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Making a BNC adapter for SNM-11 boron neutron counters

Posted: Thu Nov 02, 2017 10:37 pm
by Silviu Tamasdan
Not very exciting but had to be done. I made a plastic adapter between the SNM-11 post-and-case connections, and a more modern BNC connector so I can use it more comfortably.

The plastic piece is just a 3D-printed cylinder of ABS, the same diameter as the tube (18mm) and with an axial hole in which at one end the BNC connector is screwed, and at the other I made a connector for the tube's anode. The anode connector extends along the hole and is soldered to the central connector of the BNC. On the outside the cathode connection runs, from a collar around the tube's upper case to the case of the BNC. Nothing simpler. All metal used is 0.7mm copper sheet.

I'm attaching pictures, and the source code for the plastic piece if anyone wants to 3D print their own.

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Thu Nov 02, 2017 10:39 pm
by Silviu Tamasdan
I hit the attachment limit in that first post; a couple more pictures here (were supposed to be the first 2) and the source code.

The nice part about this size is there's a space between the anode and the BNC, just enough to add a resistor if you want to.

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Fri Nov 03, 2017 12:54 am
by Rex Allers
You might get away with it, but using a BNC connector for this tube is really pushing it on the voltage rating.

I think this tube needs a supply voltage of 1500V or greater (current limited with a 100 M resistor or so). I just looked up Amphenol specs for BNC:
Dielectric Withstanding Voltage 1500 VRMS
Voltage Rating 500V rms

In these hacker realms, people often use BNC's up to a KV or so but I wouldn't want to go up to 1500 V without realizing that this is stretching things.

For high voltage the MHV or SHV connectors are similar in size and style to BNC but are both rated for 5KV. They are harder to find and more expensive but safely within specs in the 1 to 2 Kv range here.

The larger N or C-type connectors are rated for 1500 V but you would only be pushing them a little above their ratings. They are bigger, though, and getting a flexible cable connected could be a challenge.

Just wanted to warn about how far you are stressing a BNC.

Another thing, which you didn't get into, is how you wire this up in circuits. You need to have a corona discharge in the tube with current limited by a high value resistor (100 M or so), but you need to bring the pulse signal back through a DC blocking capacitor connected at the tube anode. The signal should definitely not pass through the big resistor. How you do this with just one coax could require some thought.

If you haven't read Bob Higgins' paper, "Slow-Neutron Corona Counter Tubes: Principles and Use", you probably should. I think it is saved on these forums somewhere.

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Fri Nov 03, 2017 12:59 am
by Rex Allers
Here's a link to getting that paper.

Superb 3He Corona tube paper

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Fri Nov 03, 2017 2:31 am
by Silviu Tamasdan
I know that paper almost by heart.

However the SNM-11 works best at 700V. In fact I've had one test where the tube worked correctly at 670V. I didn't try lower voltages.

Therefore the BNC. I have SHV connectors as well but no point using them since I don't need the HV rating.

In fact even other corona tubes such as the SNM-32 which are also said to work at 1500-1700V; my experience is that they are unusable at voltages above 1275V because they start oscillating at high frequency (and I've had independent confirmation from somewhere else that it's indeed their normal and expected behavior based on a sizeable sample of them). But they work well at 700V.

I've described my setup in another thread viewtopic.php

This thread is just a corollary to the above to show how I connect these tubes. This is the third one I'm attaching connectors to.

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Fri Nov 03, 2017 9:09 am
by Dennis P Brown
That design sequence is very nice; the work you did on those tubes was outstanding - especially resolving some of the voltage issues (operating range.) That really is helpful! Thanks.

You might want to place that detector tube within a metal housing of some type. I used a thick copper tube. I then electrically sealed end caps via solder. Be aware that thin foils and non-electrically sealed metal to metal contacts will not shield from RF effectively. Of course the co-axial connector used by the detector is attached to the outer metal case or end cap in order to connect to the inner detector and keep the system RF sealed. Fusor RF can be serious for sensitive detector circuits. This may not be an issue but keep it in mind. Others will, I assume, weigh in with their experiences with detector circuits/systems around fusor's.

Your link does not work, Silviu.

Is this the link you wanted?: viewtopic.php?f=13&t=11954

Re: Making a BNC adapter for SNM-11 boron neutron counters

Posted: Fri Nov 03, 2017 11:06 am
by Silviu Tamasdan
Yes, that was it. The copy/paste function on my phone seems to be a little b0rked. Thanks!

The copper tube shielding sounds like a great idea. Fortunately that would be a simple mod to make. I have tested the detector setup with a strong gamma source, but not with a strong EMI source (because _I_don't_have_a_power_supply_ ...yet).
I plan to make a small test voltage source sometime in the next few days (nothing fancy, a small inverter from 12V/20kHz to 1kV or so with a medium-size ferrite toroid). Then I'll see what kind of effect a small plasma arc a few centimeters from the tube has on the detector SNR.

BTW the purple pen in the last picture of the first post is an essential assembly tool. In order to make the anode contact pass through the adapter hole I had to shrink its diameter a bit. The pen was used to reexpand it in place thanks to the natural curvature of its metal tip. :)