by Doug Coulter » Sun Sep 25, 2011 6:45 pm
You can of course clean your soldering up with various solvents - I do when high impedances are involved. But a little work with "normal good practice" built stuff, then finding out how much the noise of a fusor can affect that made me not-mad at how much crap I got on fusor.net about my measurements at first. They were right! I proved it to myself that they were, though I didn't like them forcing people into buying BTI's (not cheap, and die young), and them mentioning that some cargo-cult gear and 3He tubes would be trusted, but that the same guy saying that mentioned he'd cornered the market, but none were for sale at any price. That really made me mad (and is part of why this board exists).
The sparking test is the one, though you must consider DC drift up to and including any comparator as moving the threshold can let in noise, or remove real counts. Usually that's minor compared to a noise burst from an arc someplace else in the gear suddenly counting your counter up 10k counts - and you'll never know if you weren't watching it all the time (or do like std counter does, report every second so any outliers are obvious). The instant you go digital you lose all kinds of information. For example, the analog waveform, if monitored, can often show you things about what's real and what's not. It all depends on the setup of course. A "spark" vs a 50kv 1 amp arc...not quite the same thing, but at least on the same page.
I was just down running my fusor for something -- testing a new detector, actually. It's totally shielded, tight as a drum (literally airtight as well) -- all coax to the scope and digital counter, all things floated off ground except for one point -- separate power supplies, the works. Yet, a puny 20kv supply in an ion source with the ion source going in and out of "light on" was enough to make 5v positive pulses show on the scope out of something that doesn't even have one single positive power supply in it vs ground! And yes, they looked "different enough from the real thing" to be obvious on the scope, but not to the counter, which happily counted on overshoot in the "expected" direction. So, silver still rules, nothing else is even close. This other stuff is fantastic for tuning things, taken for what it's worth - don't know how I'd have got along without it. But in any case of disagreement, or any new claim of huge improvement, if I don't see silver counts, I remain a complete and thorough going skeptic, for good reasons -- I've been fooled myself too many times, and with stuff built "perfect" that some way, some how, I had a ground loop, an accidental antenna that could get enough millivolts on it to fake the millivolt sensor signal, you name it, it's happened.
The natural background is for all intents and purposes zero -- cosmics are many thousands of times higher than that (guessing, it might actually be in the millions or billions or trillions). I have two neutron sources I made with very hot staticmaster Po sources I laid Be foil over -- should be making 5-10 neuts/second by theory. Nothing can see them (other than a BTI and 48 hours getting about one more bubble with than without), maybe I could prove it with a 3He and some fancy statistics and long term counts. You definitely cannot tell by ear or by hand counting the counts. I was messing with you - I know you don't have a neutron source that's EMI free hot enough to be seen on one of these, almost no one does, they don't grow on trees. In fact, your RF driven thing might be the first practical one to exist, once it works. A mere few hundred watts of RF is nothing compared to what you can get in an arc during the peak -- there it's not impossible to break into the 10e12w region, and I've done it.
You really only need one source to test, preferably of known strength. You only need two to set scale factors.
You''ll just have to keep watching the analog, not that you can't count the thresholded version too. I feel pretty sure you'll see the things I have, or I wouldn't bother cautioning you so much -- no point getting up false hopes only to have them dashed -- I didn't enjoy it, I suspect no one else would either. But you now have this fine scope, and I hope you'll get computer screen capture going - it's really worth a bit of effort to be able to easily document your work, even if only for your own use that way. You save net effort -- it's a good investment to add one BNC or whatever to be able to do that, as you can use the human mark I eyeballs and signal processor to detect all sorts of errors that are otherwise hard to anticipate and correct before they occur -- life just can't be done perfectly feed-forward.
I've had a bit of trouble here with the Russian 3He tubes just deciding to count say 20-100 hz for seconds or minutes on end, no obvious reason for it, then settle down and work right for the rest of the day. The waveform coming out is only a little different from "the real signal" too - you really have to look close at the scope to tell (other than that you know there's no neutrons being made, so why is this thing going nuts). I'm going back to the boron ones with better behavior (damn, my spell checker doesn't like the UK spelling of that word), as I really don't need the sensitivity here and already do have a really serious 3he tube that's pretty reliable. But -- it's still not as rock-solid as the older boron tubes. Even that Hornyak, "deaf as a post" is plenty good for any real neutron production, and very hard to fool by comparison - at least here.
Hey, I'm just trying to save you trouble and disappointment here -- it feels pretty dumb to make a claim that turns out to be wrong later, that's all. These are super for tuning in real time, nothing beats a good neutron detector of some sort for that. It just better not be your only way to get it done, that's all. "Trust, but verify".
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.