Needing more than a spark test?

[WobblyHand]

Is 2.38mm lead sheet sufficient shielding? (3/32" = 0.094") The 99.9% lead sheet is $33.25 for 1 square foot/ $50.60 for 1ft x 2ft. I would think one could press this into shape, perhaps with an arbor press to persuade it? rotometals.com seems to be the cheapest source in the US, at least that I have found. https://www.rotometals.com/lead-sheet/sheet-lead-3-32-6-lbs-sq-ft/

 

[graham-xrf]

Is 2.38mm lead sheet sufficient shielding? (3/32" = 0.094") The 99.9% lead sheet is $33.25 for 1 square foot/ $50.60 for 1ft x 2ft. I would think one could press this into shape, perhaps with an arbor press to persuade it? rotometals.com seems to be the cheapest source in the US, at least that I have found. https://www.rotometals.com/lead-sheet/sheet-lead-3-32-6-lbs-sq-ft/

It's more than thick enough. The calculations have been done several times in (much) earlier posts. 1/16" inch is near 1.6mm. From memory, that blocks 99% of the photons. Going thicker, I think it was 1.8mm puts another 9 on the percentage.Go to 2mm, and It gets to more than 99.99%
3/32" is far more than is needed.

I guess those photons that hit the lead expend themselves whacking a big old lead atom, and end up dissipating as a miniscule amount of heat.

 

[WobblyHand]

It's more than thick enough. The calculations have been done several times in (much) earlier posts. 1/16" inch is near 1.6mm. From memory, that blocks 99% of the photons. Going thicker, I think it was 1.8mm puts another 9 on the percentage.Go to 2mm, and It gets to more than 99.99%
3/32" is far more than is needed.

I guess those photons that hit the lead expend themselves whacking a big old lead atom, and end up dissipating as a miniscule amount of heat.

Yes, I just figured it out. Found the earlier post and was confused. Went to the original references and worked it out.

t[mm] = 10 * ln(I/I0) /( -5.021 * 11.35). For t = 1.616mm, I/I0 = 0.0001. Or 1.6mm of lead attenuates the incident 60keV X-rays by 0.0001.
2.424mm of lead attenuates 60 keV by 1e-6.

1.6mm Pb blocks 99.99%
2.4mm Pb blocks 99.9999%

 

[graham-xrf]

I print on a thin coated steel sheet. Post print, I just remove the sheet and flex it, and the object pops off. Not all that difficult, if you let the sheet cool to ambient.

@graham-xrf, here's a rough model of the source ring, roughly to one of your early drawings. Had a bit of trouble making pockets that were tangent to the surface, but more or less solved that using a datum plane. Not sure I got it right, but that's ok. I can widen and deepen the pockets to accommodate the lead buckets that the sources sit in. Might simply make a lead ring around the center.
View attachment 434505
You can see the big pockets. They are 10.4mm wide and 4mm deep. These pockets can be stamped or cast. They are basically cups. The green can be printed. Don't know how I'd do the ring yet. Cast it? I don't have the ability to extrude it. Seems wasteful to drill it. Maybe wrap foil and clamp it?

Great stuff! It would work too.

The difference in my design is I did not have the lead tube pressed over a plastic ring. Instead, I envisioned the lead fitted down the inside of the centre ring instead, replacing the central tube you have in there. It is snugged up around the PIN diode body, with a little clearance to the diagonal of the diode. You could, if you liked, have the lead on the inside, and the plastic on the outside. I had it the other way to get the geometry as close as possible to the sample, while still having zero paths to anything except lead, and the sample under, and no way for anything from outside to get in. Also, I maintained the electrical screen all the way to the sample plane.

The diode body is 16.5mm x 14.5mm with diagonal 21.96. Allowing 0.5mm clearance brings it close to the 23mm diameter I ended up with, and making it 2mm thick gets the ring OD to 27mm.

My sources metal discs are 1/4" diameter. That 6.35mm, if given 0.5mm (about 0.020") all around so they drop in easy, and allows some tolerance in getting a lead shield in there, is what decides the inner radius of the source shield pots. 6.35 + 0.5 +0.5 = 7.35, which has the radius 3.675mm

To allow for the 2mm thick walls all around, the OD would become 11.35mm, which is where the 5.675 radius came from.

The 21.175mm radius is the radius of the circle through the centre of all the octagon ring of sources, such as to allow 2mm of support material around the little lead buckets, though this would be for a "flat" arrangement. My concept is actually the tilted version, just like your (excellent) printed piece.

I was just suggesting the central ring be the lead itself, instead of having a lead ring pushed over it. Your arrangement probably provides a better way to support and secure the lead ring.

