Electronics > Projects, Designs, and Technical Stuff

A decent photomultiplier / scintillation preamp

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ChristofferB:
Hi all!

I've been searching far and wide for a PMT preamplifier for scintillation spectroscopy - that is: a charge sensitive preamp for PMT's coupled to scintillation crystals.

The most important requirement is that the peak height of the PMT pulses are still proportional to the incident number of photons hitting the PMT.
Another point is to implement pole-zero cancellation so that there is no overshoot.

The signals are characterized by a very sharp rise-time (ns) and a slow decay (µs).

The basic amp is an op amp with a charge sensitive loop, amplification is set by the feedback resistor and time constant is set by both. The time constant of the preamplifier should be longer than that of the PMT to retain peak height information, and typically (as in the Ortec 113A amp) is 50 µs. Mine is 100 µs.

Pole zero cancellation is performed by bypassing a capacitor with a trimmer pot, and a second op amp is used as buffer/cable driver.

A test input is also furnished, to calibrate the electronics.

I wanted to share this, since it took me something like 5 design iterations to get where I wanted, and they are VERY pricey on ebay for some reason.

I've tried numerous high-end op amps but the venerable TL072 does the job quite well!¨

The DB9 matches the preamp power connector on standard NIM bin spectroscopy amp modules, and is of course not of use for standalone operation.

Hope somebody can use this!

--Chris

 

Vovk_Z:
Will this link be a useful example for you?
The whole project of a photon's counter.

andy3055:
Some 30+ years back I worked for a company that supplied Gamma counters among other laboratory instruments to universities, research institutes and hospitals. One brand we dealt with was LKB based out of Sweden, if I remember correctly. Those machines always had 2 PMTs and the noise in one of them (not visible to the sample) was used to cancel out the noise signals from the "counting" PMT, leaving the "required" signals usable. The 2 PMTs had to be factory matched and replaced together if required, for obvious reasons. Does your design take this sort of thing into consideration?

ChristofferB:
Nope, it makes sense but I'd never heard about the concept before :)

For much nuclear spectroscopy your signals are well above the noise, and there is always a window discriminator (single channel analyzer) in the measuring signal chain to sort out electrical noise (low level) and cosmic ray interactions (extremely high level)

Another possibility is to have two scintillator detectors stacked and then only accept pulses that triggers both at the same time.

Kleinstein:
The input-amplifier looks like quite high gain, that is relatively small capacitance + large resistor in  feedback.
It may help a little with accuracy as the loading to the OPs output is weaker.
Unless the OP is driven to the nonlinear range the peak high will generally be proportional to the area, as long as the shape is fixed. Slow enough an amplifier can ignore minor changes in the pulse shape at the start.

Using a second counter / detectors may help with muon showers and similar events of higher cosmic background. Just for the pulse hight measurement it would not help.  Not sure if one really needs this - maybe to get accurate low activity data, like measuring food samples.

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