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Pre-equilibrium dipole strength excitation in dissipative heavy-ion collisions
NUCLEAR PHYSICS A H s~vIH~
Nuclear Physics A583 (1995) 119-122
Pre-equilibrium dipole strength excitation in dissipative heavy-ion collisions L. Campajola °*, A. De Rosa°*, A. D'Onofrio**, L. Gialanella*, G. Inglima °*, M. La Commara*, A. Ordine*, D. Pierroutsakou*, V. Roca°*, M. Romano °*, M.Romoli*, M. Sandoli °*, F. Terrasi °* F. Amorini °^, G. Cardella +, R. Croce °*, A. Di Pietro °^, A. Musumarra °^, M. Papa ^, G. Pappalardo °^, F. Rizzo °", J.P.S. Van Shagen °^ N. Alamanos", F. Auger", A. Gillibert", D. Prosperi', G. De Angelisx, E. Fioretto x °Dipartimento di Scienze Fisiche, Universitd di Napoli Feder~co II. *INFN, Sezione di Napofi 1-80125 Napoli, Italy **Dipartimento di Fisica Teorica e smsa, Universit~ di Salerno. INFN, Sezione di Napofi 1-80125 Napoli, Italy °Dipartimento di Fisica, Universit& di Catania, +INFN, Sezione di Catania ,AINFN, Laboratorio nazionale de/Sud 1-95129 Catania, Italy "CEAJDAPNIA/SPhN Saclay , 91191 Gif sur Yvette cedex, France 'Dipartimento di fisica, Universit~ di Roma La Sapienza, 'INFN, Sezione di Roma, 1-00185 Roma, Italy XlNFN, Laboratori Nazionali di Legnaro, 1-35131 Padova, Italy
PACS numbers: 25.70-z, 24.30.Cz, 25.70.Lm The study of the evolution of the properties of the giant dipole resonance (GDR) as a function of the excitation energy has been a major field of interest in the last years 1. While the GDR resonance energy remains approximately constant over a large range of excitation energy, both its width and strength change. In the experimental data the former increases up to about twice the ground state value and the latter shows a saturation at high excitation energy. These outcomes stimulated the formulation of several models which could justify the experimental data2, 3. In this framework it was also pointed out that a significant amount of dipole strength could be excited at the very early stages of the nuclear interaction leading to preequilibrium giant dipole '~-ray emission. Two mechanisms have been proposed which could support such a possibility: the existence of a dipole moment due to the charge asymmetry between the reaction partners and the mass asymmetry in the entrance channel2, 4. In this paper we report on the first experimental evidences of preequilibrium giant dipole 7-raY emission from the intermediate systems produced in the 35C1+64Ni and 35C1+92M0 reactions at a collision energy of 271 and 260 MeV respectively. The results relative to the last reaction have to be considered to some extent as preliminary being the analysis still in progress. The 35CI pulsed beam of the superconducting heavy-ion linear accelerator of the CEA-DAPNINDPhN of Saclay (France), impinging on a self supporting 64Ni and 92Mo targets about 300 mg/cm 2 thick, was used. Coincidence spectra between 7-rays and complex fragments produced through a dissipative reaction mechanism were measured. The coincidence y-ray spectra include the photons coming from the decay of the intermediate system before its fragmentation in addition to those coming from the Elsevier Science B.V. SSDI 0375-9474(94)00644-X
120c
L. Campajola et al. /Nuclear Physics A583 (1995) 119-122
excited fragments. The dissipative complex fragments were detected and charge identified in an array of three-stage detectors consisting of a gas ionization chamber followed by a silicon strip detector and a Csl crystal. The experimental apparatus has been extensively described elsewhere5. The 7-rays were detected in coincidence with the charged ejectiles using BaF2 crystals of the TAPS standard arranged in seven pack-elements and placed at a distance from the target suitable to discriminate against neutrons using a TOF technique. In this way the laboratory kinetic energy spectra of the light fragments and the energy spectrum of the 7-rays detected in coincidence with each final light fragment have been obtained. In order to evidence in the 7-raY spectra the contribution coming from the intermediate system, the background of the excited fragments decay has to be eliminated. To this aim, being the fragments completely equilibrated, their 7-ray spectra have been calculated within the statistical model