Search

Prompt and delayed γ -ray spectroscopy of the neutron-rich 94 Kr was performed, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction 238 U ( n , f ) in combination with the ν -Ball array, a novel hybrid γ spectrometer for energy and lifetime measurements. Several new yrast and nonyrast transitions were observed for the first time, extending the previously known level scheme. Additionally, we report on the observation of a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock-Bogoliubov calculations suggests a ( 9 − ) spin and an oblate deformation for this isomer corresponding to a two-quasineutron state, indicating an isomeric structure very similar to that of the neighboring isotones 96 Sr and 92 Se

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei Dy164,166 using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an O18 beam and a gold-backed isotopically enriched Dy164 target of thickness 6.3mg/cm2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in Dy164, Dy166, and W178,179 following Coulomb excitation, inelastic nuclear scattering, two-neutron transfer, and fusion-evaporation reaction channels respectively. Gamma rays from excited states were measured using the ν-Ball high-purity germanium (HPGe)-LaBr3 hybrid γ-ray spectrometer with the excited state lifetimes extracted using the fast-timing coincidence method using HPGe-gated LaBr3-LaBr3 triple coincident events. The lifetime of the first Iπ=2+ excited state in Dy166 was used to determine the transition quadrupole deformation of this neutron-rich nucleus for the first time. The experimental methodology was validated by showing consistency with previously determined excited state lifetimes in Dy164. The half-lives of the yrast 2+ states in Dy164 and Dy166 were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q0=7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2+ state in Dy166 is consistent with a quenching of nuclear quadrupole deformation at β≈0.35 as the N=104 mid-shell is approached.

We report on the first measurement of the half-lives of K-pi = 11(-) and 12(+) four-quasiparticle states in the even-even nucleus W-178. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3(Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array. The measured half-lives are 476(44)ps and 275(65)ps for the I-pi = 11(-) and 12(+) states, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band. (C) 2019 The Author(s). Published by Elsevier B.V.

Lifetimes of excited states of the phosphorus isotopes 1533,34,35,36P have been measured by using the differential recoil-distance method. The isotopes of phosphorus were populated in binary grazing reactions initiated by a beam of S36 ions of energy 225 MeV incident on a thin Pb208 target mounted in the Cologne plunger apparatus. The combination of the PRISMA magnetic spectrometer and an early implementation of the AGATA γ-ray tracking array was used to detect γ rays in coincidence with projectile-like nuclear species. Lifetime measurements of populated states were made within the range from about 1 to 100 ps. The number of states for which lifetime measurements were possible was limited by statistics. For P33, lifetime limits were determined for the first 3/2+ and 5/2+ states at 1431 and 1848 keV, respectively; the results are compared with previous published lifetime values. The lifetime of the first 2+ state of P34 at 429 keV was determined and compared with earlier measurements. For P35, the states for which lifetimes, or lifetime limits, were determined were those at 2386, 3860, 4101, and 4493 keV, with Jπ values of 3/2+, 5/2+, 7/21−, and 7/22−, respectively. There have been no previous published lifetimes for states in this nucleus. A lifetime was measured for the stretched π(1f7/2)⊗ν(1f7/2)Jπ=(7+) state of P36 at 5212 keV and a lifetime limit was established for the stretched π(1d3/2)⊗ν(1f7/2)Jπ=(5−) state at 2030 keV. There are no previously published lifetimes for states of P36. Measured lifetime values were compared with the results of state-of-the-art shell-model calculations based on the PSDPF effective interaction. In addition, measured branching ratios, published mixing ratios, and electromagnetic transition rates, where available, have been compared with shell-model values. In general, there is good agreement between experiment and the shell model; however there is evidence that the shell-model values of the M1 transition rates for the 3/21+→1/2+ (ground state) and 5/21+→3/21+ transitions in P33 underestimate the experimental values by a factor between 5 and 10. In P35 there are some disagreements between experimental and shell-model values of branching ratios for the first and second excited 7/2− states. In particular, there is a serious disagreement for the decay characteristics of the second 7/2− state at 4493 keV, for which the shell-model counterpart lies at 4754 keV. In this case, the shell-model competing electromagnetic decay branches are dominated by E1 and M1 transitions.

The reaction of a pulsed 18O beam on a self-supporting and gold-backed isotopically-enriched 164Dy target of thickness 6.3 mg/cm2 at separate primary beam energies of 71, 76 and 80 MeV was studied at the accelerator at the ALTO facility of the IPN Orsay. The γ rays produced were detected using the newly-constructed ν-Ball spectrometer which comprised of HPGe and LaBr3(Ce) detectors. This conference paper describes the methodology and effectiveness of multiplicity/sum-energy gating, for channel selection between fusion evaporation events and lower multiplicity/energy events from inelastic nuclear scattering and Coulomb excitation of the target, and from two-neutron transfer reactions to 166Dy.

The reaction of a pulsed 18O beam on a 164Dy target was studied in the first experiment with the NuBall array at the IPN Orsay, France. Excited state half-lives were measured using the fast timing method with 20 LaBr3(Ce) detectors. The timing characteristics of the fully digital acquisition system is briefly discussed. A value for the previously unknown half-life of the first excited 4+ state in 178W is presented.

