Model predictions of the fission - product yields for 238 U Part IV : Selected case studies

The results of model calculations on nuclide yields produced in selected scenarios using reactions with U nuclei are presented. The calculation combines the modelling of the initial reaction mechanism, 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 forth 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 dedicated nuclear-reaction code ABRA which describes electromagnetic excitations (see [2]) and high-energy nuclear collisions (see [3]). The deexcitation process is calculated with the evaporation code ABLA [4]. The formulation of the level densities including collective excitations is documented in [5]. Dissipative effects in the fission process are included as described in Refs. [6,7]. The nuclide production in fission is calculated with the PROFI code [1]. Interactions of electrons and gamma rays with matter have been evaluated with the GEANT code. The figures give an overview on calculated nuclide production in fission in some selected reactions with U nuclei. The results are shown on the chart of the nuclides, as mass and nuclear-charge distributions and as isotopic distributions. Even-odd fluctuations in the isotopic cross sections are not realistic. They originate from a slightly inconsistent treatment of pairing effects in binding energies and level densities. 1. Fission induced in the passage of 1 A GeV U projectiles through a lead target. In this scenario, the U projectiles may either be excited by electromagnetic interactions or may be involved in nuclear collisions with the target nuclei, depending on the impact parameter of the collision. Electromagnetic interactions mostly excite the giant dipole resonance and lead to fission with strong signatures of shell structure. Nuclear collisions induce appreciable higher excitation energies, leading to symmetric fission with a broad mass distribution and to the production of more neutron-deficient nuclides. Fig. 1: Yields of fission-fragments produced from projectile-like fragments in the passage of U projectiles with a kinetic energy of 1 A GeV through a lead target. Fig. 2: Mass and nuclear-charge distributions from fission after the passage of 1 A GeV U through a lead target. Fig. 3a: Isotopic production cross sections from fission after the passage of 1 A GeV U through a lead target. Fig. 3b: Isotopic production cross sections from fission after the passage of 1 A GeV U through a lead target. 2. Fission induced by the spallation of U with 200 MeV protons. In the spallation of U, many highly excited fissile nuclei are produced. This results in a broad distribution of fission fragments in mass and neutron excess. The bombarding energy has a decisive influence on the nuclide production. In interactions with protons at 200 A MeV relative energy, the shell structure still partly survives. Fig. 4: Yields of fission-fragments produced in the spallation of U by 200 MeV protons.. Fig. 5: Mass and nuclear-charge distributions from fission induced in the spallation of U by 200 MeV protons. Fig. 6a: Isotopic production cross sections from fission in the spallation of U by 200 MeV protons. Fig. 6b: Isotopic production cross sections from fission in the spallation of U by 200 MeV protons. 3. Fission induced by the spallation of U with 1 GeV protons. In the spallation of U with 1 GeV protons, shell effects have almost disappeared. A broad distribution of fission fragments in mass and neutron excess is obtained. Fig. 7: Yields of fission-fragments produced in the spallation of U by 1 GeV protons. Fig. 8: Mass and nuclear-charge distributions from fission induced in the spallation of U by 1 GeV protons. Fig. 9a: Isotopic production cross sections from fission in the spallation of U by 1 GeV protons. Fig. 9b: Isotopic production cross sections from fission in the spallation of U by 1 GeV protons. 