using the code CASCADE. The code was also used to determine all the primary fragmentations which could populate the same final light fragment, in the assumption of a binary reaction. The input parameters required for the calculations have been determined in the following framework. The angular momentum transferred to the exit channel has been determined according to the sticking model. The partition of the final angular momentum between the fragments has been performed according to their moment of inertia. The used spin values ranged from 3h to 16h and from 18h to 34h for the ligth and heavy fragment respectively. The total thermal energy has been shared among the fragments according to their mass in the assumption of an equilibrated intermediate system with respect to thermal degrees of freedom. The excitation energies ranged from 16 to 64 MeV and from 60 to 93 MeV for light and heavy fragment respectively. A linear combination of the CASCADE spectra, relative to all the contributing primary fragmentations, has been used to fit the experimental spectrum of ~/-rays in coincidence with each detected light fragment. The outcome of such a procedure is the calculated statistical '~-ray spectrum which best fits the corresponding experimental one. The theoretical spectra have been folded with the experimental set-up response function using the code GEANT3. To increase the statistics, all the experimental coincidence y-ray spectra and all the calculated ones have been summed. The cumulative experimental (points) and calculated spectra (solid lines) are reported in Figure l a and 2a for the 35C1+64Ni and 35C1+92Mo reactions respectively. Then the ratio between the two corresponding pairs of cumulative spectra have been determined to evidence any y-ray excess with respect to the expected statistical emission from the fragments. The results of the above procedure are reported in Figure lb and 2b. In both cases a resonance-like bump is evidenced between 8 and 12 MeV. By fitting the bumps using a Lorentz curve, the resonance parameters have been determined to be Eres= (10.6 _+0.2) MeV with a FWHM F= (3.0 _+0.6) MeV and Eres= (11.0 _+0.1) MeV with a FWHM F= (2.2 _+ 0.7) MeV respectively. Being the contribution from the compound nucleus prevented by the coincidence technique, the resonances have been attributed to the excitation of dipole strength in the deformed intermediate system. Gamma-rays from these collective modes of the intermediate system before its fragmentation can be observed as the mean lifetime for fragmentation has been found 6 to be 10-21 s and the time the GDR needs to be excited is 10 -22 s. The displacement of the observed resonances with respect to the GDR of the compound nuclei indicates a deformation parameter of the intermediate systems ~ =0.72 and t3=0.54 respectively in the assumption of a prolate deformation.
L. Campajola et al. / Nuclear Physics A583 (1995) 119-122
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In conclusion dipole strength has been observed for the first time in the very deformed intermediate systems formed in the 35C1+64Niand 35C1+92Mo dissipative heavy ion collisions at incident energies of about 7 A MeV. This statement is supported by the results of microscopic calculations 4 based on the Vlasov equation performed for the 35C1+64Ni dissipative reaction at impact parameter b-- 6 fm and 270 MeV incident energy which showed that dipole strength is concentrated in the energy interval between 8 and 12 MeV. This strength corresponds to oscillations along the rotating elongation axis of the dinuclear intermediate system. Future experiments will be dedicated to the study of the entrance channel influence on the pre-equilibrium dipole strenght excitation in the intermediate system.
References 1 J.J. Gaardhoje Ann. Rev. Nucl. Part. Sci. 42,483 (1992) and references therein. 2 Ph. Chomaz, M.Di Toro and A.Smerzi Nucl. Phys. A563, 509 (1993). 3 R.A. Broglia, P.F. Bortignon, and A. Bracco, Prog. Part. Nucl. Phys. 28,517(1992). 4 Cai Yanhuang et al. Proceedings of the Int. Conf. on Dynamical Features of Nuclei and Finite Fermi Systems, Barcelone 1993, X. Vinas Editor (in press). 5 A. De Rosa et al., NIM A342, 534(1994). 6 A. De Rosa et al. Phys. Rev. C44, 747 (1991) and references therein.
