Plutonium pit

The pit, named after the hard core found in fruits such as peaches and apricots, is the core of an implosion nuclear weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.

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) half-spheres of 9.2 cm diameter, with a 2.5 cm internal cavity for the initiator. The gadget's pit was electroplated with 0.13 mm of silver; the layer, however, developed blistering and the blisters had to be ground and plated with gold leaf before the test. The Fat Man pit, and those of subsequent models, were all plated with nickel. A hollow pit was considered and known to be more efficient but ultimately rejected due to higher requirements for implosion accuracy.

Later designs used TOM initiators of similar design but with diameters of only about 1 cm. The internal neutron initiators were later phased out and replaced with pulsed neutron sources, and with boosted fission weapons.

The solid-cores were known as the "Christy" design, after Robert Christy who made the solid pit design a reality after it was initially proposed by Edward Teller. Along with the pit, the whole physics package was also informally nicknamed "Christy['s] Gadget".

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