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 Joe Jarski » Sun May 29, 2011 3:01 pm

Doug, I was looking through some more datasheets and found this bit of information on the SNM-12 tube datasheet, which uses the same scheme & values as the SNM-14.

The translation goes something like this:
Amplifier should be the upper limit of the bandwidth not less than .5 mc. Sensitivity threshold of the recording scheme locally input must be equal to 18 mV. Changing the threshold of 1 per cent gives the corresponding change in neutron count rate at 1 percent. Increased sensitivity threshold detecting circuit to 15 mV gives rise in the number of background counts in the presence of gamma radiation. Further increase in the threshold sensitivity of the recording scheme to 5-10 mV, making it impossible to service counters for registration of noise pulse corona discharge.

I don't know if this would give you any insight into the 3He going berserk, but thought I'd throw it out there.
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Re: First light on SNM-17 3He tubes

Postby Doug Coulter » Sun May 29, 2011 5:26 pm

When it's doing that -- most of the pulses look just like they do for the real thing, though some are "louder" and get clipped. I did an input voltage sweep while it was acting nuts and it starts right at the corona onset voltage (6-700 v), and just gets bigger and faster with more HV applied up to the limit on these of 2500v or so. Then it gradually cleans itself up (so far). If you start out at 2500, and turn the volts down, it quits for awhile, but them comes back. It's just like HV cleanup/conditioning in how it seems to act. Like dirt is flying around in there under electrostatic forces and making counts. It's real random, with bursts of faster and slower for awhile.

In fact, I just went down and tried it again. It counted like crazy for 2-3 minutes, then settled down to what I assume is the cosmic background rate -- every few seconds. The "fake" counts tend to be larger and flat topped, and wider. Then it seems to just go away. I suppose what I should do it hook it to an audio amp and monitor it over some hours to make sure this is merely a turn-on phenomenon. It'd really mess you up if you were integrating total counts and it happened during a fusor run - the fakes are faster than from all but a very hot fusor with the thing in close. This time the thing hadn't been touched since the last run other than the power switch, so it's not due to being shaken up.

When we first saw elevated counts after a run, I made some mods. I painted the inside of the coffee can, used a real tube type insulated plate cap for the tube, added a quartz tube insulator over the 50 meg R chain, and added a HV bypass cap (mainly used as a strain relief for the HV wires) coming in. Further I made a 400 pf coupling cap out of two 200 pf ones as stated in the paper you found. None of that made any difference at all - but did eliminate the theory that the extra counts were corona noise off the sharp pointed connector I'd used on the tube end to begin with, and now I know "it's as right as my hands can make it". None of these mods had any particularly noticeable effect -- other than I know I did it right now. The shielding we did makes this the most impervious to EMI detector in the house, BTW. Very nicely silent when it should be (and the tube is right). Jon made those nice flanges for the coffee can bottom to screw down to the grounded copper top of the moderator, and that works a charm.

This precise-same setup also works with the B10 tubes, only change you need is to set the HV correctly for whatever tube you're using. I really like the resulting signal quality coming out, nicely out of any noise (a few volts signal) and a good flat baseline. And the low power drain....this is getting really close to "as good as it gets" if the doggone tube will just act right. Drives plenty of feet of coax just fine, no artifacts (on this fairly slow signal, anyway). There is in general no need to impedance match the coax until you start having risetimes that make the coax a good part of a wavelength, till then, it just looks like a small capacitor across the output of the thing, which it seems to drive just fine.
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: First light on SNM-17 3He tubes

Postby Doug Coulter » Thu Jun 02, 2011 1:07 pm

Here are some scope traces of the strange fake pulses vs what you see normally:
new3heRunon.gif
Fake pulses

There's this flat place in the fake ones.
new3henorm.gif
Normal or cosmic hit

That isn't there for a normal pulse.

So far, this has cleaned itself up in some single digit number of minutes after turn-on of the HV, but I'm watching it close as I want this for my second fusor, and I want it reliable.
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: First light on SNM-17 3He tubes

Postby Joe Jarski » Tue Jun 28, 2011 11:43 pm

This could go any number of places, but since this preamp was built based on the schematic in the first thread I'll give an update here for now and maybe start another once it's complete.

