The pulsed nuclear reactors listed in Table 2.1 were used outside of the USSR to search for and to study the characteristics of NPL active media. The bulk of the studies were done in the United States, at the Sandia Laboratory (reactors SPR-II, SPR-III) and the University of Illinois (TRIGA reactor).

Fig. 2.20 Schematic view of the SPR-II reactor [2]:

(1) support column for upper half of reactor core,

(2) upper half of reactor core, (3) flange of protective tube in cavity, (4) shielding enclosure, (5) axial cavity for irradiation, (6) lower half of reactor core

A report concerning the first successful experiment outside of the USSR in a pulsed reactor to pump a laser with nuclear radiation appeared in 1975: a molecular CO-laser (к = 5.1-5.6 qm) was pumped by uranium fission fragments in experiments using SPR-II reactor [18, 60]. Figures 2.20 and 2.21 show a diagram of the fast neutron-pulsed reactor SPR-II [2] and the scheme of experiments on this reactor with a CO-laser [60]. The cylindrical reactor core consists of six annular discs and four rods made of uranium alloy (93 % enriched 235U) with molybdenum (10 %). The external diameter and height of the reactor core are 20.3 cm and 20.8 cm, respec­tively, and the total mass of the uranium-molybdenum alloy is 105 kg. Detailed characteristics of the SPR-II reactor can be found in monograph [2].

The laser cell was placed at a distance of around 20 cm from the reactor core. The neutron flux density at the reactor pulse maximum was 1 x 1017 cm~2 x s_1. Excitation of the CO at a pressure of 0.13 atm was carried out with uranium fission fragments emerging from the cylindrical 235U3O8 layer. The specific power depo­sition was around 200 W/cm3. A specific feature of these experiments was the necessity of cooling the carbon monoxide to a temperature of 77 K. The laser radiation receivers (calorimeter and Ge:Au photoresistor) were disposed beyond the biological shielding at a distance of around 15 m from the laser cell. In subsequent experiments with the SPR-II reactor with CO laser, a laser cell with a multipass cavity was used (see Fig. 2.2a).

Significantly more experiments with various NPLs were carried out at the Sandia Laboratories with the SPR-III pulsed reactor, which was put into operation in 1975

Fig. 2.21 Diagram of experiments to pump a CO-laser with uranium fission fragments [60]:

(1) SPR-II reactor core,

(2) output coupler, (3) CaF2 window at Brewster angle,

(4) aluminum tube,

(5) 235U3O8 layer,

(6) polyethylene neutron moderator, (7) 100 % reflectivity mirror,

(8) Dewar flask with liquid nitrogen, (9) region with vacuum, (10) laser radiation receivers

[2]. The reactor has a cavity 17 cm in diameter and 35-cm long that passes through the reactor core; it can be used to irradiate quite large objects. For example, this cavity accommodated a cell enclosed with polyethylene, which could be used to experimentally measure the gain in the mixture 3He-Xe-NF3 (X = 351 nm) [20]. Under such conditions, it was possible to obtain a specific power deposition of around 5 kW/cm3, the maximum possible using pulsed reactors.

In most of the experiments with the SPR-III reactor, the laser cell was positioned close to the surface of the reactor core (see studies [19, 61]). Most often a rectangular laser cell of 60 x 7 x 1 cm3 was used, placed inside a polyethylene neutron moderator (Fig. 2.22). The cell walls were coated with a layer of 235UO2 that was 3-^m thick. To register the shape of the thermal neutron pulse, a 1-m long cobalt-based neutron detector was placed inside the moderator. The distribution of the fluence of the thermal neutrons (specific energy deposition) along the 60-cm length of the cavity is non-uniform. At the ends of the moderator (30 cm from the center), the fluence of the thermal neutrons is around 25 % of the maximal value in the central part.

To study NPLs operating on the transitions of rare gas atoms, which can operate at lower specific power depositions of <100 W/cm3, the pulse duration of the SPR-III reactor was increased to several milliseconds. For this purpose, a “pulse

extender”—several grams of 235U—was placed in a cavity inside the reactor core in a polyethylene moderator with a wall thickness of a few centimeters. This device increased the effective lifetime of the neutrons in the reactor core, resulting in a threefold increase in the pulse duration [19].

To study the NPL characteristics, as a rule the experimental configurations shown in Fig. 2.23 were used [62]. Using these configurations, NPLs were studied

Fig. 2.24 Configuration of laser experiments on the TRIGA reactor [2, 30, 45]: (1 and 5) instruments registering the characteristics of the laser radiation, (2) system for supplying laser cells with the gases being studied,

(3) protective plug,

(4) thermal column (arrangement for withdrawal the beam of thermal neutrons),

(6) experimental chamber, (7 and 9) cavity mirrors,

(8) laser cell, (10) neutron detector, (11) reactor core, (12) graphite neutron reflector, (13) concrete shielding

at the transitions of atoms Ne (X = 585.3, 703.2, 724.5 nm), Ar (X = 1.27, 1.79 ^m), and Xe (X = 1.73, 2.03 |im).

The laser pulse energies were measured with calorimeters, while the shape was measured using silicon or InAs photodiodes. The energy deposition to the gas medium was determined from the jump in pressure, measured using a piezoelectric pressure sensor. To transport the laser radiation to and from the cell over a distance of around 20 m, lightguides—tubes with a diameter of 2.54 cm with a gold reflective coating—were used. Luminescent radiation emerging through the side surface of the cell was withdrawn beyond the biological shielding using a quartz optical fiber and was registered using a photodiode or a multichannel spectrometer with a registration range of 450-750 nm. Auxiliary lasers—a tunable F-center laser excited by radiation from a solid-state YAG:Nd3+ laser or a gas discharge laser— were used to measure the gain of the NPL.

TRIGA reactors are thermal neutron pool-type reactors, whose cores are made of the triple alloy UZrHx. These reactors are the most common of the research pulsed reactors—the number of their modification in the world in 1987 was 53 [2].

One of the reactors of the TRIGA family, namely the TRIGA Mark II, was used for many years to study NPL problems at the University of Illinois [29, 30]. The characteristic parameters of TRIGA pulsed reactors are: total number of fissions in the reactor core ~1 x 1018 fissions (~30 MJ), half-height pulse width ~10 ms, neutron fluence at the reactor center (0.5-1.0) x 1015 cm-2 [2]. The reactor can operate in stationary mode at a power of up to 1.5 MW with a thermal neutron flux density of 3 x 1012 cm-2 x s-1 [42].

The configuration of experiments on the TRIGA reactor is shown in Fig. 2.24. The typical length of the laser cell is 70-100 cm. The gas mixtures He-CO(CO2)

(X = 1.45 urn), Ne-N2 (X = 862.9; 939.3 nm), Ar-Xe (X = 1.73 |im), and 3He-Ne-H2 (X = 585.3 nm) were pumped by the nuclear reaction products 10B(n, a)7Li or 3He(n, p)3H. Various types of instruments were used to register the laser radiation (photomultipliers, photoresistors, monochromators, and power meters).

Apart from the studies of NPLs with direct pumping of active media by nuclear radiation, the TRIGA reactor was used to pump a photodissociation iodine laser (X = 1.31 um) with radiation of excimer molecules XeBr*, which are formed upon irradiation of the mixture Xe-CHBr3-3He by the neutron flux [29].