Do not miss the outer lead shield around the whole thing, which would have to be pushed inside the outer body tube. Add that lead tube around your plastic part, tall enough to reach to the surface of the stuff being tested. Then, you need the final outer body tube around that.

Do not take SmokeZilla too seriously. It was just a quick 'n dirty little geometry to estimate the overall size of things, and, inspired by the Pratt & Whitney R2000 zig-zag cylinder mountings, I threw in another row. I am now getting into making something less aggressive.

 

[WobblyHand]

Great stuff! It would work too.

The difference in my design is I did not have the lead tube pressed over a plastic ring. Instead, I envisioned the lead fitted down the inside of the centre ring instead, replacing the central tube you have in there. It is snugged up around the PIN diode body, with a little clearance to the diagonal of the diode. You could, if you liked, have the lead on the inside, and the plastic on the outside. I had it the other way to get the geometry as close as possible to the sample, while still having zero paths to anything except lead, and the sample under, and no way for anything from outside to get in. Also, I maintained the electrical screen all the way to the sample plane.

The diode body is 16.5mm x 14.5mm with diagonal 21.96. Allowing 0.5mm clearance brings it close to the 23mm diameter I ended up with, and making it 2mm thick gets the ring OD to 27mm.

My sources metal discs are 1/4" diameter. That 6.35mm, if given 0.5mm (about 0.020") all around so they drop in easy, and allows some tolerance in getting a lead shield in there, is what decides the inner radius of the source shield pots. 6.35 + 0.5 +0.5 = 7.35, which has the radius 3.675mm

To allow for the 2mm thick walls all around, the OD would become 11.35mm, which is where the 5.675 radius came from.

The 21.175mm radius is the radius of the circle through the centre of all the octagon ring of sources, such as to allow 2mm of support material around the little lead buckets, though this would be for a "flat" arrangement. My concept is actually the tilted version, just like your (excellent) printed piece.

I was just suggesting the central ring be the lead itself, instead of having a lead ring pushed over it. Your arrangement probably provides a better way to support and secure the lead ring.

Do not miss the outer lead shield around the whole thing, which would have to be pushed inside the outer body tube. Add another lead tube around your plastic part, tall enough to reach to the surface of the stuff being tested. Then, you need the outer body around that.

Do not take SmokeZilla too seriously. It was just a quick 'n dirty little geometry to estimate the overall size of things, and, inspired by the Pratt & Whitney R2000 zig-zag cylinder mountings, I threw in another row. I am now getting into making something less aggressive.

I was looking at the printed unit and was thinking how I could make a perhaps multi-step method of forming the lead sheet over this single use die. I could envision a surface (without the pockets) that forms the lead to the right profile. Then a separate unit with a steel punch that pushes the lead into the pockets forming the cups. So the whole surface is lead, up to the inner diameter. I know nothing about dies and forming lead, but with 1/16" lead, this seems possible. I will attempt to make a mating surface and see if I can first do the first forming, and then make a second form that allows a punch to be inserted. The pockets are wide enough so that after the lead is formed, there is enough room for the sources. If I am right, this would be equivalent to coating the whole surface with lead.

I haven't fully thought through all this, especially shielding everything else. At the moment, I'm confining myself to making or forming the source unit, and perhaps a shielded place to store them. Although these sources are not particularly dangerous, they do increase your exposure. I might make a small lead lined box, at least in the interim. I probably have a month to do that. I could print something and then line it with lead, which means I need to get some...

My Sparkfun order was to arrive today. Of course, it didn't arrive. It's now delayed until Wednesday, so they say...

 

[graham-xrf]

@WobblyHand
Hi Bruce. Stamping forming seems like a lot of trouble to go to when you can heat up some lead in an ex-tuna tin, and pour it into a hole in a chunk of aluminium. Or just get a chunk of lead. Turning the round bits looked easy on the YouTube video.

Drilling the tilted holes is OK if one has a tilt-able mill head, like mine, or a rotary table mounted tilted, but to be honest, one could do a reasonable job with a DIY hand drill. The last trick is drilling out the middle. Possibly boring out the final hole. I guess some might make the hole first, then put a mandrel rod up it, and do all the turning with it completely supported both ends. However it goes, I am thinking an all-lead front with holes drilled to set the sources down into is the simplest way.

The attractions of 3D printing is not lost on me. The trouble is, the plastic only has the properties of plastic, and while it provides a dandy way to get the shape, it then has to be fitted up all the extra bits of shielding as tubes and little mugs to make it effective. One can turn stuff out of lead. It may be soft, and need some care in the approach, but it gets you what you want.