4. Fission induced by the spallation of U with 1.4 GeV protons. By increasing the bombarding energy from 1 GeV to 1.4 GeV, the nuclide distribution is only slightly broadened. Fig. 10: Yields of fission-fragments produced in the spallation of U by 1.4 GeV protons. Fig. 11: Mass and nuclear-charge distributions from fission induced in the spallation of U by 1.4 GeV protons. Fig. 12a: Isotopic production cross sections from fission in the spallation of U by 1.4 GeV protons. Fig. 12b: Isotopic production cross sections from fission in the spallation of U by 1.4 GeV protons. 5. Photo-induced fission of U by bremsstrahlung from 30 MeV electrons. Recently it has been proposed by Y. Oganessian et al. to use bremsstrahlung, produced by stopping an electron beam in a converter target, for inducing fission of U. The electron beam hits a thin tungsten converter target and is absorbed. The photons, mostly emitted in forward direction, hit a thick uranium target. Part of them are absorbed by the uranium nuclei and induce fission. We used GEANT to calculate the bremsstrahlung spectrum in a tungsten converter target and the propagation of the photons through the converter target as well as their absorption in the uranium target. Nuclear absorption and the consecutive decay including fission was calculated with the ABLA code. In our calculations we obtained the following parameters for optimum conditions: The thickness of the tungsten converter target is 2 mm, corresponding to one radiation length. The uranium target is 30 mm thick, corresponding to approximately 10 radiation lengths. In our calculation we use a pure uranium target, although this might not be optimum for extracting the radioactive nuclei with the ISOL method. The energy of the electrons is a critical parameter for the fission yield per beam current. Its influence on the isotopic distribution is rather weak. In the present chapter we present the results for a beam energy of 30 MeV. The rates reported in the figures correspond to the production in the target. Any extraction losses are not considered. Fig. 13: Rates per second of fission-fragments produced by bremsstrahlung in a thick U target. The bremsstrahlung originates from the bombardment of a 2 mm tungsten converter with a beam of 1 mA electrons with an energy of 30 MeV. Fig. 14: Mass and nuclear-charge rate distributions from fission of U induced by bremsstrahlung from a 1 mA electron beam of 30 MeV. Fig. 15a: Isotopic production rates from fission of U induced by bremsstrahlung from a 1 mA electron beam of 30 MeV. Fig. 15b: Isotopic production rates from fission of U induced by bremsstrahlung from a 1 mA electron beam of 30 MeV. 6. Photo-induced fission of U by bremsstrahlung from 50 MeV electrons. The scenario used for the calculations in this section are identical to those in the preceding one with one exception. The energy of the electron beam was increased to 50 MeV. This results in a higher yield for a given beam intensity. Compared to the previous case of 30 MeV, the fission yield has almost doubled. The changes in the isotopic distribution are minor. Fig. 16: Rates per second of fission-fragments produced by bremsstrahlung in a thick U target. The bremsstrahlung originates from the bombardment of a 2 mm tungsten converter with a beam of 1 mA electrons with an energy of 50 MeV. Fig. 17: Mass and nuclear-charge rate distributions from fission of U induced by bremsstrahlung from a 1 mA electron beam of 50 MeV. Fig. 18a: Isotopic production rates from fission of U induced by bremsstrahlung from a 1 mA electron beam of 50 MeV. Fig. 18b: Isotopic production rates from fission of U induced by bremsstrahlung from a 1 mA electron beam of 50 MeV. [1] J. Benlliure, A. Grewe, M. de Jong, K.-H. Schmidt, S. Zhdanov, Nucl. Phys. A628 (1998) 458 [2] K.-H. Schmidt, S. Steinhäuser, C. Böckstiegel, A. Grewe, A. Heinz, A. R. Junghans, J. Benlliure, H.-G. Clerc, M. de Jong, J. Müller, M. Pfützner, B.. Voss, Nucl. Phys. A 665 (2000) 221 [3] J. Benlliure, K. Helariutta, M. V. Ricciardi, K.-H. Schmidt, GSI Preprint 2000-41 [4] J.-J. Gaimard, K.-H. Schmidt, Nucl. Phys. A 531 (1991) 709 [5] A. R. Junghans, M. de Jong, H.-G. Clerc, A. V. Ignatyuk, G. A. Kudyaev, K.-H. Schmidt, Nucl. Phys. A 629 (1998) 635 [6] A. V. Ignatyuk, G. A. Kudyaev, A. Junghans, M. de Jong, H.-G. Clerc, K.-H. Schmidt, Nucl. Phys. A 593 (1995) 519 ]7] 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