Nuclear fission. The division of a heavy nucleus into two (or, rarely, more) parts with masses of equal order of magnitude; usually accompanied by the emission of neutrons, gamma rays, and, rarely, small charged nuclear fragments.

Last fission. Fission caused by fast neutrons.

Thermal fission. Fission caused by thermal neutrons.

Spontaneous fission. Fission which occurs without the addition of particles or energy to the nucleus.

L’issionable. Of a nuclide, capable of undergoing fission by any process.

Eissile. Of a nuclide, capable of undergoing fission by interaction with slow neutrons. (In reactor physics, slow neutrons are frequently defined as those of kinetic energy less than 1 eV.)

Eertile. Of a nuclide, capable of being transformed, directly or indirectly, into a fissile nuclide by neutron capture. Of a material, containing one or more fertile nuclides.

Tission fragments. The nuclei resulting from fission and possessing kinetic energy acquired from that fission.

h’ission products. The nuclides produced either by fission or by the subsequent radioactive decay of the nuclides thus formed.

Eission yield. The fraction of fissions leading to fission products of a given type.

Prompt gamma radiation. Gamma radiation accompany­ing the fission process without measurable delay.

Prompt neutrons. Neutrons accompanying the fission process without measurable delay.

Delayed neutrons. Neutrons emitted by nuclei in excited states which have been formed in the process of beta decay. (The neutron emission itself is prompt, so that the observed delay is that of the preceding beta emission or emissions.)