Still not desired resolution (new scint from GEO)

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myID
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Still not desired resolution (new scint from GEO)

Post by myID »

Hello-

first a big thank you to Geo: had trouble with german customs when I ordered the scint from him and he was VERY helful!
So this is the device I am using with my MCA:
http://cgi.ebay.com/3-NaI-Tl-sodium-Iod ... 0069886481
My problem now is: I still do not have the spectrum I d like to see. I attached some pictures. I measured a radium source (old watch), my settings were:
850V, LLD almost 0, amplification really low, 1024 channels, no offset
I used a rolled 4.6 nF cap and 990k for decoupling (see drawing)
Now:
1. is this the max I could get from this scint when it has a bad resolution (I know- squared ones not too suited...)
2. are cap and resistor value OK?
3. Would shielding make a big change?
4. perhaps settings in general wrong?

Thanks- any help would be great because it really gets frustrating....
Attachments
CCF06252008_00001.jpg
CCF06252008_00000.jpg
cap and resistor.JPG
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square pmt 2 radium.JPG
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Richard Hull
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Re: Still not desired resolution (new scint from GEO)

Post by Richard Hull »

I can see what looks like goldilocks and the three bears and the Bi214, (radium C), peak near 609kev. The low energy separation 180kev - 400kev doesn't look all that great. At least it does resemble a real radium gamma spectrum.

The calibration would demand a pulser to allow you to set the gain though the 609 peak by itself would help

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

Post by Carl Willis »

Roman,

These things are impossible to troubleshoot effectively from a distance. I get the impression that the best advice is for you to become familiar with the principles of your equipment so that you'll know what's going on. A good book like Glenn Knoll's "Radiation Detection and Measurement" will be enormously beneficial. You have a bench full of useful, high quality test gear and are inherently in a better position to determine your cause of difficulty than some people just looking at a couple photos, so it would make the most sense if you went ahead and learned some useful background on what you're doing.

Anyway, for what it's worth, here are a few things to check.

1. The PMT should be supplied with positive HV. Last we heard from you, your power supplies were said to be negative supplies.

2. The output pulse will be a negative pulse if the HV supply is a positive supply like it should be. Can your MCA's amplifier handle this? If not, the system is just analyzing the "undershoot" from the AC coupling, which will indeed give a pulse-height spectrum albeit much degraded from the ideal.

3. The last advice you were given included using a pulser to check the MCA. That is a very good idea. With it, you could eliminate the electronics as the cause of poor resolution in one simple experiment.

4. How long is the PMT cable, and how long is the cable from your capacitor to the MCA? Both should be very short. No 50' rolls of cable right from the store.

5. The values of R and C look usable to me. C should ideally be silver mica or an NPO ceramic cap. In the interests of eliminating noise, these components should be placed in a metal project box. You can also place a filter capacitor across the HV input to this circuit.

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

Post by myID »

Hello-

thank you very much for again looking at my outcome.
I guess I have a basic knowledge on what is going on (confused that supply thing...)
My supply is POSITIVE, the pmt is set up for that, cap is highest quality I had because I do not want it to eat my tiny charges, resistors are not metallic film so kind of noisy but the only ones I had on hand....
I used short high quality BNC cables (and did not connect my multimeter like on the picture) I will order better parts now because I guess the values are ok and should not make that big of a difference (from my understanding the cap is definitely big enough because I only need a small amplification to have counts on all channels?)
I understood that a pulser would be a wonderfull tool- obviousely- but getting this stuff over here in germany is expensive and time consuming. I am building the photodiode pulser described in the files section but I guess it is not really suited and I am not sure how to modify it- just use a variable voltage devider at its output to check different channels??
Sure I understood a shielded case would be better because we are talking about small fast pulses but I was wondering if it lowers resolution in general (from my understanding I would not expect this too much).
One big problem indeed is documentation- I still do not have a manual for my MCA and figured out how to use it so far by experiments. I THINK the input amplifier inverts the signals when set correctly and "looks" at negative pulses and my experiments so far seem to proove that.....
Looking at Richards "bears" (even though I know he is probbably one of the most experienced persons in this forum...) I was a little diappointed about "all bears in one" and was wondering if there would be something simple I did not try or look into...
Thanks for the book tip- I will try to get it somehow because mo knowledge mo better

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

Post by homebrew »

hi Roman,

I bought several of those same 3 inch Bicron detectors and found out
the hard way what was wrong. first thing is that you need a dual-tapered
divider rather than the one supplied. The divider as supplied causes
compression of pulse height vs gamma energy because the last few
stages have too low a voltage to overcome space charge. Because
NaI has a decay time longer than typical pmt interelectrode transit time,
space charge limiting can be a problem as energy increases if voltages
are too low. The original design I think starves the last few stages and
especially the anode in an attempt to get semi-constant pulse amplitude
for counting applications. At the other end (cathode) the voltage is also
too low for good resolution.

