Standard counters

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

Re: Standard counters

Postby Doug Coulter » Wed Feb 02, 2011 4:53 pm

I think what you're getting translated as "pressure" really means voltage (or electromotive FORCE), which is pressure in the electronics<>plumbing analogy, and makes sense with what I am seeing here in testing. I find that analogy useful for teaching beginners -- current is flow, resistance is an orifice, a valve is a valve, a capacitor is a tank, and so on. Easy to model once you realize that for electricity the world is some sort of solid substance that needs holes (pipes<>wires) through it for it to flow.

A normal tube will usually act as a proportional tube up to a certain voltage, with varying pulse heights depending on the energy deposited in it, then go to a plateau mode where all the pulses are the same height, that height being more or less the difference between the voltage at the beginning of the plateau and where you are operating the thing. For example, a more "normal" tube might have the beginning of the plateau at 600v, and if you run at 700, you get 100v pulses. After the plateau region, running the voltage up further results in at first, false counts, then simple glow discharge (which wrecks the tube in short order under most conditions). Could be I didn't set the scope up sensitive enough to see the stuff in the proportional region. What was kind of a surprise is that above that threshold voltage, I got 100v pulses -- even at only two or three volts above the threshold. 320 volts seems comfortably above that threshold, but I'll check the other 8 tubes to get a good design center for that.

These tubes either don't do that, or I need to look more closely. Right around 260v, nothing comes out I can see. Just above that, you get 100 to 150v or so output pulses, all about the same height unless two are close in time, during the "dead time" when the ions from the last pulse haven't drifted off and been collected yet (possibly the function of that extra terminal is to accelerate that process) -- the geiger version of pileup. I'll put up a movie on this at some point, because it's worth knowing about, and it's enlightening to see how often it happens even with a nominal 300 or so cps rate (off a 1b22 spark gap tube that has radium in it).
Obviously, this isn't counting alphas from the Ra, but gammas, betas, etc from all the daughters of this WW II era radar spark gap tube.

So, even with a "safe" source -- this tube is already almost too sensitive! It's a little less so than the super good, expensive, really 2" ones we got from Geo, but since it's less than 1/4 the price, not bad at all. Torbermite ore need not apply at close range for this guy, nor a smoke detector source! Either will flat out saturate this (or most phototube/scintillator arrangements).

At any rate, fairly wide voltage variations don't affect much but the output pulse size, and that only a little bit, which is what makes this so nice for this job. It either counts, or it doesn't, the threshold is determined by physical construction, so there's no way to mess it up. It's obviously quite easy to get a cmos input to count with that kind of pulse size as well(!). As I said above, more a question of not frying one or the other due to CMOS clipping at the supply rails. And we get a cubic crap-ton of noise resistance, unlike millivolts out of a phototube.

////////////////////

So, what I'm working on here as a first go, is a pic based counter, with the pic used to monitor/control the power supply (takes less parts to use the stuff built into the pic, and it's more accurate anyway), one of the pic hardware counters to do the counting, and I/O initially through rs232 only. That will be the cheapest/easiest possible thing to make. If I have enough pins left over, I could add *either* a LCD display and some moderately expensive boost power supply parts, and a rotary encoder for UI on the box -- that will add maybe 50-70 bucks to the price, though, as it will also require a bigger box, bigger batteries, and so on. If I don't use up all of port B for that (and a couple other pins for the encoder) then I can use two port B pins as interrupts for things like anti-coincidence or other counters, or some A/D inputs for logging. The idea is to spit out RS-232 on the second, with accurate time-stamping, for use in a PC, in human readable (read, easy to parse for software too) form. Having already built and used our much fancier multi-geiger, this seems like a good way to go, and if all you have is a laptop or something, there are USB dongles for serial around pretty cheap. Since everyone here, by definition has a computer, and most of us don't have a ton of spare change, that seems reasonable.
I will use the PIC 18lf2325 chip (they're cheap) as a 28 p through hole, a MAX-232 for rs-232, a watch crystal for accurate timing, a few bypass caps, and very little else but a box and power switch. Plan A is to use 3 alkaline AA cells to power this, as it's just a little more power needed than we can suck out of the RS-232 control signals from a PC (which of course also takes software on the PC to make sure they're on and in the right state). I will use my standard 9 pin female connector for a straight (not null modem) cable or direct plug to the back of the PC, with my usual wiring of things like data set ready<>data terminal ready and CTS<>RTS so no matter the PC software, it will just work -- we are ready if you are.
The point is, doing it this way means anyone can simply buy the parts and kludge one up, wirewrap if they want to -- no need to buy it from me, and then program the software into the pic, which I'll post up here free as source and binary. I'll probably skip the fancy RJ-11-6 onboard programming connector, and go with stakes (far cheaper) for that, or nothing and just use a board with that connector here to program them.

