• The neutron multiplicity is the average number of neutrons produced by fission of a particular nuclide, v.
• The multiplication factor is defined as the ratio of the neutron concentration in one generation to that of the preceding generation; the symbol k is used.
• An assembly of fissionable materials is said to be critical, when, on average, exactly one of the several neutrons emitted in the fission process causes another nucleus to fission. Criticality thus depends on the geometrical factors as well as the distribution and kinds of material present.
• If N_i thermal neutrons are present in a system, their absorption will result in a certain number N_i+1 of next generation thermal neutrons.
• k_eff, 'k_effective' is a practical k for a real finite detector

k_¥, 'k_infinity' is a more theoretical concept which assumes that there is no leakage of neutrons from the system, i.e. the reactor is infinitely large.

• k_eff < 1 Þ subcritical

k_eff = 1 Þ critical

k_eff > 1 Þ supercritical

The effective multiplication factor is defined as

k_eff = N_i+1 / N_i+1