A new data acquisition system for a backscattering neutron spectrometer

For the new neutron source "FRM-2" at Garching a backscattering spectrometer is constructed by a group located at the Research Center Julich. The backscattering technique allows for highest energy resolutions because monochromators and analysators are hit perpendicular to the crystal plane, where angular errors have the least influence. The energy of the incident neutron beam is modulated by a moving monochromator. Because flight times are not negligible the data acquisition system needs to keep track of the monochromator position and speed at some time before a detected event; additionally, the time difference is not constant due to the changing flight length. Former implementations used a pure DSP approach to reconstruct the real neutron energy and build a histogram in the frontend. We are using a combination of DSP based software in the VME frontend and higher level corrections in user software running on a PC. Complete timing information is streamed to the PC at measurement time, allowing for more accurate and flexible algorithms to correct the data