Verification of the GREAT total data readout system using pseudo random pattern generator

Total data readout (TDR) is a powerful data acquisition tool developed for use with recoil decay tagging (RDT) experiments on the GREAT spectrometer. RDT experiments correlate prompt radioactive decays detected at the target position with subsequent decays of recoiling reaction products implanted in GREAT, located at the focal plane of the RITU recoil separator, several microseconds later. TDR uses free running channels associated in a software event builder based on timestamps to form a single time ordered data stream. It is crucial to maintain the accuracy of the timestamps over long periods (microseconds to hours) for the study of exotic nuclei. The TDR system cannot be tested fully by use of calibrated sources with GREAT and a novel method of verifying the system is required. Our solution is to use the predictable randomness of a pseudo random number generator based on linear feedback shift registers (LFSR) implemented in a 16 channel VME module. The random value defines the delay between pulses into the TDR system. The recorded results are sorted and checked for correspondence with the LFSR sequence. We show the method, design of the module, and discuss the results and benefits of the tests.