Nuclear Physics A583 (1995) 119-122
Pre-equilibrium dipole strength excitation in dissipative heavy-ion collisions L. Campajola °*, A. De Rosa°*, A. D'Onofrio**, L. Gialanella*, G. Inglima °*, M. La Commara*, A. Ordine*, D. Pierroutsakou*, V. Roca°*, M. Romano °*, M.Romoli*, M. Sandoli °*, F. Terrasi °* F. Amorini °^, G. Cardella +, R. Croce °*, A. Di Pietro °^, A. Musumarra °^, M. Papa ^, G. Pappalardo °^, F. Rizzo °", J.P.S. Van Shagen °^ N. Alamanos", F. Auger", A. Gillibert", D. Prosperi', G. De Angelisx, E. Fioretto x °Dipartimento di Scienze Fisiche, Universitd di Napoli Feder~co II. *INFN, Sezione di Napofi 1-80125 Napoli, Italy **Dipartimento di Fisica Teorica e smsa, Universit~ di Salerno. INFN, Sezione di Napofi 1-80125 Napoli, Italy °Dipartimento di Fisica, Universit& di Catania, +INFN, Sezione di Catania ,AINFN, Laboratorio nazionale de/Sud 1-95129 Catania, Italy "CEAJDAPNIA/SPhN Saclay , 91191 Gif sur Yvette cedex, France 'Dipartimento di fisica, Universit~ di Roma La Sapienza, 'INFN, Sezione di Roma, 1-00185 Roma, Italy XlNFN, Laboratori Nazionali di Legnaro, 1-35131 Padova, Italy
PACS numbers: 25.70-z, 24.30.Cz, 25.70.Lm The study of the evolution of the properties of the giant dipole resonance (GDR) as a function of the excitation energy has been a major field of interest in the last years 1. While the GDR resonance energy remains approximately constant over a large range of excitation energy, both its width and strength change. In the experimental data the former increases up to about twice the ground state value and the latter shows a saturation at high excitation energy. These outcomes stimulated the formulation of several models which could justify the experimental data2, 3. In this framework it was also pointed out that a significant amount of dipole strength could be excited at the very early stages of the nuclear interaction leading to preequilibrium giant dipole '~-ray emission. Two mechanisms have been proposed which could support such a possibility: the existence of a dipole moment due to the charge asymmetry between the reaction partners and the mass asymmetry in the entrance channel2, 4. In this paper we report on the first experimental evidences of preequilibrium giant dipole 7-raY emission from the intermediate systems produced in the 35C1+64Ni and 35C1+92M0 reactions at a collision energy of 271 and 260 MeV respectively. The results relative to the last reaction have to be considered to some extent as preliminary being the analysis still in progress. The 35CI pulsed beam of the superconducting heavy-ion linear accelerator of the CEA-DAPNINDPhN of Saclay (France), impinging on a self supporting 64Ni and 92Mo targets about 300 mg/cm 2 thick, was used. Coincidence spectra between 7-rays and complex fragments produced through a dissipative reaction mechanism were measured. The coincidence y-ray spectra include the photons coming from the decay of the intermediate system before its fragmentation in addition to those coming from the Elsevier Science B.V. SSDI 0375-9474(94)00644-X
120c
L. Campajola et al. /Nuclear Physics A583 (1995) 119-122
excited fragments. The dissipative complex fragments were detected and charge identified in an array of three-stage detectors consisting of a gas ionization chamber followed by a silicon strip detector and a Csl crystal. The experimental apparatus has been extensively described elsewhere5. The 7-rays were detected in coincidence with the charged ejectiles using BaF2 crystals of the TAPS standard arranged in seven pack-elements and placed at a distance from the target suitable to discriminate against neutrons using a TOF technique. In this way the laboratory kinetic energy spectra of the light fragments and the energy spectrum of the 7-rays detected in coincidence with each final light fragment have been obtained. In order to evidence in the 7-raY spectra the contribution coming from the intermediate system, the background of the excited fragments decay has to be eliminated. To this aim, being the fragments completely equilibrated, their 7-ray spectra have been calculated within the statistical model using the code CASCADE. The code was also used to determine all the primary fragmentations which could populate the same final light fragment, in the assumption