Re[3]: PMT preamp
Index Previous in Thread Next in Thread

It turns out that PMTs all see pulses in the 5ns range, but the current is limited due to the response time. This is where the preamp comes in. By the time the signal comes out of any preamp it is usually stretched to between 50 and 200ns or more, either through delays or electronic sluggishness (capacitive limitations). If you re-amplifiy it (Nim SCA or NIM amp) or do anything beyond here the pulse will again be stretched. I prefer a NIM pulse stretcher ( a form of adjustable monostable multivibrator with gain.) I usually set up my Ortec stretcher to have a nice pulse width of 5us. This way, I can count with the most crude of old 1Mhz nixie counters (my favorite - bright, large view angle).

Now don't anyone tell me that I raped a fine tight PMT signal which was pure and had value. I am counting neutrons from an amateur fusor!! The pulse rate is at most 300cpm. The pulse height tells me nothing, the pulse shape or width also carries no real info which is codifiable. I need to count pulses with a crummy 1960s Hewlett Packard nixie counter that I got for $25.00 at a hamfest!

Likewise I don't have to worry about "count pile up" or "coincidence" crap. The "dead time" between pulses even stretched out to 5us is nearly 100%!!!

So there..........a nice, sullied, but useful signal with two NIM modules.

Richard Hull


Created on Friday, June 15, 2001 11:02 AM EDT by Richard Hull