In the choice of suitable photomultipliers for photon counting, photomultipliers with Venetian blind structures have been found to be not suitable since they do not show a good SER pulse height distribution. The reasons attributed for this is the possibility of some electrons not undergoing the full multiplication and get transmitted to the final stages. One way to improve the response is to connect a pulse squaring circuitry which will enhance the light induced pulse amplitudes while reducing the noise or dark pulse amplitudes. What would you recommend?
Dr. Ganesan Ramanathan
National Physical Laboratory
Dear Dr. Ramanathan,
Yes Venetian Blind photomultipliers do not have good SER distributions. For this reason focused dynode structures are now commonly used instead. These have been designed with the aid of electron optical simulation software to have high collection efficiency dynodes with low losses between stages. Because multiplication spread is governed by Poisson statistics a high first dynode gain improves the single electron peak allowing better discrimination between real signal and small noise pulses. The Venetian Blind structure also suffers from space charge saturation with larger pulses and the superior linear focused structure has largely replaced it in recent years.
Regards,
Photomultiplier Doctor.
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