Atomic Energy, Vol. 88, No. 5, 2000 INTEGRAL CROSS SECTIONS OF PHOTONUCLEAR REACTIONS NEAR THE GIANT DIPOLE RESONANCE A. G. Belov, 1 Yu. P. Gangrskii, 1 K. K. Gudima, 2 and P. Zuzaan 3 UDC 539.172.2 The total cross sections of (7, n) reactions were measured for 12 nuclei from 45Sc to 2~ using an activational method on bremsstrahlung from a microtron with maximum energy 25 MeV. Calculations of the total cross sections were performed on the basis of a modified preequilibrium decay model. Good agreement was obtained between the experimental and computed cross sections in the mass number range 100-208. 1 figure, 3 tables, 13 references. Photonuclear reactions have a special place in nuclear physics. This is due to their special features. In the first place, the interaction of'/radiation with nuclei is an electromagnetic interaction, which makes it possible to give a correct description of the process [l]. As a result, the data on the structure of nuclei studied in such reactions are determined best and are virtually independent of model concepts. In the second place, the high intensity and high penetrating power of ~/radiation from modem electron accelerators makes it possible to achieve high yields for the nuclides being studied. Consequently, photonuclear reac- tions find wide applications in applied fields also - activation analysis and production of medical preparations. The purpose of the present work is to perform theoretical calculations and measurements of the integral cross sections of (~/, n) reactions of nuclei using an activational method with excitation energy near the giant dipole resonance. Such measure- ments expand the systematics of the cross sections of photonuclear reactions, and their comparison with calculations gives new information about the interaction with electromagnetic radiation. Although photonuclear reactions are being intensively studied, data on the integral cross sections of (y, n) reactions, measured with high accuracy using the same procedure under identical conditions, are clearly inadequate. In addition, many of these data (they were systematized in the review [2]) were obtained using a method of neutron detection that does not make it possible to take into account accurately the contribution of neutrons from more complex reactions, such as (y, 2n), (~/, pn), and others with energy above threshold (15-18 MeV). The activational method, where the final nucleus is clearly detected and which is used in the present work, excludes the contribution of these reactions. Experimental Procedure. The total cross sections of (y, n) reactions were measured in bremsstrahlung whose maxi- mum energy could be varied over wide limits (from 5 to 25 MeV). The bremsstrahlung source is an electron beam extracted from the MT-25 microtron in the Laboratory of Nuclear Reactions at the Joint Institute of Nuclear Research. A description of the microtron and its basic characteristics are presented in [3]. The electron energy was selected either by a transition from one orbit to another or by varying the magnetic field, and it was measured by measuring the magnetic-field intensity of the microtron by the nuclear magnetic resonance method and the frequency of the accelerating electric field. The measurements showed that the variance of the electron energy over the time of the experiment (several hours) did not exceed 50 keV. To obtain bremsstrahlung a 5 mm in diameter electron beam was directed onto a water-cooled, 2 mm thick, tungsten disk (stopping target), behind which a 30 mm thick aluminum absorber was positioned. The stopping target also served as a col- lector for the electron current, which was measured using an electric-charge integrator. 1 Joint Institute of Nuclear Research. 2 Institute of Physics, Moldavia. 3 University, Ulan-Bator, Mongolia. 2000. 408 Translated from Atomnaya l~nergiya, Vol. 88, No. 5, pp. 391-396, May, 2000. Original article submitted February 23, 1063-4258/00/8805-0408525.00 9 Kluwer Academic/Plenum Publishers