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. The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock wave.

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 the right time. If the chain reaction starts too soon ("predetonation"), the result will be only a 'fizzle yield', well below the design specification. If it occurs too late, the core will have begun to expand and disassemble into a less-dense state, leading to a lowered yield (less of the core material undergoes fission) or no yield at all (the core is no longer a critical mass). Therefore, low spontaneous neutron emission of the pit material is crucial.

For boosted fission weapons, the size of the centrally placed initiator is critical and has to be as small as possible. The use of an external neutron source allows more flexibility, such as variable yields.

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. Polonium-208 and actinium-227 were also considered as alpha sources. The isotope used must have strong alpha emissions and weak gamma emissions, as gamma photons can also knock neutrons loose and cannot be so efficiently shielded as alpha particles. 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.

The initiator used in the early devices, located at the center of the bomb's plutonium pit, consisted of a beryllium pellet and a beryllium shell with polonium-210 between the two. The pellet, 8 mm (1⁄3 in) in diameter, was coated with nickel and then a layer of gold. The beryllium shell was of 20 mm (4⁄5 in) outer diameter with wall thickness of 6 mm (1⁄4 in). The inner surface of that shell had 15 concentric, wedge-shaped latitudinal grooves and was, like the inner sphere, coated with gold and nickel. 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. The whole urchin weighed about 7 grams (1⁄4 ounce) and was attached to mounting brackets in a 2.5 cm (1-inch) diameter inner cavity in the pit.

When the shock wave from the implosion of the plutonium core arrives, it crushes the initiator. Hydrodynamic forces acting on the grooved shell thoroughly and virtually instantly 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 at a rate of about 1 neutron every 5–10 nanoseconds (See Beryllium). These neutrons trigger the chain reaction in the compressed supercritical plutonium. Placing the polonium layer between two large masses of beryllium ensures contact of the metals even if the shock wave turbulence performs poorly.

The 50 curies of polonium generated about 0.1 watts of decay heat, noticeably warming the small sphere.