Wind, Mobile Rad Lab, Home Rad Lab etc.

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Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby George Dowell » Mon Sep 05, 2011 5:07 pm

Yes I record the wind speed etc. all the time Bill F.
Missouri has a program that if you give them a good reason, like
say an online Geiger Counter, they will set up a free windmill to test the idea.

Only a few projects per year but I have been so busy I have not even put in for it.

Right now I'm waiting on a custom modified URSA II from Paul S and a Recon hand computer for testing in the Mobile Rad Lab.

The MCA itself will be the main one on the big lead pig in the Home Lab too.

I've tried a dozen or more MCA's in the Home Lab over the last few years including an URSA II, UCS-20, UCS-30, Lynx, Inspector 1000, 2000 etc
and after evaluating all the different systems have decided on the URSA II as the permanent instrumentation.

It will be backup in the Mobile Rad Lab behind the GR-135. Also primary in the Mobile Rad Lab for fixed position analysis
(i.e. not in motion- the GR-135's single button capture/analyze feature is hard to beat for safety in a moving vehicle!)

It's always something.

Geo
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby Doug Coulter » Sat Sep 10, 2011 3:22 pm

I really like the USRA we have here on loan. Nice piece of work, but I have a question. Here, on the really good "gallon jug" NaI it seems the energy drifts all over the place during a day. I know NaI has a tempco, but I'm seeing like ~10% from noon to evening, and that thing has enough thermal mass not to change much indoors when it's not changing temperature much (less than 5F over the period). Is it that bad, or does the URSA probably have a ton of gain drift? I know back when I designed such things for a living that many implementations of programmable gain amps were that bad, easily - you had to know your stuff to create a truly good one. And the URSA is really trying to pack a 200 lb woman into a 50 lb box to say the least - they obviously didn't use a premium DAC in the feedback loop of an opamp or it would have different settings available than it does. (Same for the shaping options, a little strange, and the vernier gain doesn't work right and causes time constants to change when adjusted).

I'm using it as a comparison standard for the one I'm developing myself. I can't see a reason why they should cost so darn much, and it keeps people out of gamma spectrometry who would benefit by having it.
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: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby George Dowell » Sat Sep 10, 2011 4:06 pm

Sodium iodide probes drift a lot. Some more expensive systems incorporate a phantom "channel" to which the system is locked in a feedback loop.. The calibration channel is monitored and system gain/HV is automatically adjusted to keep the phantom channel withing it's expected window. This channel is generated by LEDs, Am-241 implanted into the probe itself, or any number of external isotopes being applied continuously. Ba-133 is one that has been popular, being replaced with plain 'ol K-40 in the newest generation gear ( Ludlum 702i, PGT SAM 940 etc).

All "downhole" well logging probes contain such a feedback channel.

I've seen other issues cause what looks like drift too, such as having too much preamplification in the head end.

One MCA drove me nuts because it turned on it's own internal preamp, even though my setup uses a base mounted external preamp. No 2 readings were ever the same!!

Check out Steve Sesslemans Gamma Spectacular, GS1100A . Only a few hundred dollars and is opening up the spectrum analysis area of our hobby, using freeware software along with Steves interface/HV modules. Very successful and promising.

http://www.beejewel.com.au/research/Bee ... /home.html

Different versions are available to take advantage of whatever type probe you already have.

Robert G8RPI in England makes a similar high quality interface. robert8rpi@yahoo.co.uk
All these devices rely on PHA and Fitzpeak software (free).

One of our friends in France is collaborating on a handheld portable MCA driven by a BGO probe.

http://f1.grp.yahoofs.com/v1/8LRXThDwTu ... r2QuXkLZ7n\
A74EnOAUFTFclXJm9Z_nEmNd4EwKUl9HejtIedUV8uwFDA6KT/Gamma%20Grapher/Prototype%20be\
ta%20Version/Build_your_own_MCA_2.pdf



Amateurs scientists are once again expanding the envelope. Exciting.

