Hi there,
This is my first post on the forum so I hope I'm not doing anything wrong and sorry in advance for being a bit verbose below, just didn't want to skip over anything that may be important.
I have been researching different ways to figure out how to properly interface with a old photomultiplier tube I found online to be able to attempt Gamma Spectrometery. I found that they need roughly around 1 to 1.6kV to operate but at low currents and it appears that when a photon originating from a gamma ray that excited a scintillator at the front of the tube enters the tube, it is amplified through 9 stages of dynodes and hits what may be a photodiode allowing a small current to flow through. I believe this current is then put through a resistor of high resistance and then to the ground rail where a voltage drop is measured across it, however I have found some conflicting schematics.
The tube:
http://lampes-et-tubes.info/pm/FEU-35a.pdfA design I found with a similar tube:
https://www.instructables.com/Multi-Channel-Analyzer-for-Gamma-Spectroscopy-With/ I was planning on generating these high voltages using either a boost converter I found on ebay that is adjustable to 1.2kV or as the instructables suggested, a CCFL tester.
From the instructables article, It seems as though the signal can last 100us to ms and is of amplitude 50mV but may be floating on a high voltage rail. I think the article actually uses a negative high voltage so that the signal is not floating on hv but ground referenced with negative amplitude. The part that's been stumping me is when an issue comes up in the build/design stage of the circuit and I need to diagnose it, I don't really want to cook my scope (SDS1104X-E). After digging around and speaking with Dave a bit, I think I have a much better understanding of using a differential scope or a high voltage probe to address this issue but there are still some concerns bugging me a little. In terms of using a HV probe, I found that the Hantek T3100s were pretty affordable and had some relatively ok reviews on this forum, I figured two of them may work for a differential measurement but the signal would probably be drowned out by using very little of the hv dynamic range. So looking at the differential probes, I came across the HVP70 here, some no name brand 20v ones on ebay and Micsigs catalog of their hv differential probes which looked promising. The Micsigs have probes with 500x and 2000x attenuation for measuring up to 1.3kV and 5.6kV respectively which seem great but I figure that I'm not going to even see the 50mV signal which may be well into the uV by that point if present at all. My fear of just using a standard low voltage differential across the circuit is that some of these probes have a common mode rating of 700v or 1kV which may be exceeded even if only the differential part is small, am I correct in believing this?
I did plan on running my circuit during testing off some 12v SLA batteries to easily get a negative voltage if needed and I hear that this may actually render my concerns above null as the ground referenced can be tied to any point of the battery circuit if its floating but I still am concerned that even with a HV probe in this case, I may cook something.
If a differential probe is the way to go, is there a way that I can find one that i can use for both relatively low voltages (10s or 100sv) and this application at the same time to avoid purchasing one in the future?
Since differential probes will probably set me back a few hundred, is it practical to just purchase a cheap handheld scope meter instead which isn't earth referenced?
Also on a similar note, I was thinking, maybe this wont work at all, but could i use a high turn ratio transformer in series with the anode resistor to generate a higher voltage than 50mV thats isolated every time a pulse comes through?
Also for reference, this was another article I read through while researching that does discuss the same tube but with a different circuit:
https://www.fusor.eu/gammaspec.html Tldr; what's the best way to measure a low voltage signal on a hv rail and will a hv differential probe work?
Thanks for your patience with me, any help is greatly appreciated!