Shake (thing)
The speed of this reaction, and the process of fission, becomes clear if you work the math outwards from here. It takes roughly 2*10^24 atoms splitting to release 20 kt of energy, or the amount of energy released by a small atomic bomb. Since fission generations increase in size exponentially (1 neutron results in 2 neutrons emitted which results in 4 neutrons emitted and so forth - the term for this is k, a coefficient which indicates the number of neutrons which not only are produced per available neutron, but are restricted to those that do not escape or undergo capture by a non-fissioning atom. In a weapon, k tends to be around 2) the total number of atoms that have undergone fission at any point can be expressed as 2^(n-1) where n is the number of generations that have occurred. Therefore, solving for n, we get (I hope, my maths are horrible):
2*10^24 = 2^(n-1)
log 2 (2*10^24) = n-1
log 2 (2*10^24) +1 = n
81.7 = n ..or less than 82 fission generations are required to split all the atoms necessary to release 20 kilotons of energy. Even if we were to assume a purely mechanical and linear process (it isn't; generations overlap in time in the 'real world' meaning the actual number will be smaller) that's still only 82 shakes, or 820 billionths of a second. It gets worse; since the energy release is exponential, that means that the most recent 4-5 generations have produced (at any point) 99 percent of the total energy released thus far! That means that realistically, 99 percent of the actual energy release of a 20 kt atomic bomb happens in less than 5 shakes, or 50 billionths of a second. So the shake is extremely important when designing, describing, or predicting the outcome of atomic fission, whether the low-k 'critical' type such as is found in nuclear reactors, or the high-k 'supercritical' type found in nuclear explosions.
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