New gamma spectroscopy

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New gamma spectroscopy

Postby Doug Coulter » Thu Apr 21, 2011 4:21 pm

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.
URSA_Har.jpg
Test bench setup

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.
HSCs137.jpg
Cs-137 from big harshaw

Detectors matter. Here's about the same thing but from a smaller, thinner NaI head (STOE)
STOECs137.jpg
Same source, different head


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!
hstorb24lin.jpg
Torbernite U ore spectrum

On the other hand, more recently refined U metal has a simpler spectrum -- fewer daughters.
HsUmetal.jpg
U metal, no daughters to speak of

An old Ra sample has plenty of daugthers:
Hs1893vRa.jpg
Radium in spark gap tube, packed in WW II


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!
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.
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Re: New gamma spectroscopy

Postby Joe Jarski » Sun Apr 24, 2011 6:09 pm

Doug, do you have any idea what the piper gets paid for a unit like that? It looks like a pretty cool unit, especially with the pocket PC and isotope library. Makes for a nice portable gamma spec system.
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Re: New gamma spectroscopy

Postby Doug Coulter » Mon May 02, 2011 7:44 pm

Some more plots. This is a different Harshaw (same model, same source for it) that I rewired as suggested by Paul over at radiation safety, which added a 2r resistor in the focus electrode bias, where it had simply been shorted to the first dynode before. Calibrated a little differently of course, I did it this time on the Cs-137 from one of those spark gap tubes Geo is selling.
HS2BkCs137.jpg
Background green, spark gap tube white (Cs-137)



Now Bill found a guy using a plastic scint and a digital camera to see radiation from KCl salt substitute. I think, hey, I have this somewhat nicer thing -- and happened to have a pound of stump remover to push up to the tube. Same plan, background is green for a similar exposure time ( 5 min in both cases).
HS2Bkkno3.jpg
Potassium nitrate -- watch it, you're using radio-actives on your garden!


Just a little fun till I build this up right so it doesn't drift and is handier to use. No castle was used. In one test, it made enough extra junk from primary cosmics to not really help much, and with a smaller NaI did cut down the background. Seems it's better to have those up at the top where you can ignore them rather than down-scattered energy getting into the crystal and mucking up things there.
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.
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Re: New gamma spectroscopy

Postby Doug Coulter » Wed May 25, 2011 5:38 pm

One of the advantages of this site is that we can use it to store a little information where it's hard to lose (or at least, it's in another place to look in). I just completed a rebuild of the phototube chain for the new Harshaw big NaI:Tl, and took a background and a Cs-137 spectra, overlaid, with all the setup numbers written down, so here it is:
HS2NewCal.jpg
New recalibration with new socket parts and reduced noise setup.

These numbers might not mean much to everyone, but if I ever need to get back to my baseline with this, well, I have them stored here too.

Conditions:
Benchtop, no castle.
Room temp 85 deg F
CCFL input 9.4v, 130 ma, output 1834v DC-
Anode load 10k ohm
Gain 17.637 (FS is 5v on this URSA II unit)
Shaping 4 us
threshold 10 mv
ch numbers for Cs-137 lines 76,991
Detector - Harshaw, our #2, socket rewired for focus electrode 2r and bypass caps

Compare this to the one on Wiki:
Cs137_Spectrum.PNG
NaI spectra from HPaul on Wikipedia


The reason I have a bigger high energy peak is that with this huge thing I have a lot less loss to Compton scattering, not that I 'm not seeing the smaller peak as well, as the comparison with my background on the upper one should show. These things are really outstanding as NaI detector go (eg, hard to lift). In this case, size matters, and these are big.

So, nothing really special to see here, just a "note to myself" for the next time I need to make sure I'm set up the same for repeatable results. Putting that shield can on there, and the extra supply filtering let me set the threshold a lot lower but still out of the noise, and run lower HV so I could see higher energy without clipping on the URSA. Those peaks way up high are cosmics, which I assume are not resolved -- just stuff that clipped at the limit of the A/D -- because they look about the same no matter where the other parameters are set.

And now I can go have some fun with this, and know it's telling me the truth for sure.
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.
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