Basics of Atomic Weapons

Saturday, February 04, 2006

The Use And History Of The "Shake" Unit


An informal unit of time equal to 10^-8 seconds or 10 nanoseconds (ns). This unit originated in nuclear physics. In an atomic explosion, fast-moving neutrons break apart atoms of uranium or plutonium; the fission of these atoms releases additional neutrons which keep the reaction going. The shake is the approximate lifetime of an individual neutron. The word shake and the expression "shake of a lamb's tail" have long been used in English to mean a very brief period of time.

The amount of time taken by each link in the chain reaction is determined by the speed of the neutrons and the distance they travel before being captured. The average distance is called the mean free path. In fissile materials at maximum normal densities the mean free path for fission is roughly 13 cm for 1 MeV neutrons (a typical energy for fission neutrons). These neutrons travel at 1.4x10^9 cm/sec, yielding an average time between fission generations of about 10^-8 sec (10 nanoseconds), a unit of time sometimes called a "shake". The mean free path for scattering is only 2.5 cm, so on average a neutron will be scattered 5 times before causing fission.

10 nanoseconds = one shake, approximate time of one generation of a nuclear chain reaction with fast neutrons


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