Prompt-fission $\ensuremath{\gamma}$-ray spectra (PFGS) have been measured for the $^{238}\mathrm{U}(n,f$) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission $\ensuremath{\gamma}$ rays were measured using a number of ${\mathrm{LaBr}}_{3}$ scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission $\ensuremath{\gamma}$ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for $^{238}\mathrm{U}(n,f$) and also for $^{252}\mathrm{Cf}(sf$) as a reference. Spectral characteristics of PFG emission, such as mean $\ensuremath{\gamma}$ multiplicity and average total $\ensuremath{\gamma}$-ray energy per fission, as well as the average $\ensuremath{\gamma}$-ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at ${E}_{n}=1.9\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were found to be the mean $\ensuremath{\gamma}$ multiplicity ${\overline{M}}_{\ensuremath{\gamma}}=6.54\ifmmode\pm\else\textpm\fi{}0.19$, total released energy per fission ${E}_{\ensuremath{\gamma},\mathrm{tot}}=5.25\ifmmode\pm\else\textpm\fi{}0.20$ MeV, and the average $\ensuremath{\gamma}$-ray energy ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.80\ifmmode\pm\else\textpm\fi{}0.04$ MeV. Under similar conditions, the values at ${E}_{n}=4.8\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were measured to be ${\overline{M}}_{\ensuremath{\gamma}}=7.31\ifmmode\pm\else\textpm\fi{}0.46, {E}_{\ensuremath{\gamma},\mathrm{tot}}=6.18\ifmmode\pm\else\textpm\fi{}0.65$ MeV, and ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.84\ifmmode\pm\else\textpm\fi{}0.11$ MeV.

Fusion cross sections of 28Si + 28Si have been measured in a range above the barrier with a very small energy step (DeltaElab = 0.5 MeV). Regular oscillations have been observed, best evidenced in the first derivative of the energy-weighted excitation function. For the first time, quite different behaviors (the appearance of oscillations and the trend of sub-barrier cross sections) have been reproduced within the same theoretical frame, i.e., the coupled-channel model using the shallow M3Y+repulsion potential. The calculations suggest that channel couplings play an important role in the appearance of the oscillations, and that the simple relation between a peak in the derivative of the energy-weighted cross section and the height of a centrifugal barrier is lost, and so is the interpretation of the second derivative of the excitation function as a barrier distribution for this system, at energies above the Coulomb barrier., Comment: submitted to Physics Letters B

Background: The fusion reaction 12C+30Si is a link between heavier cases studied in recent years, and the light heavy-ion systems, e.g., 12C+12C, 16O+16O that have a prominent role in the dynamics of stellar evolution. 12C+30Si fusion itself is not a relevant process for astrophysics, but it is important to establish its behavior below the barrier, where couplings to low-lying collective modes and the hindrance phenomenon may determine the cross sections. The excitation function is presently completely unknown below the barrier for the 12C+30Si reaction, thus no reliable extrapolation into the astrophysical regime for the C+C and O+O cases can be performed. Purpose: Our aim was to carry out a complete measurement of the fusion excitation function of 12C+30Si from well below to above the Coulomb barrier, so as to clear up the consequence of couplings to low-lying states of 30Si, and whether the hindrance effect appears in this relatively light system which has a positive Q value for fusion. This would have consequences for the extrapolated behavior to even lighter systems. Methods: The inverse kinematics was used by sending 30Si beams delivered from the XTU Tandem accelerator of INFN-Laboratori Nazionali di Legnaro onto thin 12C (50μg/cm2) targets enriched to 99.9% in mass 12. The fusion evaporation residues (ER) were detected at very forward angles, following beam separation by means of an electrostatic deflector. Angular distributions of ER were measured at Ebeam=45, 59, and 80 MeV, and they were angle integrated to derive total fusion cross sections. Results: The fusion excitation function of 12C+30Si was measured with high statistical accuracy, covering more than five orders of magnitude down to a lowest cross section ≃3μb. The logarithmic slope and the S factor have been extracted and we have convincing phenomenological evidence of the hindrance effect. These results have been compared with the calculations performed within the model that considers a damping of the coupling strength well inside the Coulomb barrier. Conclusions: The experimental data are consistent with the coupled-channels calculations. A better fit is obtained by using the Yukawa-plus-exponential potential and a damping of the coupling strengths inside the barrier. The degree of hindrance is much smaller than the one in heavier systems. Also a phenomenological estimate reproduces quite closely the hindrance threshold for 12C+30Si, so that an extrapolation to the C+C and O+O cases can be reliably performed.

International audience; The theoretical and experimental determination of superdeformed states in nuclei in the mass region A≤40 has been since a long time one of the major challenges of nuclear structure studies. Despite the considerable experimental and theoretical work dedicated to this topic, up to now superdeformed bands have been found in only two nuclei, $^{36}$Ar and $^{40}$Ca. While the experimental signature of the superdeformed nature of those states is irrefutable, their theoretical interpretation is still uncertain. In particular, it is not clear whether clusterisation is responsible of the onset of superdeformation. For this reason, we wanted to investigate an even lighter system, $^{28}$Si, where a number of theoretical calculations predict the presence of superdeformation as an effect of the cluster structure of the nucleus.

