Needing more than a spark test?

[graham-xrf]

My interest has been piqued enough to take the plunge. What specifically do I need to start with? Is it the Sparkfun Pocket Geiger Radiation Sensor Type 5? This is to access the large PIN diode? Is any of the rest worthwhile, or is this just a way to get the detector? Or is it better just to buy the diode and make a TIA?

Speaking of analog hackery anyone know where to get little glass standoffs like old analog guys used to use 30+ years ago? You would drill a hole in the PCB and solder them in and they had a terminal to solder a wire. They had low capacitance. I have a circuit with a few of these. Don't know what the proper name is for these terminals. Are they made any more? Used to do galvanometer like work with them, low leakage and capacitance.

I think we might be getting around to posting some passing pictures to help folk get started. Sure, take the plunge. We are pretty sure it will work, but I guess some will want to wait until we get past the possibility of "egg-on-face" stage.

Yes - you do need the PIN diode. It comes on a tick-counting thing called Pocket Geiger, which is less than useless as it is, although Mark, with commendable skill, took off components, and used the tracking, modified, to do lots of circuit exploration work. While you can, get a Pocket Geiger. I would not be tempted to even bother to power it unless you want to play Geiger ticking. Throw the little plastic case and other junk that comes with it. Carefully remove the copper foil tape covering, and clean up with some IPA.

See Mark's post #318 on page 32.
Also my post #428 on page 43.
Here you get pictures about getting the things apart.

You need some smoke detector elements. These should be available from eBay.
They come in all sorts of little tin boxes, but what goes on inside is basically the same. It's little space where the the stream of alpha particles, which are atoms of helium with the two electrons stripped off, get to find and charge smoke particles, and every one that makes it to the tin box causes a little current that trips the alarm. We ignore the alpha particles, but maybe we should not.

Do not be afraid to take these apart. Just don't lose the (tiny) source, and don't let it get mixed up with your lunch. Don't eat the food until find it after you dropped it!

See my post #427, on page 43
The real detail stripdown is #431 on page 44.
I found it easy enough to get the little mounted pellet away from it's peened over metal spots. I used a very small file to take out one, and then I just bent the connection metal carrier with pliers to make it let go. I did have a paper towel over to guard against it being "launched".

The final circuit is not so defined yet that you need get in IC hardware, but if you choose Raspberry Pi, the supply of these is still slow. I am definitely going with the Pi, because I have some, but the final design would use a tiny Pi Zero-2W. If you use Mark's ADC design, then you get a Teensy.

Before getting too far into electronics, there is the physical design. @homebrewed is currently finding out what it takes to shield it effectively. I have my own ideas on that, and I will incorporate whatever Mark discovers. I almost have to start over with the FreeCAD to incorporate the lead in the right places, but as soon as I have that as a model, you can fabricate or 3D-print at will.

 

[graham-xrf]

@homebrewed

Re: Unwanted noise counts
Hi Mark.
I just had an off-the-wall thought about the Am421 sources and their potential to create a big interference racket.
We "aim" the sources directly onto the material, stopping photons going in unwanted directions with 2mm or so lead.
BUT..
By far the biggest pile of stuff streaming out of them is alpha particles. These would normally have been charging smoke particles. Each one of many (millions?) is a 2+ conductive ionized gas, filling up the entire box, and discharging onto anything it encounters. Here is where I am thinking the inputs of a spectacular gain amplifier can do without these being around.

Should we consider "collecting" them with a negative biased something, or maybe simpler, shorting them out with a little aluminium foil or carbon plug over the sources?

OK - it may be a mad notion, but I was thinking it might help lose that high count group of buckets.

 

[rwm]

So you are thinking the alpha particles may create charged particles in air and then float into the electronics? I suppose in theory that is possible but I think the interaction between alpha and pure air is negligible over these short distances. I doubt there is any significant charged particle production. The alphas would just hit the sample along the same path as the gammas and this would not result in much.
Now it might be interesting to blow smoke into the gap and see what happens then!

Edit: I retract the above. See this curve:

5.5 MeV alpha is close to a typical decay alpha. Alpha particles may collide with a nucleus but may also steal electrons from air in their path becoming neutral He. However, that would leave a wake of + charged particles. So really, I don't know if this could impact the system! Seems possible. Would a small fan ventilating the electrical space be enough to physically displace the ionized gas? I wonder how much ionized air a smoke detector sees at equilibrium?

