Martian lead or the terrestrial isotope?"
"Invaders from the Infinite"
John Wood Campbell
Merely adding neutrons would not be enough; that would make only a heavier isotope of the already known heaviest elements, uranium.
"A Brief History of Element Discovery, Synthesis, and Analysis"
Glen W. Watson
Two or more correctly sized pieces of plutonium or uranium isotope, when brought together, formed what was known as a critical mass, which would fission.
"Rip Foster in Ride the Gray Planet"
Harold Leland Goodwin