I Activated my Feedthrough!

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
Jon Rosenstiel
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I Activated my Feedthrough!

Post by Jon Rosenstiel » Mon Mar 22, 2004 3:42 am

I had removed the HV feedthrough stalk’s alumina insulator tube to see what effect it had on my fusors operation.

0900… After pumping all night I fired up my fusor to check neutron output. Was 12mrem at 28kV, 19mA. (With alumina insulating tube in place neutron output was 20mrem with same input power)

0915… Cranked up power to around 25mA, 35kV. Neutron output was 25 to 30mrem.

0920…. While watching TV monitor noticed my tantalum cathode was “drooping”. Shut down fusor immediately.

0930…. Removed feedthrough. Found hole burned into stainless steel stalk tube next to the cathode attachment point. Also noticed that the end of the feedthrough insulator had a real nice sputter coating of stainless steel. (See attached photo)

0950… On a whim I checked the feedthrough’s sputter coating with my pancake detector. Was floored when the count was TRIPLE background!

1000…. Started count on feedthrough with both pancake detector and gamma detector. With the pancake detector took 2-minute counts for 2 hours. With the gamma detector took a 2-hour count. (Max energy, 3keV)

1200…. Pancake detector data indicated a 28-minute half-life for the radioactivity on the feedthrough. The gamma detector found nothing!

I'm stumped on this one! Ideas anyone?

Jon Rosenstiel
[attachment=0]Radioactive Feedthrough 002.jpg[/attachment]
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Frank Sanns
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Re: I Activated my Feedthrough!

Post by Frank Sanns » Mon Mar 22, 2004 5:02 am

Jon,

Neutron activation of materials will give isotopes that will beta decay. Your pancake probe will detect the beta while your gamma probe will not.

Looking up the elements that would be present in the flange, insulator, and grid would be Fe, Co, Ni, Cr, Al, and Ta. Adding one neutron to any of the naturally occuring isotopes will not give a half life of 28 minutes. Most of thier half lifes are many days.

It may though, be tritium that is the casue of your beta counts. Tritium has a half life of 12 years but it may not be the actual half life that you are measuring. It may be the diffusion out of the feedthrough that you may be detecting.

Before somebody (Richard) screams that we do not PRODUCE measurable quantities of tritium in our fusors to detect, there is another source. The deuterium that we are all using contains quite a bit of tritium. The analysis that came with my ultra high purity deuterium says that the tritium content could be as high as 0.1 milli curies per liter. That should be detectable. I do not know if it is the answer but it is a possibility.

Frank S.

Jon Rosenstiel
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Re: I Activated my Feedthrough!

Post by Jon Rosenstiel » Mon Mar 22, 2004 5:49 am

Frank,

For a simple test I let a small quantity of deuterium flow over my pancake detector... there was no increase in activity above background.

Don't know if you had this in mind or not, but we need to remember that inside our fusors all neutrons are fast neutrons. (Along with a equal number of protons)

By 1215 (3 hours after the "drooping cathode" event) the activity on the feedthrough had decayed completely, so my 28 minute half-life number may be a little low.

Jon Rosenstiel

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Re: I Activated my Feedthrough!

Post by Richard Hull » Mon Mar 22, 2004 3:02 pm

This is a most interesting post and thanks for sharing the info Jon. You are right in being amazed as there is just little way in which anything in the chamber can activate.

The tritium diffusing out of the ceramic might be a thought as Frank mentioned. I had no idea that one had as much as 100uCi of T in a liter of D2! Probably a result of concentration during the fractionating process in boil off capture of the deuterium. That is 3.7 million disentegrations per second per liter at STP. In a 20 liter cylinder that becomes over 70 million super weak betas per second!

The beta from Tritium is so weak that is will barely pass through the mica window on the pancake. 6-8kev average, 18kev max. This could be tested in future with apiece of aluminum foil. with foil between the detector and the insulator you will see zero beta from tritium. If the betas are activation betas they will zip right through. So many clever ways to skin the cat and narrow the field.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: I Activated my Feedthrough!

Post by 3l » Mon Mar 22, 2004 4:20 pm

Hi Jon:

Congratulations!

We now are getting good indication that the group is on the right track. I particularly like the fact the event occured at 35 kv.
At 40 kv which is just a little higher voltage , a consistant raising of the neutron numbers seem to occur as the amperage is raised.

Jon,I'm chomping at the bit to get accurate neutron measurement of this stuff.

