Sampling Rate and ADC Resolution Requirements in Digital Front-End Electronics for TOF PET

In the context of the development of an in-beam time-of-flight positron emission tomography demonstrator dedicated to the in vivo monitoring of delivered dose in hadrontherapy, we evaluate the potential performance of front-end architectures based on sampling and digital pulse processing to reconstruct the energy and time of event. In this paper, we evaluate the requirements for sampling frequency and analog-to-digital converter (ADC) resolution. The timing algorithm is a digital adaptation of the constant fraction discriminator principle, with a step of interpolation using a low-pass filter based on the cubic spline technique. We demonstrate the interest of the interpolation to lower the sampling frequency requirement, by improving the signal reconstruction compared with a simple linear interpolation. Experimental tests were performed on pulse libraries acquired from a set up composed of LYSO and LaBr3 scintillators coupled to H6533 photomultipliers, and a sampling oscilloscope operating at 10 GHz. By offline processing of the signals with variable parameters (initial frequency, interpolator bandwidth, resampling frequency, and ADC resolution), we examined the impact of these parameters on time resolution. Results for the tested detectors suggest a minimal required sampling rate of 1.5 GHz, while the ADC resolution can be as small as 5 b. A logarithmic ADC could be more efficient, with a strict minimum of 4 b.