Fusor Running, No Neutron Detection

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Alan Sailer
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Fusor Running, No Neutron Detection

Post by Alan Sailer » Sun Jan 26, 2020 1:59 am

So after spending more time trying to get precipitator supplies working than anything else* I finally got a chance to try a fusion run today.

I spent much effort trying to make sure that the D2 was good. I partially filled a syringe with the gas, then vented it after 10cc (to push out
the air originally in the syringe)and filled to 90 cc overall. I pinch off the tube with a hemostat clamp and attach it to the vacuum system.
The vacuum line has a cylinder of DryRite to remove moisture. I then start the turbo/fore-pump and bring the system down to less than
1 micron. The needle valve and the main valve are opened. This clears all air from the DryRite/valve/tubing assembly up to the clamp.
I then close the needle valve and remove the clamp. This lets D2 into the line.

At this point I adjust the needle valve to get about 40 microns of D2 flow. Turn on the precip and run up the voltage. It it tricky to get the thing
stable (as many have noted). But I had several stable runs with 25-30kV and between 20-40 microns of pressure. Current was 2-4 mA
depending on the run. I have looked at other successful fusion runs and these numbers are in the same region.

However the neutron count is stone cold background. I have counted background neutrons (moderated and unmoderated) using this tube
and a scope so I know the tube is working. During my fusor runs I'm using a Ludlum 12 meter. I'd love to use the scope because I know it
works but the electrical noise during operation makes this impossible.

So three questions.

Does anyone have any pointers for making sure my counter is working without a check source?

All I have done is use the basic technique using a hot U source. Turn down the discrimination. Crank up the bias until the tube goes nuts counting gammas.
Then turn up the discriminator until the counts mostly go away. I have measured the bias being supplied to the tube and it's right in the
high end of the tubes bias spec. That's all the information I've found.

I have read hints that people claim some sort of conditioning of the set-up is required. Is this true?

Does the visual appearance of the grid give any information?

Cheers.

* I know that they have worked well for others other, but results may vary.

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Mark Rowley
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Re: Fusor Running, No Neutron Detection

Post by Mark Rowley » Sun Jan 26, 2020 4:09 am

Things are looking good. Being a 2.75” system, those numbers should provide a very healthy neutron count rate.

The only difference in your system is the addition of a drierite column in the high vacuum portion of the line. I haven’t heard either way if those plastic columns are rated for high vacuum OR the high vacuum is pulling some other type of gaseous crud from the drierite itself. Typically, the columns are put in-line immediately after the electrolysis or PEM cell which then feeds dry D2 into the syringe. The syringe is then moved to the fusor leaving the drierite column behind. It may be possible you’re poisoning the D2 with the drierite under high vacuum. Try removing the entire column from the picture and see if that changes anything.

Aside from that, I can’t add much regarding the Russian H3 tube. Conditioning isn’t anything I’ve heard of nor have any of mine needed that to operate. You may be able to run it in corona mode which may help if the drierite removal doesn’t help.

Regarding your input power...
There’s no need for you to go beyond 2.5mA at 35-40mTorr. Pushing it up to 4mA is just overtaxing your supply in this early stage of learning and getting it going. I have found that letting it churn out neutrons at 30-35kV, 35mTorr, 2mA, for about 5 minutes will stabilize the conditions and allow you to slowly raise the current flow (and subsequent neutron production). These things just don’t fire up and spit out good numbers. They really need to be run for awhile before the inner chamber conditions become optimal.

You’re getting close! Just proceed slowly and expect more hiccups. In time you’ll get the neuts.

Mark Rowley
Last edited by Mark Rowley on Sun Jan 26, 2020 4:26 am, edited 1 time in total.

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Mark Rowley
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Re: Fusor Running, No Neutron Detection

Post by Mark Rowley » Sun Jan 26, 2020 4:18 am

Almost forgot, what diameter is your grid? Too big could pose some issues.

Appearance wise, it should be glowing brightly with some rays stemming from it. The 2.75’s generally don’t produce the picturesque stars seen in the larger systems.

Mark Rowley

Alan Sailer
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Re: Fusor Running, No Neutron Detection

Post by Alan Sailer » Sun Jan 26, 2020 3:03 pm

Mark,

Thanks for the reply and the encouragement.

The DryRite is in a small stainless steel column I built. Threaded brass barbs sealed with TorrSeal epoxy.
Quite vacuum tight. Outgassed under vacuum with a heat gun. But I will try a run with the column
mounted on the PEM cell.

The grid is ~3/8" modeled after your later efforts. It uses a stainless steel stalk insulated with a length of alumina.
The technique used was the Rosenthal (SP?) construction.