This contribution reports on the results of measurements of near- and sub-barrier fusion cross sections in the system $^{30}$Si+$^{30}$Si performed at the Laboratori Nazionali di Legnaro of INFN. The $^{30}$Si beam of the XTU Tandem accelerator in the energy range of 47 - 90 MeV, was delivered on a metallic $^{30}$Si target (50 $\mu$g/cm$^{2}$) enriched to 99.64 $\%$ in mass 30. A beam electrostatic deflector was used for the detection of evaporation residues. The excitation function obtained for $^{30}$Si+$^{30}$Si has been compared with the previous data on $^{28}$Si+$^{28}$Si and Coupled Channels (CC) calculations using the M3Y+repulsion potential, taking into account the low lying 2$^+$ and 3$^-$ excitations. Reproducing the low-energy $^{28}$Si+$^{28}$Si excitation function was only possible by using a weak imaginary potential, probably simulating the oblate deformation of this nucleus. On the contrary, the data on $^{30}$Si+$^{30}$Si are nicely reproduced by the CC calculations without any imaginary potential ($^{30}$Si has a spherical shape). The astrophysical S-factor does not show a maximum, so that there is no evidence for hindrance in the measured energy range. The logarithmic derivative of the two excitation functions highlights the difference between the two systems. Even above the barrier the two systems behave differently. This is best seen by comparing the two barrier distributions where the high energy peak observed for $^{28}$Si+$^{28}$Si is not found for $^{30}$Si+$^{30}$Si. This is presumably due to the stronger couplings present in $^{28}$Si and further theoretical analyses are in progress.

Relatively neutron-rich thorium isotopes lie at the heart of a nuclear region of nuclei exhibiting octupole correlation effects. The detailed level structure of Th223 has been investigated in measurements of γ radiation following the fusion-evaporation channel of the Pb208(O18,3n)Th223 reaction at 85 MeV beam energy. The level structure has been extended up to spin 49/2, and 33 new γ rays have been added using triple-γ coincidence data. The spins and parities of the newly observed states have been confirmed by angular distribution ratios. In addition to the two known yrast bands based on a K=5/2 configuration, a non-yrast band has been established up to spin 35/2. We interpret this new structure as based on the same configuration as the yrast band in Th221 having dominant K=1/2 contribution. At the highest spin a backbending occurs around a rotational frequency of ℏω=0.23 MeV, very close to the one predicted in Th222, where a sharp transition to a reflection-symmetric shape is expected.

The near- and sub-barrier fusion excitation function has been measured for the system 30 Si+30 Si at the Laboratori Nazionali di Legnaro of INFN, using the 30 Si beam of the XTU Tandem accelerator in the energy range 47 - 90 MeV. A set-up based on a beam electrostatic deflector was used for detecting fusion evaporation residues. The measured cross sections have been compared to previous data on 28 Si+28 Si and Coupled Channels (CC) calculations have been performed using M3Y+repulsion and Woods-Saxon potentials, where the lowlying 2+ and 3− excitations have been included. A weak imaginary potential was found to be necessary to reproduce the low energy 28 Si+28 Si data. This probably simulates the effect of the oblate deformation of this nucleus. On the contrary, 30 Si is a spherical nucleus, 30 Si+30 Si is nicely fit by CC calculations and no imaginary potential is needed. For this system, no maximum shows up for the astrophysical S-factor so that we have no evidence for hindrance, as confirmed by the comparison with CC calculations. The logarithmic derivative of the two symmetric systems highlights their different low energy trend. A difference can also be noted in the two barrier distributions, where the high-energy peak present in 28 Si+28 Si is not observed for 30 Si+30 Si, probably due to the weaker couplings in last case.

Carbon burning plays a crucial role in stellar evolution, where this reaction is an important route for the production of heavier elements. A particle-γ coincidence technique that minimizes the backgrounds to which this reaction is subject and provides reliable cross sections has been used at the Argonne National Laboratory to measure fusion cross-sections at deep sub-barrier energies in the 12 C+12 C system. The corresponding excitation function has been extracted down to a cross section of about 6 nb. This indicates the existence of a broad S-factor maximum for this system. Experimental results are presented and discussed.

International audience; This contribution is focused on the STELLA project (STELlar LAboratory), which aims at the measurement of fusion cross sections between light heavy ions like 12C+12C, 12C+16O or 16O+16O at deep subbarrier energies. The gamma-particle coincidence technique is used in order to reduce background contributions that become dominant for measurements in the nanobarn regime.The experimental setup composed of an ultra high vacuum reaction chamber, a set of 3 silicon strip detectors, up to 36 LaBr3(Ce) scintillators from the UK FATIMA collaboration, and a fast rotating target system will be described. The 12C+12C fusion reaction has been studied from Elab = 11 to 5.6 MeV using STELLA at the Andromède facility in Orsay, France. Preliminary commissioning results are presented in this article.

International audience; An experiment was performed at LNL to obtain information concerning the yield distributions in both the 132Sn (N = 82) and the 208Pb (N = 126) regions of the nuclide chart. The experimental set-up was composed of the large-acceptance magnetic spectrometer PRISMA to detect the light partner of the reaction, in conjunction with a recently installed ancillary set-up to detect the heavy partner.

We performed a γ -particle coincidence experiment for the 60Ni+116Sn system to investigate whether the population of the two-neutron pickup channel leading to 62Ni is mainly concentrated in the ground-state transition, as has been found in a previous work [D. Montanari et al., Phys. Rev. Lett. 113, 052501 (2014)]. The experiment has been performed by employing the PRISMA magnetic spectrometer coupled to theAdvanced Gamma Tracking Array (AGATA) demonstrator. The strength distribution of excited states corresponding to the inelastic, one- and two-neutron transfer channels has been extracted.We found that in the two-neutron transfer channel the strength to excited states corresponds to a fraction (less than 24%) of the total, consistent with the previously obtained results that the 2n channel is dominated by the ground-state to ground-state transition.

