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Modulated neutron initiator
A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation. It is a crucial part of some nuclear weapons, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration is prompt critical. It is also known as an internal neutron initiator. The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock wave.
One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield, sufficient neutrons must be present within the supercritical core at the right time. If the chain reaction starts too soon ("predetonation"), the result will be only a 'fizzle yield', well below the design specification, therefore low spontaneous neutron emission of the pit material is crucial. If it occurs too late, the core will have begun to expand and disassemble into a less-dense state, leading to a lowered yield (less of the core material undergoes fission) or no yield at all (the core is no longer a critical mass).
For boosted fission weapons, the size of the centrally placed initiator is critical and has to be as small as possible. The use of an external neutron source allows more flexibility, such as variable yields.
==Design==
The usual design is based on a combination of beryllium-9 and polonium-210, separated until activation, then placed in intimate contact by the shock wave. Polonium-208 and actinium-227 were also considered as alpha sources. The isotope used must have strong alpha emissions and weak gamma emissions, as gamma photons can also knock neutrons loose and cannot be so efficiently shielded as alpha particles.〔(Nuclear Weapons FAQ, Section 4.1, Version 2.04: 20 February 1999 )〕 Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mixing of the metals.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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