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3d printed shield?

Posted: 18 Oct 2016, 13:57
by Stanford
Hi all...

Any thoughts on using a 3d printed shield to house copper coated lead shot? is it reasonable for the lower section to have a smaller shield thickness? and, what's a reasonable sample chamber thickness? is 25mm sufficient?

Thank you,
Stanford
rad shield 003.jpg
rad shield 003.jpg (83.7 KiB) Viewed 4253 times

Re: 3d printed shield?

Posted: 18 Oct 2016, 16:13
by Steven Sesselmann
Stanford,

Novel idea, but copper plated lead shot isn't going to do it. You need to look up half value layer for 80 kV x-rays, it will be thicker than the plating on lead pellets.

You can do a simple experiment using a piece of lead in front of your detector on a 45˚ angle with a shielded source emitting gamma above 80 Kev out to the side, the source will induce 80 keV x-rays in the lead so you can try inserting copper plates between the detector and the lead, and see how the thickness affects the counts.

For this experiment you could also use an Am source directly facing the detector which emits 59 keV gamma but the half value layer will be somewhat thinner.

http://www.sprawls.org/ppmi2/RADPEN/

Steven

Re: 3d printed shield?

Posted: 19 Oct 2016, 13:18
by Stanford
Thank you, Steven.

Seems like I'm going about this backwards. If my goal is to be able to detect trace elements in food and water then I imagine I need to work out a way to dehydrate and concentrate samples, a method of determining optimal sample container surrounding the detector (which I imagine is mirrored by marinelli beaker dimensions?), and determining (as wraith_oz has shown in his or her spreadsheet) the thickness and type of shielding to support the level of sensitivity I'm after. There isn't exactly a book available that lays it all out is there?

Learning :-)
Thanks all,
Stanford

Re: 3d printed shield?

Posted: 06 Mar 2017, 06:37
by Stanford
Thank you Steven,

When you say
You can do a simple experiment using a piece of lead in front of your detector on a 45˚ angle with a shielded source emitting gamma above 80 Kev out to the side, the source will induce 80 keV x-rays in the lead so you can try inserting copper plates between the detector and the lead, and see how the thickness affects the counts.
does that imply that the copper need only shield the detector?

Re: 3d printed shield?

Posted: 06 Mar 2017, 15:57
by Steven Sesselmann
Stanford,

The x-rays generated by your lead shield show up as a peak at around 80 keV, if you are looking for Cs137 at 660 keV then you need not even worry about the Pb x-ray, you know what it is so you just ignore it. On the other hand if you are looking for peaks around 70-90 keV the Pb x-ray can be annoying, and you need to screen it out.

Generally materials of high Z are the best shields but they also generate higher energy secondary x-rays, so to build an effective shield one has to create layers of increasing density material.

The first layer is generally copper, it emit's secondary x-rays at around 5 keV which you don't worry about because your NaI(Tl) detector can't see it anyway. For the second layer it is common to use pewter it emits secondary x-rays at around 25 keV, and finally the lead.

So the general idea is when your radioactive isotope is inside the shield, it will induce secondary x-rays in all three materials, but the Pb x-rays are attenuated by the Sn and the Sn x-rays are attenuated by the copper, and the Cu x-rays are attenuated by the Al casing on your detector.

With a well constructed shield you can measure levels of radiation to a standard well below the recommended maximum level as set by the WHO. (See Karl Brehwens recent post)

Re: 3d printed shield?

Posted: 02 May 2017, 02:54
by Stanford
Steven thank you

Will small quantities of lead-free solder (Sn-Ag-Cu) be a problem in assembling the inner shield?

Re: 3d printed shield?

Posted: 04 May 2017, 12:48
by Steven Sesselmann
Stanford,

I can't imagine backscatter from a few tacks of lead free solder is going to show up in your spectrum.

Best of luck with the build.

Steven