Lead, because of its large Z number
, high density and low cost, is a popular shielding material in radiation labs
. It is used to protect the lab and its environment from highly radioactive objects and processes, but it is also used to protect sensitive detectors and counting chambers
from background radiation
. For the latter purpose it is very important that the lead itself not be sigificantly radioactive
-- that is, it contribute a low(er than usual) amount of background radiation
This sounds trivial, but in fact, it isn't. Many radioactive heavy metals are chemically very similar to lead (or identical, such as lead-210) and tend to easily contaminate it. On July 16, 1945, the United States detonated the world's first atomic bomb (the Trinity test). This relased large amounts of radioactive heavy metals into the environment. All lead mined or processed since contains substantial amounts1 of these radioactive contaminants and is therefore completely unsuited to use in shielding sensitive detectors2. (This is also the reason why carbon-14 dating is useless for objects dating from the 20th century).3
Thus, there is a significant industry in obtaining lead produced before 1945 and selling it to radiation labs. This is known as low background lead and is significantly more expensive than ordinary lead (because, obviously, no more is being made!). It is usually sold in bricks, but is also available in sheets.
One sigificant (and fascinating) source of low-background lead is shipwrecks. Many cargo ships used large quantities of lead as ballast when travelling unloaded or lightly loaded. There are companies and individuals who will dive to sunken ships and harvest the lead ballast and sell it as shielding material.
1 Please note that we are talking about 'substantial' in terms of the noise limits of the detectors in question. Some of these are noticably affected by as little as 10 Becquerel of contamination -- and 10 Bq of lead-210 is about 1.2 picograms, barely detectable by most methods.
2 RPGeek states that the real contributor to lead having a low background level, and thus being valuable, is lead-210 depletion after chemical separation and that nuclear fallout makes an insignificant contribution. The above explanation is what I was tought in my radiochemistry course, but may be erroneous or incomplete.
3 Here I am referring to an inability to reliably carbon-14 date 20th-century items in the far future. Carbon-14 is always useless for recent items because the margin of error is at least +/-50 years at best. RPGeek disagrees and informs me that bomb-contributed carbon-14 is accurately known and subtracted out; this is not my understanding but carbon dating is not my field.