A decent photomultiplier / scintillation preamp - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 20, 2020, 10:50:31 pm
content: 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

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 21, 2020, 04:02:46 pm
content: Will this link be a useful example for you?The whole project of a photon's counter.

author: andy3055
author_rank: Super Contributor
timestamp: July 21, 2020, 04:28:40 pm
content: 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?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 21, 2020, 04:48:04 pm
content: Nope, it makes sense but I'd never heard about the concept beforeFor 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2020, 04:56:58 pm
content: 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.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 21, 2020, 05:10:19 pm
content: It's in the upper end of the scale of preamp gain. Maybe a lower gain would be nice in that it could allow for higher pmt voltages.Right now the scintillation probe runs at 600-700v which is quite low.Muon interactions are usually so energetic that the signal just saturates completely. I see a few per minute, but they dont make it to the multichannel analyzer, so I assume the pulse pileup rejection recognizes the messed up peak shape and vetoes the count.

author: jmelson
author_rank: Super Contributor
timestamp: July 21, 2020, 06:35:06 pm
content: Quote from: ChristofferB on July 20, 2020, 10:50:31 pmTL072? Yikes, how 1980's. That will only work for very slow scintillators. There are much better and faster op-amps now available.Of course, if the TL072 is good enough for your application, then fine. For general-puepose op-amps, I've used the OP275 in a lotof stuff.Jon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 21, 2020, 06:59:58 pm
content: Well this is specifically made for NaI(Tl) scintillators which are some of the slowest in use. I'd previously used EL2006 op amps but had some stability issues.The OP275 looks nice, though, and is pin compatible. Maybe I should upgrade.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2020, 07:06:12 pm
content: The op275 is more like an odd one - kind of hybrid between BJT and JFET. It is not really suited for high impedance. For direct upgrade of the tl072 I would more consider OPA2172, OPA1642 or OPA2134. The speed and noise is comparable to the op275, but without the high bias.

author: calzap
author_rank: Frequent Contributor
timestamp: July 22, 2020, 12:01:16 am
content: quote from 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?===================================================================================================LKB still makes radiation counters. They were based in Sweden, but are now in Australia. The concept you were referring to is called coincidence counting. AFAIK, it has always been used in liquid scintillation counting. In crystal scintillation counting, it is used in high-end counters, but not low-end. The most massive example of coincidence counting is the LIGO system of gravitational wave detection.Mike in California