VNIITF began studying NPL problems in 1979, and developed the EBR-L exper­imental complex for these purposes [2, 3, 31]. At present, apart from the EBR-L pulsed reactor, the complex includes a set of laser cells, a system of evacuation and

Fig. 2.15 Diagram of EBR-L reactor core: [3, 31]:

(1) mechanism for fine adjustment of reactivity,

(2) upper copper reflector,

(3) hemispherical parts of reactor core, (4) cylindrical copper reflector,

(5) polyethylene moderator,

(6) lower reflector,

(7) safety unit, (8) pulse rod. (9) stop unit filling of laser cells with gas mixtures, an arrangement to withdraw optical radiation from the reactor hall and register its parameters. Ten types of NPLs were studied with this complex, and lasing was achieved at 21 laser transitions [10, 55], including metal-vapor lasers (Cd, Zn, Hg) and laser at UV transitions of the molecular ion N2+.

The EBR-L reactor (Figs. 2.14 and 2.15), which is a modified version of the pulsed reactor EBR-200, was put in operation in 1981. The reactor core consists of two hemispherical parts made of 235U-Mo alloy (3 % molybdenum by mass), enclosed by a copper reflector. Between the two parts of the reactor core there is a cylindrical copper reflector with a through channel 12.5 cm in diameter. A polyethylene moderator 60 cm in long is placed in this channel, and it accommo­dates the laser cells. Some characteristics of the EBR-L reactor are shown in Table 2.6.

The average flux density of the thermal neutrons along the length of the uranium layer at the pulse maximum is 1.3 x 1017 cm~2 s_1.

A scheme of one of the laser cells for metal-vapor NPLs is shown in Fig. 2.16. The central part of the cell is in the form of a stainless steel tube 80-cm long, with a wall thickness of 3 mm. A three-section wire heater is wound around the tube and is used to heat the tube up to 850°K. Water-cooled extension pipes and adjustment units are connected to the ends of the tube. The total length of the cell is 120-

140 cm. Inside the cell there are tubes with an internal diameter of 2.8 cm, on whose surface a layer of U3O8 is deposited (90 % 235U enrichment) at a thickness of 2­3 mg/cm2 and a length of 60 cm. For experiments with NPLs, laser cells are also used without heaters, with uranium layer diameters of 1.2 and 4.8 cm. The basic characteristics of the cells are shown in Table 2.7.

To measure the characteristics of laser or luminescent radiation emerging from the cell, optical equipment (Fig. 2.17) is used, which can determine the laser spectra, energy and threshold characteristics of the laser pulse. In experiments to study the luminescent characteristics of gas and vapor media, the spectral composition of radiation, the temporal pulse shape, and the intensity of luminescence were mea­sured for several lines. The spectral range for registration of optical signals was 0.2-5 qm.

Fig. 2.17 Diagram of the withdrawal and registration of optical radiation [3, 31]:

(1) laser cell, (2) adjustment laser, (3) calorimeter,

(4) semitransparent plate,

(5) focusing lens,

(6) spectrograph,

(7) deflecting mirror,

(8) photoreceivers,

(9) monochromator