My cure was to 1) cut the metal tube 2.75 inch from the cable/pot end -
(the dead end) and 2)replace the divider resistors with 1/4 watt resistors
as follows starting at the photocathode: 5.1,3.3,2.2,1.5,1,1,1,1,1,1.5,2.2,
and finally 3.3 Meg. That final resistor is from last dynode to HV, and 3)
loose the cable capacitance by putting a buffer or preamp right at the
assembly. The final 3.3 Meg to HV needs to have 1000p-0.01 u added
across it. Leave the other three original caps in place across the last few
resistors. When all is done, the last four divider resistors will have caps
across them. Also leave the original 4.7 u supply bypass. Loose the sens
pot or leave it tuned full in direction that puts it at a dead short = best
sensitivity. (cw??) If you have to leave the pot there put a 0.01u cap
across it that can withstand the voltage.

My load was 1.5 Meg anode to HV w 8pf in parallel to bring the total cload
to 12 pf ,3300pf 1.5 kv from anode to preamp input.

My preamp was an LF356 with: 330k from 3300pf to -in, 1 Meg from out to
-in, 6pf from out to -in, and +in went to a pair of 2.2k divider resistors off +12
and gnd and a 0.01u to gnd. Lf356 powered by +12 with output leading to
MCA (4.25 v full scale) thru 0.1u. Be sure to bypass power on the LF356.

HV was +510 and scaling was 5.5 Mev = 4.25 volt. Values above will give
a nice gaussian positive output pulse to MCA with 2us risetime and 3.9 us
time to peak value and about 10-20us decay time.

Compression went away and PHR at 1.45 Mev was very good - as I
remember it ended up at 5.5% . This is now the best gamma det I have
and I especially like the 3 inch depth for stopping power but the smaller
dimensions the other way for fewer backround counts.

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

Post by Carl Willis »

Hello wk,

Good information.

I recently replaced the dead factory crystal in a TSA-460 portable MCA with one of these portal monitor 3" jobs from Geo, complete with its as-supplied dynode chain minus the gain pot, and I have noticeable compression at the high-energy end of the pulse-height spectrum. I don't care too much about it, because with only a 250-channel memory, there's limited ability for quantitative energy measurement on those old units anyway.

But if I had to suggest a solution to improve the linearity, it would be to decrease the resistance of the entire chain while holding the ratios roughly constant and adding more capacitance on the last couple stages. The problem with that in my application would be an increased current demand from the battery-hungry HV supply. So I may reconsider your re-proportioning idea.

I don't do too much with surface-mount parts. Do you know if the parts on that dynode chain are "805s" or some other series? If you know what size they are then I'll have an easier time ordering replacements. Thanks for your contribution.

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

Post by homebrew »

hi Carl,

I think there were two sizes, 805 and another. I bought surface mount
resistors but it was so tight I could not use them and I didn't want to
unsolder the board from the pmt leads. I ended up using 1/4 watt
leaded resistors standing up. good news was that I kept the center
of the string at 1 meg so 5 of the resistors could stay the original
surf mounts that were on the board, and only 7 had to be removed
and replaced w leaded parts. As I mentioned previous msg I think
I added 1 more string bypass so that the last 4 were bypassed
instead of the last 3.

The lack of compression might be nice even if you only have 256
bins because for 0.5 mev trig and 5.5 mev full-scale that would be
abt 20 kev per bin where the inherent scint/pmt avg analog reso
mid scale is about 80-100 kev so you still have several "stairsteps"
up and down each side of a peak. Ironically, even tho I could have
as many as 4096 bins to cover 0-5.5 or 0-11 mev, I settled on 256
as the best compromise between counts per bin, how long it took
to gather enuf counts for good statistics, how good the spectrum
"looked" on the pc screen in mathcad, etc etc.

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

Post by homebrew »

hi All,

Looked inside the box today - replacement resistors in divider string
for 3 inch Bicron were 1/8 watt - not 1/4 watt
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Re: Still not desired resolution (new scint from GEO)

Post by myID »

Sorry-

I do not have much time to work on my fusor due to my move- and I have not much time to follow the forum due to my new job
Anyways- this is great info! I am really looking forward to modify my detector and get the desired resolution- finally!!
I love this forum and all the great tips I get from the experts here!!!

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

Post by hjerald1 »

I also have one of these Geo probes and would like to improve its performance.

Questions:
Has anyone by now (e.g.Carl or Roman) tried wk's modification? How
did it work in your hands?

For folks like me (electronically challenged) could someone (wk,
Carl, others) supply some more details (pictures, components
used etc....sorry I am not even familiar with what "805s" are for example)
of the mods so that my chances of doing it correctly are improved?

Did I not see somewhere that Geo provided some info on opening the
probe (mild heating to soften the thermoplastic adhesive?) or am I having
a senior moment?

If you wizards of all things electrical will humor me in this project, perhaps I can help someone in the future with questions on chemistry, or the like .