Geo
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby Doug Coulter » Sat Sep 10, 2011 4:31 pm

Well, we can all afford to remember here how much the Amateur Radio guys contributed. Amateurs rock fairly often - nothing beats loving a subject for getting good at it. I was involved with Steve's thing a bit. After telling him what needed to be done to make it work for 6 months, then giving up, once it was "his idea" it started being tried, and worked. But computer soundcard a/d's absolutely stink for this job, and it will only work with very time-smeared pulses and very low count rates - there is no way to implement any dead time with one, when the FIR filter in it is at least 50 samples long (and rings for that long off an impulse). So as long as you're content counting millisecond pulses, it can work, but...I'm not. (that whole debacle is on fusor.net) I want URSA-like performance, but for a few hundred bucks, and better than that almost-ok thing BTI was selling for a couple $k. I'm sure I can get that, and not need to lowpass 5-10ns pulses from fast scints too much to get it, because I can put in a real peak catcher in fast, discrete parts. That's where an external CPU that doesn't have to run a dog and pony show opsys and eye candy can really do some good for you, and we've now got a platform going that will do this - even has a built in fast A/D, all we need to do is a good front end for it now, and a little software (which I did for a living for many decades, no sweat). It will be able to handle 100k cps worth of conversions, and properly handle cases where the pulses are faster than that via actually being able to have actual dead-time, rather than having all that garbage get into a built in filter that all the PC sigma-delta A/D's have to have that then rings for another millisecond after.

Now, my drift on the URSA was pretty big, and I was using only its preamp, and a separately regulated supply (our loaner one is blown for that). But I still saw huge drift...does 10% for 5 deg F sound right to you? I knew NaI had a bit of tempco, but heck, that's a thermometer! This was like, forget doing an hour count on anything, all the lines would smear way out, even on a Cs-137 cal source. If you had a loud source (I do) and could do a short count (I did) you got nice narrow lines, and it looked better than the NaI example on Wikipedia. But not if you counted the same thing for an hour.
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: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby George Dowell » Sat Sep 10, 2011 5:07 pm

That is a lot of drift. I don't have my charts here. What is the HV set to? Is the HV supply rock stable?are you using a 3 point calibration routine?
Is your an URSA or URSA II?
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby Doug Coulter » Sat Sep 10, 2011 8:59 pm

Ursa II. Power supply 1834.0 volts, continuously monitored as well as regulated. I used a negative supply with the anode lightly loaded and put into the input of the URSA unit, so no variation from possible drift in the dynode chain resistors in that current (other than if they did drift, the gain of the tube might have changed, but the current was constant and the voltage stayed within .5v the entire time). I had experimented with various supply voltages and gains in the URSA to get the skinniest lines at the higher energy Cs peak. Some pics and words here: viewtopic.php?f=11&t=337 Yes, I know it's a terrible setup for real measuring...but nothing there should cause drift.

I only had two point cal with Cs-137 for the drift testing, and with the big tube, the lower peak was just out of tube noise with this huge crystal, it took some fiddling with the threshold and other parameters to make it clear and obvious out of the sort of "1/E background", but I got that to happen on both of the big guys we have here.

It's interesting that I also have several other NaI's but the only other type that seems worth it really is one I have with a very tiny crystal, thin window, built in preamp. It really gets that low energy stuff clean and neat, but of course fails miserably on the higher energy stuff that comptons out of the tiny xtal. The midsize ones I have, about 1" sq by about 3.5" deep seem like a compromise that gets everything wrong. For that matter, one we have 1/2" sq by ??? deep seems as good or better than the 1" guys. I've not yet hooked up one of my homebrew BGO's or plastics to the URSA, I suppose I should. Even with the lousier resolution the amount of drift I'm seeing should show and tell me which end of the signal chain it's coming from.

I'd like to kill off that drift so I can do neutron activations to measure other things with some confidence (minerals, soil samples)...gotta get the test gear around 10x better than the thing you're measuring, isn't that the good old rule? And it's not like a random gamma spectrum is all that easy to decode - it's at least as bad as a mass spectrum with "illegal" chemistry that can exist long enough to register a mass line funny stuff like COH, H3 and H3O all show up in those, stuff that "can't exist" in normal conditions. I don't mention it much but one of the fusors here is plenty hot enough to activate "you name it" as far as something as sensitive as this is concerned. Dunno if I have any "record output" but it's in the millions neuts/second somewhere. Still don't have anything but a relative calibration on that off Richard's data. Comparing silver activations, his 20-something minute run at 2009 HEAS did silver to 497 cpm, a good 5 min timed run here does the same size sample to the 1500's -- we are both using that nice pancake counter we got from you for counting those, but we plan to substitute our new standard counter as a reference for that, it will be a little less sensitive, but the tube we got for it (a Russian 3 element tube, claimed to be beta optimized) seems much more relatively beta sensitive than the Geo detector -- so it counts less background but seems to count about the same on a beta emitter in tests here. Yet it only counts 3/5 as much on a pure gamma source. Of course, that means they won't read the same ratio on all possible sources -- which was the point behind coming up with a standard counter design in the first place - so we could get good cross-lab comparisons and calibrations.
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: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby George Dowell » Sat Sep 10, 2011 9:09 pm

