When using NOCs, the laser element may be removed from the source of the nuclear energy. This reduces the need for radiation resistance of the laser medium. For example, some publications [39, 40] report on experiments using the TRIGA pulsed reactor. In these experiments, light radiation from the NOC was transmitted using 5-m long bundles of light-guide fibers beyond the biological shielding and directed to the Y3Al5O12:Nd3+ crystal. Gas as well as condensed media can be used as NOCs.

Gas NOCs may be divided by emission spectrum into two types: sources with a continuous spectrum close to the spectrum of black-body radiation and sources with a line spectrum consisting of separate luminescent lines or bands. In the first case, the source of the light radiation is nuclear-excited plasma heated to sufficiently high temperatures. As the data show [41, 42], when gas media based on rare gases (krypton, xenon, etc.) containing the isotopes 3He, 10B, or 235U at atmospheric pressure are irradiated with neutron fluxes from pulsed reactors with specific energy depositions up to ~10 J/cm3, the media temperature can reach 104K. In this case, the spectra of the luminescence closely correspond to the emission spectrum of an absolutely black body. Intensity lines belonging to the atom transitions of rare gases in the 800-1,000 nm spectral range are sometimes superimposed on these spectra.

With specific energy depositions <1 J/cm3, the main contribution to the emis­sion spectra at X < 1 pm is made by luminescent radiation. Naturally, in this case, gas media with maximum conversion efficiencies should be used for NOCs. In the UV spectral range, these media are mixtures based on the excimer molecules of the rare gases R2* (R = Xe, Kr, Ar) and the halides of rare gases RX* (X = F, Cl, Br, I) [40, 43—47]. With the use of excimer molecules, there is no radiation reabsorption because when a photon is emitted, the excimer molecules transit to a lower dissociable state or a weakly linked ground state.

A simple expression was used to evaluate the maximum values for the conver­sion efficiency nX (Table 11.4): nX max = hv/wa, where hv is the photon energy of a luminescent transition, and wa is the average energy consumed for the formation of one primary active particle (an ion or excited atom of a buffer gas). During subsequent relaxation processes, this active particle can produce maximum of one excited excimer molecule. Using Platzman’s formula (see Chap. 4, Sect. 2), a link may be obtained between the wa and ionization potential of the buffer gas V;: wa к 1.2V;-. It follows from Table 11.4 that the conversion efficiency for some excimer molecules can reach 50 %. The bottom line of the table gives experimental data for some excimer molecules at excitation of the gas media by proton [48] and electron [49] beams, and also nuclear radiations [50, 51].

For effective pumping of active laser elements based on the condensed media, their absorption spectra must correspond to the emission spectra of the NOC. The most intense absorption bands of ruby and neodymium lasers are located in the visible and near IR spectra. These laser media can be pumped using NOCs based on excimer alkaline molecules [40, 44—46] or rare gases [40, 46, 52, 53]. According to estimates [40, 45, 46], the maximum conversion efficiency for alkaline molecules Li2* (Amax = 458 nm), Na2* (A^ = 436 nm), K2* (Amax = 575 nm), Rb2* (Amax = 601, 603, 606 nm), and Cs2* (Amax = 703, 713, 718 nm) may reach 40­50 %.

Experiments [40, 52, 53] have shown that in NOCs based on mixtures of rare gases, almost all of the most intense lines are located in the near-IR range of the spectrum, 800-1,000 nm, and belong to the transitions (n + 1)p — (n +1)s of the Xe, Kr, and Ar atoms (n = 5,4,3 with respect to Xe, Kr, Ar). Table 11.5 gives the results of a study of the luminescent characteristics of He, Ne, Ar, Kr, Xe, and their binary mixtures in the spectral region 740-1,100 nm when excited by uranium fission fragments [53]. The VIR-2 M pulsed reactor with a pulse duration of approximately 3 ms was used as the neutron source [16]. For the most intense lines, 912.3 and 965.8 nm (Arl) in the He-Ar mixture, 892.9 nm (KrI) in the He-Kr mixture, and 980.0 nm (Xel) in the Ar-Xe mixture, the conversion efficiency was 0.1-0.15 %. With excitation of 3He-Ar(Kr, Xe) mixtures with the products of the nuclear reac­tion 3He(n, p)3H (q = 20 W/cm3), similar values qA < 0.5 % were obtained for the spectral range 740-840 nm [52].

The active substance for NOCs based on condensed media is phosphor as liquid or solid. The characteristics of some liquid luminescent media are considered in the previous section. Of the other liquid media for NOCs, one study [54] proposes a saline solution of the isotope 245Cm in heavy water. The luminescence of this solution when excited by a-particles occurs in the 560-620 nm region at a com­paratively low nA ~ 0.1 %.

In VNIIEF, some characteristics of alkali halide crystals and luminescent plas­tics were studied in experiment [2] at the VIR-2 and at the BIR-2 pulsed reactors

[16] . The CsI(Tl), CsI(Na), and NaI(Tl) crystals were considered in the most detail. Earlier these crystals were mainly used at low excitation levels in the nuclear particle counting mode. The VNIIEF experiments showed that the luminescence spectra of these crystals change insignificantly at the absorbed doses of the

y-radiation DY < 5 x 103 Gr and the neutron fluences F < 5 x 1014 cm~2 (Fig. ). Other important parameters for the CsI(Tl), CsI(Na), and NaI(Tl) crystals were studied as a function of the activator concentration and the dose rate of y-radiation: the specific output of luminescent radiation, and the conversion efficiency. It is possible to conclude from the results that the light output, for example, of the CsI (Tl) crystal increases nearly linearly with the growth in the absorbed dose up to

4.5 x 103 Gr. At this maximum dose, the specific light output is 1.1 J/cm3, and the conversion efficiency is about 6 %.