Well, John, there's no question you know your stuff - for others' benefit, John works at Ion Beam Technologies, which builds all sorts of interesting beamline types of things, implanters, nano-material investigation, and so on. John's an ace at this stuff and has produced at least one patent I know of for his company, inventing a new way to make zinc oxide nanorods and in quite a clever (and inexpensive) fashion. We've met - John's no dummy, rather the opposite. And unlike many, he's got hands-on experience with all this stuff, though more in a situation where you just buy things new than what we hobbyists encounter more often.
But 100% of every detector I've gotten - including the bicron "gallon jug" ones which are obviously designed for spectrometry and which have 2 bnc's, one for power, and one for signal - use positive voltage unless you re-wire them, and still capacitive couple the signal out as a negative pulse. In the case of the big guys, the signal isn't all mixed up with the phototube divider chain - it has its own load, usually a couple hundred k ohms, separate. All of the other, smaller ones are two wire (EG one coax), and those are what are available to the hobbyist, likely having been stripped from PET scanners, so that's what one designs for if one is designing a thing for hobbyists. Since the shield on the coax is universally connected to the metal case, you can't float that end and use it for signal without picking up all the ambient noise in the lab (and creating a possible safety issue).
Remember, in the case of phototubes, we are ALWAYS talking to the anode - that's where the high level signal comes out after the electron multiplier gain. The pulses will therefore always be negative-going. It's a good deal that a simple opamp inverter will flip them to positive polarity, and nicely linear, no step function glitch with a fast pulse like a non-inverting lashup will give with many opamp designs (national semiconductor application handbook, no longer in print, but it's also shown on a lot of data sheets)
All of the smaller heads (including a few home built) we've encountered here have a single BNC (not the high voltage flavor) for a positive input, you have to provide your own load, which in my case has usually been 10k ohm. The lower impedance seems to work OK with reasonable lengths of coax (we don't have to get as far away from things as for John's big megavolt setups). A little shaping by the coax capacity isn't always a bad thing.
The
URSA II we borrowed had facility for 2 wire setups also, though it also provided another input for cases where you had your own supply, though Paul there recommended we use the inbuilt one (which fried itself while we were not using it, too much stuff in a too-tiny box? Type C connector for that, ugh, we had to search for one, but while searching, I simply built a connector myself). One of the setup parameters was cable length, for the reasons John mentions. In the case of Phillip's design, we probably need upfront shaping anyway, as it will slew-limit on fast pulses as is, and there's no handier way than using cable capacity to get it - it tends to be nicely high Q - a good quality capacitor, and the price is right - all you have to do is provide the R and you have an RC lowpass.
The "modern" (eg microprocessor based) Canberra MCA that was donated, unfortunately dead on arrival, didn't even have facility for anything but a single coax input with a positive supply built into the unit.
A lot of people use 50 ohms, in a somewhat misguided attempt to make the coax look like transmission line, then require a huge-gain preamp. I believe that's simply silly, these signals from NaI's at least, are so slow you'd need quite the length of cable to see a reflection from a mismatched impedance, they look like a pure C at any reasonable length with 1us and so forth type pulses. I admit, I have zero experience with HPGE, as I can't afford either those or the cryo ongoing expense to run one. I know BGO's are faster, but...they have worse resolution and output than the NaI's do, so I don't use them.
In PET scanners, they are looking for coincidence of 512 mev gammas, and nothing else - it's a one-bin situation (really two - the one we want, and everything else, which is discarded). So they don't have to be all that great, but in reality, most of them are pretty good used for spectroscopy. Most of the extant surplus is stripped out of these, where they have 256 to 1024 detectors, and really cannot afford to do it any other way - they're all single-coax affairs. They accept that there will be a little extra noise from including the divider chain current in the signal, and a little nonlinearity. Our experience here is that it's not bad enough to notice,
given that NaI lines are broad compared to HPGE anyway. Only our "gallon jug" NaI's are even a little better than the ones shown at the link, and at that, only a little (those were ~~$10k new, we scored them cheap as surplus).
Here's a typical signal from a couple of NaI's I plan to send on to Phillip for his testing. These are 2 wire, and I used in the neighborhood of 820v with a 10k load driving the 1' provided (hardwired at the sensor end) cable and a .01 uf coupling cap to the ten meg scope probe to capture these.
- From the .5" crystal bicron Cs 137 662 kev line
- From the 1x1" by 4" deep bicron, looks like we caught a double here. (pileup)
I suspect most units do it this way to keep the photocathode at ground level. I've had issues trying to run them at the full supply voltage negative to their surroundings, as sometimes a little corona noise gets into things and so on. You have to be careful to "do it right" and get that nice, floating anode as your (still negative going) output.
I had to rewire BOTH of our gallon jugs for this mode. In that case I was lucky and there was no noise due to running the photocathode at negative 1500v or thereabouts. The divider chains, as shipped (acquired surplus of course) were twin BNC, but still positive supply, the only difference being the isolation of the anode signal from the resistive divider so you could not have any more Johnson noise on that signal. They DO work better rewired for a negative HV input and my own load, which now no longer needs capacitive coupling and the issues that come at high pulse rates when the coupling cap picks up charge due to a finite impedance load. We produced slightly better spectra with the URSA than is displayed on the wikipedia page this way - but only a little better.
We also learned that with either their power supply or ours (which was better/quieter), that NaI rarely needs more than about 1k resolution, due to other issues in the detector itself. As the Wiki article shows, HPGE can use a lot more resolution as it is intrinsically better as a spectrometer head. Too bad I live far from anywhere LN2 is shipped. But I can't afford a dewar, the rate of required buys of LN2, and I like things that just plain work all the time. You never know when inspiration will strike and you want to test something. FWIW, I have yet to encounter a postive going pulse in anything whatever, surplus or new, that didn't have a built in inverter/preamp, or was transformer coupled (so you have a choice which way you hook up the seconday). And in that latter case, I was the guy who designed and wound the transformer (small ferrites do well at this speed range, only a few turns required).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.