Still not desired resolution (new scint from GEO)

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Carl Willis
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Re: Still not desired resolution (new scint from GEO)

Post by Carl Willis » Fri Mar 05, 2010 8:37 pm

Hi Jerry,

Sorry I missed your message.

I have NOT tried wk's mod.

The part I like is the new dynode supply resistances he has figured out. According to his report it improves the energy linearity of the assembly. I still would like to try that when and if I get the 'round-tuits. I don't really have a need for the onboard charge-sensitive preamplifier he suggests; NaI scintillators have a huge output pulse and I would just as soon put a preamp somewhere more accessible, or omit it altogether depending on the use.

"805" is a package type for surface-mount components. I don't know what it means other than denoting a particular size and shape of package. I suspect you could Google around for the dimensions and specifications for the common surface-mount packages, and compare the sizes of the packages on the dynode chain, and then know what kind of parts to buy if you were to replace them with different values. I THINK most of the resistors on that socket assembly are 805s but I have not gotten out my calipers to be sure. My experience with surface-mount construction is minimal.

You are right that George at one time had instructions for removing the housing that involved softening the sealant with a hot-air gun. That does work pretty well, much better than trying to liberate the assembly with a hobby knife. I cringe to think of the crystal changing temperature so quickly, but can report taking apart five of these assemblies with a hot-air gun and having no crystals crack on me.

-Carl
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Doug Coulter
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Re: Still not desired resolution (new scint from GEO)

Post by Doug Coulter » Sat Mar 06, 2010 1:17 am

Roman,
That's not real bad (compared to what I get with the same stuff, fairly optimized circuits). I note something that no one else mentioned -- geometry.

To get this right, you have to have the gammas coming straight in only -- nothing oblique, as those may deposit only part of their energy into the NaI and the rest skips out. This broadens all the peaks.

So what you do is make a colimator. One way is to put the source in a lead block with a hole a few times longer than the diameter in it to let only gammas out that are going straight into the crystal end, the other way is to do the same thing at the tube end. Big difference in results over just putting a source near the tube.

Doug
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup » Sat Jun 20, 2020 10:24 am

Happy 10 year anniversary for this thread!

Sorry to revive such an old thread but there are still a LOT of those square bicron probes kicking around - orders of magnitude cheaper than their round equivalents.

I wanted to document the modification progress as suggested by user 'Homebrew' and I thought this thread was the best place to do so.

Here goes:

Modification of the Bicron/Saint-Gobain probe 1.12x1.12m3/1.12L and equivalents:

Disassembling the probe:

There is no need to cut the casing, the back cap pops off with a knife blade or screwdriver, allowing for the removal of the potentiometer and cable.
On the crystal end (furthest from cable) a black resin can be seen between probe housing and aluminium inner housing, can be gently scraped out with a box cutter blade or equivalent.
Once a knife blade can be inserted under all four sides, one should be able to gently push the entire assembly out the back end by pressing on the end of the crystal.
bicronsquare11.jpg
As it is clearly seen, the NaI(Tl) crystal is sitting in its own inner housing, which is epoxied onto the PMT. The pmt is wrapped in a mu-metal shielding, and the divider sits on a little board at the back.

Next step is to reverse engineer the divider to verify what it actually looks like, then replace resistors, pot and capacitors.

Confirming original circuit:

Ohming out the circuit shows only one difference from the schematic posted earlier in this thread, the series R before the potentiometer:
bicron square original.jpg
The 750K resistor in the upper end of the divider looks strange to me. I wonder if it was just sized experimentally for high count rates.

Either way, implementing 'Homebrew' 's taper mod, the new circuit should look like this. Note the little circles to indicate modifications.
bicron square mod.jpg
More to come when I've made the modification!

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Richard Hull
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Re: Still not desired resolution (new scint from GEO)

Post by Richard Hull » Sat Jun 20, 2020 6:41 pm

Wow, This is pulled back from the dead! I actually had about 3 of these little Bicron PMT systems way back in 2008. In all but one, the hermetic seal around the crystal had, in some manner, failed and the thin aluminum casing around it had reacted with the sodium iodide and blistered. I did manage to save the little PMT in those two. The other one worked as is and I have it packed away somewhere around here. As I now have a formal Bicron 3" PMT and matching cased 3X3 Xtal and a similar 5X5 Bicron cased tube and mated xtal, I have not fiddled with my old 2X2 Bicron unit, much less, these little units.

By the way, the giant 5X5 bicron chromed beauty is rather valueless here as it is too sensitive for my environment. It needs a 1000lb castle to really perform. So I muddle along with the 3X3 with long background subtraction runs during gamma spec runs. I rely heavily on my friend Bill Kolb and his 5X5 system with homemade, limited castle for work that tickles the limit of my PMT work and on good ole Jon Rosenstiel and his germanium, liquid nitrogen system for picking out the "needle peaks" that a PMT based gamma spec smears out when the peaks are close together and small.

A very long count here in the upstairs lab is 5000 seconds under the best of conditions. At 3000 seconds I start recording the uranium equilibrium background here. If you are in a high background area with no castle, yet a good gamma spec system by Canberra, a good 2X2 or 3X3 pmt head is about the limit.

