Part III: Multi-chance fission

The results of model calculations on nuclide yields produced by fission of excited U nuclei are presented. The calculation combines the modelling of the deexcitation process including the fission competition with the prediction of the nuclide production in the fission of excited U and all daughter nuclei produced in the evaporation cascade. The calculations are relevant for the design of future secondary-beam facilities. The present work is the third of a series of reports on predicted fission-product yields for U. In the first part, we describe the details of the semi-empirical fission code PROFI used for the calculations. The model is a more advanced version of the statistical saddle-scission model described in ref. [1]. In the second part, we present the predictions for the nuclide production by first-chance fission of U at different excitation energies. In the third part, we calculate the complete nuclide production obtained from fission, when U is excited to different excitation energies. This calculation includes multi-chance fission. The forth part presents predictions on nuclide production by fission reactions, using U as a target or projectile nucleus, under a few selected experimental conditions. In these cases, many fissioning nuclei in a large excitation-energy range may contribute to the fission yields. Finally, in the fifth part we compare the model predictions with available data. The nucleus U is of prime importance for technical applications due to its high abundance. Further, it is the most neutron-rich fissile nucleus found in nature and, therefore, it seems to be well suited for the production of neutron-rich nuclides by fission in secondarybeam facilities. The calculations have been performed using the evaporation code ABLA [2]. The formulation of the level densities including collective excitations is documented in [3]. Dissipative effects in the fission process are included as described in Refs. [4,5]. The nuclide production in fission is calculated with the PROFI code [1]. The figures give an overview on calculated nuclide production in fission after excitation of U. The results are shown on the chart of the nuclides and as mass and nuclear-charge distributions. Predictions are given for a series of excitation energies. Fig. 5: Fission-fragment yields from U at an excitation energy of 10 MeV, that is 4 MeV above the fission barrier. The yields are normalised to 200%. Fig. 6: Fission-fragment mass distribution and nuclear-charge from U at an excitation energy of 10 MeV, that is 4 MeV above the fission barrier. The yields are normalised to 200%. Fig. 7: Fission-fragment yields from U at an excitation energy of 10 MeV, that is 14 MeV above the fission barrier. The yields are normalised to 200%. Fig. 8: Fission-fragment mass distribution and nuclear-charge distribution from U at an excitation energy of 20 MeV, that is 14 MeV above the fission barrier. The yields are normalised to 200%. Fig. 9: Fission-fragment yields from U at an excitation energy of 50 MeV, that is 44 MeV above the fission barrier. The yields are normalised to 200%. Fig. 10: Fission-fragment mass distribution and nuclear-charge distribution from U at an excitation energy of 50 MeV, that is 44 MeV above the fission barrier. The yields are normalised to 200%. Fig. 11: Fission-fragment yields from U at an excitation energy of 100 MeV, that is 94 MeV above the fission barrier. The yields are normalised to 200%. Fig. 12: Fission-fragment mass distribution and nuclear-charge distribution from U at an excitation energy of 100 MeV, that is 94 MeV above the fission barrier. The yields are normalised to 200%. [1] J. Benlliure, A. Grewe, M. de Jong, K.-H. Schmidt, S. Zhdanov, Nucl. Phys. A628 (1998) 458 [2] J.-J. Gaimard, K.-H. Schmidt, Nucl. Phys. A 531 (1991) 709 [3] A. R. Junghans, M. de Jong, H.-G. Clerc, A. V. Ignatyuk, G. A. Kudyaev, K.-H. Schmidt, Nucl. Phys. A 629 (1998) 635 [4] A. V. Ignatyuk, G. A. Kudyaev, A. Junghans, M. de Jong, H.-G. Clerc, K.-H. Schmidt, Nucl. Phys. A 593 (1995) 519 ]5] A. Heinz, B. Jurado, J. Benlliure, C. Böckstiegel, H.-G. Clerc, A. Grewe, M. de Jong, A. R. Junghans, J. Müller, K.-H. Schmidt, S. Steinhäuser, GSI Scientific Report 1999, GSI 20001, p. 30