See Urchin disambiguation page for other uses of the word Urchin.

Urchin was the code name for the neutron generating device that triggered the nuclear detonation of the earliest plutonium atomic bombs such as Fat Man, once the critical mass had been "assembled" by the force of conventional explosives. It was also known as the screwball design.^[1]

A different initiator (code named ABNER) was used for the Little Boy uranium bomb.

Contents 1 Purpose 2 Design 3 Development 4 See also 5 References

[edit] Purpose

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 just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle yield', well below the design specification; if it occurs too late, there may be no yield whatsoever.

[edit] Design

The detonator, located at the center of the bomb's plutonium pit, consisted of a beryllium pellet coated with a thin layer of gold and nickel. The coated pellet was surrounded by a beryllium shell with a grooved inner surface that was also coated with gold and nickel. The grooves were filled with polonium-210, which emits alpha particle radiation. The thin layers of gold and nickel maintained the slight separation between the beryllium and polonium necessary to shield the beryllium from alpha particles emitted from the polonium. When the shock wave from the implosion of the plutonium core arrived, it crushed the detonator. Hydrodynamic forces acting on the grooved shell thoroughly mixed the beryllium and polonium, allowing the alpha particles from the polonium to impinge on the beryllium atoms. Reacting to alpha particle bombardment, the beryllium atoms would emit neutrons that then triggered the chain reaction in the compressed supercritical plutonium.

[edit] Development

The urchin initiator was developed as part of the Dayton Project under the leadership of Charles Allen Thomas. The Dayton Project was one of the various sites comprising the Manhattan Project

Urchin style initiators were later superseded by other means of generating neutrons such as pulsed neutron emitters that don't use polonium.

In 1949 Mound Laboratories in nearby Miamisburg, Ohio opened as a replacement for the Dayton Project and the new home of nuclear initator research & development.

[edit] See also

• Nuclear weapon design
• Dayton Project

[edit] References