The atomic bomb created isotopes that help identify fake paintings. How?

Nuclear weapons are not generally thought of having any purpose other than for annihilating ones enemy. When exploded, they tend to create huge amounts of destruction, also leaving the area polluted and totally unfit for human occupation for years.

Thankfully, only two nuclear bombs have been detonated as acts of war, both during WWII. However, quite a number of devices have been detonated all over the world as part of various nations testing plans, and this has had some interesting side effects.

For instance, until 1945, it was very difficult to detect a forged painting through scientific means. Most forgeries were detected by experts examining the painting and using their knowledge and experience to determine the authenticity.

Now, however, one can simply test for the presence of either Caesium-137 or Strontium-90 in the paint. Neither of these two isotopes occurred naturally before the first atomic bomb tests, so any painting claiming to be from a period before 1945, but containing these isotopes, will be a fake.

(Source)