I suspect my neutron counter, the Ludlum 12 is the problem. If so, I have no idea what to do.

Cheers.

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Richard Hull
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Re: Fusor Running, No Neutron Detection

Post by Richard Hull » Sun Jan 26, 2020 6:57 pm

The problem is either the gas or the counter. It is reasonable to suspect the counter if it has never counted in a known neutron field. I have been loath to respond as I have no experience with cross fusors, yet.
Neutron counters are another matter.

What is your detector tube? What is its size"
What is your bias voltage?
What was your gamma source?
What kind of moderator?
Photos of the setup? Everything needs to be in the photo of your detection system.

Diagnosing, at range, is always a crap shoot.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Bob Reite
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Re: Fusor Running, No Neutron Detection

Post by Bob Reite » Sun Jan 26, 2020 8:46 pm

I've had no problems with having the desiccant in line between the over/under pressure vials and the mass flow controller leading to the fusor. So I would suspect an issue with the neutron counter first, if you have already checked for leaks causing air contamination of the D2 source.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

Alan Sailer
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Re: Fusor Running, No Neutron Detection

Post by Alan Sailer » Sun Jan 26, 2020 9:07 pm

Richard,

The detector is an SNM-18 proportional tube. The tube is about 18" longer with and active length of 10.4 inches.

The bias voltage is about 1350 volts. Since the Ludlum 12 has such a low output current my meter may be loading it down.
According to Carl Willis and electrometer style meter is best to measure. I don't have one.

The gamma source is a piece of autunite (50% natural uranium and all the daughters) with a NaI scintillator tube count of ~7000cpm.

Moderator is 30 pounds of paraffin inside a cardboard cylinder with a Lexan tube in the center to hold the tube. The wax thickness is
3 inches.

The tube and the wax were used to measure background using external bias and a scope to manually count pulses. The background was
about 7cpm outside the wax and ~14 inside. This result convinced me that I was actually counting neutrons.

I tried to attach a picture but I kept getting HTTP Error. There is nothing special, a He3 tube hidden inside a white painted cardboard
tube filled with wax. Coax cable going to Ludlum 12.

The only unknown to me is if the Ludlum 12 can count neutrons. I plan to spend an hour today manually counting background in and
out from the moderator.

Cheers.

Alan Sailer
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Re: Fusor Running, No Neutron Detection

Post by Alan Sailer » Sun Jan 26, 2020 9:47 pm

Bob,

I also suspect the counter.

It's good to know that, at least in principle, the desiccant does not contaminate D2 gas.

The D2 source is a new 100ml syringe. It's always possible that there is a leak. Since the syringe
is on the non-vacuum side of the needle valve the pressure there is pretty high. I use the syringe
right after the PEM cell fills it so there is not any real time for air to diffuse in.

It might be a useful exercise to let the vacuum system totally drain the syringe and see what
vacuum I can get to.

Cheers.

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Mark Rowley
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Re: Fusor Running, No Neutron Detection

Post by Mark Rowley » Sun Jan 26, 2020 11:26 pm

Unless they were crushed or damaged during shipping, I can’t imagine the syringes being so leaky that the D2 is contaminated to the point of not generating one neutron. I’ve used 3 cheapo syringes from Amazon without a hint of problems. I’ve also let the syringes go empty under full vacuum and obtaining sub micron levels has been 100%.

Have you pre-charged the chamber with D2 before turning on the power? Prior to any runs I’ll backfill the chamber to 500+ mTorr of D2 and then bring it down to 40. At 40 I’ll ignite the plasma and walk it down to 35 or less.

Bob, you speak of the “over/under pressure vials”. Picturing your PEM set up, I’m not exactly sure we’re referring to the same thing. Just to be clear, is the drierite in your system subjected to the same deep vacuum found within your Fusor chamber during operation? If so, and it’s not outgassing, this is good to hear. But I don’t see how water impregnated drierite wouldn’t outgas under deep vacuum conditions.

Mark Rowley.

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Richard Hull
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Re: Fusor Running, No Neutron Detection

Post by Richard Hull » Mon Jan 27, 2020 5:31 am

Your background count sounds about right. Sadly, that is only encouraging. Too bad you don't have access to a warranted neutron source. A good scintillator will always count far more counts from any U mineral source than a GM counter as gammas will be counted from the guts of the mineral, while the GM counter will only count the betas from the surface and virtually none of the gammas. When I say a blisteringly hot gamma source I am talking about a 100,000 cpm source on a GM counter and much more on a scintillation counter. I have a lot of different counters, but have never used a Ludlum model 12 or the Russian tube. Do you have data on the 3He pressure in the tube? My 4 atmosphere Reuter and Stokes works around 1450-1500 volts bias. Wish I could be of more help.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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