APH N.S., Heavy Ion Physics 18/2–4 (2003) 267–271 HEAVY ION PHYSICS c Akad´ emiai Kiad´ o Clusters as Many-Body Resonances R. Id Betan, 1,2 R.J. Liotta, 1 N. Sandulescu 1,3 and T. Vertse 1,4 1 Royal Institute of Technology, SE-10691 Stockholm, Sweden 2 Departamento de Fisica, FCEIA, UNR Avenida Pellegrini 250, 2000 Rosario, Argentina 3 Institute of Physics and Nuclear Engineering P.O. Box MG-6, Bucharest-M˘ agurele, Romania 4 Institute of Nuclear Research of the Hungarian Academy of Sciences H-4001 Debrecen, Pf. 51, Hungary Received 10 January 2003 Abstract. A formalism to evaluate the resonant states produced by two par- ticles moving outside a closed shell core is presented. The two-particle states are calculated by using a single-particle representation consisting of bound states, Gamow resonances and scattering states in the complex energy plane (Berggren representation). Two representative cases are analysed correspond- ing to whether the Fermi level is below or above the continuum threshold. It is found that long-lived resonances are mostly determined by either bound states or by narrow Gamow resonances. However, they are significantly affected by wide resonances and the continuum background itself. Keywords: two-particle resonances, continuum, proton decay PACS: 21.10.-k, 24.30.Gd, 21.60.Cs 1. Introduction Present experimental facilities allow one to measure very unstable nuclei, for which the continuum part of the spectrum may be important. Much work has already been done in this subject, particularly regarding halo nuclei [1]. Still, the role played by single-particle resonances and of the continuum itself upon particles moving in the continuum of a heavy nucleus is not fully understood. For instance, one may wonder whether two particles outside a core where the Fermi level is immersed in the continuum may produce a quasibound state. If this is the case, the question remains of whether that state is built upon narrow single-particle resonances or whether it is induced by an interplay between the two-particle interaction and the 1219-7580/03/ $ 20.00 c  2003 Akad´ emiai Kiad´ o, Budapest