So close... what gives?

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brett_kvo
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Re: So close... what gives?

Post by brett_kvo »

Thank you all for the feedback. After spending an hour conditioning the chamber and swapping our stainless egg beater for a proper tungsten grid, I believe we managed to get detectable x-rays at 30 kV @ 15 mA and 0.007 Torr. We solved the arcing issues by adding 2 non-conductive PVC and rubber hoses over the hot lead from our ballast resistor to the cathode.

I think activation will be our next step, as our neutron counter is still not registering anything (despite being right next to the chamber window).

Here is a photo of our setup (please excuse the mess--it's a work in progress):
2021-07-28 12.14.58.jpg
You can see our Maximus Energy neutron detector encased in about 1.5" of HDPE sitting in front of the glass conflat (we're about to replace this glass with an SS blank...). On the left, we have a water tank shield (overkill, I know, but essential for the University's support). We're still using a leak valve at the moment, which allows us to control the pressure and deuterium supply with a long wooden stick while under operation. The chamber is about 8-10 inches internal diameter with 8" conflats.

Here's another photo of the setup from the other side:
2021-07-28 12.14.40.jpg
A photo of the plasma at the moment when we measured radiation:
2021-07-27 16.57.37.jpg
A photo of our neutron/x-ray counter, courtesy of Carl Greninger, registering the radiation:
2021-07-27 17.45.03.jpg
(note that background is around 500 cpm)
He built this for us as a gift. It uses a plastic BC-412 scintillator disc taped to the top of a GM counter, allowing us to pick up both neutrons and x-rays.

edit: added photos inline rather than as links.
Last edited by brett_kvo on Wed Jul 28, 2021 5:17 pm, edited 1 time in total.
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Ed Meserve
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Re: So close... what gives?

Post by Ed Meserve »

Hi Brett, nice work. One comment / request:
This site is not only an excellent reference for multiple technical domains, but it is also a living, historical reference for attempts and efforts of the community. A lot of effort goes into making this site world class, and as a small token, I would ask that you consider uploading your images directly to fusor.net, to be saved for posterity.

While you and I may not be on the cutting edge like some folks here, "the ocean would still be less for missing a drop of water".

Regards,
Ed
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Mark Rowley
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Re: So close... what gives?

Post by Mark Rowley »

You mentioned that your Maximus Energy neutron detector has been tested. How so?

I think I pointed out in an earlier post that the Maximus systems are good for registering neutrons from static sources like AmBe or PoBe. However, registering thermalized neutrons in a pea soup of high voltage noise and xrays will be a formidable task. Im 99% positive they utilize the old Soviet 3He tubes. Those tubes (excluding the Soviet boron type neutron detectors) are notorious for registering electrical noise as neutron counts.

I think going with activation may be the best method at your disposal as your power supply seems to provide more than enough power for your system.

Mark Rowley
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Dennis P Brown
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Re: So close... what gives?

Post by Dennis P Brown »

First off, extremely impressive and professional setup - must have cost a great deal. Your shielding is something almost to the level of a nuclear power plant ( ;) )

Activation at low voltage could be a challenging effort. Most fusors that can't activate can produce significant neutron signals even in soviet style boron tubes.

Are you certain that the GM is producing the required voltage for the scintillator - some models don't. Could you explain why you think it even works, much less that you are confident it is operating/accurate? I assume the BG counts occur with the fusor full on (all voltage/current) at operating conditions but moderator removed. Then the counts don't change at all (on average) with the moderator in place? Aside: that photo shows the GM max'ed out; can you select a scale where it isn't? When it is pegged like that, readings could be a good bit higher and it won't show.

You mention it is also an x-ray counter - do you have any shielding to eliminate that issue for the scintillator? Those counts could easily swamp your very real neutron signal since it is likely rather low compared to noise/x-rays counts in your low voltage fusor.

Could you also provide some details on your deuterium source - certainly if the detector is working but the deuterium is compromised (some how) that could lead to issues, as well.
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Richard Hull
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Re: So close... what gives?

Post by Richard Hull »

As noted, electronic detection methods for neutrons is an art few ever need deal with to the point that the few who do are often deluded by novel neutron detection ideas as opposed to a good BF3 or 3He detection system. If anything like 100,000 n/s is coming out of the fusor a piece of sheet silver will activate in a suitable moderator and then just a plain vanilla GM counter will tell the tale. You neutron detection system sounds like a thoughtful kludge, but how good is it?? You say it has been tested....With what?

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
brett_kvo
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Re: So close... what gives?

Post by brett_kvo »

Hi all, thank you for the feedback. Mark--I think you're right. I'm also convinced that the neutron counter readings we are getting are due to electrical noise. The Maximus Energy counter only ticks when we have an arc in the system. Even when it is sitting directly in front of the window about 6" from the center of the chamber running at 30kV @ 15mA there are no neutron counts. It looks like activation will be our best bet. Does anyone have a good recommendation on where to pick up some silver foil?

Dennis-- We are very fortunate to have access to a bunch of very high-end equipment. The shielding is overkill, I agree. Most of our setup was scrounged from parts found laying around in the very old, very well-stocked physics department at the Univ. of Puget Sound. We only had to buy a vacuum gauge, a power supply from Joe Gayo, a Labjack, and the neutron counter (which appears not to be working for us). We also had to build/buy the shielding, which is overbuilt to keep the University happy (and I still have more to build!).

