NPLs excited by products of nuclear reactions occurring at the interaction of the nuclei of several isotopes with thermal neutrons were considered earlier in this chapter. There is yet another method of pumping, using fast neutrons. It was proposed at VNIIEF and was used to excite NPLs operating on transitions of the Xe atom [181]. This method is based on elastic scattering of fast neutrons on atoms (nuclei) of the medium, and makes it possible to homogeneously excite gas media at pressures of tens and hundreds of atmospheres. For the efficient transfer of

energy from fast neutrons to recoil nuclei, it is advisable to use buffer gases with a low atomic weight, for example helium or hydrogen. Direct ionization and excita­tion of the gas medium is carried out by recoil nuclei. For helium at a pressure of 100 atm, the flux of fast neutrons with an energy of 1 MeV is weakened by a factor of e over a length of approximately 60 cm, so that it is possible to realize a quite homogeneous pumping of gas media at pressures of ~100 atm with characteristic transverse dimensions of ~1 m.

Gas lasers were first pumped with fast neutrons at VNIIEF in 1981 in experi­ments with the BR-1-pulsed reactor; the results were published in 1990 [181]. Suc­cessful pumping of the lasers became possible thanks to the BR-1 reactor, in which, in one of the operating modes, the reactor core can be separated to create an internal gap (slot) up to 10 cm in height.

The experiments used a cylindrical laser cell with an internal diameter of 3.5 cm and an active length of 40 cm, placed in the gap between the halves of the reactor core (see Fig. 2.1c). The fluence of the fast neutrons with an energy of more than 0.5 MeV was equal to 5.6 x 1014 cm-2, while the neutron flux density at the pulse maximum was Фтах = 6.4 x 1018 cm-2 s-1.

The specific power deposition near the axis of the laser cell at the pulse maximum may be evaluated from the expression q = [He]a! ФтахЕа ([He] is the concentration of helium atoms, us = 6.5 x 10-24 cm[9] is the cross-section of elastic scattering of neutrons with an average energy of En = 1.3 MeV in helium nuclei, Ea is the average energy of a recoil nucleus). With isotropic scattering, Ea = 2AsEn/ (As + 1)2, where As is the mass number of the atoms of the medium. The specific power deposition is proportional to the pressure of the helium and for the conditions cited above, q « 80 W/cm[10] atm. The energy deposition owing to the absorption of у radiation was approximately two orders of magnitude lower.

The mixture He-Xe was used to prove the possibility of pumping high-pressure gas lasers with fast neutrons; it was quite well studied from pumping with nuclear reaction products (see Sect. 3.1) and involves a buffer gas of low atomic weight. In experiments, the changes in laser power and threshold characteristics of a He-Xe laser (A = 2.65 pm) as a function of the pressure and composition of the mixture were studied. An oscillogram from one of the experiments is shown in Fig. 3.8. The dependence of the laser parameters on the helium pressure at optimal xenon pressure of around 4 Torr is shown in Table 3.11. A maximal output power of 290 W (ni~0.2 %) and minimal value of ФгА = 1.9 x 1018 cm-2 s-1 were registered for the mixture He-Xe (950:1) at a pressure of 5 atm. These energy parameters evidently are not maximal, since the laser cavity parameters were not optimized.

At present, among NPL applications being considered is the possibility of creating powerful reactor-laser facility (see Chap. 10), in the reactor core of which the kinetic energy of fission fragments is converted into laser radiation. Investigations in the field of neutron pumping of lasers can be of interest for future power plants based on controlled thermonuclear fusion, in which the bulk of the energy is released in the form of kinetic energy of fast neutrons.

Fig. 3.8 Oscillograms of fast neutron pulse of reactor BR-1 (a) and laser pulse (b) for the He-Xe mixture (2,000:1) at a pressure of 5 atm [181]