ELSEVIER Nuclear Physics A73 1 (2004) 49-56 www.elsevier.com/locate/npe Particle decay of the isoscalar giant dipole resonance in “‘Pb M. Hunyadi’“’ C. B%umerb, A.M. van den Berg”, N. Blasi”, M. Csatl&, L. Csiged, B. Davids”, U. Garg”, J. Gulyjs d, M.N. Harakeh”, M. de Huu”, B.C. Junkb, A. Krasznahorkay”, S. Raker&, D. Sohlerd, H.J. Wijrtche” ‘“Kernfysisch Versneller Instituut, 9747 AA Groningen, The Netherlands bWestf?&sche Wilhelms-Universitat, Miinster D-48149, Germany “INFN, Milan0 I-20133, Italy “Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, Hungary “University of Notre Dame, Notre Dame, IN 46556, USA The direct particle decay of the isoscalar giant dipole resonance (ISGDR) has been studied in “‘Pb using the (a, cu’p) and (cu, cy’n) reactions at a bombarding energy of 200 MeV. The direct-decay channels of proton and neutron emissions were observed in coincidence with the scattered a-particles in order to study the population of proton- hole states in 207T1 and neutron-hole states in ‘07Pb The results of the partial and total branching ratios allowed us to test recently performed continuum-RPA (CRPA) calculations for the ISGDR and thereby to learn about its microscopic structure. 1. INTRODUCTION The isoscalar giant dipole resonance (ISGDR) . 1s s 1 t’ll one of the most interesting collec- tive vibrational modes of nuclei. Its curiosity stems from the fact that in first-order the isoscalar dipole transition operator is associated with a spurious center-of-mass motion and the next-order term results in a compressional vibration of the nucleus. Therefore, the excitation energy of this resonance mode is directly related to the incompressibility of nuclear matter [1,2], which plays a crucial role in astrophysical processes through the nuclear equation of state. Several single-arm measurements have been reported on the ISGDR in some medium-heavy and heavy nuclei [3-81, which focussed on the precise, systematic determination of its excitation energy, offering an alternative and complemen- tary method to the studies of the nuclear incompressibility based on the isoscalar giant monopole resonance (ISGMR) [9,10]. The primary goal of the single-arm measurements on the ISGDR is to separate the isoscalar dipole strength sitting on a huge continuum and partially overlapping in excita- “Present address: Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, Hungary 0375-9474/$ - see front matter 0 2004 Elsevier B.V All rights reserved. doi:10.1016/j.nuclphysa.2OO3.11.018