Development of a hybrid MSGC detector for thermal neutron imaging with a MHz data acquisition and histogramming system

For thermal neutron imaging at the next generation of high-flux pulsed neutron sources a large area and fourfold segmented, hybrid, low-pressure, two-dimensional position sensitive, microstrip gas chamber detector, fabricated in a multilayer technology on glass substrates, is presently being developed, which utilizes a thin composite 157Gd/CsI neutron converter. The present article focusses on the readout scheme and the data acquisition (DAQ) system. For position encoding, interpolating and fast multihit delay line based electronics is applied with up to eightfold sub-segmentation per geometrical detector segment. All signals, i.e. position, time-of-flight and pulse-height signals, are fed into deadtime-less 8-channel multihit TDC chips with 120 ps LSB via constant fraction and time-over-threshold discriminators, respectively. The multihit capability is utilized to raise the count rate limit in combination with a sum check algorithm for disentangling pulses from different events. The first version of the DAQ system uses the PCI form factor and comprises one slow control board and up to four boards for DAQ and real time histogramming with up to 2 MHz per board. Each DAQ board incorporates four TDC chips, up to 256 Mbytes of histogram memory and a 1 GFLOP DSP. The time of flight can be measured with a precision of a few LSB up to ranges >100 ms.