Z. Phys. A 354, 119–120 (1996) ZEITSCHRIFT F ¨ UR PHYSIK A c  Springer-Verlag 1996 Short note The 12 C + 12 C → 8 Be gs + 16 O gs reaction at E cm = 27 to 36 MeV M. Aliotta 1 , S. Cherubini 1, 2 , E. Costanzo 3, 4 , M. Lattuada 1, 3 , S. Romano 3, 4 , C. Spitaleri 1, 2 , A. Tumino 1, 3 , D. Vinciguerra 1, 3 , M. Zadro 5 1 INFN, Laboratorio Nazionale del Sud, Catania, Italy 2 Istituto di Fisica, Facolt` a di Ingegneria dell’Universit` a, Catania, Italy 3 Dipartimento di Fisica dell’Universit` a, Catania, Italy 4 INFN, Sezione di Catania, Catania, Italy 5 Rud er Boˇ skovi´ c Institute, Zagreb, Croatia Received: 18 September 1995 / Revised version: 7 December 1995 Communicated by C. Signorini Abstract. The 12 C + 12 C → 8 Be gs + 16 O gs reaction has been experimentally investigated at c.m. energies between 27 and 36 MeV. A resonance is present at 32.5 MeV, i.e. at the same energy of the resonance previously observed [1] in the 12 C(0 + 2 )+ 12 C(0 + 2 ) exit channel, but its width is about three times narrower. Moreover the data indicate an enhancement of the cross section at a c.m. energy close to 29 MeV. These results are discussed and compared to the predictions of the Band Crossing Model obtained for different exit channels of the 12 C + 12 C reaction. PACS: 25.70.Ef; 21.60.Gx; 27.30.+t The 5 MeV wide resonance observed in the 12 C + 12 C → 12 C(0 + 2 )+ 12 C(0 + 2 ), (1) reaction at E cm = 32.5 MeV has been related [1] to the ex- citation in 24 Mg of an α-chain configuration (six α-particles in a row). This interpretation was suggested by the fact that the second 0 + state at 7.65 MeV of 12 C can be described [2] as having a three loosely bound α-particle structure, with a component of 3 α-particles in a row [3]. The behaviour of the angular distributions [1] suggests that many J values are involved in the process. However the shape eigenstate model [4], which implies the coherent contribution of states with different J values, is successful in describing the angular distributions only around 90 o . In order to investigate further this region of the 24 Mg excitation energy, we studied the 12 C + 12 C → 8 Be gs + 16 O gs , (2) reaction at various c.m. energies between 27 and 36 MeV. The measurements were performed at the LNS Tandem ac- celerator in Catania, during several experiments. The re- sults of the first experiment at three incident energies only (E cm =27.9, 32.5 and 35.0 MeV) have already been reported Fig. 1. Differential cross section averaged over the angles between 73 0 and 106 0 , as a function of the incident energy. The empty symbols represent data already published in ref. [5]. The continuous line is the sum of two Breit-Wigner shapes and an exponential background. The two Breit-Wigner contributions have been centered at 28.7 and 32.5 MeV and have a FWHM of 1.4 MeV and of 1.2 MeV respectively [5]. An account of the experimental procedures is also given in ref. [5]. In this last experiment the uncertainty in the c.m. incident energy due to the energy loss in target was about ± 200 keV. Angular distributions were measured between 30 0 and 110 0 in the center of mass system but, in order to compare our data with the results of ref. [1], the excitation function for process (2) was obtained by averaging the experimental angular distributions in a narrower range, namely between 73 0 and 106 0 (see Fig.1). Anyway we checked that the main features of the excitation function are still present even when the whole measured angular range is taken into account.