Neutrons are introduced into the supercritical mass by a neutron initiator (also called neutron trigger/generator). A neutron initiator is not stricly required for a gun triggered nuclear device as long as the device uses "target capture," meaning that the two subcritical masses, once fired, are kept together until they explode. However, the use of a neutron initiator can ensure precise control to the millisecond over the timing of the nuclear explosion.
There are several designs for a neutron initiator:
Early neutron initiators consisted of a small pellet of polonium-210 and beryllium-9. Polonium-210 has a halflife of about 140 days, so this type of initiator needs to have its polonium stock replaced regularly. This method is appropriate for the gun triggering method, but the timing is not sufficiently precise to be used in an implosion weapons design.
The two elements are separated by foil within the fuel core. The separating foil is broken when the subcritical masses combine. Because polonium is radioactive, it spontaneously emits alpha particles.
These particles then react with the beryllium-9 to form beryllium-8 and free neutrons. For every 1 million alpha particles, 30 million neutrons are emitted.
The free neutrons then initiate the fission reaction.
A pulsed neutron emitter is a small ion accelerator with a metal hydride target. The ion source creates a plasma (high excited state) of deuterium or tritium. A large voltage is then applied across the tube, which accelerates the ions into scandium or any other tritium rich metal. The ions are accelerated so that there is a high probability of nuclear fusion occuring. The deuterium-tritium fusion reactions produce neutrons, which can then initiate the fission chain reaction.
This method has very precise timing and can be used in an implosion design. Note that the process of nuclear fusion occurs in this type of neutron initiator.