Fission of complex nuclei induced by 52-MeV monochromatic and polarized photons

Abstract

Experimental fissility data from photofission reactions of Th-232, U-238, and U-235 nuclei at 52 MeV have been obtained using monochromatic and polarized photon beams and dielectric fission-track detectors. These data, along with literature data for Np-237, U-235,U-238 Th-232, Bi-209, Pb-208, and Yb-174 nuclei have been analyzed within the framework of a simple two-step model for photofission reactions, i.e., absorption of the incident photon by a neutron-proton pair followed by an evaporation-fission competition mechanism for the excited nucleus. For nuclei in the Ta-Np region the trend of calculated fissilities clearly shows shell effects in the vicinity of Pb-208. In the case of U-238 the effect of photon polarization on fission direction has also been studied, and isotropy was observed in the fragment azimuthal distribution

Analysis of209Bi and238U photofission cross section in the quasi-deuteron region of photonuclear absorption

“…8 c); the full line is the result of the Monte Carlo calculation by Lepr~tre et al [5]. Experimental data are as follows: EJ, Bellini et al [23]; o, Bellini et al [4]; o, Lepr~tre et al [5]; o, Moretto et al [24]; o, Ivanov et al [7]; v, Ahrens et al [3]; a, Minarik and Novikov [20]; o, Bernabei et al [6]; m, Martins et al [8]; A, Tavares et al [9]; ..... , Arruda-Neto et al [25] 415 mean value of 16 • 2 rob, in good agreement with Monte Carlo calculations at energies up to 75 MeV (solid curve from [5]). For photon energies beyond ,-,60MeV, the result of the present calculation (dashed curve) agrees quite well with that reported in [5], although marked differences there exist at the lower energies.…”

“…This model, although strongly dependent on the values of some nuclear parameters (level density parameter, for instance) involved in the calculations, has shown to be capable in explaining the trend of fissility (total fission probability, f) with photon energy, k. In addition, at fixed moderate incident energies (k ~ 50-70 MeV), the model has accounted for the existence of shell effects in fission which are clearly observed in the region ofZ = 82, N= 126 shell closures when nuclear fissility is studied as a function of parameter Z2/A of the target nucleus. Such effects have been put in evidence very recently [8,9].…”

“…The use of monochromatic photon beams of energy greater than 20 MeV or so to study photonuclear reactions has produced in the last twelve years a number of reliable data on photoreactions, particularly those concerned with fission reactions (photofission) of actinide nuclei [1][2][3][4][5][6][7][8][9] and, to a lesser extent, of heavy-metal nuclei of mass number A < 210 [8,[10][11][12][13][14]. For incident photon energies in the range ,-~ 30-140 MeV, the photo fission data have been generally interpreted on the basis of a model which considers the primary nuclear photoabsorption as taking place between the incoming photon and a neutronproton pair (quasi-deuteron photoabsorption mechanism first proposed by Levinger [15]) followed by a process of competition between nucleon evaporation and fission experienced by the excited residual nucleus [5, 8, 9, 12-14, 16, 17].…”

Photofission cross section and fissility of pre-actinide and intermediate-mass nuclei by 120 and 145 MeV Compton backscattered photons

“…In recent years high-energy monochromatic photon beams produced by backward scattering of a laser light against high-energy electrons have been widely used for the study of photoreactions [1]. In particular, photofission cross section and fissility data for actinide, preactinide, and intermediate-mass nuclei have been obtained at photon energies k 100 MeV with the LADON apparatus at the Frascati National Laboratories [2][3][4][5][6][7], and at photon energy k = 100 MeV with the ROKK-1M facility at the Budker Institute of Nuclear Physics (BINP, Novosibirsk) [8]. Tagged photons produced by the ROKK-2 facility at the storage ring VEPP-3 (BINP) have been recently used for the first time to measure photofissility of the 209 Bi nucleus in the range 60-270 MeV [9].…”

Photofission of27Al nucleus in the quasi-deuteron region of photonuclear absorption

“…Such a procedure enabled us to identify tracks of etch-pit opening larger than a minimal of d = (1.8 ± 0.1) µm. Based on the current scanning methodology and a low track population recorded in the present and previous measurements [15][16][17], a counting efficiency of c = 0.79±0.07 was assumed for track loss during track analysis. Mapping of the total of 127 tracks recorded was constructed from their coordinate positions (figure 3), which shows the recorded tracks distributed over an approximately circular region of diameter value compatible with that expected from the geometry of irradiation.…”