The GM counter was tested at the NW Nuclear Lab's fusor (and built by Carl himself) so I have seen it work in a similar system. When we put a lead sheet in front of the scintillator, the counts go to zero. We're getting insane x-ray readings, but zero neutron counts regardless of the configuration. We also tested the Maximus Energy neutron detector at NWNL a few months ago and it was picking up neutrons from their fusor.

The deuterium is straight from sigma-aldrich. It's leaked in through a precision leak valve to maintain about .005-.007 Torr.

In other news, I think we need to thoroughly clean the chamber. I think this could be a major contributor. The tungsten we're using for the cathode grid changed from a dark grey to a brightish clean silver. All of that junk had to go somewhere.... Any recommendations on the best kind of acid for that job? We melted the tungsten and had to open up the chamber anyway.

Thanks again~
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Mark Rowley
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Re: So close... what gives?

Post by Mark Rowley »

Brett, you’d be surprised at how resilient a fusor is to having sputtered junk in the chamber. Unless you have globs of vacuum grease, plastics, etc in the chamber, I highly doubt you've got anything going on to cause a zero neutron yield.

Regarding silver, no need to source high purity foils. Sterling silverware, silver coins, etc will easily activate under modest neutron exposure. Foil is nice if you have it, but there’s no need to delay activation experiments for a special ordered high purity foil.

Mark Rowley
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Richard Hull
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Re: So close... what gives?

Post by Richard Hull »

Mark tells it like it is. Never crack a fusor open repeatedly it just means more time wasted getting it to some modicum of cleanliness again. With such a large fusor, you will soon realize that a good beating is more desirable than opening your fusor to air for some purpose, be it vital or trivial.

It is hard to believe you melted a tungsten grid unless it was awful fine wire. It is usually newbies who melt rapidly assembled SS wire grids. Mid-West Tungsten use to have an "orphan pile" and many here fed off of it for years. I loaded up on their twin twisted tungsten wire of a decent gauge and used it to make up 4 grids, none of which failed. (different forms and sizes).

90% silver (coin silver) or 92.5% silver (sterling silver) are just fine, but as Finn Hammer noted, a pure .9999 fine silver round is found in any city in the land or Europe at coin shops, pawn shops, gold and silver joints that buy and sell silver and gold. Foil is ideal, but in your case, for base level activation, is just not what is demanded. Today, silver coin is non-extant in change having been harvested out since 1964 when last considered to be "real money" by virtue of the fact that it carried true value within its own mass. Traditional married couples getting sterling silver cutlery as wedding gifts just is not done much any more, it is now rare and carries a markup of about 800% for its weight in silver. Go to a flea market and pay for a heavy tarnished piece of sterling cutlery where the markup is virtually zero. The pawn shops are ideal as now they are also in the biz of buying and selling silver. If they have .999 or .9999 silver buy that as it will be useful later when you pound or roll it out to a thin sheeting.

The key to activation for newbies is to have a moderator virtually pressed up against the fusor that lets you instantly remove, (within under 5 seconds of fusor shutdown), the silver and have it counting within 10 seconds under a mica windowed GM tube after you shut down the fusor. You will not get much counting if making well under 50,000 n/s, but if you are fast, it is possible to tell you have neutrons. The key is fast to the counter and counting and a mica windowed GM tube.

Your shielding is a vast overkill, but if it makes you feel warm and comfy, fine. Rad safe officers and CHIPs are institutionally taught to worship the ALARA Gods. It is as if the inverse square law and small exposure time is immaterial to them. They assume a 7 day a week worker 8 hours a day will be there lashed to the device with it running full bore.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
John Futter
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Re: So close... what gives?

Post by John Futter »

Next time you have your fusor open
And I agree with Richard ie not to open unless necessary

but to clean Tungsten deposits you use concentrated KOH (potasium Hydroxide) solution ---wear gloves unless you want new skin
Rex Allers
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Re: So close... what gives?

Post by Rex Allers »

Silver foil:
I think Rio Grande is a great source
https://www.riogrande.com/product/999-f ... h/101916gp

999 = pure silver

I suggest 30 ga (.010 inch thick). With this 6" wide listing, I ordered 2" so 6 x 2" and cut it into three 2 x 2" pieces.

Prices have gone up since my order several years back but I think still reasonable.
Rex Allers
brett_kvo
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Re: So close... what gives?

Post by brett_kvo »

Mark-- Good to know. Thank you. I have some silver coins that we'll try out on Monday. There's no grease or anything in the chamber, so perhaps my hunch is incorrect.

Richard-- Yeah, it's really cool to see the inverse square law in action. The shielding is an annoying formality (and an expensive one at that!). On the upshot, it will allow us to be a bit closer and operate in a smaller space. Also, given that this is a student-led project, it will provide a measure of safety if a student were to wander too close. The tungsten wire is 1mm in diameter. I'll check with the student who disassembled it to see if he has some insight into where it melted.

John-- we'll try some of that KOH for the tungsten. We had to open the chamber anyway to fix the tungsten that had melted out.

Rex-- thanks for the link!

I'll report back next week with our findings. Hopefully it will be a celebratory posting! Thanks all :)
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