Needing more than a spark test?

[homebrewed]

5.49MeV would be to the left of the Plot X-axis
It won't get through the paint over the diode substrate (Hmm.. would it??)
The paint might just be hydrocarbon. The more I think about it, the more I like the notion of a thin graphite short-circuiting plug over the sources.

I am just saying that we should, just for policy, remove all ionized stuff that would be in a smoke chamber, from being in our "bigger" smoke chamber. I can imagine Mark's soldering iron, and other stuff that burns around him when he is busy.

We might want to use smoke detector parts, but we don't want to be building a bigger, better, smoke detector with a nice display that shows smoke levels.

It may not be necessary for the alpha particles to penetrate the coating over the detector. There is a well-known phenomenon called charging by induction -- an opposite charge accumulates on conductive objects near a charged object. So just having an alpha particle smack into the outside surface of the detector _might_ induce a transient charge on the detector itself.

 

[WobblyHand]

I'm a bit ahead of you regarding the possibility of alpha particles contributing to the extraneous counts. I placed a little sheet of aluminum foil over the aperture hole and that didn't materially alter the count rate of my current baseline system (Am241 sources in place, copper shield between the signal conditioning board and pocket geiger board). S/W trigger level = 100mv. Darn. I was hopeful that might calm things down.....

Need to isolate the cause. Might be entirely different source than you think. Have any high power radio, tv or ham stations nearby? I had a uhf TV station get into some of my circuitry on a radar set. Does it change if you cool or heat things? Could be circuits oscillating or chirping. From the sounds of it you don't have that much gain. You have less than 30 dB total?

 

[WobblyHand]

It may not be necessary for the alpha particles to penetrate the coating over the detector. There is a well-known phenomenon called charging by induction -- an opposite charge accumulates on conductive objects near a charged object. So just having an alpha particle smack into the outside surface of the detector _might_ induce a transient charge on the detector itself.

Have the alphas hit an aluminum cover over the sources. Can you wrap the emitter ring in foil? Hopefully no alphas will get to the detector that way.

 

[WobblyHand]

Cool the detector? Protect it from light?

 

[graham-xrf]

Alpha particles don't go far, because upon collision with matter, they strip electrons from it and become helium. Unionized helium won't set off the detector. If the path between the source and detector is torturous enough, no alphas will reach the detector. I suppose one could slightly positively pressurize the detector, but who knows how effective that would be. Just cover the sources with some low atomic number material and the average alpha count will be greatly reduced. How much suppression is needed? 20dB? 60? Even a rough swag will give an idea of what is needed.

So a helium2+ hits something, and claims electrons.
That leaves the something charged. The Am241 is continuously putting in energy into that space. One way or another, the charged stuff needs to be dissipated, and when it does, it makes a noise current.

I would much rather that be into a low impedance short circuit shunt, instead of onto a guarded very high impedance electrode with enough gain to show us a 60pA pulse. The fix is, of course, easy. Even a little plug of paper would do, but I opt to short-circuit them.

I admit, I have a near-insane design philoshophy. I have been a design Devil's advocate for decades. There have been times when my stuff did not work, because of something I didn't think of, but I was the backstop. No design ever failed to work because of stuff I did think of.

 

[graham-xrf]

I'm a bit ahead of you regarding the possibility of alpha particles contributing to the extraneous counts. I placed a little sheet of aluminum foil over the aperture hole and that didn't materially alter the count rate of my current baseline system (Am241 sources in place, copper shield between the signal conditioning board and pocket geiger board). S/W trigger level = 100mv. Darn. I was hopeful that might calm things down.....

Ahh - but you don't get how pleased I am that you got there first!
It means my notion was not entirely bonkers!
Thanks for checking. That makes your diagnostic more specific.

 

[graham-xrf]

Cool the detector? Protect it from light?

It's already in the dark. When I opened it, the diode was covered over in opaque black paint. :|

 

[graham-xrf]

Need to isolate the cause. Might be entirely different source than you think. Have any high power radio, tv or ham stations nearby? I had a uhf TV station get into some of my circuitry on a radar set. Does it change if you cool or heat things? Could be circuits oscillating or chirping. From the sounds of it you don't have that much gain. You have less than 30 dB total?

Even a cable in the wall nearby running power to something, makes a magnetic 60Hz that goes right through metal shielding. It's true that as the frequency goes up beyond into RF, with shorter wavelengths, magnetic fields lose the ability to get inside a Faraday cage, but 60Hz is so low, it may as well be DC.

When I get to this, I may well be putting the amp inside a magnetic shield. If not a mu-metal cover, then some toroid cores.

 

[WobblyHand]

So a helium2+ hits something, and claims electrons.
That leaves the something charged. The Am241 is continuously putting in energy into that space. One way or another, the charged stuff needs to be dissipated, and when it does, it makes a noise current.

It makes a noise current into the structure of the source metal housing. Why? Because of the metal housing we constructed for the emitters and their aluminum covers. The detector is elsewhere and is physically isolated from the source.

 

[WobblyHand]

It's already in the dark. When I opened it, the diode was covered over in opaque black paint. :|

The purpose of my questions is merely to stimulate thought.

I'm searching (from afar) for ideas that we might use to isolate the noise or interference sources. If the diode is shorted out, or removed, does the noise change? If one changes the bias, is there a noise change? Stuff like that... Looking for ideas that produce large changes, not 0.5dB changes which we cannot distinguish from noise without significant processing.

This is all good stuff right now.