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The original simple lead (Pb)? lid worked great as a shield but was giving stray Pb XRF peaks to the detector.
To fine tune it, different combinations of aluminum topped with lead were tried and tested.

It's going to take a thick aluminum lining and a thinner lead top to get rid of the strays.. The picture shows the arrangement of materials that worked.

I discovered that there is leakage along the edges of the thick aluminum when used as a shield, probably from internal scattering. Therefore the final lid will have the aluminum layer a smaller diameter than the Pb top, so it can be completely surrounded by the Pb chamber wall. Perhaps I'll simply attach an thick aluminum disc to the bottom side of the existing lid.



A 10 minute scan to record the background radiation picked up by the Si-PIN when inserted into the chamber, but no exciter present. Only two hits were recorded,? probably random Cosmic Ray Muons





Geo>K0FF

Aluminum scrap from the shop, for some unknown past project:



Top view



The thickness is fine, works as expected to eliminate unwanted XRF, but the diameter is way to large.
Initial testing shows scattering from exposed edges, so new final version of the lid will have all the internal aluminum XRF passivation layer shielded on the outside by 1/4" of Pb. The aluminum interior is to provide clean spectrum scans, reducing or eliminating Pb XRF from the personnel safety shielding.

Lathe rough cut to dimensions for fit testing:



As expected, the plug section is 1mm oversized, and the retainer lip on top is way to thick. More measurements were taken, then back to the shop for the final machining cuts.

More test fitting, then back to lathe for touch-up and polishing.
?The finished piece:



and the fit test:


Now all that's left to do is craft the final layer of the top from 1/4" Pb and a handle.

Geo