Assembly of a He-3 Tube/System

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.
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Dennis P Brown
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Assembly of a He-3 Tube/System

Post by Dennis P Brown » Fri Nov 10, 2017 2:49 pm

After reading a number of excellent posts on the recent successes of various neutron detectors, it is time I got back and started building &/or assemble the ones I have on hand (and get my fusor running again.) The easiest would be my He-3 tube since I have a counter/display/power supply consule (the ST-360) to run it. Just a matter of assembly of wiring/connectors to the He-3 tube. And thanks again for that information on proper voltage, Silviu!

In that regards, I have found that the Russian tubes (both the Boron gas based as well as the He-3 ones) electrical end connectors are the exact size to mate/connect with american standard copper tube compression fittings (see first picture. The fittings for 1/4" or 6 mm OD tubing.) I have a metal screw that easily fits inside the compression fitting core and makes tight contact with the He-3 detector pin. I then used a std bolt/wire connector on that nut/bolt assembly.

Aside: That exposed assembly will have silicon sealant covering to protect that assembly on the tube even though the HV is low enough that that shouldn't really be an issue.

MVC-014L.JPG
He-3 tube with copper pipe connector tightened in place with soldered HV wire


I have the nearly finished He-3 neutron detector (see following picture) - all that remains is to install/seal the tube into the RF shielded case.

The ST-360 (recently and successfully repaired - thanks everyone here for that critical help!) is also shown.

The other end of the HV lead is my end cap (brass) with the coaxial HV connector connector attached. This end cap will be silver epoxied (conductive) to the copper tube. In this manner the entire system is RF tight and easy to handle.

MVC-013L.JPG
He-3 tube/connector system and ST-360

Here is the finished He-3 detector assembly fully assembled and sealed - see picture below.

Note: the copper foil tape on edges of the assembly offers little RF protection; the Ag epoxy and metal components provides the major shielding factor. The center "pin" for the coaxial reads "open" relative to the shielding (as it better had); all shield sections read 0.3 ohms or less relative to each other (again, must or there is a bad connection somewhere.) I used some crumpled aluminum foil to fill the lower section of the copper case to keep the He-3 tube in good contact with the shield. The HV wires, "wrapped around inside the tube hold the other side of the detector firmly in place.

MVC-015L.JPG
He-3 Tube w/coaxial connector and in RF protection case


Before I fire up my fusor for neutrons, I will need to clean my fusor window (my electrode feed's thru it. ) Also, I intend to add a wire mess cylinder in the main fusor body/chamber so I have both a larger area and more uniform anode/ground for my cathode cage; this, I would think, will make my plasma more uniform. Hopefully, this improves my fusor's neutron output or lowers my require power for a similar performance.

Currently, I have only one copper tube (this is special thick walled; I got it surplus) so my large boron gas detector had to be removed. I will get that back in operation in a few weeks (yes, Silviu, likely will just use regular copper tubing as you suggest) if all goes well with the He-3 detector. I also have a solid boron detector wired and ready so, if the He-3 detector works, I might try that one as well.

My final major detector project is to assemble my scintillation neutron detector - that will be my last effort/project relative to detectors.
Last edited by Dennis P Brown on Fri Nov 10, 2017 7:00 pm, edited 13 times in total.

Silviu Tamasdan
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Re: Assembly of a He-3 Tube/System

Post by Silviu Tamasdan » Fri Nov 10, 2017 3:17 pm

Good luck with the project. Copper tubing is easy to find, just swing by the plumbing section at Home Depot or Lowes. The 3He and boron SNM-series tubes are thinner than your large detector (18mm).
You're welcome, I was merely relaying information I gathered from other sources plus my minimal experience with these tubes.
I may steal your pipe fittings idea for further connectors. :)
There _is_ madness to my method.

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Dennis P Brown
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Re: Assembly of a He-3 Tube/System

Post by Dennis P Brown » Sun Nov 12, 2017 1:17 pm

I will try the He-3 neutron detector again; both because I fixed the incorrect coaxial connector and found a new circuit for this specific tube type: this circuit is now displayed below. The design is provided by someone selling a similar He-3 detector tube.

I only have a 10 k resistor (R2) and I am following the logic of using 100 M-ohm for R1 (via the paper here - being certain the current is low enough.)

I just set up the circuit (incorporating all coaxial connectors and cables) and ran a ten minute BG test (gamma from cosmic rays.) Getting around 12 counts a minute. Now trying a U-238 ore source (due to gamma from the ore; this is NOT a neutron test) - the rate appears faster (will see after the ten minutes test is complete.) If this appears to be working, maybe run the fusor for a real test - ok; the count was around 30/sec; a bit higher than the BG count.

I am also going to try and get my gas filled boron tube back on line (no shielding but should give me some results, I'd think, with and without moderator. Ditto on the He-3 tube if it appears to be working with my ST-360 counter.) The issue, is that I am not sure my counter can handle this system; however, this new circuit appears like a good methodology - like the grounding resistor (R2) since this should help with current issues and limit charge build up on the cap (never had this arrangement before.) Will see how all this progresses today.

One thing that disturbs me is the system appears very sensitive to line voltage variation (if a major appliance is turned off/on); previously, I ran the entire system using a car battery - might have to set this system back up. Maybe the arrangement is just too sensitive to be used - that can best be determined running it with a battery supply (via 12 v DC to 120 v AC converter.)

s-l500Neutrondetector.png
Possible circuit for the Detector tube

Oops, the battery is nearly dead (12.02 volts) - so I will start charging it - that could take a few hours. Well, delays are part of most fusor endeavors ...lol.

Silviu Tamasdan
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Re: Assembly of a He-3 Tube/System

Post by Silviu Tamasdan » Sun Nov 12, 2017 2:03 pm

My system is powered by my laptop's battery through USB. This eliminates mainline voltage problems.
There _is_ madness to my method.

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