 The Thin Man gun @-@ type design was at that time based on the <unk> of the very pure plutonium 239 isotope so far only produced in microgram quantities by the Berkeley cyclotron . When the Hanford production reactors came on @-@ line in early 1944 , the mix of plutonium 239 and plutonium 240 obtained was found to have a high rate of spontaneous fission . To avoid pre @-@ detonation , the muzzle velocity of the gun @-@ type design would need to be greatly raised , making it impractically long . 