 

[graham-xrf]

So you are thinking the alpha particles may create charged particles in air and then float into the electronics? I suppose in theory that is possible but I think the interaction between alpha and pure air is negligible over these short distances. I doubt there is any significant charged particle production. The alphas would just hit the sample along the same path as the gammas and this would not result in much.
Now it might be interesting to blow smoke into the gap and see what happens then!

I know that alpha particles won't get through a piece of paper, and in air, they penetrate only a few centimetres.
A "few centimetres" is all we need!

They don't go in straight lines either. They don't "float". They ping about at mean free path speed like any other gas. The connections to the photodiode and the input to the electronics is only millimetres away. Helium "normal" gas can get about really fast. For doing vacuum chamber leak testing, I released hardly more than a party balloon's worth of Helium while the roll-up door was fully open. It contaminated the building and entire car park outside in less than a couple of seconds, such that I could not test again for about 20 minutes.

We can stop them easily. I am just saying they might be discharging all over the place, within their few centimetres, and rattling our very high gain amplifier. I don't really know if any of this is actually happening, or even possible. I do know I had the anticipatory speculative thought, and nothing yet shoots it down. Maybe covering the electronics with copper foil is all one needs to do.

Right now I am into mains hum filters, and trying for one that does not introduce an offset.
Also, selectable gain range opamp stages.

 

[rwm]

Sorry, I was typing my edit at the same time! You have a good thought. See above.

 

[graham-xrf]

Sorry, I was typing my edit at the same time! You have a good thought. See above.

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.

 

[WobblyHand]

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.

 

[homebrewed]

My interest has been piqued enough to take the plunge. What specifically do I need to start with? Is it the Sparkfun Pocket Geiger Radiation Sensor Type 5? This is to access the large PIN diode? Is any of the rest worthwhile, or is this just a way to get the detector? Or is it better just to buy the diode and make a TIA?

Speaking of analog hackery anyone know where to get little glass standoffs like old analog guys used to use 30+ years ago? You would drill a hole in the PCB and solder them in and they had a terminal to solder a wire. They had low capacitance. I have a circuit with a few of these. Don't know what the proper name is for these terminals. Are they made any more? Used to do galvanometer like work with them, low leakage and capacitance.

Last time I checked, Mouser offers the detector, for less than the Pocket Geiger sold by Sparkfun (however, they currently are OOS, expecting more in February). So if you're up for making a TIA and have a little patience that would be a viable approach.

Digikey has two large-area PIN diodes made by Advanced Photonix. P/N 445-14-21-305 costs $90.76 and PDB-C110 runs $71.77. However, while they are PIN diodes I note that their capacitance is significantly higher than the X100-7, even when reverse biased. This suggests to me that their intrinsic region is narrower, so their x-ray sensitivity may not be as high. Perhaps not an issue because we're dealing with relatively low-energy xrays, but it needs to be said because they may turn out to NOT be suitable.

 

[homebrewed]

@homebrewed

Re: Unwanted noise counts
Hi Mark.
I just had an off-the-wall thought about the Am421 sources and their potential to create a big interference racket.
We "aim" the sources directly onto the material, stopping photons going in unwanted directions with 2mm or so lead.
BUT..
By far the biggest pile of stuff streaming out of them is alpha particles. These would normally have been charging smoke particles. Each one of many (millions?) is a 2+ conductive ionized gas, filling up the entire box, and discharging onto anything it encounters. Here is where I am thinking the inputs of a spectacular gain amplifier can do without these being around.

Should we consider "collecting" them with a negative biased something, or maybe simpler, shorting them out with a little aluminium foil or carbon plug over the sources?

OK - it may be a mad notion, but I was thinking it might help lose that high count group of buckets.

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.....

 

[WobblyHand]

Pocket Geiger is listing for $90 at Sparkfun right now. Based on what you've told me, it still seems ok to get and cannibalize, at least I get a known good diode (I hope).

Now if we are still counting angels on a pin, I seem to recall a NIST paper on battery noise. If I recall correctly, small cells are noisier than larger ones. I think I still have a copy of the paper. Have to go go look for it in my archives.

Edit: No longer in my archives, but found it online. Different chemistries have better or worse voltage noise. But nearly all batteries are better than any power supplies.