The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of fissile material into a prompt critical mass. This is one of the three main requirements in a nuclear weapon design to create a working fission atomic bomb. The need for a short insertion time with plutonium-239 is the reason the implosion method was chosen for the first plutonium bomb, while with uranium-235 it is possible to use a gun design.

The basic requirements are:

• To start with a subcritical system
• To create a super prompt critical system
• To make the change between these two states in a length of time (insertion time) which is shorter than the time between the random appearance of a neutron in the fissile material through spontaneous fission or by other random processes.
• Also at the right moment in time, neutrons must be injected into the fissile material to start up the fission process. This can be done by several methods.
  • Alpha emitters such as polonium or plutonium-238 can be rapidly combined with beryllium to create a neutron source.
  • Neutrons can be generated using an electrostatic discharge tube, this tube uses the D-T reaction.

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