# Insertion time

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{{refimprove|date=November 2024}}

In nuclear weaponry, '''insertion time''' is the interval required to rearrange a [subcritical mass](/source/subcritical_mass) of [fissile](/source/fissile) material into [critical mass](/source/critical_mass).<ref>{{Cite web |title=4.1 Elements of Fission Weapon Design |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html |access-date=2024-11-28 |website=nuclearweaponarchive.org}}</ref> 

Appropriate insertion time is one of the three main requirements to create a working fission [atomic bomb](/source/atomic_bomb). The need for a short insertion time with [plutonium-239](/source/plutonium-239) is the reason the implosion method was chosen for the first plutonium bomb, while with [uranium-235](/source/uranium-235) it is possible to use a gun design.

The basic requirements{{Of what|date=November 2024}} are:
* Start with a [subcritical](/source/subcritical) system
* Create a super [prompt critical](/source/prompt_critical) system
* Switch between these two states in a length of time (insertion time) shorter than the time between the random appearance of a [neutron](/source/neutron) in the fissile material through [spontaneous fission](/source/spontaneous_fission) or by other random processes.
* At the right moment, neutrons must be injected into the fissile material to start the fission process. This can be done by several methods.
** Alpha emitters such as [polonium](/source/polonium) or [plutonium-238](/source/plutonium-238) can be rapidly combined with [beryllium](/source/beryllium) to create a neutron source.
** Neutrons can be generated using an electrostatic [discharge tube](/source/discharge_tube), this tube uses the [D-T reaction](/source/D-T_reaction).

==References==
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Category:Nuclear weapons
Category:Nuclear physics
Category:Nuclear technology

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