Pulses out of SNM 18-1 (at last)

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

Re: Pulses out of SNM 18-1 (at last)

Postby chrismb » Sat Aug 27, 2011 5:33 pm

Doug Coulter wrote:Looking good! I looked up a standard 9v alkaline on digikey and the data sheet shows 100 hours (to 5.4v) at 5ma -- in other words, 5 ma is a real battery-eater. 4 days for a few bucks. Now the original 150ua I designed that to draw doesn't look so dumb. Those specs are always quite optimistic in my experience. There's a lot of air and packaging inside a 9v battery, not so much is actual battery.
You're right. Sorry. I was thinking they were around 1000mAh, but they are only a couple of hundred now I check. Well, it was reading only 2mA, actually, so I might be lucky, but anyway what I was thinking to do was have a rechargeable in there, with a power jack directly connected to power to recharge it easily. I was going to check out how much power my inverters suck as well, maybe I will pacakge up a pack of AAs to drive that too, or get some shv connectors so that I can plug in either the crate or an external 'battery' powered inverter in its own box with a lead to run to the signal amp. Keep them separate, then I can stay flexible on what plugs into what.


Yeah, those scopes are really a good thing to have, eh? Does yours have provision to connect to a PC or put things on an SD card? I find that really useful here, and you get much better pix.
Yeah, but too easy just to take a photo than to try to work out how to use that PC based stuff right now! Too many other things to juggle the concentration on!


What you might do is build the thing inside the existing crate
I'd never go there. A proprietary piece of kit will always stay that way in my hands! I can make a box-of-electronic in the comfort that if I start from strach it can all get chucked in the bin if I mess it up and I ended up with what I started with - a pile of bits. This looks far too complicated, with its HV-out/signal-in combined detector interface that I'd not dream of touching it internally.
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Re: Pulses out of SNM 18-1 (at last)

Postby Doug Coulter » Sat Aug 27, 2011 6:03 pm

I forget some here didn't spend the time I did as a repair tech for consumer gear. To me it would be the risk-free work of a few minutes to find the obvious tie between the HV out and signal in (only one wire from the jack to trace -- or maybe thats where they join and it's a zero time thing after getting into the box) and some power to steal a little of. We rarely could get schematics for what we had to fix -- fast - or not eat well. So we learned how to trace circuits real quick. Useful skill, after all, 90% of the kit on ebay is flakey, or it wouldn't be for sale there cheap (usually).

The PC stuff that came with mine was just put in a CD and click "install". Well worth the time if that's what you have. The linux stuff does take more work, but it's better stuff (and obviously, you have to have linux).

Anyway, glad you got it working. If you want ideal bias, you adjust so that there's just a little voltage beginning to appear across the NPN 10k resistor -- half a volt down to just zero or so will do it. That's what will give you max pulse height out before clipping, and clip any overshoot if you get it right close to zero (but not all the way to zero or past, then you get a free but temperature dependent threshold "free"). It's quite a versatile little circuit.
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: Pulses out of SNM 18-1 (at last)

Postby chrismb » Sun Sep 11, 2011 7:15 am

I made up a moderator for the tube. I have been cutting down to size a 2m x 1m piece of 10mm thick HDPE sheet. (cuts easy with a jig-saw, at that thickness).

P9110777.JPG


P9110778.JPG


There does appear to be a higher count. It's a bit 'bursty', but as per prior discussions on fusor.net, background neutron counts with moderator are generally muon stuff generated in the moderator, rather than 'fast neutrons' being moderated, because it's mostly thermal neutrons at sea level - presumably on account of all the water in the air.

I'd say this morning it went from around 4-6 counts per min (2~3 per min after my aggressive thresholding), to around 7-12 (4~6 with thresholding). Watching the 'scope, I got the impression it wan't quite 'double', as those figures suggest, but that was the actual count from 5 mins of each came up with. On 'rate' I'd have estimated a 50% increase.

I'll do some long runs once the circuit is made up.

