Solar Flare/ Radioactive Decay Rate Link

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Re: Solar Flare/ Radioactive Decay Rate Link

Postby chrismb » Fri Mar 04, 2011 2:26 pm

I worry that such experiments are merely measuring the sensitivity of the detector to ambient variations, and the variability of the detector behaviour. How to control for this?
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby Doug Coulter » Fri Mar 04, 2011 2:53 pm

That's always a trick, to be sure. One way is to expose a detector to whatever ambient changes you can think of, with a test source and simply see what if anything changes. Not that hard with this class of gear.

For completely other reasons, I have a nice little box, 2' by 2' by 5' tall, that I can do various environmental things in -- I control heat, humidity, and gas mix in there pretty easily, and as a side use, it's where I expose photolith things to short UV. I originally built it to accelerate the old style rust-blueing of iron things (gun parts and so on), and drying of fine finishes I put on things like gunstocks and knife handles in a dust free place. So in my case, it would be fairly easy to test that sort of thing with the normal environmental variations. Probably wouldn't be too hard to mess with magnetic or E fields in there either should I want to -- and quite easy in the range one normally encounters naturally.

Another is to use more than one technology of detector, on the theory that such things would not affect all of them the same. I'm doing that now in trying to nail down that weird time-variable balance between neutrons and gamma output in my fusor -- hence all the junk I've been posting about detector sub-projects. The eventual plan is to have 3 sorts (at least) neutron detectors, and a similar number for ionizing photons - all different technology. So if say 3 detectors, all different, agree -- I feel a lot better about making some extraordinary claim.

Plans go like this:
for neutrons:
3He tube, B10 tube, BF3 tube (at least one, I have two).
Perhaps a fast neutron scint/knockon detector if I get that perfected so it doesn't see gammas so much.
Other radiation:
two differently made NaI heads, geiger counter(s), maybe a plastic scint head as well.

All this to be logged in time-sync via my PIC uP frontend and mysql/pc backend. This is just for fusor stuff, but the same kind of idea might be pretty smart for the decay rate testing.
I for one would be particularly interested in seeing H field and E field (outdoors) added to the data taking -- any correlations (perhaps time-shifted) would be interesting. H field could itself be measured by both a huge inductor (I wound a 10 henry *air core* one for listening to ULF radio waves) and a hall effect device (noisy but goes to DC). As well as orientation of the H field.

An aside, I hung a great big NeFeB magnet on a cord from the ceiling upstairs in a place that's hard to get to -- mostly to be safe from it ( a 3" diameter by 1" thick one of these is dangerous to you and itself). It's a real good compass (!), and normally faces the south-facing wall of the building (sited it for ideal solar panel mount). So, for a few days, it didn't point at the wall, it was off by quite a lot, some degrees -- no mirror/laser pointer needed to see this, and stopped hanging straight up and down as well -- it picked up a bit of tilt too. This was a bit freaky, so I quickly canvassed the whole shop -- did some major piece of magnetic stuff get moved downstairs, for example? No, no clues there. I mentioned this to an astronomer friend (a big studier of the sun) and he said, didn't you know we just had one of the bigger solar/magnetic storms on record?

Doh!

The trick on this measurement is going to be correlating anything you see with something else, so it seems wise to measure a few "something elses" along the way in time sync unless you really trust being able to get data off the web on those things later -- and time-accurately. If your tests above show variability in the detectors with atmospheric conditions, a few weather sensors would seem good to have in there too.
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: Solar Flare/ Radioactive Decay Rate Link

Postby Barrabas » Fri Mar 04, 2011 9:43 pm

I've bought several CDV-700 6B geiger counters online (the old yellow civil defense units). These use the 6993 GM tube which is halogen quenched and largely immune to minor changes in tube voltage.

I'm putting together a precision timebase for both the timestamps and for the uController clock. Crystals are temperature sensitive, so I'm using a "TXCO" - temperature controlled crystal oscillator. Used units are available on eBay and have stability in the 10 ppm range, independent of temperature, pressure, and humidity.

I'm also building a precision voltage source for A/D measurements (see below). You can get chips which are 2ppm/'C for about $8, and a PC board specifically crafted to mount it for $9.

What a world we live in!

Anyway, my plan is to identify all systemic and environmental variables I can think of and either eliminate them or measure them. I plan to measure the GM tube voltage (for all tubes), GM tube temperature, ambient pressure, relative humidity, and some others.

I plan to have the AC voltage go through a UPS, then a medical grade isolation transformer (which I have), then into a power supply. This should eliminate AC line transients or long-term AC voltage swings.

The system is in a metal box (an old PC tower case) which should provide EM protection. The case could be isolated due to the isolation transformer - I haven't decided whether to isolate the case or ground it. Any suggestions?

One decision that I haven't made yet: I could *conceivably* put the GM tubes in a vacuum chamber and suck the air out. This would eliminate pressure and humidity entirely, and constant temperature is relatively straightforward.

