Who created the dirty bomb?

Answer 1

The original concept for something like what is now called a "dirty bomb" was proposed at Los Alamos during the Manhattan Project before it was certain that an actual atomic bomb could be made to work. When it was clear the atomic bombs would work, the idea was shelved.

As far as I know, nobody has ever actually bothered to build one. Doing so would likely be suicidal for the workers doing the assembly.

Note: another unrelated usage of the term "dirty bomb" simply refers to an ordinary hydrogen bomb. This usage came into fashion about 1958 when the US was fascinated with the idea of building "clean bombs" (i.e. reduced fallout hydrogen bombs), mostly for Project Plowshare - the project to use nuclear explosives for construction, mining, and other peaceful uses - but also to try to make high yield nuclear weapons that would produce less public outcry/opposition/protests. The US perfected "clean bomb" designs having as little as 5% of the fallout of ordinary nuclear explosives of the same yield and the USSR did even better than that. "clean bomb" designs have been generally abandoned.