Nuclear weapons* are triggered by neutron initiators, devices that produce a sudden burst of neutrons on activation. They are most often constructed from a mixture of beryllium-9 and polonium-210. The polonium emits high-energy alpha particles, and when brought into contact with the beryllium it causes the beryllium to transmute into carbon with the release of a neutron. This neutrons causes an atom of uranium-235 to split (to fission) and in the process release a huge amount of energy and more neutrons that go on to cause further fissions.
This uncontrolled chain reaction results in the production of many exotic isotopes, as the uranium atoms split to form “chunks” of other elements. For example, it was in the aftermath of the ‘Ivy Mike’ test of the first thermonuclear bomb that the elements einsteinium and fermium were discovered.
The existence of rare isotopes can be used to demonstrate that a painting or other work of art was not produced before the 1940s or 1950s, when nuclear weapons testing was at its peak. Strontium-90 and caesium-137 are isotopes that did not exist in nature before the age of nuclear weapons and which permeate soils and are taken up by plants and other living things as they are very soluble in water. If these organic materials are used in the production of paints, or binders for paint, or in other ways in a piece of art then the presence of Sr-90 or Cs-137 can be used to prove that the item in question was created after the beginning of the nuclear age.
* This paragraph details the operation of a fission bomb. Fusion (thermonuclear) bombs work differently, but all use a fission stage to initiate the fusion process.