PHYSICAL REVIEW C 71, 044610 (2005) Multinucleon transfer processes in 40 Ca+ 208 Pb S. Szilner, 1,5 L. Corradi, 1 G. Pollarolo, 2 S. Beghini, 3 B. R. Behera, 1 E. Fioretto, 1 A. Gadea, 1 F. Haas, 4 A. Latina, 1 G. Montagnoli, 3 F. Scarlassara, 3 A. M. Stefanini, 1 M. Trotta, 1,∗ A. M. Vinodkumar, 1 and Y. Wu 1 1 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, I-35020 Legnaro, Italy 2 Dipartimento di Fisica Teorica, Universit` a di Torino, and Istituto Nazionale di Fisica Nucleare, I-10125 Torino, Italy 3 Dipartimento di Fisica, Universit` a di Padova, and Istituto Nazionale di Fisica Nucleare, I-35131 Padova, Italy 4 Institut de Recherches Subatomiques, CNRS-IN2P3/ULP et Universit´ e Louis Pasteur, F-67037 Strasbourg, France 5 Ru − d er Boskovi´ c Institute, HR-10 002 Zagreb, Croatia (Received 30 December 2004; published 28 April 2005) Multinucleon transfer reactions in 40 Ca+ 208 Pb have been studied at bombarding energies close to the Coulomb barrier. Projectilelike fragments have been identified in nuclear mass and charge with a time-of-flight spectrometer. Angular and total kinetic energy loss distributions and inclusive cross sections have been compared with those of semiclassical models. The analysis shows that a successive transfer mechanism of single nucleons does not account for the data, and a direct nucleon pair transfer has to be included in the description. Nucleon evaporation effects are taken into account. DOI: 10.1103/PhysRevC.71.044610 PACS number(s): 25.70.Hi, 24.10.−i, 25.70.Bc I. INTRODUCTION Nuclear reactions where few quanta—energy, angular momentum, and number of nucleons—are exchanged between projectile and target, provide an important tool to study the properties of nuclei close to their ground states. Among the different kinds of quasielastic reactions, the exchange of nucleons is peculiar because such reactions provide specific information on single-particle levels and correlations. The extensive experimental work with light-ion reactions, for example, gave considerable inputs to the pairing model. With heavy ions, these studies can be further extended. The two colliding partners can exchange a large number of nucleons, thus enabling to probe the pair density in the nuclear medium, that is, to probe the ability of two nucleons to form pairs of zero angular momentum [1]. From the reaction mechanism point of view, the study of multinucleon transfer reactions provides an insight about which degrees of freedom have to be included in any model to describe the evolution of the heavy-ion reaction from the quasielastic to the deep inelastic regimes and to fusion. Extensive work on multinucleon transfer have been carried out during past years [2,3]. These studies allowed to extract the radial dependence of the average transfer form factors for the different channels. Enhancement factors have been introduced to explain the deviation from the naive picture that in an uncorrelated medium the probability for the transfer of two nucleons is simply given by the product of the probability for the transfer of a single nucleon. In recent studies [4,5] a complete identification of the final reaction products has been achieved up to six neutron and proton transfer channels. The comparison between data and calculations, based on semiclassical models [6–9], allowed to identify the different degrees of freedom that influence the evolution of the reaction, such as deformation, single, and pair transfer modes. ∗ Present address: INFN-Sezione di Napoli, 1-80126, Napoli, Italy. Closed-shell nuclei constitute an almost ideal case for a quantitative comparison with calculations. Therefore, we have studied multinucleon transfer processes for the system 40 Ca+ 208 Pb at energies close to the Coulomb barrier. The differential and total cross sections and the total kinetic energy loss are reported and compared with semiclassical models. The results of pure neutron transfer channels were presented in a recent publication [10]. In this article we discuss the results for all multinucleon transfer channels. II. EXPERIMENT AND EXPERIMENTAL RESULTS The experiment was performed using the Tandem+ALPI accelerator complex of the Laboratori Nazionali di Legnaro. A 40 Ca beam was accelerated onto a 208 Pb target (200 µg/cm 2 , sandwiched between carbon foils) at incident energies of 235 and 249 MeV, which correspond to 5 and 15%, respectively above the nominal Coulomb barrier. Projectilelike fragments were detected with the time-of-flight spectrometer PISOLO [11], which combines a large solid angle (≃ 3 msr) and a good mass and a charge resolution (A/A ≃ 1/100 and Z/Z ≃ 1/60). Time-of-flight signals were derived from two microchannel plate detectors, whereas a multianode transverse field ionization chamber was used for nuclear charge and en- ergy determination [5,11]. Absolute normalization of the cross sections was ensured by four silicon monitors detecting elas- tically scattered 40 Ca ions. To cover most of the transfer flux the measurements have been performed in the angular range 42 ◦ <θ lab < 115 ◦ for the lower energy (E lab = 235 MeV) and 42 ◦ <θ lab < 100 ◦ for the higher bombarding energy (E lab = 249 MeV). Examples of A-Z and E-E two-dimensional spectra obtained at E lab = 235 MeV at the grazing angle are displayed in Fig. 1. A clear identification of the nuclear charge and mass of projectilelike fragments is obtained. In the right panel, the dashed lines correspond to the pure neutron pickup (N ) and the pure proton stripping (Z) and the full line represents the 0556-2813/2005/71(4)/044610(8)/$23.00 044610-1 ©2005 The American Physical Society