Speaker: Tony Wright of Electron Tubes, UK
Host: Richard Schirato
Date: December 8

Fast Timing with Slow Scintillators.
                                      
Post and Shiff (1950) showed that timing based on the detection of the
first photon of the decay provides optimal timing. An expression for the
varience of the Qth photon out of a total of R was also given. This
treatment refers to the ideal case whereas in practice the detector
itself adds further dispersion.
                                                     
Photomultiplier transit time dispersion (jitter) and the effect of noise
in the gain process have unexpected consequences. Although the decay
time of NaI(Tl), for example, is 100 times greater than the PMT jitter
this parameter has a significant effect on timing fidelity.
                                                                   
The arrival of the first photon no longer provides the best timing which
is now obtained at much higher photon or photoelectron levels. Monte
Carlo simulations of real photomultiplier pulses highlight problems of
multiple triggering and how they can be overcome by suitable pulse
shaping. The widely used technique of constant fraction discrimination
is examined.
                                                                         
Timing and photon yield parameters of long established and of some newly
introduced scintillators are presented and discussed.