Background: Heavy-ion fusion reactions at energies near the Coulomb barrier are influenced by couplings between the relative motion and nuclear intrinsic degrees of freedom of the colliding nuclei. The time-dependent Hartree-Fock (TDHF) theory, incorporating the couplings at the mean-field level, as well as the coupled-channels (CC) method are standard approaches to describe low energy nuclear reactions.Purpose: To investigate the effect of couplings to inelastic and transfer channels on the fusion cross sections for the reactions $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ and $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$.Methods: Fusion cross sections around and below the Coulomb barrier have been obtained from CC calculations, using the bare nucleus-nucleus potential calculated with the frozen Hartree-Fock method and coupling parameters taken from known nuclear structure data. The fusion thresholds and neutron transfer probabilities have been calculated with the TDHF method.Results: For $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$, the TDHF fusion threshold is in agreement with the most probable barrier obtained in the CC calculations including the couplings to the low-lying octupole ${3}_{1}^{\ensuremath{-}}$ state for $^{40}\mathrm{Ca}$ and to the low-lying quadrupole ${2}_{1}^{+}$ state for $^{58}\mathrm{Ni}$. This indicates that the octupole and quadrupole states are the dominant excitations while neutron transfer is shown to be weak. For $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$, the TDHF barrier is lower than predicted by the CC calculations including the same inelastic couplings as those for $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$. TDHF calculations show large neutron transfer probabilities in $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ which could result in a lowering of the fusion threshold.Conclusions: Inelastic channels play an important role in $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ and $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ reactions. The role of neutron transfer channels has been highlighted in $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$.

The level structures of the N = 50 83As, 82Ge, and 81Ga isotones have been investigated by means of multi-nucleon transfer reactions. A first experiment was performed with the CLARA–PRISMA setup to identify these nuclei. A second experiment was carried out with the GASP array in order to deduce the γ-ray coincidence information. The results obtained on the high-spin states of such nuclei are used to test the stability of the N = 50 shell closure in the region of 78Ni ( Z = 28 ). The comparison of the experimental level schemes with the shell-model calculations yields an N = 50 energy gap value of 4.7(3) MeV at Z = 28 . This value, in a good agreement with the prediction of the finite-range liquid-drop model as well as with the recent large-scale shell model calculations, does not support a weakening of the N = 50 shell gap down to Z = 28 .

Fusion cross sections have been measured in the 40 Ca + 58 Ni and 40 Ca + 64 Ni systems at energies around and below the Coulomb barrier. The 40 Ca beam was delivered by the XTU Tandem accelerator of the Laboratori Nazionali di Legnaro and evaporation residues were measured at very forward angles with the LNL electrostatic beam deflector. Coupled-channels calculations were performed which highlight possible strong effects of neutron transfers on the fusion below the barrier in the 40 Ca + 64 Ni system. Microscopic time-dependent Hartree-Fock calculations have also been performed for both systems. Preliminary results are shown.

Background: No data on the fusion excitation function of $^{48}\mathrm{Ti}+^{58}\mathrm{Fe}$ in the energy region near the Coulomb barrier existed prior to the present work, while fusion of $^{58}\mathrm{Ni}+^{54}\mathrm{Fe}$ was investigated in detail some years ago, down to very low energies, and clear evidence of fusion hindrance was noticed at relatively high cross sections. $^{48}\mathrm{Ti}$ and $^{58}\mathrm{Fe}$ are soft and have a low-lying quadrupole excitation lying at $\ensuremath{\approx}800--900$ keV only. Instead, $^{58}\mathrm{Ni}$ and $^{54}\mathrm{Fe}$ have a closed shell (protons and neutrons, respectively) and are rather rigid.Purpose: We aim to investigate (1) the possible influence of the different structures of the involved nuclei on the fusion excitation functions far below the barrier and, in particular, (2) whether hindrance is observed in $^{48}\mathrm{Ti}+^{58}\mathrm{Fe}$, and to compare the results with current coupled-channels models.Methods: $^{48}\mathrm{Ti}$ beams from the XTU Tandem accelerator of INFN-Laboratori Nazionali di Legnaro were used. The experimental setup was based on an electrostatic beam separator, and fusion-evaporation residues (ERs) were detected at very forward angles. Angular distributions of ERs were measured.Results: Fusion cross sections of $^{48}\mathrm{Ti}+^{58}\mathrm{Fe}$ have been obtained in a range of nearly six orders of magnitude around the Coulomb barrier, down to $\ensuremath{\sigma}\ensuremath{\simeq}2 \ensuremath{\mu}\mathrm{b}$. The sub-barrier cross sections of $^{48}\mathrm{Ti}+^{58}\mathrm{Fe}$ are much larger than those of $^{58}\mathrm{Ni}+^{54}\mathrm{Fe}$. Significant differences are also observed in the logarithmic derivatives and astrophysical $S$ factors. No evidence of hindrance is observed, because coupled-channels calculations using a standard Woods-Saxon potential are able to reproduce the data in the whole measured energy range. Analogous calculations for $^{58}\mathrm{Ni}+^{54}\mathrm{Fe}$ predict clearly too large cross sections at low energies. The two fusion barrier distributions are wide and display a complex structure that is only qualitatively fit by calculations.Conclusions: It is pointed out that all these different trends originate from the dissimilar low-energy nuclear structures of the involved nuclei. In particular, the strong quadrupole excitations in $^{48}\mathrm{Ti}$ and $^{58}\mathrm{Fe}$ produce the relative cross section enhancement and make the barrier distribution $\ensuremath{\approx}2$ MeV wider, thus probably pushing the threshold for hindrance below the measured limit.

