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.