This is the PCB that I made for the corona neutron detectors. The plan is to stuff it and the battery in a piece of 1-3/8" x 3" long tubing with the required BNC connectors on each end and a small toggle switch, then attach them directly to my B10 tubes. I haven't been able to test it yet, but I'll probably throw something together over the next few days and maybe listen for some background Ns.
PreAmp.jpg
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Re: First light on SNM-17 3He tubes

Postby Doug Coulter » Wed Jun 29, 2011 8:09 am

Looking good - but a test is always a good idea, and you should get something "at all" from cosmic rays - you might have to be patient (a good DSO helps as it will save the last screen until the next trigger). I only get about 1 per minute here. Any new layout of parts this fast needs a test to make sure it doesn't oscillate from changed stray capacities between the connections - I took serious care in that other layout for 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: First light on SNM-17 3He tubes

Postby JonathanH13 » Sat Aug 20, 2011 8:39 am

I have recently completed the housing for the 'other' 3He tube (the one mentioned in this thread). I have had it up and running for sometime - since Bill F came to visit - he actually witnessed it registering neutron hits from the fusor during a couple of runs. But it was housed in a large vegetable oil drum for electrical shielding, which did the job, but was too large and rather smelly. So I have built a customised aluminium box for it, which is smaller and better shielded.

tube he3.jpg


noise shield.jpg


I also built the power supply for it described in the link below, and made sure it was well shielded in its own aluminium box:

viewtopic.php?f=9&t=290&hilit=power+supply

power.jpg


The tube registers a cosmic ray hit about every 10 minutes, and appears completely immune to noise or signal from anything else - all in all, a very stable and quiet detector. I will post some results soon - the next step is cooking up something to count the hits (maybe it's time to replace my 30 year old scope).

under the hood.jpg


If you look carefully at the scope you can see a captured negative-going spike.

cosmic ray.jpg
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Re: First light on SNM-17 3He tubes

Postby Bill Fain » Sat Aug 20, 2011 2:23 pm

Jon, Hi. That's quite an upgrade from the "Charles Chips" type container you were using a couple of weeks ago; nice and clean looking. I did manage to snap what I thought was a "picture" of the scope when we were at neutron conditions (reasonable pressure, voltage, and proper looking poisser). It's rather out of focus, but after the 8 hour flight, no sleep, customs, tubes not operating, and walking completely around Paddington station to look for the bus stop, this might have looked in focus to me at the time. BTW thanks for picking me up when I got off at the wrong stop. Although blurry, your scope trace looks like Doug's, but with less hits, though hits nonetheless, I think. Compare this with Doug's last static shot of his scope at the end of his comments on page 1 of this thread (new3heFusionroll.gif). Again sorry about the quality of the image. -bill
DSCF6226.JPG
a myopic view of Jon's He3 tube output
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Re: First light on SNM-17 3He tubes

Postby chrismb » Sun Aug 21, 2011 4:45 am

Could someone please clarify something for me, please. I'm a bit fuzzy on the intended meaning of this...

These tubes' data sheets say to stick a 50Mohm in series with the power. Then they say operational voltage is 1600V and typical coronal current 20uA.

Is the 1600V the voltage on the power supply, or across the tube? If I stick 1600V on these tubes I get a few 100 uA across them, but if I put 1600V across tube and 50Mohm resistor, then there is around 20uA through the circuit but this would only be around 600V across the tube.
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Re: First light on SNM-17 3He tubes

Postby JonathanH13 » Sun Aug 21, 2011 2:54 pm

This is what I used to test mine & I think Doug has the same setup...
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He3 tube test.jpg
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Re: First light on SNM-17 3He tubes

Postby chrismb » Sun Aug 21, 2011 3:57 pm

So, what voltage do you think is needed across the tube to make it work?

(..and does the 10k to ground achieve some funtion that the internal resistance of the scope can't?)
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