Carl Willis had a great "homemade" U ore sniffer built up out of those very small BGO crystals sold by Frank Sanns in the early 2000s. Carl attached a matrix of those crystals to a little 1.5" PMT that he hooked up to a Ludlum counter for field work. It was as good as or better than the professional, expensive, TSA larger plastic scintillator units that Bill Kolb and I used in the field on our Utah U hunts. Carl joined us at the Moab, Utah mining region area one year with his kludged up little BGO set up and was finding stuff "at depth" with his system. This means he was seeing smaller "hot" pieces buried deeper in overburden than we were. (All of the best ore is now long buried along the haulage roads near the mines) The intervening 70 years since the "U Boom" has seen rain and topsoil cover most of the "good stuff" that bounced out of the giant haulage trucks as the bounced along the roads near the mines. There is never much to be found actually at the mines, themselves, as they have been "picked over" by mineral hunters or blasted shut in an effort to keep the curious from entering as part of "cleanup efforts" by the states.

Related to BGO....( Bismuth Germanate)...This is a great scintillator for seeing gammas over 2mev as it will see them! However its resolution is rather poor in a gamma spec situation. I own a fully mounted 3X3 BGO crystal and mated PMT unit and when used on my Canberra gamma spec system it demonstrates the classic Thorium daughter gamma peak near 2.4mev easily, but it is wide and the lower end stuff is not crisp like the 3X3 NaI:Tl system. The 3" BGO and PMT assembly is very heavy and not suitable for field use, but I'll bet, on a push cart, it could see small stuff a meter down! However, who would want to dig a 3 foot hole in the high desert in Utah in 100+ degree heat?? The deepest I ever "gophered down" for a gem piece of scintillator sniffed ore was about 6 inches. (Well worth it too!)

Using A PMT based U hunting set up really wins the day when swung back and forth at the end of a long leather strap a couple of inches off the ground around old mine dumps. A Geiger counter is just worthless for U hunting. Leave the GM counter in the car for when you get back with your goodies to quantify and sort your finds of the day once you are back at the motel in the evenings.


Richard Hull
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup » Sat Jun 20, 2020 7:27 pm

Wow, I never considered high background could be such a nuisance. Thankfully, here in Denmark, the underground is about 1 km mud and gravel before bedrock is reached.

This 1.12x1.12 probe measures about 50 cps with no sources present, at 1000V with the modifications installed.

I've modified it as per the schematic I uploaded, and I'm sorry to say.. performance didn't improve much.

The epoxy holding the probe together is black so I assume it plays a role as light shielding. I've played it safe and taped up the finished probe, see attached.

Pre-mod:
bicronpremod.jpg
post-mod:
bicronmod.jpg
bicronmod (2).jpg
and the assembled probe:
bicronmoddone.jpg

I'm still hunting bugs in my system, resolution is pretty bad, but I think that might be my preamplifier that is pretty atrocious (the one I posted turned out to have major issues, I'll edit that later)

I think this detector is still a step up from my previous 1.5 x 0.25 !! flat-disc NaI(Tl) detector!


I'm also 3D printing a mold template of an interlocking lead brick, so I can cast myself some of them to build a little castle. Alternatively I will cast a 2-3 cm thick lead bucket in a paint bucket .

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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup » Sat Jun 20, 2020 8:11 pm

Here is a spectrum of a very pretty photopeak with compton edge taken with the new modified detector. It may be K40 but I'm unsure, difficult calibrating with such a weak source.

I'm quite pleased with that, actually.
40K with bicron.jpg

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Re: Still not desired resolution (new scint from GEO)

Post by Richard Hull » Sat Jun 20, 2020 8:46 pm

Good job there. What was your source? How long was the count? That peak is pretty vivid! You were around something, for sure. You would never see that in any background run. Got any radium around, that has a gang of peaks at the low end that a small crystal will catch easily. Am241 from a smoke alarm will have a huge rapid developing peak at 59kev.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup » Sat Jun 20, 2020 9:01 pm

Thanks! Those are the two sources I'm counting on (ha ha) being able to find. Th-Wo TIG electrodes as well.

Count time here was around 2 hours on 100 g potassium carbonate. Unfortunately, I don't have the manual for my MCA so I'm unsure how to calculate FHWM directly.

I can export a txt file of counts vs. bins that can be imported by Fitzpeaks, that's the way I manage my data at this point. Right now I'm just going by looks, though.

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Re: Still not desired resolution (new scint from GEO)

Post by Richard Hull » Sat Jun 20, 2020 9:38 pm

Then that is definitely the high energy gamma from K40. Thanks for the information. Typically when showing a gamma spec image you should always note the collection time, the source and the scintillator info. Commenting on the peaks is nice, but most folks who speak "gamma spec", once you identitfy the source, will know what the peaks are.

I have so much U ore and check sources around that I always have to do a similarly timed background subtraction count after most all low level source gamma spec runs to punch out most of the large background noise peaks.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup » Sat Jun 20, 2020 11:22 pm

Just saw your 5x5 scintillator, what a beast!

That's understandable, yeah I was a little fast on the trigger with posting.

But see, here is where the trouble starts:
uglassbicron.jpg
This is a 1H40min run of uranium glass, 1000VBias, which looks absolutely atrocious.
I can just about convince myself that the two leftmost peaks are the Th234 peaks and the wider one is U235, but this is even worse than what I saw with my tiny 1x.25 detector!

Could this loss of resolution be caused by 'compression' effects, as mentioned earlier in the thread, which would be common for non-tapered PMT divider detectors?

What other causes could it have? How could it be improved?

Thanks!

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