As noted above, NPLs include lasers with active media that are excited directly using nuclear radiation, or with the use of intermediate nuclear-optical converters. There is also another method of pumping gas lasers using nuclear radiation, in which this radiation is used only for pre-ionization of the laser medium. In this case, the nuclear radiation performs an auxiliary function (creating the conditions for uniform volume pumping the gas medium), while direct pumping occurred through gas discharge. Such lasers (sometimes they were called electroionization lasers), which radiate at transitions of CO2, CO, XeF molecules and of Xe and Ar atoms, are not classified as NPLs, and are not considered in this book. Information about them may be found, for example, in the survey papers [54, 56, 61].

There are three basic sources of nuclear radiation which can be used to pump NPLs:

1. Nuclear explosive devices.

2. Radioactive isotopes.

3. Laboratory neutron sources.