Page "Trinity (nuclear test)" Paragraph 10

Plutonium is a synthetic element not found in nature in appreciable quantities.

It also has relatively complicated physics, chemistry, and metallurgy compared to most other elements in the periodic table.

The only prior plutonium isolated for the project had been produced in cyclotrons in very minute amounts.

In April 1944, Emilio Segrè received the first sample of reactor-bred plutonium from the X-10 reactor and discovered that reactor-grade plutonium was not as pure as cyclotron-produced plutonium by a significant degree.

Specifically, the longer the plutonium remained irradiated inside the reactor — which is necessary for high yields of the metal — the greater its content of the isotope plutonium-240.

Pu-240 undergoes spontaneous fission at an appreciable rate, and that releases plenty of excess neutrons.

These extra neutrons implied a high probability that a gun-type bomb with plutonium would detonate too early, before a critical mass was formed, scattering the plutonium and producing a small " fizzle " of a nuclear explosion many times smaller than a full explosion.

The practical result is that a simple gun-type atomic bomb ( the proposed Thin Man ) would not work as had been hoped.