A Low Walk, High Resolution Timing System for Silicon Detectors

Utilizing times of flight on the order of a nanosecond for heavy particle identification with silicon detectors, it is desirable to reliably distinguish time differences of approximately 100 picoseconds or less. In addition, the required timing circuitry must not substantially deteriorate amplitude resolution capability. In the system designed for this purpose the detector charge is first collected on the capacitance of the detector and fast, low noise, preamplifier and is later transferred to the charge sensitive preamplifier. The fast signal is processed to obtain good time resolution and low walk. The system has a FWHM time resolution of 40 picoseconds, referred to 5.8 MeV deposited in a 100 pF silicon detector. An actual peak width from 3 MeV deposited in an 80 pF silicon by He3 particles was 165 psec FWHM, which is about twice that expected if more optimum pulse shaping had been employed. The instrumental walk for a 100 to 1 amplitude range is less than 150 picoseconds, and is approximately 100 picoseconds for a 30 to 1 range. The noise added to the slow system is equal to that caused by an additional 17 pF of capacitance at its input, plus an overall increase of 5% or less for commonly used slow pulse shaping.