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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.
Ultimately, polonium-based neutron initiators were used in both the gun-type Little Boy and the implosion-type Fat Man used in the atomic bombings of Hiroshima and Nagasaki respectively. The fact that polonium was used as an initiator was classified until the 1960s, but George Koval , a technician with the Manhattan Project's Special Engineer ...
Polonium and beryllium were the key ingredients of the 'Urchin' initiator at the center of the bomb's spherical pit. [44] 'Urchin' initiated the nuclear chain reaction at the moment of prompt-criticality to ensure that the weapon did not fizzle. 'Urchin' was used in early U.S. weapons; subsequent U.S. weapons utilized a pulse neutron generator ...
The polonium-beryllium "Urchin" initiator was assembled, and Louis Slotin placed it inside the two hemispheres of the plutonium core. Cyril Smith then placed the core in the natural uranium tamper plug, or "slug". Air gaps were filled with 0.5-mil (0.013 mm) gold foil, and the two halves of the plug were held together with uranium washers and ...
So there might be a third basic component to their atomic bomb besides nuclear core and confining tamper: an initiator - a Ra + Be source or, better, a Po + Be source, with the radium or polonium attached perhaps to one piece of the core and the beryllium to the other, to smash together and spray neutrons when the parts mated to start the chain ...
The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2 kg of solid hot pressed plutonium-gallium alloy (at 400 °C and 200 MPa in steel dies – 750 °F and 29,000 psi) half-spheres of 9.2 cm (3.6 in) diameter, with a 2.5 cm (1 in) internal cavity for the initiator.
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Although the weapon's designers thought that simply bringing a critical mass together would be sufficient, Serber suggested that the design should also include an initiator. A polonium-210-beryllium initiator was chosen because polonium 210 has a 140-day half life, which allowed it to be stockpiled, and it could be obtained from naturally ...