NPLs with direct pumping absorb the kinetic energy of nuclear particles (neutrons, Y-quanta, fission fragments) and convert it into laser radiation in the laser medium itself. Especially interesting are laser media with fissile elements uniformly distrib­uted in material (for example, 235U), because, in this case, higher specific energy depositions are possible. The main obstacle on the road to creating condensed — media NPLs is their radiation damage: the formation of displaced atoms and color centers in solid-state lasers (see, for example, [7]); radiolysis [8] and the formation of gas-vapor bubbles [9] on the tracks of nuclear particles in liquid lasers.

Solid-State Laser Media

To our knowledge, there have been no experiments on the direct pumping of solid — state laser media with nuclear radiation of pulsed reactors, although one study [2], for example, was devoted to investigation of the luminescent characteristics of some laser-active materials (ruby, glass with neodymium, CaWO4:Nd3+, Y3Al5O12: Nd3+, etc.) and the radiation resistance of ruby and neodymium glass lasers under the effect of the n, y-ray radiation of pulsed reactors. Some materials had rather high energy conversion efficiencies па, where nA is the ratio of the specific output of luminescent radiation to the specific energy deposition. Thus, with high-energy electron beam excitation the following values of ща were obtained: ща k 10 % (A k 611 nm) for Y2O3 doped with europium [10], and nA = 4 % (A = 900­1,100 nm) for the crystal Y3Al5O12:Nd3+ [11]. The value of nA depends on the type of charged particle. When excited by heavy charged particles (a-particles and fission fragments), the crystal Y3Al5O12:Nd3+ has lower values of ца: nA = 2 % in the range 900-1,100 nm, and nA = 0.28 % for the more intense laser line 1,063 nm [12]. We note that in another study [11], when the crystal Y3Al5O12:Nd3+ was excited by an electron beam, laser action or, at least, superluminescence was obtained.

Yet another study [13] examined the possibility of lasing when neodymium- doped, uranium-containing media is irradiated by the pulsed neutron flux of the BARS-6 reactor [14]. The Y3Al5O12 crystal and GLS2 silicate and phosphate (GLS22, GLS27, and LGS40) glasses were selected as the dielectric matrices. With U concentrations in these media at 5 x 10 cm, the specific energy deposition was 31 J/cm3 (the neutron pulse duration was 140 ps). We note that the possibility of introducing uranium into these media and its influence on laser characteristics have not yet been verified in practice. In the authors’ opinion [13], under these conditions and using direct pumping with fission fragments, a laser effect is possible for active media based on Y3Al5O12 and GLS27 and LGS40 glasses. Of the effects arising during a pumping pulse and resulting to additional losses, the calculations only took into account light radiation scattering on the fission fragment tracks. In fact, a substantially larger contribution to the total detrimental loss factor may be caused by radiation-induced (neutrons, y-ray emis­sion, and fission fragments) light absorption due to formation of color centers (see Chap. 2, Sect. 1). Some media (for example, Y3Al5O12 without 235U impurities) completely lose their transparency on the 1.06 pm line at the maximum of the reactor pulse when the dose rate is about 2 x 105 Gr/s. Therefore, conclusions of study [13] concerning the direct pumping of solid-state neodymium media with nuclear radiation in experiments on pulsed reactors are excessively optimistic.