I am trying to keep the total cost under $200 here and still have something that does this one job quite well -- EG, our cheaper TCO version of a BTI detector (these should last a good bit longer than BTI's do). Other designs can be made off this basic thing (and have already, and the software for one is already posted on this board for a fancy model), but that's too expensive for what I have in mind here - a more or less universal way to compare results from fusion runs -- even share moment by moment data as simply as having hyperterminal (windows) or GTKterm (linux) or ??? (apple) save a file to upload so we can all see it and make any interpolation or extrapolation ourselves. In the case of silver, it's crucial to time the interval between fusor-off and sample measure start, for example, and this gives you that since just about any fusion device will make this count pretty fast during the run. I have to put my current tube behind a 2" thick lead block, some lead sheet, lead glass, and the lead on the tank walls with the actual grid about 2 feet away to get the count rate down to about triple background for example. Without that stuff, it will saturate hard -- just removing the 2" lead brick pushes this up to the thousands counts per minute, to the point where it is losing some counts due to pileups.

Maybe most people have very little engineering experience, but consider that the setup charge for a PCB build is more than the cost of two tubes, and a quality box costs nearly what the contents does...for a small quantity run, the detector doesn't dominate the total cost - it's almost in the noise.

I will probably have this put out a line per second of text that will look something like:

Runtime:00:00:23 Count1:10 Count10:112 CountPM:672 AD:1234 <cr><lf>

With Time being pic runtime (accurate, from a watch xtal and not subject to idiotic PC opsys latencies/errors), counts for the last 1 and 10 second intervals, and a CPM based on the previous 10 seconds worth of counts (updated every second by dropping the oldest and adding in the newest 1 second counts). That seems like it will do it. We have a lot of options on the MultiGeiger stuff, but we never use them. Background subtraction is pointless since it varies so much and would produce negative numbers often as not. Other time intervals are too short or too long to be practical counting silver -- and you can always add data up from this format if you want long periods anyway. I'm a big fan of providing unsmoothed raw data so you keep the option for any kind of analysis later on (this is pretty basic science-religion, actually, though often violated these days in papers). Internally, the PIC will extend the 16 bit counter in software via interrupts when the hardware counter rolls over, to a 32 bit number -- plenty! It will derive the output numbers via 32 bit subtractions and of course multiplications by 6 for CPM derived from the 10 sec interval. I'll use the pic opsys I've already posted up here -- it works fine.

Possibly desirable would be to keep track of total counts out of the tube over all time -- these do have a lifetime, and it's not crazy big - I would not let one sit and count torbermite for a week. We could put out that number on every bootup -- the pic has some eeprom to store things like that, and that would let you know when to stop trusting the thing, though there will be other ways to test it. We use a thoriated mantel from a gas lamp for a calibration source here, as it gives on the order of 10k cpm on our Geo tube, nice round number -- and we have tons and can mail those around with the units.

This format allows for changes and upgrades in the future, as any parsing software can simply look for the defined labels, then pick out the next number. If we add things, all we have to do is make sure to use unique new labels for it. Most scripting languages do this easy (they were really meant for things like this).
If desired, I suppose one might have a mode for CSV here, with just one header at the top to make it easier to import to spread sheets, but frankly, it would be less hassle to write a 2-3 line perl file format converter for that.

Down the road I can add things like commands from the PC to put the thing in some other mode if we can think of a reason to do that. For now, the KISS methodology rules. The hardware will be ready at any rate, no point not hooking it up.

