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.
The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock wave.
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.
[1] Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mixing of the metals.
Urchin was the code name for the internal neutron initiator used by the Los Alamos Laboratory as a neutron generating device to trigger the nuclear detonation of the earliest plutonium atomic bombs such as The Gadget and Fat Man, once the critical mass had been 'assembled' by the force of conventional explosives.
[2][3] A small amount of polonium-210 (50 curies, 11 mg) was deposited in the grooves of the shell and on the central sphere: the layers of gold and nickel served to shield the beryllium from alpha particles emitted by the polonium.
Placing the polonium layer between two large masses of beryllium ensures contact of the metals even if the shock wave turbulence performs poorly.
[9] It has been estimated that the initiators used in the wartime weapons produced only on the order of 100 neutrons during the critical ~1 microsecond of assembly time.
[12] An improved construction of the initiator, probably based on conical or pyramidal indentations, was proposed in 1948, put into production by Los Alamos in January 1950, and tested in May 1951.
It is believed the polonium was deposited on lotus-shaped platinum gauze to maximize its surface and enclosed in a tantalum sphere surrounded by uranium shell with embedded beryllium pellets.
The polonium used in the urchin initiator was created at Oak Ridge National Laboratory and then extracted and purified as part of the Dayton Project under the leadership of Charles Allen Thomas.
In 1949, Mound Laboratories in nearby Miamisburg, Ohio opened as a replacement for the Dayton Project and the new home of nuclear initiator research & development.
[17] Polonium from Dayton was used by the G Division of Los Alamos in initiator design studies at a test site in Sandia Canyon.
[19] The sealed neutron initiator, brought into inventory in late 1954, still required a periodic disassembly to access its capsule for maintenance checks.