First light on SNM-17 3He tubes

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

Re: First light on SNM-17 3He tubes

Postby Doug Coulter » Sun Aug 21, 2011 5:23 pm

You don't really need the 10 k to ground other than to avoid being shocked if you'd let it float, then touched it (I didn't use it, but I had the 10 meg scope probe on there before powering up). You should never put any volts on that tube without series resistance, as it can "fire" (like any other gas tube) and current rise without limit if the supply is stiff -- till it burns out the .0005" wire in the middle. You can't measure the volts right on the tube either -- the added load of any normal meter (including a HV probe) will mess up the reading, and the additional capacity to ground there will likely make it into a relaxation oscillator, which is enough of a problem without that, actually. The spec sheet is telling you the power supply volts, the tube, due to that current drain, kind of self-regulates to something lower than that of course.

If you watch on the scope, you can see it start to get into corona noise as the power supply volts come up to the data sheet spec for the corona, and it gets a little quieter with a little higher voltage as the corona kind of smooths out (less shot effect as the current comes into range). But you'll see some noise from corona. The actual pulse is a goodly amount bigger as the scope traces on this thread show.

FWIW, the B10 types seem to have a better SNR than the 3He, but are a little less sensitive to a given neutron flux. I attribute that to being able to use a better gas for a counter in them. Neither 3He or BF3 have especially nice characteristics for a proportional counter. Not a big deal IMO - either would do for me just fine with a real fusor. Any of them count pretty furiously long before you get into any danger from the neutrons on your body.

I'm surprised you only see one per 10 min on cosmics, Jon. I'd set the trigger to a little less than twice the normal noise peaks, and you should then see more than that. Not all the "real" pulses go to the max amplitude. It depends on where in the tube they originated. Only ones from ideal spots make the full amplitude. So that might just be a threshold issue (and the reason we're making standard counters for activation -- no doubts or questions with those). The tubes are best for relative/tuning kinds of things, where the real time feedback is a huge advantage, but even the pros have real issues with using them for absolute measurements if I read the literature right. It's just too hard to get them calibrated, and compensated for all the things that can drift. (and EMI has been an issue as well). Most of the literature contains late corrections to original papers once they found out they weren't as calibrated as they thought.

I see very little if any difference with a hot rock on one. My wimpy Po-Be neutron source barely makes an increase over the cosmic count here. On the other hand, it counts over 500hz from the fusor -- with it right on there (the B10 gets to 350 or 400 cps). Just right for audio evaluation. Our super big 3He tube counts much too fast to hear variations on the same source and farther away -- it's just a shhhhhhh that doesn't give the ear much useful info. For fusor use, our big (1.5" by 15") and very old surplus GE B10 tube, about 2 feet from the action, is the best in this house for those reasons. It has no noise, it counts about the right speed for getting the info into you via the ear, and it's rock solid reliable. The 3He's are a little flakey and noisy by comparison, and the BF3's have so little signal and need such high voltage that they tend to have "issues" with regular old corona outside the tube and vary with the weather unless all else is perfect.
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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