I already have most of a PCB layout, I'd done one for a 28 pin pic with rs232 a long time ago, and making the new one will mostly involve removing things from that which aren't needed here.
Somewhere along the way, the RJ-11-6 connector I used then became completely unavailable, and all the new ones reverse the pin footprint, which is a shame as that board was almost single sided (would be with two jumpers). Turning all the pins around would make quite a mess, though. The plan is probably to make at least the first one here to check it, before sending it off to AP Circuits in Canada for the real ones. Converting my output to gerber files for them will be most of the total work! It can be a nightmare to get the aperature and drill files all correct and lining up, and the reason they only charge what they do for setup is they don't check -- you get what you sent them, else setup would be $300-$600 instead of whatever it is right now, usually under $100 for proto-1 service which is plenty high quality. Of course, if there's only going to be 3-5 ever built, I'll make the boards here and put in any jumpers needed. I can kind of do double sided, but not plated through holes, and for something this simple, there's no point in that.
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: Standard counters

Postby Doug Coulter » Wed Feb 02, 2011 5:10 pm

Chris, all I know about this tube is on page two of this thread, the data sheet in Russian which JoeJ has helpfully been translating (Thanks Joe!).

There is no point being more sensitive than being able to count background. If you're not making enough activation to get well above that, you're not doing any sort of serious work, and you need something a lot more exotic and expensive to get background separately; and even that isn't perfect because there's some intrinsic in the counter -- of any type -- that can't be seen and removed by anti-coincidence with another counter that can't see something like a decay in the materials the other one was made of. For example, I have an exquisitely sensitive NaI head that got contaminated on the way to me. There's nothing I can do for that, it reads "zero" from outside -- and the noise looks just the same as any legitimate input creates.

Being more sensitive won't improve that ratio. Using a more sensitive and faster counter makes the background more variable than it is already, because a fast one can sometimes resolve individual particles in a cosmic-ray shower-burst if the burst was initiated well overhead (say, the roof on the floor above) -- they don't all go the same speed.

You need to really understand this!
Cranking up sensitivity does not improve signal to noise ratio, here, or anywhere. It might result in loss of data due to "clipping" though.

I'd be very surprised if your lab background isn't significantly higher than mine is due to where you live alone -- and that's your confidence limit, not the detector. No matter where you are, there are cosmic rays, and in most cities, a plethora of other sources -- building blocks are hot, kitty litter is hot, coal ash is hot, dust has radon daughters on it, and so on. If anything, that might be better as it's probably also more uniform. As I said above, here we see a huge range because we keep the lab extremely clean of radio-actives and see only cosmics which are the basic perfect random noise source -- with a range from "undetectable" to "dangerous" in amounts, depending on solar and magnetic storms etc.

There's no way to get around that, short of going down into a salt mine, which I don't think any of us have (and my cave is granite, and hot itself). You simply have to get above the background to get to any confidence in any measurement. That's one reason I like the big area format -- takes less neutrons to get there with that, as any moderator is going to spread the neutrons around a good bit by it's nature, you may as well catch as many as you can in a big sample, then measure that whole sample. 100% of your sensitivity worries are aimed at the wrong end of the process!
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: Standard counters - scope traces

Postby Doug Coulter » Wed Feb 02, 2011 6:16 pm

Here's some data that illustrates what I am talking about here.
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Notice the frequency of triggers on the scope. On the calibrated 2" tube dowwnstairs, this source (a lamp mantle) reads right around 6k cpm, but of course due to the random nature of radioactivity, during two minutes returned counts from under 5500 to over 6700 -- that's just how radioactivity is. Even at this extremely high count rate, much less what you'd get off slightly activated silver! Note also that pulses right behind one another are pretty frequent -- truly random timing, eh? Further, ones inside the "tube ion sweep time" are smaller then ones in isolation.

This has really messed up some programmers here, who make rotten assumptions -- just like the scope frequency estimator does in these examples. No, you can have 1000 cpm and all 1000 counts come in 100 us on a phototube, you can't depend on them being anything like evenly spaced. I didn't select pictures for this, I just hit "snapshot" 4 times -- a decent sample.

The source in this case is a lot less hot than say, some of the U ore I have, or a smoke detector source, a staticmaster source, and quite a few other things considered safe enough to sell on the open market to anyone. I'd say this is plenty sensitive enough!