I just don't know if it's worth the trouble of doing this. My current plan is not to use vacuum, but to characterize the temperature and pressure behaviour of [other] 6993 tubes once the experiment is running. This will measure experimentally the effects of temperature, for instance, and allow me to state how much of an effect that has.

I've got some Mu metal foil, but I don't know a lot about magnetics and I also don't know what type of shielding to build (example: should it be layered?). This would need to be designed, but I don't have the necessary background. I'll probably just ignore it.

The timestamp of the uController (accurate to a microsecond or so) is disconnected from real-time. The data logging PC (separate from the data-taking uController) will be running NTP, which is real-time accurate to 10 milliseconds. The logging PC will insert it's own measurements of real-time in the data stream at intervals, and processing software can count backward or forward using the [highly stable] timestamp.

I dislike the real-time/timestamp disparity, but I haven't come up with a good solution either. RTC chips will require crystals which can drift (NTP takes care of drift), and GPS has variations due to the speed of light in the atmosphere with changing in humidity. I'm still thinking through the issue.

I've got specific questions about things that I'll ask in another post, but I wanted to get some of the details out in the open and let people kibitz.

If you (the reader) have observations or suggestions, please let me know.
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby chrismb » Sat Mar 05, 2011 6:12 am

One of the effects I *think* I have seen for tubes left for a while (while checking background) is an internal 'self-drift', which even a vacuum chamber won't fix.

It goes like this - low count rates cause lower discharges in the tube causes lower gas temps therein, reduces sensitivity and so the count rate reduces further. Higher rates lead to higher temps which increases sensitivity. I cannot say I have noticed this with GM tubes, but I tend to think it is what is happening with a 'new' [old stock] Russian pancake I have bought recently. So my logic is that if we are talking about fractions of percent variations, I think we'd need to know if this observation of mine is a genuine effect, and if it can happen in GM tubes.

The upshot of this effect would be a slow modulation up and down, as the background rate probabilistically varies around its median on timescales similar to this effect. e.g. if self-heating/cooling inside the tube does have a small effect over a 20 minute timescale, then if in 'a first' 20 min period the sum total background count is lower than 'the background mode' then the tube will have some hysteresis towards under-reading, whereas if in the second 20 min period it is higher, then you'd see hysteresis towards a higher sensitivity so it would over read the background.

Well, this is just a hypothesis, FWIW, of one factor that might contribute to long time-scale fluctuations.

...extra note: In fact, the opposite to a vacuum might be needed to identify this - maybe put it into a pressurised helium cell with a fan in it, so as to maximise heat equilibrium of the tube.
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby Doug Coulter » Sat Mar 05, 2011 5:52 pm

There isn't really a solution to the time vs running timestamp problem, even NSA had me build "something" to help with that for them, but getting them locked in super close when there's more than one timing source, say better than a uS, just isn't going to happen without "heroic" work on that problem, which I can describe but it takes a lot of words. I use a watch xtal in my uP data logger, but that only has to be decent for 5-10 minutes at a time. There are some nice timing quality standards out there surplus, we've got one from NBS (before they renamed themselves NIST) that is very very good. I think Charles Wenzel's outfit makes some things that are very very good, but I'm not sure if he's into absolute rate-accuracy, but more like stability and low sideband noise. It won't matter if you can compare with NTP once in awhile anyway.

I'd say injecting the NTP time once in awhile should do it (with some back tracking in software later). It kind of depends on what you're looking for, you may not need sub millisecond accuracy if you think it's related to solar activity, but you'd sure like it better than 10 minutes!

I think those tubes are really stable. If they have that "grille" structure on the side like mine do, they will let alphas in there - the grille protects a part of the tube that is a thin Be window for that.

They are completely uncritical about supply voltage and gas pressure, and are run a very much sub micro-watt powers and do not drift or self-heat with counting unless something else is very broken. Having built a few tubes from scratch, a rather large factor of gas pressure variation doesn't have a lot of effect either. The physics in this case is in your favor all the way -- they either avalanche or not, and there's enough energy put in there by a beta or gamma to make them trigger quite reliably under a very wide range of conditions.

You will have the usual statistical false alarms -- John Strong's book and Haliday's book (see listings elsewhere on the board) both have some very good explanations of how much you have to count to get a number you can trust to some percentage, and it tends to be crazy big to get to single digit percents, much less fractions of percent.

The main issue you may have with those is their response time. After a tube counts, it's dead for a little while, then puts out smaller sized pulses for another little while after that. Whether you count those will make a big difference when the count rate is high. Typical times are in the 100's of microseconds, which means at 10k cpm, you'll be hitting that "all the time" -- check with a scope and a good HV coupling cap right at the tube signal. If counting 1/4 or 1/2 height pulses (or not) is going to affect your accuracy (and it will) -- you'll have to go to a much less hot test source to avoid that happening many times a second. Remember, radioactive decay is (at this point) the very most random thing known to man -- you can have two near simultaneous pulses even at very low average count rates, and that's going to be most likely the most important error source using these.

I think Haliday said it best. In U decay, alpha's present themselves at the coulomb barrier of the nucleus at about 10e22 times a second, yet U has a half life of what? In the billions of years, depending on the isotope. You can see how such a process would be pretty darn random, making brownian motion look very regular by comparison.

