Neutron and X-Ray

[Alan Sailer]

Hello,

I have a question about the SNM-18 tube. I am trying to identify exactly which type tube this is. Mark Rowley and the
Russian tube site both say the tube is a proportional tube. Lukas Springer (who seems very knowledgeable) says that the
SNM-18-1 is a corona tube.

What Is the difference between SNM-18 and SNM-18-1? Is the SNM-18 truly a proportional tube?

Second and more important question is about X-ray production. I have done basic x-ray shielding after running the fusor for
the first time in air. I wrapped 1mm lead sheet around the viewport to collimate the beam and added a 45 angled front surface mirror
to view the grid. This really helped. The x-ray rate inline with the viewport is high but where I am located it is very small.

However when I was experimenting around yesterday I got a very different result under some conditions. With the radiation detector off
axis from the viewport and about two feet away, several times the counter went crazy, hundreds of counts per second. I was running at about
20kV, maybe 50microns and 4-5 mA. Using air contaminated deuterium. The grid was also white hot.

Don't like this. I was expecting a moderate soft x-ray flux that would be mostly shielded by the stainless steel CF cross and my job
would be making the viewport safe. I investigated the feedthru a few days back and it was much, much quieter than the viewport.
As far as I can tell, this high flux was going through the CF walls. I didn't spend any time investigating the source radiation because
frankly it scared me.

Does any of this sound familiar to anyone?

Cheers.

[Richard Hull]

The typical Spherical fusor used hemispheres from Braun and these are at least double or maybe triple the thickness of the cross tubing walls. The cross walls will have x-ray shine through at a much lower voltage than the spherical fusor with the 0625 wall thickness. I would suggest a 1/16 -1/8- inch lead shadow shield for all cross operations over 20-25 kV. A normal GM counter is a great thing to have to sniff out x-ray leaks. Beyond this, an ionization chamber is even better.

Neutrons will never be the danger with most all amateur fusors, it is the x-radiation that can get nasty once you cruise well over 20kV applied. Regardless of chamber thickness, the skin burning x-rays will all be stopped with no shielding at all. Only a GM counter will tell when you start to have "shine through". Every one should learn the Shine through voltage of their fusor. Fusor IV goes transparent at about 32kV.

Richard Hull

[Mark Rowley]

It’s a proportional He3 tube. You can try to operate it in the corona region if you’d like.

This is a good writeup:

http://physicsopenlab.org/wp-content/up ... 1v10-1.pdf

Mark Rowley

[Alan Sailer]

Richard,

Thanks. I ran across your description of radiation shadow earlier and it makes good sense.

This x-ray problem has me spooked. Fusions is all fun and games until your eyeballs start glowing.

Mark,

Thanks for the verification. I have the Higgins article and am re-reading it more carefully.
I also talked with Carl Willis (he helped me with a plasma globe project a few years back) and
he didn't think I was crazy trying to use the SNM-18 tube for neutrons. And since it's my only
tube I have no choice.

Cheers.

[Mark Rowley]

I dont think you're making a mistake by attempting to use one. But I do feel it's important to let you know about the potential issues with the old Soviet He3 tubes being operated at proportional voltages. Like I said, It may work for you or you may have to experiment with running it at corona mode voltages. It's a fact that these tubes are very different from a modern LND variant and can easily test ones patience to the max. Btw, boron corona tubes are dirt cheap and much easier to get operational. The SNM11 and SNM14 are comparatively easy to dial in and have outstanding gamma/xray rejection. I've recently seen the 14's going for around $35 so no need to throw in the towel if the Soviet He3 gets testy.

Although it didn't help in my case, I have to second what Richard said regarding the star grounding arrangement. Every system is different so it may help.

Mark Rowley

[Alan Sailer]

Mark,

I am using a slight variation on the star grounding technique. It's a short (2") piece of copper bar with holes tapped
every 0.375". Screws attach each spade lug.

Star grounds are great but I dislike the one screw for multiple lugs.

And thanks for the advice regarding the tube. I'll have to read more about neutron detection. At this point I am just
confused as to why a tube that was designed to detect neutrons (He3 proportional) is not a good choice for measuring neutrons
from a fusor.

And at the risk of sounding like a broken record, my only real concern at this moment is x-rays.

Cheers.

[Richard Hull]

The ideal is a good 3He proportinal tube with 4 atm pressure. I have no knowledge of any Russian tubes as I have a Reuter and Stokes 4 atm tube that has done beautiful work since 2003 and I'm still using it.

Richard Hull