Well, I'm learning, and we've wanted to have a decent gamma spec around here for quite some time now. Bill has managed to get us an evaluation unit of an URSA-II MCA, which is what I'm working with. We also have an older NIM thing, that never gave us good results, but given what I've been learning with a known good unit, it might come back out of storage too.
The URSA is a nice thing, compact, possible of battery operation, and pretty versatile. The unit I have was modded for someone who then returned it, and doesn't have full functionality, but what it has is pretty darn nice. Around here, that tiny size is pretty important too. Here it is on my tinker bench with a nice NaI head Bill found on ebay. The URSA is the aluminum box under the scope probe -- nicely portable, even with built in batteries. It will provide +/- 12v to a preamp, and has it's own programmable HV supply as well.
I'm just learning how to get the most out of something like this at present, twiddling the many adjustments and seeing how they interact and things like that. As I get a little smarter, I'll add on to this thread. At the moment, I'm running the phototube off a separate CCFL based supply which in turn is running off a bench supply, which isn't as stable as a closed loop system, but stays pretty good (inside a volt) according to my HV probe. If I rewired the tube socket, I could run it off the URSA built in supply, which is probably a better one than this currently is.
I am having some drift issues with temperature or time of power on, and not sure where they are coming from just yet -- I only know it's not my cheesy HV supply as I checked for that right off.
But, once warmed up, this produces some nice looking spectra. Here's one with a Cs-137 source on the big Harshaw NaI.
Detectors matter. Here's about the same thing but from a smaller, thinner NaI head (STOE)
Just a note of intrest. The lower peak on the STOE head IS the right peak for the low energy Cs-137 line. On the upper one, that lower peak is just noise. The STOE is super quiet, but can't handle the high energy 662 kv peak -- it's the most far right one, and that larger one below it is from photons that had some energy scatter out of the thinner crystal. In the first picture, from the Harshaw (5" diameter by 3" or so deep) that high energy peak is correct.
So, we learn something here every day, or try to. Maybe with a decent preamp, better wiring, I can get the correct low energy peak out of the otherwise bigger and better detector head. I'll be trying that. PaulS over at the manufacturer wants me to rewire the phototube base and try it off the URSA power and I'll try that when I find the parts to do it, maybe this weekend, maybe I'll have to order some. I don't usually stock tens of high value high wattage resistors in high precision around here, and that's what it needs.
For pure eye candy, this is a lot of fun. Richard H says with U ore you look for goldilocks and the three bears. Well, on this Cadillac head, we have 5 bears!
On the other hand, more recently refined U metal has a simpler spectrum -- fewer daughters.
An old Ra sample has plenty of daugthers:
As usual, click the pix for a better view of the details.
Significantly, the author of this software is contemplating Linux support -- that alone might sell me the thing when the time comes to pay the piper.
Thorium is showing me different spectra depending on if it's from a lamp mantle, the metal, or in welding rods....but that's another post. Happy weekend everyone!