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

Post by Carl Willis »

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

Post by Doug Coulter »

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
Why guess when you can know? Measure!
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup »

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

Post by Richard Hull »

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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TSA Annot.jpg
TSA Annot1.jpg
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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 »

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 »

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 »

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 »

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 »

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup »

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

Post by Richard Hull »

Like Carl Willis noted long ago, in this very thread, it is tough to diagnose "at range". I am not there to see touch and adjust.

If you truly have 2048 channels, I see what looks like a fixed, uncontrolled amplifier gain there before hitting the ADC. Some radium or Uranium ore would tell the tale due to the large number of easy peaks. In all gamma spec systems, amplifier gain and windowing play the real role of putting what you really want to see on the screen.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Still not desired resolution (new scint from GEO)

Post by ChristofferBraestrup »

I'm very much aware how big a mess tele-troubleshooting is, That also wasn't my purpose posting in this thread, but I really appreciate the effort!
[+HV]
V
My setup is something like [scint]->[bias tee]->[preamp]->[ortec 572 spec amp]->[Canberra Accuspec MCA]

1kV bias
x6 gain in preamp
3µS shaping time
500x gain in spec amp
2048 bins adc gain
lld=0 uld=2048

What is meant by "fixed, uncontrolled amplifier gain before hitting the ADC"?
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Re: Still not desired resolution (new scint from GEO)

Post by Richard Hull »

Most ADC signal input amps in spectro work allow windowing. This, for initial convenience, is usually zero on the lower level and the upper limit set to the max height. This allows for a 1kev/channel spead over the 2048 channels. You have done this.

This displays a 0kev to 2048kev range. (1kev per channel) the gain will determine pulse height positions within the spectrum. With the smoothly variable gain control and the window set as above, You can calibrate the system with known isotopes.

Am241 59kev gamma. adjust gain so the huge peak top is centered on channel #59
Cs137 610kev adjust gain so that the peak top is centered over channel #610

I assume you have a movable channel blinking cursor as you seem to have your system setup nicely as above. Move the cursor to the channel peak desired and calibrate as below. The amp gain can't just be set to an even number (500)! You much also have a widely variable gain pot to move the spectrum peaks!

In time, with experience, you can set the LLD and ULD windowing and gain to attempt to separate nearby peaks to within the resolution of the scintillator.
Note, that slight non-linearities can exist such that if you calibrate solely at 59KEV the 610kev peak might be at 605 or 618kev. Often, a balance is needed to allow for total span accuracy that is close. One can fine tune, naturally, for one hyper accurate point anywhere in the spectrum for a specific energy expected or any peaks near it.

A full fledged, all-in-one gamma spec usually has a vertical range selector switch that has ranges like 1k,2K,4k, 8K, 16k and so on counts to the top of the screen. Set to 4k counts, small slow peaks on weak sources can be seen to develop quickly. as the peak grows you can move up to 8k or 16k counts at the top of the screen with much finer peaks displayed.

Calibration demands hot isotope sources so that a 20 second acquisition will produce the desired peak easily seen. The gain can be tweaked and re-run for another 20 seconds as the peak is now moved. I use 10 uCi isotopes and adjust the gain on the fly as the little nipple peak can be actively gain tweaked and the fast rising nipple peak slides along the channel numbers. In short, what works, works.
with weak sources you are lost on calibration. You have to wait a long time to see the result of our calibration efforts.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
ChristofferBraestrup
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Real name: Christoffer Braestrup

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

Post by ChristofferBraestrup »

What a great summary of MCA setup, thank you!

I will keep at it optimizing the setup, but for now I think I will conclude that the square bicron probe with mods behave decently.

One thing that hinder the performance of these probes are the very long RG174 cables built in as standard. RG174, like RG58 has a capacitance of around 100 pf/m,
so half a metre of cable yielding 50 pf to ground from the detector might be significant!

If there is room I would like to put the SHV and BNC connector directly on the back plate (grounded, of course), and put the bias resistor directly in the probe.
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Rich Feldman
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Real name: Rich Feldman
Location: Santa Clara County, CA, USA

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

Post by Rich Feldman »

Very interesting to read about tapered PMT voltage dividers, I guess related to linearity of the charge multiplication.
Nice idea about embedding the bias tee to eliminate some cable and connectors.

This is a chance to point out (for noobs) a simple thing about transmission lines.
No coincidence that RG174 and RG58 both have 100 pF/m, because they are 50 ohm cables with PE dielectric.
Velocity factor = 66%, so a 1 meter section has delay of 5 ns. C = delay/impedance = 5000 ps / 50 Ω = 100 pF.

When low capacitance is desirable, and cable can't be made shorter, it can be made with higher impedance and/or higher velocity stuff.
50 and 75 ohm are popular for RF and video transmission, but 93 and 125 ohm are available, as discussed in this forum.
A TDR measurement of my BF3 neutron detector tube revealed that the gas-filled section has Z0 of about 320 ohms!
All models are wrong; some models are useful. -- George Box
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