Modulated Neutron Initiator - Design - Urchin

Urchin was the code name for the internal neutron initiator, a neutron generating device that triggered 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.

The detonator used in the early devices, located at the center of the bomb's plutonium pit, consisted of a beryllium pellet 0.8 cm in diameter coated with nickel and then a layer of gold. The coated pellet was surrounded by a beryllium shell of 2 cm outer diameter with walls 0.6 cm thick. The inner surface bore 15 concentric, wedge-shaped latitudal grooves and was, like the inner sphere, coated with gold and nickel. A small amount of polonium-210 (50 curies, 11 mg) was deposited on the grooves inside 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. The whole urchin weighed about 7 grams and was attached to mounting brackets in a 2.5 cm diameter inner cavity in the pit.

When the shock wave from the implosion of the plutonium core arrives, it crushes the detonator. Hydrodynamic forces acting on the grooved shell thoroughly mix 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 emit neutrons in a rate of about 1 neutron each 5–10 nanoseconds. These neutrons trigger the chain reaction in the compressed supercritical plutonium. Placing the polonium layer between two large masses of beryllium ensured contact of the metals even if the shock wave turbulence performed poorly.

The amount of polonium generated about 0.1 watts of decay heat, noticeably warming the small sphere.

The grooves in the inner surface of the shell shape the shock wave into jets by the Munroe effect, similar to a shaped charge, for fast and thorough mixing of the beryllium and polonium. As the Munroe effect is less reliable in linear geometry, later designs used a sphere with conical or pyramidal inner indentations instead of linear grooves. Some initiator designs omitted the central sphere, being hollow instead. The advantage of a hollow design is its possible smaller size while retaining reliability.

The short half-life of polonium (138.376 days) requires frequent replacement of initiators and continued supply of polonium for their manufacture, as their shelf life was only about 4 months. Later designs had shelf life as long as 1 year.

The US government used Postum as a code name for Polonium.

Use of polonium for the neutron initiator was proposed in 1944 by Edward Condon. The initiator itself was designed by James L. Tuck.