Z. Phys. A 355, 35-39 (1996) ZB'[SCHRIFT FOR Pt-I IK A (~) Springer-Verlag 1996 High-energy gamma rays emission in coincidence with light charged particles from the 328 -I- 74Ge reaction at 210 MeV F. Lucarelli l, N. Gelli I, P. Blasi I, M. Cinausero 2, E. Fioretto 2, E.M. Fiore 4, L. Fiore 4, V. Paticchio 4, P.F. Bortignon 5 t Istituto Nazionale di Fisica Nucleare and Dipartimentodi Fisica dell'Universita di 2 Laboratori Nazionali di Legnaro, 1-35020 Legnaro (Padova), Italy 3 Istituto Nazionale di Fisica Nucleare and Dipartimentodi Fisica dell'Universith di 4 Istituto Nazionale di Fisica Nucleare and Dipartimentodi Fisica detl'Universit~ di 5 Istituto Nazionale di Fisica Nucleare and Dipartimentodi Fisica dell'Universit~ di G. Prete 2, D. Fabris 3, G. Nebbia 3, G. Viesti 3, Firenze, 1-50125 Firenze,Italy Padova, 1-35131 Padova, Italy Bat'i, 1-70126 Bat'i, Italy Milano, 1-20133 Milano, Italy Received: 6 November 1995 / Revised version: 20 December 1995 Communicated by B. Herskind Abstract. High-energy ")'-rays from the 32S + 74Ge reaction at 210 MeV bombarding energy were measured in coinci- dence with light charged particles detected in a large area hodoscope. Experimental results show that energetic "/-rays in coincidence with light charged particles are essentially emitted in the compound nucleus decay. The parameters of the giant dipole resonance (GDR) have been extracted from a lineshape analysis of the experimental y-ray spec- trum. The derived values of mean energy ED, width F and strength S are in good agreement with results from previ- ous experiments on Sn isotopes obtained by using different experimental techniques. PACS: 24.30 Cz;25.70 Gh 1 Introduction The measurement of the high-energy "/-rays emitted in heavy-ion reactions provides informations on the properties of nuclei at high temperature. Special interest is set on the energy range E7=10+20 MeV, being related to the decay of the giant dipole resonance (GDR) built on highly excited states of nuclei [1]. In fact, the parameters characterizing the GDR (mean energy ED, width F and strength S) depend on fundamental collective nuclear properties such as the sym- metry energy, the size and the shape of the nuclei and the coupling of the collective motion to the surface vibration [2]. Furthermore, even at moderate temperature (T ~ 2+3 MeV), -,/-emission from GDR states mostly occurs in the first steps of the de-excitation chain and thus represents a direct probe of the initial compound system. A systematic study of the GDR parameters, as done in the A ~ 110 mass region (Sn isotopes)[2-8], makes possible, in principle, to follow the evolution of collective nuclear properties as a function of angular momentum and temperature. The main experimental difficulty in detecting high-energy ?-rays is related to their low multiplicity (approximately 10 -3 high-energy photons per nuclear decay). For low bom- barding energy, hard "),-rays are essentially produced in the decay of the compound nucleus (CN), therefore ')'-emission from GDR states has been studied in inclusive experiments [2]. When the bombarding energy is increased, contributions to the total "3'-ray spectrum coming from different reaction mechanisms start to be significant and more complex exper- imental techniques are required. A widely used technique consists in the measurement of the low-energy "),-rays mul- tiplicity in coincidence with the hard y-rays [3-5]. In this way, the contributions from central collisions are magnified by the rejection of the low multiplicity events, which are of- ten associated with peripheral reactions. Some high-energy y-rays measurements in coincidence with the evaporation residues (ER) were also performed at bombarding energies near or above the threshold (,-~10 MeV/amu) for the onset of the incomplete fusion [7-10]. Exclusive GDR measure- ments in coincidence with isomeric states or discrete y-lines in residual nuclei are also available in the literature [6, 11, 12]. An alternative way to tag fusion-evaporation events is the detection of the light charged particles emitted during the CN decay. This technique has been successfully used in the past for lighter nuclei such as Cu and Ca isotopes [13, 14] and turned out to be more efficient compared to an evaporation residues coincidence measurement. In this work, the same experimental method has been applied in the A ~ I10 mass region to study the GDR at moderate temperature. 2 Experimental details The experiment was performed at the XTU Tandem facility of the Laboratori Nazionali di Legnaro (Italy). A 210 MeV 32S beam was focussed onto a 280 #g/cm 2, 95% enriched, self-supporting 74Ge target. The l°6Cd compound nucleus was formed at total excitation energy Ex=134 MeV. The critical angular momentum for the fusion-evaporation chan-