4 pags., 4 figs. -- Open Access funded by Creative Commons Atribution Licence 3.0, C-12 is synthesised in stars by fusion of three a particles. This process occurs through a resonance in the C-12 nucleus, famously known as the Hoyle state. In this state, the C-12 nucleus exists as a cluster of a particles. The state is the band-head for a rotational band with the 2(+) rotational excitation predicted in the energy region 9 - 11 MeV. This rotational excitation can affect the triple-alpha process reaction rate by more than an order of magnitude at high temperatures (10(9) K). Depending on the energy of the resonance, the knowledge of the state can also help determine the structure of the Hoyle state. In the work presented here, the state of interest is populated by beta decay of radioactive N-12 ion beam delivered by the IGISOL facility at JYFL, Jyvaskyla.

Measurements of fusion excitation functions of Ni-58 + Sn-124 and Ni-64 + Sn-124 are extended towards lower energy to cross sections of 1 mu b and are compared to detailed coupled-channels calculations. The calculations clearly show the importance of including transfer reactions in a coupled-channels treatment for such heavy systems. This result is different from the conclusion made in a previous article which claimed that the influence of transfer on fusion is not important for fusion reactions of Ni + Sn. In the energy region studied in this experiment no indication of fusion hindrance has been observed, which is consistent with a systematic study of this behavior.

Measurements of fusion-evaporation cross-sections for the system 48Ca + 154Sm have been performed in the sub- and near-barrier energy range. Barrier-passing cross-sections have been obtained by adding recently measured capture-fission cross-sections at the same energies, and the barrier distribution for capture has been extracted. The data have been analyzed within a coupled-channel model, and a large subbarrier cross-section enhancement is observed, due to the ground-state prolate deformation of 154Sm. The 48Ca + 154Sm capture cross-sections are compared to existing data on 16O + 186W fusion, leading to the same CN, where a few higher-energy points have also been measured. The evaporation residue cross-sections for the two systems above the barrier indicate that complete fusion is inhibited for 48Ca + 154Sm by ≈ 40% in that energy region, with respect to 16O + 186W.

16 pages, 2 tables, 4 figures.-- PACS nrs.: 23.40.-s; 23.20.-g; 27.20.+n.-- Printed version published May 17, 2004., The γ-ray spectrum following β-decay of Li-11 has been remeasured at the ISOLDE facility at CERN. Two new transitions were observed through the use of large Ge-detectors. Most γ-decays will follow β-delayed neutron emission. Information on the energy of the neutron is derived from analysis of the γ line-shape and used to construct a partial decay scheme for Li-11. Lifetime values for the 1(-) and 2(-) levels in Be-10 are also derived. A new partial decay scheme is presented., This work has been partially supported by the Swedish Research Council VR.

The shell closure N=20 has been studied by β-decay spectroscopy experiment performed at the GANIL facility. The experimental setup is described and first results on the beta-decay of 32 Mg are presented. The extracted level scheme of 32 Al is compared to Shell Model calculations and discussed in the light of the structural changes observed for the nuclei of the so-called island of inversion. Finally, neutron time-of-flight spectrum from the β-delayed neutron decay of 32 Mg is presented.

4 pages, 3 figures.-- PACS nr.: 23.40.-s.-- Printed version published Apr 2004.-- et al., Beta-decay experiments are a primary source of information for nuclear-structure studies and at the same time complementary to in-beam investigations of nuclei far from stability. Although both types of experiment are mainly based on γ-ray spectroscopy, they face different experimental problems. The so-called Pandemonium effect is a critical problem in β-decay if we are to test theoretically calculated transition probabilities. In this contribution we will present a solution to this problem using total absorption spectroscopy methods. We will also present some examples of experiments carried out with the Total Absorption Spectrometer (TAS) at GSI an describe a new device LUCRECIA recently installed at CERN., This work was partially supported by C.I.C.Y.T. (Spain) under contract AEN96-1662, by MCYT (Spain)contract No. FPA2002-04181-C04-03, by C.S.R. (Poland) grant KBN-2Pp03B-039-13, by R.F.B.R. (Russia)-D.F.G. (Germany) contract 436 RUS 113/201/0(R). Support from the HPMF-CT-1999-00394 project as well as from the European Large Scale Facility program at CERN is also aknowledged.

The fusion excitation function of 48 Ti + 58 Fe has been measured in a wide energy range around the Coulomb barrier, covering 6 orders of magnitude of the cross sections. We present here the preliminary re- sults of this experiment, and a full comparison with the near-by system 58 Ni + 54 Fe where evidence of fusion hindrance shows up at relatively high cross sections. The sub-barrier cross sections of 48 Ti + 58 Fe are much larger than those of 58 Ni + 54 Fe. Significant differences are also observed in the logarithmic derivatives, astro- physical S-factors and fusion barrier distributions. The influence of low-energy nuclear structure on all these trends is pointed out and commented. Coupled-channels calculations using a Woods-Saxon potential are able to reproduce the experimental results for 48 Ti + 58 Fe. The logarithmic derivative of the excitation function is very nicely fit, and no evidence of hindrance is observed down to around 1 μb. The fusion barrier distribution is rather wide, flat and structureless. It is only in qualitative agreement with the calculated distribution.

The multinucleon transfer reaction 40Ar+208Pb has been measured at Elab = 260 MeV with the large solid angle magnetic spectrometer PRISMA. Mass and charge yields, differential and total cross sections, total kinetic energy loss distributions of different channels were simultaneously studied. Angular distributions were measured in a wide angular range by matching three different angular settings of the spectrometer. Absolute cross sections were obtained after the response function of the spectrometer was evaluated. These cross sections for different transfer channels allow the discussion of the role played by nucleon-nucleon correlations.