My HV/Gain looks like 850V and more gain I think. Nother over 1K, ever. Geo
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby Doug Coulter » Sat Sep 10, 2011 9:36 pm

I'll try that next time I fire it up. My "theory" and for sure, that's all it is, is that more output from the sensor, and less gain required in something else would generally be better (certainly helps with noise pickup issues), but I can't be sure about that. I tend to be a little skeptical about twitchy stuff I didn't design myself, for which I don't have a schematic. I've seen too much dumb stuff out there to trust much I can't analyze myself. I did start at the originally recommended 1100-1200v, and things just got better and better re line width and signal to noise as I went up in volts and down in preamp gain - which made me suspect slew-rate limiting in the preamp, all too common a thing. I stopped when the dynode chain started drawing about a watt or so (which is about the ratings on the phototube in there), it's all made of 2w precision resistors. I rewired it as Paul suggested, as the way it came it had the focus electrode wired to the first dynode directly. It all seemed to match up. With the additional 2R drop for the focus electrode, the inter-dynode volts on the rest of the chain came out about the same as before when it seemed to be working the best.

Note, I wouldn't worry one bit about a 10 or 30 minute warmup if it got stable after that, but this seems to drift all day long. Annoying to say the least.
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby George Dowell » Sat Sep 10, 2011 9:56 pm

1800V is he range you will need for a gas proportional tube like He3, PLUS a lot of gain. On a handheld meter it would be equiv. to 2 milliVolts input sensitivity.

Be sure to measure the applied HV with an ultra hi Z test meter, I use 1000 Megs

Anything lower loads down the supply, gives false readings.
Once set up, I find the URSA II as a stable and very well thought out instrument package, with most any kind of probe I've thrown at it. The ONLY thing I didn't like was the fan that runs all the time when power is applied.

Paul S. is addressing that in a special mod update, what we used to call at Motorola an SP ( for SPecial)


I'm looking forward to really working out the new version with many different types of probes and scenarios.

Just about everything except N2 cooled,


Geo
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Re: Wind, Mobile Rad Lab, Home Rad Lab etc.

Postby Doug Coulter » Sat Sep 10, 2011 10:25 pm

Yeah, I live too far out for cryo stuff to be worth it. I did see someone was making something self-contained at one point (peltier?) but it was one of those hopeful press releases from a university about something they were going to commercialize...which I suppose failed to happen (as always - academics don't understand business -- you can't vaporware your first outing, you have to actually have something to sell). I'd love to be getting spectra like JonR gets...

That picture looks a lot like the ones I've got here, and boy they are nice for non-cryo. Yes, they're heavy, though I've not weighed one, they're about 20 lbs give or take. I'm guessing the smaller ones I've managed to get (BillF always gets some credit in this - he's the master at finding cool things) were for something like a PET scanner, where all they really needed was to discriminate between 511kv and "all else". Not that sure how useful they would be for real forensic measuring and discovering what something was made of. Better than nothing to be sure. One interesting thing I've seen on those is a line in the 2 mev range when running the fusor - not sure what exactly makes that, but I see it on them (the big fusor is lead shielded 1/4" all over, more on the operator side except for one tiny deliberate leak to look at stuff like that). I won't be taking one of the big gems near a hot neutron source!

I just left the 100 meg HV probe on there the whole time, so taking it off wouldn't change the volts -- that's how I monitored it to see it not drift while the spectrum was drifting. The power supply is 12 watts worth...shouldn't care much anyway. I'm sure it's not that, it's something else -- just gotta figure out what. Maybe I'll try one of my fast optimized preamps with lower tube voltage and all else the same on the URSA. Yeah, that fan is a pain. Paul said it was only needed for charging, things get hot for that. I don't run it off the batteries anyway.

Maybe I should just make a phototube simulator off a pulse generator and test with that.
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