First experience with the focusing neutron guide on IN1OC

MIt is the extinction-limited extension of the Bragg peak in reciprocal space3 and a is the divergence (full width) ofthe beam. In numbers, we obtain At/t= 1.86 • iO for the 111 reflection of Si. Thus, a beam divergence of .700 would balance both terms in eq. 1. The neutron flux for such a highly monochromatic beam is very low. Consequently, backscattering spectrometers can only be operated at high flux reactors, and indeed the backscattering spectrometer IN1O at the ILL has been the only widely used crystal spectrometer with sub-jieV resolution. In most cases only strong scattering samples containing H could be measured in spite of this favorable implementation. Recently the question arose whether a backscattering spectrometer could be designed which would offer improved performance. A higher count rate should result from an extensive use of focusing neutron optical elements throughout the instrument, while it was considered mandatory to maintain the unique energy resolution of IN1O. (The measured resolution on IN1O is \E = O.3ieV, i.e., AXJA = 7.2 • iO. This indicates that the resolution is dominated by the beam divergence).