Detective Quantum Efficiency and Deadtime Losses in Compton Imaging Systems

The Compton camera is an imaging system where the vertex and the angle of the scattered gamma ray are reconstructed by measuring in coincidence the Compton electron and the scattered gamma ray. If one of the detectors has a modest time resolution a hardwired coincidence unit will lead to unsatisfying results at elevated rates. Therefore new concept of a software based coincidence system systems has been developed. This allows to record most events and to use additional information to solve possible ambiguities. The key elements are time stamps added to all signals and to process data in a first unit in order to rearrange the signals to good events. Monte Carlo simulations have been carried out in order to assess the effects of detector dead time and dead time introduced by these necessary processing stages. It could be shown that the new data acquisition system behaves as if at high rates it is limited by an equivalent paralysable deadtime, which then translates to a rate dependent detective quantum efficiency (DQE). More detailed studies show that the time duration for the processing of an event in the processor of the system and the event buffer depth are controlling factors. This is important for system design to optimize the DQE for real time high data rate applications. Specifically, Monte-Carlo (MC) simulations are performed for highly efficient thick Si(Li) detectors where event reconstruction is compromised by the long drift times of the order of microseconds. Time stamping and realtime processing of complex event information remedies the drift problem which is demonstrated with the developed concept of DQE and equivalent time coincidence window width.