Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Photomultiplier
Date: Oct 04, 8:39 am
Poster: Richard Hull

On Oct 04, 8:39 am, Richard Hull wrote:

>I was going to have a fet buffer and feed a comparator for discrimination and then some new super duper low noise op amp for the output.
>Just how fast a slew rate are we looking at??.

For spectroscopy 1000v/us is none too fast! However for just simple detection. a few hundred volts per microsecond is adequate. Most pulses are in the 35 nanosecond rise time range or shorter.

Most traditional preamps use only direct coupled transistors (~3-5 of them) and a huge coupling cap would be 12pf.

Jim Lux just recently posted the official Hamamatsu preamp diagram which I reverse figured on his website. ( He got one of my tubes) If constructed with virtually zero lead length, it is a superb way of preserving the pulse image while getting the PMT output into the 100mv range.

Modern video opamps can take it from there.

I often use HP counter's inputlevel control as a discriminator, but it requires a scope to be effective. I sort of eyeball the pulses and adjust accordingly. This allows a lot of freedom in discrimination and at the same time keeps the human in the control loop.

Richard Hull