P9110774.JPG


My original plan was to have 4 layers with an aim of 4cm worth of moderator, which I might move towards, but there again if I point the 'diagonal' towards a neutron source, then any neutron reaching the tube would've had to go through 4 cm worth of moderator for most paths to there. So, for now, I think I'll see how this goes and chop up more HDPE only if needed/feel like experimenting.
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Re: Pulses out of SNM 18-1 (at last)

Postby Doug Coulter » Sun Sep 11, 2011 7:34 pm

Most people use them end on (according to the books). The moderator acts like a scatterer - as many neutrons bounce straight back as any other way. To get to thermal, you need much more than 2" of thickness, more like 4-5 inches -- and it will make the tube more sensitive to do that, as it will also pick up more neutrons being bigger.

You do get muons - with or without the moderator. However, you also get knocked out neutrons from any in the moderator, the air, your roof, the bench, etc. Double isn't a surprise, I'd expect about that, maybe a little more.
As far as the tube goes, a muon is about like a beta ray, and high energy ones don't leave much energy behind -- they might not count this tube, dunno. Betas have low counting efficiency in geiger tubes unless they're enhanced for that -- special gas mix and so on (our standard counter tubes have that, which is good for that use, since most activated things are beta emitters).

Some metals eat neutrons at some energies, Al being among those - if that's a metal tube in there, make it steel, but better, make it nothing.

I've since come up with a somewhat better version of that preamp, posted in analog electronics, if you care. The new one likes a much wider power supply range just fine, otherwise, a little more gain and just quieter and more stable. A good physical layout is shown, and it should drive TTL kinds of things more or less directly.
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Re: Pulses out of SNM 18-1 (at last)

Postby chrismb » Mon Sep 12, 2011 1:00 am

Doug Coulter wrote:As far as the tube goes, a muon is about like a beta ray, and high energy ones don't leave much energy behind -- they might not count this tube, dunno.

There is also a negative muon + proton -> neutron + neutrino reaction when it occurs in lumps of higher Z stuff.
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Re: Pulses out of SNM 18-1 (at last)

Postby chrismb » Mon Sep 12, 2011 1:05 am

Doug Coulter wrote:To get to thermal, you need much more than 2" of thickness, more like 4-5 inches -- and it will make the tube more sensitive to do that, as it will also pick up more neutrons being bigger.

Better than nothing. I can only afford so much, and there is also a space issue if there is to be another foot of space required around my device.

I was looking at trying to judge the effectiveness of such thicknesses of moderator, but there doesn't seem to be a straight-forward plot of neutron distribution heading away on the other side, for a given input distribution and thickness.
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Re: Pulses out of SNM 18-1 (at last)

Postby Alex Biersack » Tue Sep 11, 2012 5:13 am

Hi, I have these russian tubes too (snm 9 snm 13 and snm 14), what requirements go to a scope to do this kind of work? Do I need 10Megasamples/sec or 100Ms/sec?

Will something like this do? http://www.ebay.de/itm/HANTEK-PC-USB-2C ... 667wt_1173

Is 1mOhm 25pF impedance okay?

Or is something like this sufficient? http://www.ebay.de/itm/DSO-2090-100Msa- ... 5584wt_964

What would you guys recommand?
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Re: Pulses out of SNM 18-1 (at last)

Postby Doug Coulter » Tue Sep 11, 2012 9:08 am

Either should work. The pulses are several uS wide in most cases. However, the scopes and probes won't take the couple kv dc at that point, and you'll have to still provide your own coupling capacitor before the scope to protect it and the probe (and allow the couple kv on the tube to be there). Most scopes/probes won't do HV anymore. The pulses themselves are only a few millivolts, so you need to make sure you've got the gain to even see that.

Note, even 100mhz sample rate is on the slow side for a DSO. You'd get more other use out of the faster one, and good probes are never a waste (or cheap).
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Re: Pulses out of SNM 18-1 (at last)

Postby Jerry » Wed Sep 12, 2012 5:42 am

At the prices of real scopes now I would look at that before I would get one of the USB things. I have a Tektronix TDS340A and they sell for about $100 to $200 used. Thats two channel 100MHz at 500 megasamples
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