Chris, I very much doubt your theory on that one. These run pretty high pressure, plenty temp conductive as a pirani gage would show (doesn't even read much less than atmospheric). The powers involved for a pulse are very tiny if you're doing this right -- megohms in series with half a kv and for a few microseconds at a time (during which the tube voltage is near zero, so very little power is involved - it's a switch) -- these tend to run in the region of 1/10 atmospheric pressure to A: be near the bottom of the Paschen curve and B: have enough gas in there so radiation is more surely going to interact at all with it. Strong (with 1930's tech) did notice a difference in tubes made with rubber stoppers, due to outgassing, with temperature. That's not going to be a problem in a tube already sealed up (with no rubber) for a few decades. That's pretty far in the past. I would bet any money your background variation is simply a real thing -- I see it here and it does not matter whether I'm using a geiger tube, or a scintillator, or for that matter, a neutron tube -- they all count "up" during heavy cosmic ray periods which are themselves pretty doggone random, but also tend to have a time of day component if you stack counts over 24 hour runs. Also depends on what the sun is doing to the earth's magnetic field just then.

This big background variation IS the big issue in counting silver activation, of course, since you're not all that far "out of the noise" there at a mere 1k cpm or so.

We see on a particular sensitive geiger a range of 6 cpm to about 120 -- quite normally, and when it's counting hot, so are all the other detectors. It's just the way that is.
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby charleswenzel » Sat Mar 05, 2011 8:46 pm

I found an "kludgy" way to post the output of my experiment in nearly real time: http://www.techlib.com/solarflare.htm . I also slowed down the chart such that it holds a week's worth of data. I don't think timing is particularly important, frankly. If there's 24 hours from the event to the flare, there's bound to be hours and hours of variability. Note the diurnal wave - what the heck is that?

I'm still in the skeptic's mode, but I did see a nice dip before that last big flare, so my mind has opened a little. The dip was a full day earlier. Unfortunately, I lost a week of data when I was fooling around with this technique to put the plot on the web in real time. I do have that dip and the following flare on a composite plot on the same page linked above (second image on the page). That plot was before I slowed everything down to get a full week on one plot.

The interesting thing about my lost week plot is that there was a clear diurnal sine wave in the data. It's becoming clear in the new plot, too. It isn't pressure or temperature; those curves have been all over the place, lately. It's below the threshold I arbitrarily assigned to this "giggle test" experiment (0.1 %), but I can't think of what it might be. It looks pretty smooth, too.

(By "giggle test," I mean that I want to see something happen above 0.1% before I go to much more trouble. The obvious next step would be to build two systems, in two locations. Plus, there are lots of ways to improve this system. But, tilting at windmills is unappealing, so I'll wait for another big flare or two.)
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby charleswenzel » Sat Mar 05, 2011 8:49 pm

I should have mentioned that the sine wave peaks at noon, local time. Sure seems like the sun.
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby Doug Coulter » Sat Mar 05, 2011 9:04 pm

Diurnal variations of cosmics is fairly old news -- all the old books have it, and it varies with lattitude, which makes sense after looking at some pictures of the magnetosphere. It's why the aurora happens near the magnetic poles instead of elsewhere. The earth's field isn't real strong, but it's real big, and can deflect charged particles in the cosmic background in a variable way. It's as though the solar wind "pushes" on it. In fact, though I don't have a quick pic for it, it's known to sometimes "bleb off" on the backside of the earth -- the magnetic lines can rejoin and a "bleb" of magnetic field is just ejected "downwind". When this happens you can see it with either a big inductor or some other magnetic field sensor, in the few Hz region as the part that stays with us rocks back and forth at (roughly) the speed of light.

To the extent cosmics are part of the measurement (kinda have to be unless you own a real deep salt mine or something) they'll show up here. So the trick is to have a separate sensor for that, and have the "correct" strength of source you're testing. Too loud might incur some errors in the sensor, too soft, and the background variations have too much effect.

Hey, if this was easy....many would have done it and there'd be definitive results, right?

Here's wiki on this. http://en.wikipedia.org/wiki/Magnetosphere

EarthMagnetosphere.jpg
One of many pix on the web on this


FWIW, and this is anecdotal, here I usually see the highest backgrounds in morning and evening, but then I'm well north of Charles....I'll have to make a log over a few weeks sometime soon. That standard counter project I'm working on should be perfect for the purpose.
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby Doug Coulter » Sat Mar 05, 2011 11:19 pm

Charles, just got around to checking your link. That's some awesome work, dude :D
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Re: Solar Flare/ Radioactive Decay Rate Link

Postby charleswenzel » Sat Mar 05, 2011 11:41 pm

Well, it didn't even occur to me that it was the background radiation! I'll be surprised if the background variation adds up to 0.05%, but that is a straightforward explanation. The chamber is a small almond tin and it has two smoke detector elements inside, so I imagined the background is trivial in comparason. I have the older experiment that I can test, if I can't come up with the numbers.
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