Performance of a Photomultiplier with a Porous Transmission Dynode

The root-mean-square deviation in number of secondary electrons from a normal secondary emitter increases with the square root of the secondary-emission ratio. The constant of proportionality is slightly greater than unity. The pulse-height resolution of a multiplier is therefore improved by the use of a high-gain first dynode. The secondary-emission ratio from a porous potassium chloride transmission dynode has a much higher value than that from conventional dynodes. Ratios of fifty to a hundred are easily obtained. If the statistics of emission obeys the same laws as for conventional secondary emission, porous dynodes would be advantageous as the first stage of a photomultiplier for scintillation counters used for energy resolution. A multiplier incorporating a porous transmission first dynode was built and tested. The pulse-height resolution was poor compared with that of a normal photomultiplier. The pulse-height resolution can be expressed in terms of FWHM as follows: FWHM =2.35 =1/√Ne√1 + M where Ne is the average number of photoelectrons per pulse and M is the degradation of resolution by the multiplier structure. A conventional photomultiplier operated at normal gain has an M value in the neighborhood of 0.5. A multiplier with a normal first dynode of gain 30 would have an M value of 0.06. The porous-dynode multiplier with a gain of 30 for the first stage had an M value of the order of 4.3.