No abstract

In the present paper we describe a target for the "Fakel" linear electron accelerator that serves as a source of neutrons for neutron-spectrometric investigations, conducted by the time-of-flight method in a wide energy interval from thermal energies up to several MeV. The neutrons are produced in a photonuclear reaction, produced in the target by bremsstrahlung formed in the same target by the accelerated electrons. The target is designed to work with electrons with energy up to 100 MeV and an average electron-beam current of up to 100/~A. A moderator (water) is used to increase the intensity of the resonance neutrons. To maximize the ratio of the flux of moderated neutrons to the dispersion of the time of flight of the neutrons from the moderator, characterizing the quality of the source in time-of-flight experiments, the neutron yield from the target must be maximized and the volume of the target must be minimized.The maximum neutron yield is achieved by using a converter material with a large nuclear charge Z. This is due to the fact that, first of all, the bremsstrahlung energy in a thick target (the thickness is greater than the radiation length) for high electron energies is proportional to ZE e, where E e is the energy of an electron [1]. Second, the critical electron energy (in MeV) above which the radiation losses predominate over ionization losses is -800/Z [2]. In addition, the position of the giant-resonance peak, characterizing the photoabsorption of 3,-rays, is displaced into the region of lower energy for heavier nuclei [3]; the photoabsorption cross section, integrated over the energy in the region of the giant resonance, is proportional to NZ/A, where N is the number of neutrons and A is the mass number [4]. A uranium target is preferable. It should be noted that the use of fissioning material increases the neutron yield from the target due to photofission. As one can see from Fig. 1 [5], the photofission cross section for 238U is comparable in order of magnitude to the cross section of (-g, n) and (3', 2n) reactions. The yield of neutrons per 50.5 MeV incident electron, which are produced in a 238U target with thicknesses g/cm 2 (cm) of 24.3 (1.25), 48.6 (2.5), and 72.9 (3.8) is equal to (%) 2.25 5: 0.18, 2.47 5: 0.22, and 2