of a binary reaction. The input parameters required for the calculations have been determined in the following framework. The angular momentum transferred to the exit channel has been determined according to the sticking model. The partition of the final angular momentum between the fragments has been performed according to their moment of inertia. The used spin values ranged from 3h to 16h and from 18h to 34h for the ligth and heavy fragment respectively. The total thermal energy has been shared among the fragments according to their mass in the assumption of an equilibrated intermediate system with respect to thermal degrees of freedom. The excitation energies ranged from 16 to 64 MeV and from 60 to 93 MeV for light and heavy fragment respectively. A linear combination of the CASCADE spectra, relative to all the contributing primary fragmentations, has been used to fit the experimental spectrum of ~/-rays in coincidence with each detected light fragment. The outcome of such a procedure is the calculated statistical '~-ray spectrum which best fits the corresponding experimental one. The theoretical spectra have been folded with the experimental set-up response function using the code GEANT3. To increase the statistics, all the experimental coincidence y-ray spectra and all the calculated ones have been summed. The cumulative experimental (points) and calculated spectra (solid lines) are reported in Figure l a and 2a for the 35C1+64Ni and 35C1+92Mo reactions respectively. Then the ratio between the two corresponding pairs of cumulative spectra have been determined to evidence any y-ray excess with respect to the expected statistical emission from the fragments. The results of the above procedure are reported in Figure lb and 2b. In both cases a resonance-like bump is evidenced between 8 and 12 MeV. By fitting the bumps using a Lorentz curve, the resonance parameters have been determined to be Eres= (10.6 _+0.2) MeV with a FWHM F= (3.0 _+0.6) MeV and Eres= (11.0 _+0.1) MeV with a FWHM F= (2.2 _+ 0.7) MeV respectively. Being the contribution from the compound nucleus prevented by the coincidence technique, the resonances have been attributed to the excitation of dipole strength in the deformed intermediate system. Gamma-rays from these collective modes of the intermediate system before its fragmentation can be observed as the mean lifetime for fragmentation has been found 6 to be 10-21 s and the time the GDR needs to be excited is 10 -22 s. The displacement of the observed resonances with respect to the GDR of the compound nuclei indicates a deformation parameter of the intermediate systems ~ =0.72 and t3=0.54 respectively in the assumption of a prolate deformation.
L. Campajola et al. / Nuclear Physics A583 (1995) 119-122
105 E
. . . .
u
. . . .
i
.
.
.
.
.
.
.
.
u
. . . .
.
. . . .
|
. . . .
g
. . . .
121 c
I
,,o, I 100
I
4
. . . .
n
. . . .
|
o
I
100I
. . . .
4"
n 0
5
10
. . . .
o 15
0
5
10
E~, (MeV)
E1, (MeV)
Figure 1
Figure 2
15
In conclusion dipole strength has been observed for the first time in the very deformed intermediate systems formed in the 35C1+64Niand 35C1+92Mo dissipative heavy ion collisions at incident energies of about 7 A MeV. This statement is supported by the results of microscopic calculations 4 based on the Vlasov equation performed for the 35C1+64Ni dissipative reaction at impact parameter b-- 6 fm and 270 MeV incident energy which showed that dipole strength is concentrated in the energy interval between 8 and 12 MeV. This strength corresponds to oscillations along the rotating elongation axis of the dinuclear intermediate system. Future experiments will be dedicated to the study of the entrance channel influence on the pre-equilibrium dipole strenght excitation in the intermediate system.
References 1 J.J. Gaardhoje Ann. Rev. Nucl. Part. Sci. 42,483 (1992) and references therein. 2 Ph. Chomaz, M.Di Toro and A.Smerzi Nucl. Phys. A563, 509 (1993). 3 R.A. Broglia, P.F. Bortignon, and A. Bracco, Prog. Part. Nucl. Phys. 28,517(1992). 4 Cai Yanhuang et al. Proceedings of the Int. Conf. on Dynamical Features of Nuclei and Finite Fermi Systems, Barcelone 1993, X. Vinas Editor (in press). 5 A. De Rosa et al., NIM A342, 534(1994). 6 A. De Rosa et al. Phys. Rev. C44, 747 (1991) and references therein.