RISETIME.  WHAT RISETIME IS BEST FOR MY APPLICATION?

The risetime performance of charge sensitive preamplifiers is important in many applications.  Some applications are:

The time of flight measurement.  In a time of  flight measurement, a detectable particle travels through one detector, producing a signal, and then to the other detector, also producing a signal.  Fast shaping amplifiers (typ. TC > 10 ns.) are used to amplify and filter the preamplifier signal.  The signal is routed to a constant fraction discriminator and then to a time to digital converter (TDC).  Maximizing the signal to noise ratio divided by rise time of the detector system results in the greatest time resolution.  However, using a preamplifier with a risetime excessively shorter than the electron collection time will not improve performance.

Time resolution can be maximized by improving the system signal to noise ratio divided by rise time.  This can be achieved through component selection and system design choices.  Using preamplifiers with the risetime adjusted to maximize the signal to noise ratio divided by rise time helps.  Donít select a peamplifier based solely on a risetime specification or other design feature.  A separate fast time and energy output is not a measure of quality.  Remember that optimum system energy and time resolution is the goal.  Two different preamplifier designs with equivalent risetime specifications can exhibit  different time resolution performance.  A preamplifier with a very fast risetime response may perform poorly compared to a preamp with a longer risetime.

Risetime discrimination.  Risetime discrimination is used to improve the energy resolution of some types of detectors.  Some detector materials such as Cadmium Telluride (CdTe), Cadmium Zinc Telluride (CZT) and Mercuric Iodide (HgI) suffer from a relatively low positive charge mobility.  This results in an increased probability of recombination resulting in incomplete charge collection and a reduction of the detector energy resolution.  By using risetime discrimination to reject the longer risetime (bad) signals, the apparent resolution can be improved.  Of course the detector efficiency will be reduced.  In this application the preamplifier  risetime can be just short enough to allow full risetime discrimination.  Decreasing the preamplifier risetime excessively beyond the electron collection time will not improve perfomance.