The sub-barrier fusion excitation function of Ca-40 + Zr-96 has been measured down to cross sections similar or equal to 2.4 mu b, i.e. two orders of magnitude smaller than obtained in the previous experiment, where the sub-barrier fusion of this system was found to be greatly enhanced with respect to Ca-40 + Zr-90, and the need of coupling to transfer channels was suggested. The purpose of this work was to investigate the behavior of Ca-40 + Zr-96 fusion far below the barrier. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with Ca-48 + Zr-96 is very useful in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon Q > 0 transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few mu b. Locating the hindrance threshold, if any, in Ca-40 + Zr-96 would require challenging measurements of cross sections in the sub-mu b range.

International audience; Cluster resonances in light heavy-ion systems like $^{12}$C+$^{12}$C and $^{12}$C+$^{16}$O may have a major impact on astrophysics stellar scenarios. Resonant radiative capture reactions have been studied for these systems at energies at and slightly below their Coulomb barriers to investigate the possible $^{12}$C-$^{12}$Cand $^{12}$C-$^{16}$O molecular origin of the resonances. Spins have been attributed to the resonances and specificities of their γ-decay have been identified. At deep sub-barrier energies, a fusion cross section measurement using the particle-γ coincidence technique is discussed for the $^{12}$C+$^{12}$C system. A new project is presented to possibly extend the $^{12}$C+$^{12}$C low-energy S factor study.

Background: The nuclear structure of colliding nuclei is known to influence the fusion process. Couplings of the relative motion to nuclear shape deformations and vibrations lead to an enhancement of the sub-barrier fusion cross section in comparison with the predictions of one-dimensional barrier penetration models. This enhancement is explained by coupled-channels calculations including these couplings. The sub-barrier fusion cross section is also affected by nucleon transfer channels between the colliding nuclei.Purpose: The aim of the present experiment is to investigate the influence of the projectile and target nuclear structures on the fusion cross sections in the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ and $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ systems.Methods: The experimental and theoretical fusion excitation functions as well as the barrier distributions were compared for these two systems. Coupled-channels calculations were performed using the ccfull code.Results: Good agreement was found between the measured and calculated fusion cross sections for the $^{40}\mathrm{Ca}+^{58}\mathrm{Ni}$ system. The situation is different for the $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$ system where the coupled-channels calculations with no nucleon transfer clearly underestimate the fusion cross sections below the Coulomb barrier. The fusion excitation function was, however, well reproduced at low and high energies by including the coupling to the neutron pair-transfer channel in the calculations.Conclusions: The nuclear structure of the colliding nuclei influences the fusion cross sections below the Coulomb barrier for both $^{40}\mathrm{Ca}+^{58,64}\mathrm{Ni}$ systems. Moreover, we highlighted the effect of the neutron pair-transfer channel on the fusion cross sections in $^{40}\mathrm{Ca}+^{64}\mathrm{Ni}$.

Background: The fusion excitation function of the system $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{28}\mathrm{Si}$ at energies near and below the Coulomb barrier is known only down to $\ensuremath{\simeq}15$ mb. This precludes any information on both coupling effects on sub-barrier cross sections and the possible appearance of hindrance. For $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{30}\mathrm{Si}$ even if the fusion cross section is measured down to $\ensuremath{\simeq}50$ $\ensuremath{\mu}\mathrm{b}$, the evidence of hindrance is marginal. Both systems have positive fusion $Q$ values. While $^{28}\mathrm{Si}$ has a deformed oblate shape, $^{30}\mathrm{Si}$ is spherical.Purpose: We investigate 1. the possible influence of the different structure of the two Si isotopes on the fusion excitation functions in the deep sub-barrier region and 2. whether hindrance exists in the $\text{Si}\phantom{\rule{4.pt}{0ex}}+\phantom{\rule{4.pt}{0ex}}\text{Si}$ systems and whether it is strong enough to generate an $S$-factor maximum, thus allowing a comparison with lighter heavy-ion systems of astrophysical interest.Methods: $^{28}\mathrm{Si}$ beams from the XTU Tandem accelerator of the INFN Laboratori Nazionali di Legnaro were used. The setup was based on an electrostatic beam separator, and fusion evaporation residues (ER) were detected at very forward angles. Angular distributions of ER were measured.Results: Fusion cross sections of $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{28}\mathrm{Si}$ have been obtained down to $\ensuremath{\simeq}600$ nb. The slope of the excitation function has a clear irregularity below the barrier, but no indication of a $S$-factor maximum is found. For $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{30}\mathrm{Si}$ the previous data have been confirmed and two smaller cross sections have been measured down to $\ensuremath{\simeq}4$ $\ensuremath{\mu}\mathrm{b}$. The trend of the $S$-factor reinforces the previous weak evidence of hindrance.Conclusions: The sub-barrier cross sections for $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{28}\mathrm{Si}$ are overestimated by coupled-channels calculations based on a standard Woods-Saxon potential, except for the lowest energies. Calculations using the M3Y+repulsion potential are adjusted to fit the $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{28}\mathrm{Si}$ and the existing $^{30}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{30}\mathrm{Si}$ data. An additional weak imaginary potential (probably simulating the effect of the oblate $^{28}\mathrm{Si}$ deformation) is required to fit the low-energy trend of $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{28}\mathrm{Si}$. The parameters of these calculations are applied to predict the ion-ion potential for $^{28}\mathrm{Si}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{30}\mathrm{Si}$. Its cross sections are well reproduced by also including one- and successive two-neutron transfer channels, besides the low-lying surface excitations.

