Rare gases are a component part of virtually all gas NPLs. Helium, neon, or argon are used as buffer gases, while lasing is achieved at a number of transitions of Xe, Kr, Ar, and Ne atoms, so the luminescence spectra of rare gases are of the greatest interest. Unfortunately, in the scientific literature one can to find information only about the luminescence spectra in the visible and near IR spectral ranges (X < 1.2 pm), which may be explained by the absence of sensitive photodetectors for the longer-wave radiation. Table 4.12 shows a list of the studies in which the most significant results were achieved.

Despite the fact that study of the luminescence characteristics of rare gases excited by ionizing radiation (electron and ion beams, nuclear reaction products) began more than 50 years ago, investigations in this area cannot be deemed complete. As a rule such investigations were conducted under different experimen­tal conditions for a limited number of gas media and pressures, which makes it difficult to compare results. Studies [40, 82, 87, 88,91] carried out measurements of the absolute intensities of spectral lines.

The most complete information is contained in [89—91], in which all the rare gases and their binary mixtures that were of most interest from the standpoint of NPLs were studied under identical experimental conditions. The gas mixtures were excited by U fission fragments escaping from a thin uranium layer when the U nuclei interacted with neutrons. A VIR-2M-pulsed reactor was used as the neutron source. As a result of the investigations, information was obtained on the intensity of the roughly 300 spectral lines belonging to the transitions of Hel, Nel, ArI(II), KrI(II), and XeI(II). Also observed were lines belonging to transitions of the molecules N2 and N^, and the atoms O, N, and C, which were there because of the presence of impurities N2, CO2, CH4, O2, H2, and H2O in the rare gases, with a total concentration of no more than 5 x 10~4 %. The intensities of certain of them (for example, the 777.4 and 844.6 nm lines of the O atom) were comparable with the intensities of the basic lines of rare gas atoms.

The calibration of spectral apparatus performed in study [91] by using a refer­ence source made it possible to determine the absolute values of radiating powers on individual lines in the range of 740-1,100 nm, while the conversion efficiencies obtained for the mixtures He-(Ar, Kr, Xe) conform well with the results of studies [87, 88]. Virtually all of the most intensive lines belong to the transitions (n + 1)p-(n +1)s of the atoms Ar, Kr, Xe (n = 3,4, 5 for Ar, Kr, Xe respectively). The levels (n + 1)p are the lower levels for numerous laser IR transitions in the 1-4 pm region, arising on excitation of rare gases by nuclear radiation (see Chap. 3, Sect. 3.1). The maximal conversion efficiencies (~0.1 %) were registered for the lines 912.3; 965.8 nm (Arl, mixture of He-Ar); 892.9 nm (KrI, mixture of He-Kr) and 980.0 nm (Xel, mixture of Ar-Xe).