Hi!
I'm hoping you can help me with this, I'm going absolutely crazy.
I'm building a setup for gamma spectroscopy and other particle detection stuff.
Pretty basic in the world of particle/nuclear physics, but my setup consists of a scintillation probe (scintillator+PMT), high voltage bias tee, preamp (home built), and a 'spectroscopy amplifier'.
The spectroscopy amplifier is a pulse shaping amplifier, taking the long-tailed sharp rise pulse from the PMT (preamp) and making a short, gaussian pulse from that.
Here's the problem: I get a bipolar pulse out of my spectroscopy amplifier. see attached. Here are the observations:
- I'm 90% the spec. amplifier (Ortec 572) is functioning, I have 2 identical and a third and they all behave identically.
- PMT and divider is a finished bicron module.
- Bias tee for the PMT is copied from the one the PMT came with, and HV PSU is good too.
-The preamp is home made. A so-called parasitic capacitance amplifier. Loosely based on the block diagram in:
http://courses.washington.edu/phys433/equipment/Ortec_113.pdf- I have tried all combinations of 50 ohm terminations on in/out of all cables. No difference in undershoot whatsoever.
- AC or DC coupling of the preamp doesn't change anything.
- Spectroscopy amp. input impedance is 500 ohms.
- Spectroscopy amp. pole-zero potentiometer changes nothing.
What can I be doing wrong? I've built 3 different preamps all with this problem. I'm stumped. Should my preamp have its own P/Z cancellation network and a buffer before going to the spectroscopy amplifier? Have i messed up the impedance matching grossly?
I hope you can help! Thanks!