Outside of Russia, lasers operating on transitions of rare gas atoms were first pumped by nuclear radiation in 1975 at LANL in the United States, with excitation of the mixture He-Xe (A = 3.51 pm) by uranium fission fragments [38]. Further investigations were aimed at finding and studying parameters of NPLs operating on various transitions of Xe, Kr, and Ar atoms. The basic results of the experiments are shown in Table 3.5.

It is shown that in these studies, carried out mainly in the United States, lasing was obtained on the same transitions nd-(n + 1)p of the Xe, Kr, and Ar atoms (except for the 3.65 pm line of the Xe atom and 1.27 pm line of the Ar atom). Maximal output powers up to 1 kW were registered in the experiments [51] with the

APRF fast pulsed reactor (Фтах = 4.3 x 1016 cm-2 s-1) for a laser with the mixture 3He-Ar (A = 1.79 ^m) when a multiple-path laser cell with a volume of 3.6 l was used (see Chap. 2, Sect. 2.1). However, the laser efficiency was not great, and amounted to ~0.01 %.

In studies of American researchers published prior to 1989, the efficiency for IR lasers operating on the transitions of Xe, Kr, and Ar atoms did not exceed 0.1 %, which may be explained by the design features of the laser cells, which were sealed at the ends by windows arranged at the Brewster angle. In these windows, owing to the reduction of light transmission under the effects of the reactor n, y-radiation, additional losses occur inside the resonator (see Chap. 2, Sect. 2.2). In addition, the presence of Brewster windows prevented lasing at the intensive 2.63 and 2.65 ^m lines of the Xe atom, because of absorption of radiation in atmospheric water vapor in the cavity areas located between the windows and the mirrors.

Significantly higher values of щ = 3.3 and 5.6 % were obtained at the Sandia Laboratories for lasers using the mixtures He-Ar-Xe (A = 2.03 ^m) and Ar-Xe (A = 1.73 ^m) when cells with windows made from radiation-resistant optical materials were used, and with use of comparatively low specific power depositions q < 10 W/cm3 [45]. At higher specific power depositions and specific absorbed energies, there was a reduction in efficiency, and lasing was observed at the leading edge of the pumping pulse, which could be caused by the significant deteriorations of optical uniformity of the medium (especially in the Ar-Xe mixture) or by plasma effects. Of the research done on the xenon NPL at Sandia, one should note the studies [46, 55], which examined the influence of helium additions on the charac­teristics of an Ar-Xe laser. The introduction of helium to the Ar-Xe mixture leads to a change in the laser spectrum (instead of the 1.73 ^m line, the 2.03 ^m line appears), and to an increase in the laser pulse duration.

Sandia carried out research into the amplification properties of NPL active media operating on transitions of the atoms Xe [45, 46, 56, 57] and Ar [53, 54]. It should be noted that the first experiments [58] to determine the gain of NPLs operating on transitions of the Xe atom were carried out in 1981 for a 3He-Xe- laser at the 2.65 ^m line (in study [58], the 2.63 ^m line was mistakenly given as the lasing line). In the studies [45, 53, 54], the Rigrod theory [25] was used to determine the small-signal gain, while in the other studies, direct measurements were made by the “oscillator-amplifier” scheme using cw tunable color-center lasers, or low-pressure gas-discharge lasers using an He-Xe mixture.

Of the studies in the United States, one should mention [42], where the influence of additives of 235UF6 on the characteristics of the Ar-Xe laser (A = 2.6 ^m) was investigated. These experiments were aimed at determining the possibility of using the gaseous compound 235UF6 as a volume source of fission fragments. It was shown that reduction in the energy parameters is observed at a low concentration of uranium hexafluoride of ~0.5 %, which testifies to the effective “quenching” of the excited atoms Xe* by the molecules of UF6. An analogous conclusion follows from calculations of characteristics of a laser based on the mixture 235UF6-Xe [59] and luminescence investigations [60], which detected a more intense “quenching” of the ArI lines in the 235UF6-Ar mixture than in 3He-Ar. Thus at present there are no data that would allow us to hope for the possibility of creating lasers based on 235UF6 as the buffer gas.