A neutron source is some material or device that emits a steady stream of neutrons for research or transmutation. There are five primary types of neutron sources, which are detailed below:
These sources utilize nuclear spallation, a reaction in which heavy nuclei such as tungsten, lead, or uranium are blown apart by protons accelerated to energies about 1 billion electron volts. About 25-40 neutrons are released for every high energy proton. Spallation is an easy, energy efficient, and effective source with a high neutron flux (amount of neutrons emitted per unit area per unit time). However, it requires a quite large particle accelerator to accelerate the protons.
Alpha emitter + Beryllium Source
In this scheme, a radioactive isotope that emits alpha particles is coated with beryllium. The beryllium absorbs the alpha particles, and emits a small amount of neutrons. Such a device can be very compact, and requires no external source of power. However, the neutron flux is so low that it cannot be used for any large-scale project.
Photon capture Source
Some nuclei (such as beryllium-9 or deuterium, when hit by an ultra high energy photon, such as a gamma ray, emit a neutron. Theoretically, this is the best source, because it requires no exotic isotopes, is so energy efficient and requires only modest external equipment. However, since the photons are much more likely to react with electrons than nuclei, only bare nuclei can be used. Even then, the photon has such a small probability of reacting with the nucleus that this scheme is little more than an academic curiosity.
Spontaneous fission Source
Some artificial elements, such as Californium, decay via spontaneous fission. They emit neutrons spontaneously. This scheme is very similiar to the alpha emitter + beryllium source, except with a much higher flux. If the spontaneous fission isotopes weren't so rare and expensive, it would be an excellent source.
Nuclear reactor Source
This source nuclear fission in a nuclear reactor to create neutrons. This source has the highest flux by far, and is used for mass production of transmuted elements. A nuclear reactor is a big investment, however, ruling out its use in most laboratories.
Spontaneous fission and alpha emitter + beryllium sources are most preferred for small scale lab work, while nuclear reactors are used for the large progjects. Spallation is used for medium size projects.