The majority of studies dedicated to examining the characteristics of nuclear — excited plasma are computational-theoretical, and the data presented previously are essentially a brief review of them. The number of experimental studies in this area is very small, especially as regards experiments performed with the help of reactors.

Experiments with stationary reactors were devoted primarily to measuring of electrons and the electron temperature using microwave diagnostics [28, 66, 67] and electrical probes [68—70]. Measurements were carried out for 4He [66]; 3He [69]; Ne [67, 70]; Ar and Xe [70]; and for the Ne-Ar mixture [28, 68, 70] at ionization of gas media directly by n, y radiation of a reactor [66, 67], as well as by uranium fission fragments [28, 68, 70] and products of the nuclear reaction 3He (n, p)3H [69].

The most interesting are the optical methods of plasma diagnostics, which do not introduce any perturbations in the characteristics to be measured. Information about the properties of the plasma in this case can be obtained from investigation of the characteristics of its radiation or absorption—the intensity and wavelength of individual lines, the width and shape of contours of the lines, etc. [71, 72]. We know of only one study [73] in which, during reactor experiments, the concentra­tions of metastable atoms in He, Ne, and the He-Ne mixture were measured using the optical method. Excitation of gas media at pressures of up to 1.2 atm was carried out by products of the nuclear reaction 10B(n, a)7Li in the process of irradiation of a gas cell having a thin layer of 10B by a neutron beam of a TRIGA-pulsed reactor. To determine the concentrations of metastable atoms He*(21S), He*(23S), Ne*(3s [1/2]0), and Ne*(3s[3/2]2), absorption of radiation by these atoms at transitions

with wavelengths of 501.6 nm; 388.9 nm; 626.6 nm and 621.7 nm, respectively, was used. The concentrations of metastable atoms, depending on the pressure of the gas media and the neutron flux density, can be significant. Thus for helium at PHe = 0.2-0.5 atm and a neutron flux density Ф = 4 x 1013 cm-2 s-1, the concen­tration of atoms He*(23S) was (2-4) x 1011 cm-3, which is comparable in size to the concentration of electrons (~1012 cm-3).