Measurements of the excitation function for the fusion of $^{24}\mathrm{Mg}+^{30}\mathrm{Si}$ $(Q=17.89\text{ }\text{ }\mathrm{MeV})$have been extended toward lower energies with respect to previous experimental data. The $S$-factor maximum observed in this large, positive-$Q$-value system is the most pronounced among such systems studied thus far. The significance and the systematics of an $S$-factor maximum in systems with positive fusion $Q$ values are discussed. This result would strongly impact the extrapolated cross sections and reaction rates in the carbon and oxygen burnings and, thus, the study of the history of stellar evolution.

Fusion excitation functions of light heavy-ion systems show oscillatory structures above the Coulomb barrier, caused by resonances or due to the penetration of successive centrifugal barriers well separated in energy. In heavier systems, the amplitude of oscillations decreases and the peaks get nearer to each other. This makes the measurements very challenging. We have performed a first experiment for 28Si + 28Si, by measuring fusion cross sections (σ) in an energy range of ≃15 MeV above the barrier, with a small ΔElab = 0.5 MeV step. Three regular oscillations are clearly observed, which are best revealed by plotting the energy-weighted derivative of the excitation function. The excitation function has been recently measured down to cross sections ≤1μb with larger energy steps. Coupled-channel (CC) calculations based on a shallow potential in the entrance channel are able to reproduce the oscillations. A further analysis will provide a stringent test for the calculations, in particular for the choice of the ion-ion potential, because the subbarrier excitation function has to be reproduced as well. Coupled-channel (CC) calculations based on a shallow potential in the entrance channel are able to reproduce the oscillations. A further analysis will provide a stringent test for the calculations, in particular for the choice of the ion-ion potential, because the subbarrier excitation function has to be reproduced as well.

The magnitude of the Coulomb mixing parameter δ 1 has been experimentally deduced, for the first time, for the β decay of 74 Rb. The estimated magnitude is derived from the feeding of the non-analog first excited 0 + state in 74 Kr. The inferred upper limit of 0.07% is small compared to theoretical predictions. The half-life was measured to be 64.90(9) ms.  2001 Published by Elsevier Science B.V.

The sub-barrier fusion excitation function of 40 Ca + 96 Zr has been measured down to cross sections ≃2.4 μb, i.e. two orders of magnitude smaller than obtained in a previous experiment, where the sub-barrier fusion of this system was found to be greatly enhanced with respect to 40 Ca + 90 Zr, and the need of coupling to transfer channels was suggested relying on coupled-channels calculations. The purpose of this work has been to investigate the behavior of 40 Ca + 96 Zr fusion far below the barrier, thereby disentangling the elusive interplay of effects due to inelastic couplings, transfer couplings and, possibly, the appearance of the fusion hindrance. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with 48 Ca + 96 Zr is illuminating in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon Q>0 transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few μb. Locating the hindrance threshold, if any, in 40 Ca + 96 Zr would require challenging measurements of cross sections in the sub-μb range.

An excitation function of one- and two-neutron transfer channels for the $^{60}\mathrm{Ni}+^{116}\mathrm{Sn}$ system has been measured with the magnetic spectrometer PRISMA in a wide energy range, from the Coulomb barrier to far below it. The experimental transfer probabilities are well reproduced, for the first time with heavy ions, in absolute values and in slope by microscopic calculations which incorporate nucleon-nucleon pairing correlations.

We measured excitation functions for the main transfer channels in the 116 Sn+ 60 Ni reaction from above to well below the Coulomb barrier. The experiment has been performed in inverse kinematics, detecting the lighter (target-like) ions with the magnetic spectrometer PRISMA at very forward angles. The comparison between the data and microscopic calculations for the present case and for the previously measured 96 Zr+ 40 Ca system, namely superfluid and near closed shells nuclei, should significantly improve our understanding of nucleon-nucleon correlation properties in multinucleon transfer processes.

We measured excitation functions for the main transfer channels in the 116 Sn+60 Ni reaction at different bombarding energies from above to well below the Coulomb barrier. The experiment has been performed in inverse kinematics, detecting the lighter (target-like) ions with the magnetic spectrometer PRISMA at very forward angles. Good mass, nuclear charge and kinetic energy resolutions have been achieved. The comparison between the data and microscopic calculations for the present case and for the previously measured 96 Zr+40 Ca system, namely superfluid and near closed shells nuclei, should significantly improve our understanding of nucleon-nucleon correlation properties in multinucleon transfer processes.

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects.Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the $A=23$, $T=1/2$ mirror nuclei ${}^{23}$Na and ${}^{23}$Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei.Methods: Excited states in ${}^{23}$Na and ${}^{23}$Mg were populated using the ${}^{12}$C(${}^{12}$C,$p$) and ${}^{12}$C(${}^{12}$C,$n$) reactions at beam energies of 16 and 22 MeV, and their resulting $\ensuremath{\gamma}$ decay was measured with Gammasphere.Results: Level schemes for ${}^{23}$Na and ${}^{23}$Mg have been considerably extended; highly excited structures have been found in ${}^{23}$Na, as well as their counterparts in ${}^{23}$Mg for previously known rotational structures in ${}^{23}$Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2${}^{+}$. Excited states in the region above the proton threshold have been studied in both nuclei.Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent.

International audience; Multineutron and multiproton transfer channels, populated in the inverse kinematics reaction 197Au+130Te at Elab=1.07 GeV, were measured at Laboratori Nazionali di Legnaro using the presently heaviest ion beam delivered by the PIAVE-ALPI accelerator complex and detecting both projectile-like and targetlike ions. To this end the large solid angle magnetic spectrometer PRISMA was coupled to a second arm for the detection of the heavy fragments in kinematic coincidence with the light ones selected and identified with the spectrometer. The data analysis is still in progress and will allow to compare the yields of both light and heavy partner with theoretical predictions performed with the GRAZING code to get quantitative information on transfer channels and the effect of evaporation and fission on the production rate of primary fragments. The mass integrated Z distribution, extracted from the experimental data, evidenced the population of proton pickup channels that, in conjunction with the neutron stripping ones from the 130Te, open the path for the production of neutron-rich heavy nuclei. In the following, we will present some preliminary results as well as details on the experimental configuration and perspectives for future investigations in the neutron-rich heavy region.

