20 November 2010

Old lead

For modern, state-of-the-art science you need old church roofs. I didn't know that! We have a radionuclide lab here at university, and it can be used for things such as 137Caesium and 210Lead dating. Lead has lots and lots of isotopes, and most of them are unstable. The decay of 210Pb, which has a half-life of 22 years, is what we measure in the lab for dating purposes. If you try to measure the very weak signal you of course don't want background radiation to get in the way. And how do you shield something from radiation? With a big slab of lead. Lead? Didn't that have many isotopes, most of which unstable...

The gamma counter. Notice the thick slab of lead on top!

If you simply use lead to shield yourself from radiation that you use for something such as medical treatment you don't care if that lead sometimes emits a gamma "particle". But if that is exactly what you want to measure in your samples you do care! So how do they solve that? They use lead that's so old (many times the half-life of 22 years) it doesn't emit anything anymore. And that's where the church roofs come in! And there seems to be a company that does nothing other than sniff out old lead and sell it to the manufacturers of gamma counters and the likes! And church roofs seem to be their main source, but it seems one day the wreck of an old Spanish gallion that had been transporting, among other things, lead ingots was found... so they ended up in the laboratoria around the world! I thought that was a brilliant story.

Just a random find of a shipwreck with lead ingots. Source: http://www.culture.gouv.fr/fr/archeosm/archeosom/en/plouma-s.htm. Picture: Photos : Yves Gladu, Michel L'Hour and Gérard Réveillac.


Anonymous said...

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Anonymous said...

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