar to the current detector. These wires would have to be on an insulated "comb" that replaces the current anode plane. The latter solution requires development of electronics to determinate the lateral centre of gravity of detection events.
The project is split in two phases, an exploratory phase of two years to select the better solution and to test a prototype, and a construction phase for producing such a detector for D22. A similar but smaller detector for D11 would follow as a separate project. The cost of phase I, wich is to be completed by December 2001, is estimated as 560 kF plus 76 weeks of manpower, that phase II, to be completed by December 2003 or June 2004, to 3640 kF plus 30 weeks of manpower.
Risks : after two years R&D the required 2 MHz count rate may not be achievable whilst retaining all other characteristics of the present detector. At this point, a decision would be made on whether to proceed with a reduced specification of the detector.
Project status in November 2003
The solution of independant PSDs has been rapidly prefered to the Multi-wire single vessel detector for reasons of simplicity and cost. In the zig-zag configuration, the shadowing effect of detectors located in the front plan requires a calibration which depends on the distance between the sample and the detector, and of the neutron wavelength. For this reason, the zig-zag configuration, which was first considered to maximise detection efficiency, will not be implemented . Several prototypes of the D22 PSD have been produced by Reuter Stokes, and extensively studied at the ILL. Specifications are now freezed.
Detection performances are matching the D22 specifications:
70% efficiency at 6 Angstroms (taking into account the dead zones between tubes)
6 mm FWHM position resolution along the tubes
100 KHz/PSD counting rate.
The mounting of the detector mechanics will start in December 2003, and we plan to start the commissioning phase at the begining of the next reactor cycle, in March 2004.
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