The fusion excitation function of ${}^{32}$S + ${}^{48}$Ca has been experimentally studied in a wide energy range, from above the Coulomb barrier down to cross sections in the sub-$\ensuremath{\mu}$b region. The measurements were done at INFN--Laboratori Nazionali di Legnaro, using the ${}^{32}$S beam from the XTU Tandem accelerator. The excitation function has a smooth behavior below the barrier, and no evident hindrance character shows up in the measured energy region. The fusion barrier distribution has a peculiar shape with two distinct peaks of similar height, lower and higher than the Aky\"uz-Winther barrier. Coupled-channels calculations using the M3Y + repulsion potential are presented for this system and for ${}^{36}$S + ${}^{48}$Ca. The results of these calculations give a good account of the data, and indicate the influence of one- and two-nucleon transfer channels with positive $Q$ values, which are only open for ${}^{32}$S + ${}^{48}$Ca.

We performed a study of the behaviour of the main transfer channels in the 116Sn+60Ni system at different bombarding energies from above to well below the Coulomb barrier. The experiment has been done in inverse kinematics, detecting the lighter target-like ions with the magnetic spectrometer PRISMA at very forward angles. Good mass, nuclear charge and kinetic energy resolutions have been achieved. Sufficient statistics has been accumulated to extract angular distributions for different bombarding energies, requiring a study of the response function of the spectrometer. The comparison between the data and microscopic calculations for the present case and for the previously measured 96Zr+40Ca system, namely superfluid and near closed shells nuclei, should significantly improve our understanding of nucleon-nucleon correlation properties in multinucleon transfer processes.

International audience; Neutron-rich chlorine isotopes were populated in the Ar-40 + Pb-208 multiple transfer reaction via the -1p channels in a fragment-gamma coincident measurement employing the Prisma-Clara setup. New gamma transitions have been identified and, together with already available data, level schemes have been revised and compared with the results of large-scale sd-pf shell-model calculations. The evolution of the energy splitting between the s(1/2) and d(3/2) orbitals and the increased mixing of different proton configurations in the populated Cl isotopic chain have been deduced by inspecting the shell-model wave functions.

The fusion excitation function of 32S + 48Ca has been experimentally studied in a wide energy range, from above the Coulomb barrier down to cross sections in the sub- b region. The measurements were done at INFN-Laboratori Nazionali di Legnaro, using the 32S beam from the XTU Tandem accelerator. The excitation function has a smooth behavior below the barrier with a rather flat slope, and no maximum of astrophysical factor S vs. energy has been observed. However, other interesting features of the dynamics of this system can be noted. In particular, the fusion barrier distribution has an unusual shape with two peaks of similar height, lower and higher than the Akyuz-Winther barrier. Preliminary coupled- channels calculations and a comparison with nearby systems have been performed to get information on the possible influence of nucleon transfer channels with positive Q-value.

International audience; The revival of transfer reaction studies benefited from the construction of the new generation large solid angle spectrometers, coupled to large gamma arrays. The recent results of gamma-particle coincident measurements in Ca-40+Zr-96 and Ar-40+Pb-208 reactions demonstrate a strong interplay between single-particle and collective degrees of freedom that is pertinent to the reaction dynamics. The development of collectivity has been followed in odd Ar isotopes populated in the Ar-40+Pb-208 reaction through the excitation of the 11/2(-) states, understood as the coupling of single particle degrees of freedom to nuclear vibration quanta. Pair transfer modes is another important degree of freedom which is presently being studied with Prisma in inverse kinematics at energies far below the Coulomb barrier. First results from the Zr-96+Ca-40 reaction elucidate the role played by nucleon-nucleon correlation.

The heavy-ion radiative capture reaction ${}^{12}$C(${}^{16}$O,$\ensuremath{\gamma}$)${}^{28}$Si has been studied at three energies ${E}_{\mathrm{c}.\mathrm{m}.}=8.5$, 8.8, and 9 MeV which are close to the Coulomb barrier. The weak radiative capture process has been identified by measuring the ${}^{28}$Si recoils in the highly selective 0${}^{\ensuremath{\circ}}$ spectrometer DRAGON at TRIUMF (Vancouver). The coincident $\ensuremath{\gamma}$ rays have been recorded in the associated BGO array. This has allowed a complete measurement of the $\ensuremath{\gamma}$ spectrum and the relative strength of all decay pathways. An important part of the decay through quasibound states close to the particle threshold and the feeding of bound states with particular deformation have been identified for the first time. Comparisons with Monte Carlo simulations allowed the extraction of the full experimental radiative capture cross section. Our results suggest an important contribution of spins ${J}^{\ensuremath{\pi}}={5}^{\ensuremath{-}}$ and 6${}^{+}$ in the entrance channel. The surprisingly large cross sections from 12 $\ensuremath{\mu}$b at ${E}_{\mathrm{c}.\mathrm{m}.}=8.5$ MeV to 25 $\ensuremath{\mu}$b at ${E}_{\mathrm{c}.\mathrm{m}.}=9.0$ MeV for the heavy-ion radiative capture process are discussed in terms of the interplay between statistical and structural aspects of the process.