Making it out of lead! How?

[rwm]

You could keep the Pb part simple. Print a source holder out of plastic per your design. Then use a 2 mm think Pb sheet disc on the back side and a short lead tube to line the center hole. You could make that all out of flat Pb sheet. No machining. The plastic would hold the sources in the correct geometry and you would minimize the Pb to where it is needed.

 

[graham-xrf]

Well, it weighs a kg if no hole in it. Honestly, if that what it takes, that's ok. A kg of lead isn't that expensive. 5 lbs of 99.5% pure lead goes for $13.29 at Rotometals. It will be a lot more expensive if you want chemically pure lead.

In this trick, one has to ask what might be the impurities?
I don't expect there would be any problem with ordinary lead sheet, but..

Common impurities would be Antimony, Arsenic, and Tin.
Tin has X-ray four X-ray responses, including two big ones at 25.27keV and 28.48keV which can be seen by the detector
Arsenic has responses at 10.54keV and 11.72keV, where the detector probability of seeing it is near 100%
Antimony is a big atom No 51. It has four responses, 26.36keV, 29.72keV, 3.60keV, and 3.84keV, all of which can be seen.

You can tell that trying to do this sort of stuff is packed with traps for the unwary. It may be we get away with humdrum stuff that happens to be at hand, but only if we think about every kind of atoms that are in there.

 

[rwm]

Even though the lead has impurities, the original gamma will not get to the far wall of the lead and therefor the characteristic xrays of the lead impurities will not reach the detector. OK, some small fraction will but it will be a very low number of events.

 

[WobblyHand]

In this trick, one has to ask what might be the impurities?
I don't expect there would be any problem with ordinary lead sheet, but..

Common impurities would be Antimony, Arsenic, and Tin.
Tin has X-ray four X-ray responses, including two big ones at 25.27keV and 28.48keV which can be seen by the detector
Arsenic has responses at 10.54keV and 11.72keV, where the detector probability of seeing it is near 100%
Antimony is a big atom No 51. It has four responses, 26.36keV, 29.72keV, 3.60keV, and 3.84keV, all of which can be seen.

You can tell that trying to do this sort of stuff is packed with traps for the unwary. It may be we get away with humdrum stuff that happens to be at hand, but only if we think about every kind of atoms that are in there.

Well that's really the issue, isn't it? You want really really pure, then it won't cost $5.84 / kg. For what it is worth, the Rotometals page I checked showed the following certification. This is for 99.92% lead, I can't imagine common lead sheet to be purer than that. And yes, there are measurable impurities that are in your list. Does your lead sheet come with an assay?

Certification of Compliance / MTR
PO# PO21-02309

We certify that the Lead Nuggets Pure 99.9% min supplied to you were made to the following specs
Nickel 0.001 %
Silver 0.005
Zinc < 0.001
Copper 0.006
Bismuth 0.015
Iron < 0.002
Antimony 0.015
Tin < 0.001
Arsenic < 0 .001
Cadmium < 0.001
Aluminum < 0.001
Lead 99.92

Based on the above results, it is certified that this material meets specifications of QQ-L-171 Grade A

Signed: Rocky Gibson
Date: 10/22/2021
RotoMetals.com

 

[graham-xrf]

You could keep the Pb part simple. Print a source holder out of plastic per your design. Then use a 2 mm think Pb sheet disc on the back side and a short lead tube to line the center hole. You could make that all out of flat Pb sheet. No machining. The plastic would hold the sources in the correct geometry and you would minimize the Pb to where it is needed.

Yes - I was very attracted to this concept. Plastic is OK, because carbon X-ray 277eV response is feeble, almost nonexistent, and below what the diode can see. It won't contribute to the noise. The 60,000eV gamma rays will pass straight through it, except where we selectively stop it with lead sheet, fitted into slots in the plastic.

I think we know enough now to try and avoid difficult stuff in the making of it. Tuned out of plastic, or even aluminum, with lead inserts, bits of sheet, pads, whatever, all in the right place, makes it something most of us can make in our shop space.

 

[graham-xrf]

Even though the lead has impurities, the original gamma will not get to the far wall of the lead and therefor the characteristic xrays of the lead impurities will not reach the detector. OK, some small fraction will but it will be a very low number of events.

You hit lead with gamma, and what sprays out comes out all over the place, including out the other side. That said, lead is so good at stopping it that pretty well zero will get to the detector going sideways, simply because of the geometry.

The whole design is about creating shadows, and exploiting the fact the gamma is like light. It cannot turn corners. You think of the stuff in the structure as if everything was somewhat transparent, with various color light coming off the surfaces.

 

[graham-xrf]

Well that's really the issue, isn't it? You want really really pure, then it won't cost $5.84 / kg. For what it is worth, the Rotometals page I checked showed the following certification. This is for 99.92% lead, I can't imagine common lead sheet to be purer than that. And yes, there are measurable impurities that are in your list. Does your lead sheet come with an assay?

Certification of Compliance / MTR
PO# PO21-02309

We certify that the Lead Nuggets Pure 99.9% min supplied to you were made to the following specs
Nickel 0.001 %
Silver 0.005
Zinc < 0.001
Copper 0.006
Bismuth 0.015
Iron < 0.002
Antimony 0.015
Tin < 0.001
Arsenic < 0 .001
Cadmium < 0.001
Aluminum < 0.001
Lead 99.92

Based on the above results, it is certified that this material meets specifications of QQ-L-171 Grade A

Thanks so much for the handy breakdown. I think that even common roofing flashing lead from the hardware store will be OK. X-rays provoked by the gamma still has to make it out of the lead anyway, and as you know, lead stops X-rays. Basically, it is the gamma sideways route through radio-transparent stuff like plastic and aluminum, direct into the detector that we have to stop. Pretty much everything else in the gamma path is either too feeble to glow X-rays, or it's the stuff we want to see.

Related to this, I am thinking that Delrin plastic is free machining and reasonably available. POM (Polyoxymerthylene) or acetal plastic as it is called, is made up of carbon, hydrogen, and oxygen only, none of which can make X-rays we can see.

 

[rwm]

"You hit lead with gamma, and what sprays out comes out all over the place, including out the other side" This is not correct. It would be attenuated due to the thickness of the lead and the lower energy of the secondary xrays.

 

[graham-xrf]

"You hit lead with gamma, and what sprays out comes out all over the place, including out the other side" This is not correct. It would be attenuated due to the thickness of the lead and the lower energy of the secondary xrays.

You have to get it 2mm or so thick. 1mm is not enough. This is something we calculated way back in the thread.
Lead has L-shell responses 10.55keV and 12.61keV, and it has got me thinking..

 

[graham-xrf]

Copper has an atomic no. of 29. Carbon is 6.

Duh.. I was looking at C for carbon at the time.
Copper delivers 8,08keV and 8.9keV right where the detector can see 100%
BUT
The trick may still work because the gamma can't get through the lead to the copper jacket, and even if it could, the end of the copper is at a place where it can't glow into the detector. This is why I like the back-scatter arrangement.