Laboratory Neutron Sources

Of the powerful neutron sources, the most widespread are the nuclear reactors, which were used to perform the bulk of investigations to find active NPL media and to study their characteristics. Nuclear reactors are sources not only of neutron radiation, but also of у radiation; however, for pumping NPLs, neutrons are used, since in this case it is possible to obtain a specific energy deposition to the laser medium approximately two orders of magnitude greater than the energy deposition from у radiation.

In experiments investigating NPLs with help of reactors, as a rule direct pumping of active media is carried out using not neutron radiation, but the products of exothermal nuclear reactions, which take place during interaction of neutrons with the nuclei of 235U, 10B and 3He (Table 1.4).

For effective excitation of the gas medium, it is necessary for the isotopes that interact with the neutrons to be in immediate contact with the laser medium. When nuclear reactors are used as neutron sources, two basic types of laser-medium excitation are utilized (Fig. 1.1): (1) a gaseous isotope or compound thereof (3He, 235UF6) is a component part of the laser medium; (2) the internal surface of the gas-filled laser cell is coated with a thin layer of isotope (10B, 235U) or compound thereof (235UO2, 235U3O8). In studies published up to this time, both methods are used. From the comparison made in the first section of Chap. 7 of the efficiency of pumping gas NPLs using the isotopes He, U, and B, it follows that approx­imately identical energy contributions to gas media can be obtained using the

Isotope (energy of reaction, MeV)

Natural

composition of isotope

Cross-section of reaction for thermal neutrons, barns

Reaction

products

Kinetic energy of reaction products, MeV

Path length of reaction products in air at 1 atm pressure, cm

3He (0.76)

4He (100 %)+

5,400

!H

0.57

1.0

+3He (0.00014 %)

3H

0.19

0.2

10B (2.3)

nB (80.4 %)+

3,800

4He

1.5

0.9

10B (19.6 %)

7Li

0.8

0.4

235U (167)

238U (99.28 %)+

580

Light

99

2.3

235U (0.72 %)

fragment

68

1.8

Heavy

fragment

gaseous isotope 3He and a thin layer of 235U. However, in the first case, it is necessary to use only 3He as the buffer gas; this substantially limits the possibilities of gas mixture selection. When thin uranium layers are used, the maximal specific power deposition (q ~5 x 103 W/cm3 for gas media) is achieved in experiments with pulsed reactors with a minimal pulse duration of ~100 ^s. This pumping method is interesting because on its basis, it is possible to create powerful nuclear-laser units (reactor-lasers) in the core of which uranium layers are used not only to excite the laser medium, but also as nuclear fuel.

The specific power deposition and uniformity of excitation of laser media depend on the magnitude and duration of the neutron flux, the pumping method, the geometry and dimensions of the laser cell, the type of nuclear particles, and the gas pressure.

In the case of a volumetric source of pumping using 3He, the non-uniformity of pumping comes from the absorption of slow neutrons in 3He and from the reduction of the energy contribution in the region near the wall owing to the removal of reaction products (1H, 3H) to the walls of the cell. Results of computation of the total energy deposition and spatial distribution of the deposited energy depending on the 3He pressure and the diameter of the cylindrical cell are given in [67, 68], while [69] shows the results of computation of the energy deposition for a 235UF6- He mixture. The use of the gaseous (at comparatively low temperatures) compound 235UF6 makes it possible in principle to obtain an efficiency of nuclear energy deposition in the gas mixture of up to 100 %, but the search for nuclear-pumped gas media based on 235UF6 to date has not yielded a positive result owing to the high rates of “quenching” of excited atoms by molecules of UF6.

In the case of a surface pumping source, the area of uniform excitation is determined by the transverse dimension of the laser cell and the path length of fission fragments or a particles, which for various gases at atmospheric pressure is 1-10 cm. Computations of the total energy deposition and its spatial distribution for laser cells in the shape of a cylinder and a rectangular parallelepiped with layers of 235U applied to the internal surface, depending on the thickness of the uranium

Подпись: b

Подпись: Neutron Подпись: Neutron Thin layers of 235U or 10B

a

Fig. 1.1 Methods of exciting NPL gas media: (a) volumetric method of pumping, (b) surface method of pumping layer, dimensions of the laser cell, and the gas pressure (as well as experimental methods of determining the energy deposition and the results of its measurement) are provided in Chap. 7. Here we note that when uranium layers are used, the efficiency of energy deposition from fission fragments in the gas medium does not exceed 50 % (an infinitely thin layer) and as a rule is 15-20 % when a thickness of uranium layer equal to one half of fragment path length in layer material (~5 mg/

cm2).

As was noted above, basically nuclear reactors were used as neutron sources in experiments with NPLs. Among nuclear reactors, pulsed aperiodic reactors [70, 71] with a pulse duration ranging from ~50 qs to ~10 ms possess the maximal possible neutron fluxes; they provide repeated and controlled fission bursts of uranium nuclei. The organization of experiments with NPLs using pulsed reactors is con­sidered in Chap. 2.

Apart from aperiodic pulsed reactors, for the pumping of NPLs it is also possible to use periodic pulsed reactors [72], although the neutron fluxes which they supply are roughly 10-50 times less than in the case of aperiodic reactors. The review [28] mentions an experiment conducted in 1985 by associates of MIFI and IOFAN using the pulsed periodic reactor IBR-30 (Joint Institute of Nuclear Research (OIYal, Dubna)). This experiment, carried out using gas mixtures He-Ne-Ar and He-Ar-Xe, did not yield a positive result, which may be explained, the authors believe, by the low power deposition.

Stationary nuclear reactors have substantially lower neutron fluxes than pulsed reactors. In stationary research reactors such as the IRT-2000 and the VVR, the thermal-neutron flux densities are ~1013 cm-2 s-1 (specific power deposition of gas media up to ~1 W/cm3), which is not sufficient for researching most NPLs, especially in the stage of the search for new laser media. Therefore, experiments with stationary reactors were directed chiefly at studying the spectral-luminescent plasma characteristics and electroionization lasers (see, for example, [54]). It should be noted that there are special SM-type stationary reactors [73], in which thermal-neutron flux densities in the central cavity of the reactor core reach

2 x 1015 cm-2 s-1. However, experiments with NPLs with an SM reactor are hampered owing to the limited volume of space with such a high neutron flux.

Of the other possible laboratory neutron sources for NPL pumping, it was proposed that the neutron radiation from tokomaks [74] could be used. Devices based on high-density plasma (plasma focus, Z-pinch) [75] could also be used for this purpose.

The options for pumping NPLs using neutron sources that were mentioned previously use nuclear reactions that take place with the interaction of nuclei of certain isotopes with thermal neutrons. To increase the flux of thermal neutrons, laser cells were surrounded by a fast-neutron moderator (Plexiglas, polyethylene, graphite). The maximal pressures of gas media do not exceed 5-6 atm, which is due to the appearance of excessive non-uniformity of pumping with a growth of pressure because of the shortening of the path length of the nuclear reaction products or weakening of the thermal neutron flux. One method of uniform pumping of gas media (at pressures of tens and hundreds of atmospheres) and of condensed media is the use of the elastic scattering of slow neutrons on atoms (nuclei) of the medium. In this case, ionization and excitation of the gas medium are carried out by recoil nuclei. This NPL pumping method was first used at VNIIEF [76] and is considered in the third section of Chap. 3 of this book.

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