There are much more radiation from Thorium and Uranium, sometimes K40, than from Co60 in metals, such as stainless steel, nickel and titanium. Copper purified b

There are much more radiation from Thorium and Uranium, sometimes K40, than from Co60 in metals, such as stainless steel, nickel and titanium. Copper purified by electroforming is surprisingly clean. It is usually used inside the lead shielding to reduce background further.