HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Measurement of the B(E2, 0_1 → 2_1) in the N = 16 nucleus Ne J. Gibelin, D. Beaumel, T. Motobayashi, N. Aoi, H. Baba, Y. Blumenfeld, Zs. Dombradi, Z. Elekes, S. Fortier, N. Frascaria, et al.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. The N=14 shell closure in O viewed through a neutron sensitive probe E. Becheva, Y. Blumenfeld, E. Khan, D. Beaumel, J.M. Daugas, F. Delaunay, C.E. Demonchy, A. Drouart, M. Fallot, A. Gillibert, et al.

The evolution of the giant dipole resonance (GDR) properties in nuclei of mass A = 120 to 132 has been investigated in an excitation energy range between 150 and 270 MeV through the study of complete and nearly complete fusion reactions using (11)6Sn beams at 17A and 23A MeV from the cyclotron of the Laboratorio Nazionale del Sud impinging on C-12 and Mg-24 targets. gamma rays and light charged particles were detected using the multi-element detector array MEDEA in coincidence with evaporation residues detected by using mass and charge identification spectrometry with telescope (MACISTE). Light-charged-particle energy spectra were analyzed within the framework of a multiple-source-emission scenario by using a fitting procedure to determine the amount of pre-equilibrium emission and deduce the excitation energies reached in the compound nuclei. A detailed analysis of the gamma-ray spectra and their comparison with statistical model calculations is presented. Evidence of a quenching of the GDR gamma yield was found at 270 MeV excitation energy. The quenching effect becomes progressively more important with increasing excitation energy, as observed when the comparison is extended to data from the reaction Ar-36 + Mo-96 at 37A MeV where hot nuclei were populated up to 430 MeV excitation energy. A coherent scenario emerges indicating the existence of a limiting excitation energy for the collective motion of about E*/A = 2.1 MeV for systems of mass A = 105 to 111 while a slightly lower value was observed for nuclei of mass A similar to 132. The existence of a possible link between GDR disappearance and the liquid-gas phase transition is discussed.

A study of hot Giant Dipole Resonance (GDR) in nuclei of mass A=120~130 in an excitation energy range from 150 to 330 MeV, where the GDR quenching is expected to arise, has been undertaken using the MEDEA multi-detector system. Hot nuclei were populated using complete and incomplete fusion reactions. The characterization of hot system was performed through the study of residue time of flight combined with the analysis of light charged energy spectra detected in coincidence. Gamma-ray energy spectra show an evolution of the GDR main features both in terms of width and multiplicity. Evidences of a saturation of gamma multiplicity appear at high excitation energy at variance with predictions of statistical model calculations. Gamma-ray energy spectra can been reproduced in a phenomenological way introducing in the statistical model a sharp suppression of the gamma-ray emission above E* = 240 MeV. A comparison of experimental data to models describing the GDR disappearance will be presented.

Coulomb excitation of the exotic neutron-rich nucleus (26)Ne on a (208)Pb target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+/-0.16e(2) fm(2) is deduced, corresponding to 4.9+/-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

The evolution with excitation energy of the Giant Dipole Resonance features in nuclei of mass A approximate to, 108 - 136 is reviewed. We first discuss the results of the experiments performed with MEDEA studying the GDR gamma decay from hot nuclei populated at excitation energies above 300 MeV. The focus of the paper is on the excitation energy region between 160 and 290 MeV. This region has been investigated through the study of the reactions Sn-116 + C-12 at 17 and 23A MeV, and Sn-116 + Mg-24 at 17A MeV. Gamma-rays were detected using MEDEA in coincidence with evaporation residues detected in MACISTE. The analysis of the gamma-ray spectra and their comparison with statistical calculations are presented. The comparison with gamma-ray spectra from the reaction Ar-36 + Mo-98 at higher excitation energies shows a coherent scenario where a progressive reduction of gamma multiplicity relative to predictions for 100% of the Energy Weighted Sum Rule is observed above 200 MeV excitation energy. Finally, the existence of a link between disappearance of collective motion and the liquid-gas phase transitions is discussed.

Differential cross section of the inelastic scattering of a 54 MeV/nucleon Ne-26 beam on a lead target has been measured by detecting the deexcitation gamma-rays. Analysis of the first 2(+) state angular distribution of the inelastically scattered nuclei shows that the process cannot be considered as a pure Coulomb excitation, and nuclear contribution must be taken into account. The charge deformation deduced, beta(C)(2)=0.392 +/- 0.024, corresponds to a B(E2)=141 +/- 18 e(2) fm(2) in agreement with a N=16 subshell closure.

Measurements of the C-14(C-14,C-14[Be-10+alpha]) breakup reaction have been made at a beam energy of 98.2 MeV. The studies were performed with two charged-particle telescopes that permitted the energy, mass, charge, and emission angle of each detected particle to be determined. A series of C-14 excited states at energies of 14.3, 14.8 15.6, 16.4, 17.3, 18.6, 19.8, 20.6, and 21.6 MeV was observed to decay to either the Be-10 ground state or the first excited states. Angular correlation measurements suggest an assignment of J(pi)=3(-) for the 15.6 MeV state.

Coulomb excitation of the exotic neutron‐rich nucleus 26Ne on a 208Pb target was measured at 58 A.MeV in order to search for low‐lying E1 strength above the neutron emission threshold. Data were also taken on an Al target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV Ar beam delivered by the RIKEN Research Facility. The set‐up included a NaI gamma‐ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross‐section, a reduced dipole transition probability of B(E1)=0.49±0.16 e2fm2 is deduced. For the first time, the decay pattern of low‐lying strength in a neutron‐rich nucleus is measured.

