Einzel-Puls Neutronen Diffraktometrie

While high data rates are primarily associated with experiments at synchrotron radiation sources, new opportunities offered by the MW spallation neutron sources will lead to extremely large data rates also for neutron scattering experiments. One example is given by the DiffractomEter for Non-equilibrium States of condensed matter DENS, which JCNS proposes to build at the SNS (beamport 8). This single pulse diffractometer for kinetic experiments will allow studies of time dependencies on a ms time scale after disturbance of a system by an external pulse (field / temperature / pressure etc). This is a typical application for a MW spallation source, since the full peak flux can be utilized, which is about two orders of magnitude higher than the flux of the best continuous sources. Simulations show that by appropriate focussing techniques an incident neutron current on the sample of about 109 neutrons per pulse can be obtained. Larmor labelling techniques will be employed to achieve good resolution despite the rather large beam divergences./p>

The peak count rate can be estimated, if we assume a source repetition frequency of 60Hz (SNS), a neutron current per pulse of 109 neutrons on the sample (simulation for SNS beamport 8) and a detection efficiency of maximal 1% on the entire detector area. For the histogram mode with a detector area of 2·5 m2, we have 200·500=105 position channels (assuming 1 cm position resolution) and about 103 time channels, corresponding to 400 Mbyte per pulse. For list-mode counting we have to encode 1% of 109, i.e. 107 neutrons per pulse. With 20 byte encoding, this corresponds to 200 Mbyte of data per pulse, or 60 · 200 Mbyte/sec = 12 Gbytes/sec. Therefore a single pulse diffractometer at a MW spallation source can produce maximal data rates as high as 40 Tbytes/h! Depending on the specific experiment, the average count rate will be significantly lower but can still exceed several 100 Gbytes/h.