A METHOD TO INCREASE OPTICAL TIMING SPECTRA MEASUREMENT RATES USING AMULTI-HIT TDC

A method is presented for using a modem time to digital converter (TDC ) to increase the data collection rate for optical timing measurements such as scintillator decay times. It extends the conventional delayed coincidence method, where a synchronization signal ''starts'' a TDC a nd a photomultiplier tube (PMT) sampling the optical signal ''stops'' the TDC. Data acquisition rates are low with the conventional method b ecause epsilon, the light collection efficiency of the ''stop'' PMT, i s artificially limited to epsilon almost-equal-to 0.01 photons per ''s tart'' signal to reduce the probability of detecting more than one pho ton during the sampling period. With conventional TDCs, these multiple photon events bias the time spectrum since only the first ''stop'' pu lse is digitized. The new method uses a modern TDC to detect whether a dditional ''stop'' signals occur during the sampling period, and activ ely reject these multiple photon events. This allows epsilon to be inc reased to almost 1 photon per ''start'' signal, which maximizes the da ta acquisition rate at a value nearly 20 times higher. Multi-hit TDCs can digitize the arrival times of n ''stop'' signals per ''start'' sig nal, which allows epsilon to be increased to almost-equal-to 3n/4. Whi le overlap of the ''stop'' signals prevents the full gain in data coll ection rate to be realized, significant improvements are possible for most applications.