Improved timing resolution using small side-on photomultipliers in single photon counting

The time-resolution capabilities of two 28-mm-diam side-on photomultiplier tubes, Hamamatsu R928 and RCA 931-A, were studied by point illumination of the photocathode surface with the attenuated beam of a synchronously pumped dye laser (10-ps pulse width). Using the time-correlated single photon counting technique, an apparatus response of 112 ps FWHM was obtained. Contour maps giving the time shift and the width of the response were constructed as a function of the point of illumination of the photocathode surface. These data permit selection of the optimized photocathode region to be illuminated during fluorescence decay measurements.