This tube counts about 1/sec with the usual wide variation when no source is present other than the rest of the universe. Nice to have a way to be sure it works. We find significant noise in 10 second counts here...but silver is so quick to decay, longer periods are right out, so we use the good old eyeballs to average over the noise. I'll take a decay curve next time I have some hot silver so you can see this. There basically isn't any point at all in being able to detect less than 100 cpm activations in ten second counts - at that point the SNR is pretty horrible, and you have to strain to see an exponential in there.
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Re: Standard counters

Postby chrismb » Wed Feb 02, 2011 6:49 pm

Doug, Is the extra grid for quenching? Some early tubes appear to have had no quench gas (so I have read) and relied on electrostatic suppression of secondary electrons at the cathode.

In regards sensitivity, yes I do recognise all that you are saying, wrt signal/noise, and perhaps my question isn't actually about a standard neutron measurement approach, if it makes little difference/sense to have greater sensitivity than this, but is actually more asking "what is the minimum neutron flux that silver activation can statistically discriminate"?
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Re: Standard counters

Postby Doug Coulter » Wed Feb 02, 2011 8:02 pm

I suspect that extra terminal is for either that or shielding, there's nothing extra visible inside the tube at all -- only one feedthrough, but it might connect to the ring around the mica window. I'll have to get out an ohm meter and check. Could just be shielding inside the bakelite container for all I know. Works fine with it floating, about what you get out of any normal geiger tube.

Edit: This is a hydrocarbon quenched tube, which is why it has a measurable lifetime (some billions of counts under power) as the hydrocarbon molecules get broken up. Unquenched tubes need 100 meg like limit resistors and are far too slow for much of anything. I've made them here (ala John Strong's book). Halogen tubes last forever, but aren't on sale in large pancakes.

We have been making, dunno, a few million neuts/second* (see note), and in 5 minutes, that gets you to somewhere around 500+ cpm on silver/million. (our record is 1536 cpm) Background is from 6-130 cpm, and varies all over, instantly. This much is very easy to see if you get to the counter pretty quick (inside 20 seconds). Theoretically, we should run more like 12-20 minutes to get to "secular equilibrium" which should produce about two or three times that count for the same neutrons/sec -- weighted towards the most recent output if things are varying there. This makes the 3He and B10 tubes count good and quick during a run, haven't put an actual counter on them but it's pretty frequent (maybe a few hundred hz) depending on a lot of things (like where they are re the setup). Those tubes have about the same background as the geiger, for the same reason - cosmics, which make everything count.

I'd say the lower practical limit, given the above, is about 1/10 that or so -- less for a longer run. You could go a little lower with a 22" long 3He tube of course, but that's real money (and truthfully, no two are the same). The B10 tube is less resistant to other things (gammas) so...not as low there, but it sings nicely sitting on the floor 3 feet away when the fusor is running. It's really a question of how much you want to visually smooth the decay curve with your eyes and try to back fit it to a number -- at some point the variation in background swamps it badly, and you'd have to be lucky to have the background sit still long enough to not make variations in it bigger than the actual activation. But that's a pretty low fusion rate for a tabletop thing -- anything that's going to get anywhere close to useful output will make this work fine.

If you're willing to run a lot longer, a few hours into indium would probably be more sensitive, as though you'd get less CPM out of it in general, you get to average the counting for longer - easier to curve fit to good accuracy with one of the softwares that will do that. In not many minutes the silver is basically cooled back down, where the In stays hot far longer. That in turn makes short term background variations less important. Grubby old statistics!

I'm sitting here watching this Russian tube count background on a rolling ten second scope sweep (the DSO has a peak detect function so you can do that even at slow sweep speeds and still see skinny pulses). I sometimes see no counts in 10 seconds, sometimes maybe up to about 20, and even at that some are within about 200us of one another. I'd guess that's from showers that started high in the atmosphere so that some particles get here that much later due to being heavier/slower in transit.

* note:
Our estimates are based on Richard H's and Carl W's run at HEAS 2009, where they figured about half a million neutrons per second (for a million fusions/sec) based on their estimates of their tube efficiency, and they got 497 cpm off the silver after about an hour of continuous running doing that. Next year, they claimed well over twice that for the exact same setup, but refused to run another silver run to verify that one -- I'm still pissed off about that -- that's no way to do science -- so we don't know if the change was magic, or if they're using "new math" or what, as I've never seen a 2::1 improvement without improving something in the setup, and they claimed no changes at all -- just turned it back on after a year of sitting there. I did witness the 2009 run myself to verify the techniques used, and duped it as closely as possible, though my neutron oven is maybe a little better than they used -- that's where the game is in sensitivity I think, on the input side.