Take apart the SparkFun kit carefully. Then look back at all that Mark @homebrewed discovered about the power supplies, and the switcher bias noise maker. Mine is still in one piece, but so far, other than carefully discovering all about it, all I have thought to do was cut off the end with the PIN diode on it, to save me having to unsolder it. Then to fix it at right angles at the end of my own amplifier PCB using two fillets of JB-Weld. Hot melt glue might do at a pinch.

MrPete shows us how..

He said he used a "harder" lead, whatever that is.

Also, someone who wanted a workshop pencil made out of lead! (Good Lord why?? !)

This one definitely had soft lead that he melted and cast.

 

[WobblyHand]

Turning the round bits looked easy on the YouTube video.

It always seems easy on YouTube. They rarely show what happens if you aren't doing it perfectly.

My forming idea is to use lead sheet and plastic forming tools. It may not appeal to you. That's ok, I'm still going to try it, or something like it. Casting lead has it's own issues. I've done it in the past.

 

[homebrewed]

Today I sort of did a "reset" in terms of evaluating my setup. I've been seeing some unexpected spectrum shapes and had the thought that I'm not really using enough of the Teensy's ADC range so the pulses have a lot of digitization noise. My signal conditioning board provided 1X, 10X and 100X gain ranges, and I've been using the 10X range -- but the pulses of interest have been in the 500mV range, or a bit less. That's not using much of the ADC's dynamic range. Going to the 100X gain setting would overdrive the ADC, so I modified the board so the max gain is 60X. Doesn't sound like a significant reduction but it looks like it will be about right in terms of getting as much as I can out of the ADC. The 16 bit ADC board should be arriving soon so it will be interesting to see how that works out.

I also revisited the power supply voltages going to the signal conditioning board and discovered that I was exceeding the LMC662's maximum-specified power supply range, so I changed the -10V input to -5V. That didn't affect the circuit offset much. I'm evaluating the overall system noise level now to see if it's improved or not..

 

[graham-xrf]

Today I sort of did a "reset" in terms of evaluating my setup. I've been seeing some unexpected spectrum shapes and had the thought that I'm not really using enough of the Teensy's ADC range so the pulses have a lot of digitization noise. My signal conditioning board provided 1X, 10X and 100X gain ranges, and I've been using the 10X range -- but the pulses of interest have been in the 500mV range, or a bit less. That's not using much of the ADC's dynamic range. Going to the 100X gain setting would overdrive the ADC, so I modified the board so the max gain is 60X. Doesn't sound like a significant reduction but it looks like it will be about right in terms of getting as much as I can out of the ADC. The 16 bit ADC board should be arriving soon so it will be interesting to see how that works out.

I also revisited the power supply voltages going to the signal conditioning board and discovered that I was exceeding the LMC662's maximum-specified power supply range, so I changed the -10V input to -5V. That didn't affect the circuit offset much. I'm evaluating the overall system noise level now to see if it's improved or not..

Oh dear!
If the little chip was taken to it's max and beyond, would you maybe expect it took some damage and should be replaced.

I am confused about how much of the noise you see is quantization noise, and why you have only 500mV signal. That said, as much as 500mV would be a fully expected good return that you would want to measure amplitude to high resolution. That would be about 16,000 in a 16-bit counter, but I don't know how many effective bits you have right now. I think the 16-bit ADC board would be much more than "interesting". I think perhaps you under-state things. Probably, you can hardly wait for the package to arrive!

The signal conditioner provides a x1.001 gain buffer, and then a (1+9) = 10x in U1. Then another (1+9) = 10x in U2. The other U2 output is not used. Thus, the board has gain about x100 already. Going back into the Pocket Geiger, before the TIA, there is another x100.
So far, it's 80dBV, before we take in the gain of the TIA stage.
Taken at face value, as published, the 66MΩ, for a current input amplifier has gain 66 million. Total gain now 6.6e+11
A full sized maxed out 2V at the ADC would have been made from a current of 3pA !

At this point, I run out of ways to think about it, because that little current is not what I would equate to a "maximum".

 

[RPMWorks]

I know this is an old post, but, I think my answer may help others going forward. Graham, your answer was in your question. You said, “only cost-effective if they are identifying exotic stuff all all day long.” so, bring it to a place that checks “all day long”. We always relished any opportunity to help the “general public” who were brave enough to come in and ask for help. It was just good public relations. Any decent aerospace defense company will have one to verify every stick of material they receive. I’d imagine material handling companies and mills also have one.
even if they did choose to charge you for it, it wouldn’t cost much as it takes literally about 5 seconds of their time.