>>>>> Gentlemen ,Elvis is leaving the building! :^) <<<<<

Here's my theory on this stuff:

I had noticed a very similar event in pulsing that I had chalked up to anomolous behaivor but later I had my mind changed.
I too use ceramic feedthrus....If I pulse say 10 times then on the eleventh or twelveth pulse heck breaks loose. It could be we are
plating the chamber and insulator with tritium. At a critical level the gas readmits itself to the chamber giving the "runaway" reaction. By increasing the pre ignition glow field above 50 ma
the regularity of high neutron events can be repeated with fair regularity. I for one turn off the pumps at the desired vacuum level so any gas would stay put and it would now seem that a sputtering process retains some too. It could be that a rampaging electron beam could disrupt the metal hydride Tantulum Tritide and Tantulum Deuteride coating on the walls producing a Puff of tritium +d gas.

Happy Fusoring!
Larry Leins
Fsuor Tech

Jon Rosenstiel
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Re: I Activated my Feedthrough!

Post by Jon Rosenstiel » Tue Mar 23, 2004 4:31 am

Larry,

Hmmmm, starting to get quite interesting. I may have to attempt an re-enactment. Then try Richard's suggestion of using Al foil to see if the betas can be blocked. (Would that be considered a beta-blocker)?

Man, this fusion stuff sure is fun!

Jon Rosenstiel

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Re: I Activated my Feedthrough!

Post by 3l » Tue Mar 23, 2004 1:16 pm

Hi Jon:

You bet!

This is all new and fairly mysterious but doable.
It beats the heck out of fishing!
Golf! LOL!
I'm bidding on a huge transformer pretty much like yours.
How did you contain it?
Was it a do it yourself or did your tranni have a box with it?

Happy Fusoring!
Larry Leins
Fusor Tech

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Re: I Activated my Feedthrough!

Post by Starfire » Tue Mar 23, 2004 5:23 pm

If the pressure keeps rising Jon - we may all be on beta-blockers. :)

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Carl Willis
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Re: I Activated my Feedthrough!

Post by Carl Willis » Wed Mar 24, 2004 3:10 am

Hi Jon,

Wow, this is an interesting result! I'm stymied as well.

For a guess, your background was probably in the 50 cpm range, and so you were seeing ~150 cpm, or ~200 cpm, at the beginning of the measurement? Did you see any evidence of brehmsstrahlung on the gamma spec?

Do you use a backfill or continuous flow method? I did some calculations and am almost certain it is not tritium. However, nothing else makes as much sense to me yet! If you did make a lot of tritium, the positively charged tritons would tend to get buried along the feedthrough and in the cathode and would concentrate there. The brehmsstrahlung from T betas IS detectable by a pancake GM. Unless you were producing thousands of times as much tritium as a normal continuous fusor, though, the tritium explanation falls apart. I'd often monitored my vacuum pump outlet for spurious counts above background with a pancake GM during operation, and never noted anything.

Is the feedthrough itself made from alumina? In that case you should expect to see the 1.7 MeV gamma from Al-28 (half life 2 min.) due to neutron absorption, but would undoubtedly have an easier time detecting the betas. The feedthough is certainly large enough to moderate and reflect many neutrons to thermal energy although it is far from ideal in this respect.

Finally, I think it's inconcievable that a (D, non-D) reaction, or a (P,x) reaction, is occuring and giving rise to a radioisotope. The energies involved are just too low.

Don't know what to say!! Keep up the good experimental work and try to repeat this most interesting experiment.

-Carl
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Jon Rosenstiel
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Re: I Activated my Feedthrough!

Post by Jon Rosenstiel » Wed Mar 24, 2004 4:44 am

Larry,

About my transformer, you may have me confused with someone else. I have an old 100kVp, Kenotron rectified, x-ray power supply. It came complete with tank, oil, etc.

Carl,

You're real close, my background is normally 47cpm. My initial count rate was in the 150 to 160cpm range. And that was a good half hour after the "drooping cathode" incident. I'd guess that if I had checked the feedthrough immediately I would have seen around 200cpm!

I’m away from home right now, but when I get back later this week I’ll take another look at the gamma spec. I took a 7000 second gamma spec of the insulator and also took a 7000 second background gamma spec. When I overlaid them there were no obvious differences. (Although I did not look very closely at the low end of the spec).

I use the continuous flow method. I’ve also done the same as you (check vacuum pump exhaust with pancake detector) and found nothing.

My feedthrough is an eBay item, so I’m not 100% certain of its pedigree, but dimensionally it matches Ceramaseal’s 70kV model, which they say is alumina.

Yeah, what happened is very unusual. I checked the feedthrough for activity at least three separate times, once using a different counter (but same probe). All with the same result. When the probe was brought up close to the feedthrough ratemeters went up, speakers clicked faster. It looked, sounded, and felt like radiation, so it must be radiation! (Well, I didn’t actually “feel” the radiation)!

Thanks for your input. Thoughtful as usual.

Jon Rosenstiel

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