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Lawrence's Radiation Laboratory in Berkeley, California. Because of the presence of the isotope Pu-240, reactor-bred plutonium had a much higher rate of spontaneous neutron emission than was previously thought, and if a gun-type device was used it would biography/autobiography most likely pre-detonate and result in a messy and costly "fizzle". The spontaneous fission rate of Pu-240 is 415,000 fissions/(s·kg), and the amount was 0.9% of 6.2 kg, is 56 g. Thus biography/autobiography there were 23,000 spontaneous fissions per second. This means that the last few centimeters would have to be travelled in preferably much less than 40 microseconds. It is theoretically possible to build a plutonium gun-type device, but it would need to be 19 feet long in order to allow the sub-critical masses to be fused into a critical mass before a fizzle occurs. The mass of a plutonium gun-type device would have been beyond the payload of the B-29.
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