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The most stable isotope is 231 Pa with a half-life of 32,760 years, 233 Pa with a half-life of 26.967 days, and 230 Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds.
Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by: This isotope decays by: Alpha emission : the decay energy is 5.2 MeV and the decay product is protactinium-231 .
Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years. [1]
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Perrin assumed that the mean free path was much larger than the radius of the sphere. Peierls did not agree, and commenced his own calculations. A key insight came from Frisch, who wondered what would happen if, instead of natural uranium, a sphere of the uranium-235 isotope was used. [42] By definition, the mean free path is:
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The nominal spherical critical mass for an untampered 235 U nuclear weapon is 56 kilograms (123 lb), [6] which would form a sphere 17.32 centimetres (6.82 in) in diameter. The material must be 85% or more of 235 U and is known as weapons grade uranium, though for a crude and inefficient weapon 20% enrichment is sufficient (called weapon(s)-usable).
750 mm by 600 mm (30 in by 24 in) Pennsylvania shield, made to the specifications of the Manual on Uniform Traffic Control Devices (MUTCD), 2003 Edition (sign M1-5). Uses the Roadgeek 2005 fonts. (United States law does not permit the copyrighting of typeface designs, and the fonts are meant to be copies of a U.S. Government-produced work anyway.)