It would be a worthy project to calculate this from scratch, ourselves, for a standard setup. But it would take considerable effort to do so, and I think that might explain the divergence above. You'd have to account for neutrons scattered out of the oven with no effect, among other things, and ones that are captured in the moderator, as well as those caught in the sample to get anywhere close.

I believe they were using simple math based on square law and the angular coverage of their 3he tube, knowing its quantum efficiency and with some assumptions about neutrons scattering out of the moderator they used -- it's a lot of assumptions in series, for my taste. But like silver, the QE of a tube is a variable depending on neutron energy spectrum, so to do it right gets you into a lot of calculus real quick, with some monte carlo on top. And in practical terms, not real important so far -- by the time absolute numbers are important, we'll be measuring them in watts, not neutrons! By the time we get to milliwatts, hey, we're cooked if in the same lab with one of these! Neutron detection is just hard, not just for us, but for everyone.

My goal is to be able to tell "is it better this time" or not, for now. That's what you really need to see if something you changed worked for you or not.

http://www.eljentechnology.com/index.ph ... &Itemid=57 claims to be pretty good. We paid about $150 for one of those hornyak detectors, haven't seen a pip out of it so far, all tests inconclusive, but you know, hours in the day - could just need more testing, or more time in the dark before testing. I have the stuff to dupe their 6Li/ZnS:Ag so I might try that. Either one -- completely useless for comparison across labs -- anything with a phototube basically won't fly for this job. Heck, not even that good within one lab, as 2::1 variations are hard to eliminate over time.
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Re: Standard counters

Postby chrismb » Sat Feb 05, 2011 1:56 pm

Doug,

Focus Fusion/Lerner have put up a pre-print of the publication they've got into 'Journal of Fusion Energy';

http://lawrencevilleplasmaphysics.com/i ... an2011.pdf

You might like to note at the bottom of p.10;

we have built a silver activation detector exactly following a previously calibrated design. [29]
29. D. R. Slaughter and W. L. Pickles, Nucl Instrum Methods, 160 (1979) 87-92.


Can you pull that reference?
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Re: Standard counters

Postby Doug Coulter » Sat Feb 05, 2011 2:13 pm

Bill might be able to. Anyone else? It'd be nice to at least know the range there.

Edited to add:

Well, I'd have to see the conditions of calibration in the first place. A point source will calibrate differently than something intended to be immersed in flux coming from all sides, for just one example. I'd guess Lerner can fairly safely assume a near point source, I'd question at this point if a fusor is, or that the point is where we think, perhaps. So any transferral from something designed for say, a fission reactor is going to be very suspect. I still don't understand the fascination with absolute numbers that seems like a disease to me. When we have to care about that, we'll be using thermocouples (or a steam engine/dynamometer!), not neutron detectors. We don't even know if the assumptions for the 3 possible reactions in DD translate from thermal conditions to the ones we have! All we know is we subtend a certain angle of the flux with the input face of our oven, and that it's likely nearly all the neutrons are coming in that way, not all over the oven.

Or, as my fast neutron physics books all say "to reduce scattering, first remove all the air from the laboratory".
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Re: Standard counters

Postby johnf » Sat Feb 05, 2011 6:02 pm

Bet our library at work has got this in print i'll get it tomorrow and pdf it
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Re: Standard counters

Postby chrismb » Sat Feb 05, 2011 6:44 pm

I had a quick look on amazon for the book - sometimes they go for a single cent. I did a search for "Nuclear Slaughter".... well, you can imagine.....

Edit: Apart from the unfortunate search terms of subject and author, I think the other reason not to find it by author is that other references give a book by the same name and year to different authors. Therefore, I think it is a book composed of a series of author-contributions, so searching by author name may not help.
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Re: Standard counters

Postby chrismb » Sun Feb 06, 2011 3:12 am

I think it is actually; "Nuclear Instruments & Methods by K Siegbahn-Uppsala (Hardcover - 1979)" which appears to have been re-printed in 1984 and 1990. Well, whether it is or not, I just bought a 2nd hand copy of the 1979 print off of Amazon for $10, so we'll see soon enough.
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