Fission of²⁷Al nucleus by 69 MeV monochromatic photons

Abstract

A b s b d . Monochromatic photon beams have been used to investigate lhe photofission of "AI at 69 MeV with plastic nuclear-track detectors. The measured fissility was found to he (5.7 * 1.1) X lo-'. Wis result, along with the available data for "' Sm and '74Yb nuclei taken at the same energy, has been interpreted within the frameaork of a two-step model for the photofission reaction. According to this model the incoming photon is considered to he absorbed by a neutron-proton pair, and then followed by an evaporation-fission competition for the exiled residual nucleus The measured fissility values are seen IO be in satisfactory agreement with the calculated ones, and the predictions from the model are confirmed, specially in the region of light fissioning systems.

Fissility of Bi, Pb, Au, Pt, W, Ta, V and Ti nuclei measured with 100 MeV Compton backscattered photons

“…These latter data represent the unique available information in the region of intermediate-mass nuclei. As expected it is found that, within the referred uncertainties, the set of data points obtained at k = 100 MeV lies between the fissility trends calculated for monoenergetic photons of 69 MeV [25,27] (full line) and of 600 MeV [39] (dotted line). Moreover, the data appear closer to the fissility trend calculated for 69 MeV than to that for 600 MeV.…”

“…As far as the 51 V and nat Ti targets are concerned, it is worth noticing that, for nuclei lighter than silver, both a study of nuclear fissility throughout the periodic table [38] and Monte Carlo calculations of photon-induced reactions based on the cascadeevaporation model [39] have indicated a clear trend of increasing fissility with decreasing Z 2 /A. Recently, in a study of 27 Al fission by 69 MeV monochromatic photons [27], such calculations have been extended towards less massive nuclei (left portion of the full line in figure 4). Although no trend of calculated fissility is at present available for incident photons of 100 MeV, the experimental values here determined for 51 V and nat Ti (∼ 10 −2 ) seem to confirm the predicted increasing of fissility for fissioning systems with Z 2 /A 20.…”

“…Since the total area to be scanned was very large (∼ 1440 cm 2 ), track identification on each detector was done by one observer (single scanning) and checked by a second one. Track loss during the optical analysis has been estimated from previous double scanning photofission experiments performed with the same methodology [25][26][27]. In this way, a counting efficiency of (79 ± 7)% was evaluated and thoroughly adopted in converting track counting into fission yields.…”

“…In some instances photofission cross sections for pre-actinide and intermediate-mass nuclei were obtained by unfolding the electrofission yields with a virtual photon spectrum [8][9][10][11]. In recent years the development of new techniques has allowed the use of monochromatic photon beams and/or tagged photons [12][13][14][15][16][17][18][19] to obtain more reliable photofission cross section data at energies k 140 MeV [20][21][22][23][24][25][26][27].…”

“…The current research represents an extension towards higher energies of that performed by some of us by using monochromatic photons of 69 MeV (LADON facility at the Frascati National Laboratories) [25][26][27]. Following the same approach, the present paper reports fissility data collected at a fixed energy for various target nuclei.…”