Measurements of the $^{14}\mathrm{C}$($^{14}\mathrm{C}$,$^{14}\mathrm{C}$[$^{10}\mathrm{Be}$+$\ensuremath{\alpha}$]) breakup reaction have been made at a beam energy of 98.2 MeV. The studies were performed with two charged-particle telescopes that permitted the energy, mass, charge, and emission angle of each detected particle to be determined. A series of $^{14}\mathrm{C}$ excited states at energies of 14.3, 14.8 15.6, 16.4, 17.3, 18.6, 19.8, 20.6, and 21.6 MeV was observed to decay to either the $^{10}\mathrm{Be}$ ground state or the first excited states. Angular correlation measurements suggest an assignment of ${J}^{\ensuremath{\pi}}={3}^{\ensuremath{-}}$ for the 15.6 MeV state.

Investigation of unbound nuclear systems He-9, H-7 and (4)n was performed at GANIL-SPIRAL using the sHe beam at 15.3 A MeV and a CD2 target. The missing mass spectra were deduced from kinetic energies and emission angles of light ejectiles detected by the Silicon array MUST. In addition to previously known low-lying narrow resonant states in He-9, the d(He-8,p) reaction displays a structure just above neutron emission threshold, identified with the "true" ground state of He-9. The analysis of angular distributions shows that the inversion of s(1/2) and P-1/2 neutron shells previously observed in Be-11 and Li-10 also exists in the lightest N=7 isotone He-9. The d(He-8,He-3) and d(He-8,Li-6) reactions were used to search for resonant states in the t+4n (H-7) and 4n systems, respectively. The missing mass spectrum of the 4n system does not give evidence forthe existence of a bound '' tetraneutron ''. However the comparison with the results of 5-body phase-space calculations emphasizes the existence of correlations in the 4n system. The broad structure observed at similar or equal to 2 MeV above the t+4n emission threshold is proposed to be the ground state of H-7.

Coulomb excitation of the exotic neutron-rich nucleus Ne-26 on a Pb-208 target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an A1 target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the Ne-25+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross-section, a reduced dipole transition probability of B(E1)=0.49 +/- 0.16 e(2)fm(2) is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured.

Investigation of unbound nuclear systems 9He, 7H and 4n was performed at GANIL‐SPIRAL using the 8He beam at 15.3 A MeV and a CD2 target. The missing mass spectra were deduced from kinetic energies and emission angles of light ejectiles detected by the Silicon array MUST. In addition to previously known low‐lying narrow resonant states in 9He, the d(8He,p) reaction displays a structure just above neutron emission threshold, identified with the “true” ground state of 9He. The analysis of angular distributions shows that the inversion of s1/2 and p1/2 neutron shells previously observed in 11Be and 10Li also exists in the lightest N=7 isotone 9He. The d(8He,3He) and d(8He,6Li) reactions were used to search for resonant states in the t+4n (7H) and 4n systems, respectively. The missing mass spectrum of the 4n system does not give evidence for the existence of a bound “tetraneutron”. However the comparison with the results of 5‐body phase‐space calculations emphasizes the existence of correlations in the 4n system. The broad structure observed at ≃2 MeV above the t+4n emission threshold is proposed to be the ground state of 7H.

The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be from the GANIL facility on a 48Ti target by measuring correlations between the 10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to the emission of a neutron at large angle in the laboratory frame is identified with the towing mode through its characteristic n-fragment correlation. The experimental spectra are compared with a model where the time dependent Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A good agreement is found between experiment and theory for the shapes of neutron experimental energies and angular distributions. The spectroscopic factor of the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission from the 1p and 1d orbitals is also studied.

To investigate the behavior of the N = 14 neutron gap far from stability with a neutron-sensitive probe, proton elastic and 2(1)+ inelastic scattering angular distributions for the neutron-rich nucleus 22O were measured using the MUr à STrip detector array at the Grand Accélérateur National d'Ions Lourds facility. A deformation parameter beta(p,p') = 0.26 +/- 0.04 is obtained for the 2(1)+ state, much lower than in 20O, showing a weak neutron contribution to this state. A microscopic analysis was performed using matter and transition densities generated by continuum Skyrme-Hartree-Fock-Bogoliubov and quasiparticle random phase approximation calculations, respectively. The ratio of neutron to proton contributions to the 2(1)+ state is found close to the N/Z ratio, demonstrating a strong N = 14 shell closure in the vicinity of the neutron drip line.

To investigate the behavior of the N = 14 neutron gap far from stability with a neutron-sensitive probe, proton elastic and 2(1)+ inelastic scattering angular distributions for the neutron-rich nucleus 22O were measured using the MUr à STrip detector array at the Grand Accélérateur National d'Ions Lourds facility. A deformation parameter beta(p,p') = 0.26 +/- 0.04 is obtained for the 2(1)+ state, much lower than in 20O, showing a weak neutron contribution to this state. A microscopic analysis was performed using matter and transition densities generated by continuum Skyrme-Hartree-Fock-Bogoliubov and quasiparticle random phase approximation calculations, respectively. The ratio of neutron to proton contributions to the 2(1)+ state is found close to the N/Z ratio, demonstrating a strong N = 14 shell closure in the vicinity of the neutron drip line.

Coulomb excitation of the exotic neutron-rich nucleus ^26Ne on a ^natPb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an ^natAl target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV ^40Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the ^25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV. The reconstructed ^26Ne angular distribution confirms its E1 nature. A reduced dipole transition probability of B(E1)=0.49±0.16 e^2fm^2 is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is obtained. The results are discussed in terms of a pygmy resonance centered around 9 MeV.