Search

Excitation functions of quasielastic scattering at backward angles were measured for the weakly bound ${}^{6}$Li and ${}^{7}$Li projectiles on a ${}^{64}$Zn target at energies around the Coulomb barrier. The corresponding barrier distributions were derived from the experimental cross sections. The experimental data were analyzed within the coupled-channel model using a double-folding potential as the bare potential. Inelastic excitations of the target, the ${}^{7}$Li first-excited state, and ${}^{6,7}$Li resonant state(s), corresponding to sequential breakup, were included in the calculations. The comparison between the data and coupled-channel predictions shows that the effects of channels not included in the calculations, such as direct breakup and transfers, are much larger for ${}^{6}$Li than for ${}^{7}$Li.

High-precision measurement of half-life and ${Q}_{\ensuremath{\alpha}}$ value of neutral and highly charged $\ensuremath{\alpha}$ emitters is a major subject of investigation currently. In this framework, we recently pushed half-life measurements of neutral emitters to a precision of a few per mil. This result was achieved by using different techniques and apparatuses at Istituto Nazionale di Fisica Nucleare Laboratori Nazionali del Sud (INFN-LNS) and GSI Darmstadt. Here we report on ${}^{213}$Fr half-life determination [${T}_{1/2}$(${}^{213}$Fr) = 34.14$\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}$0.06 s] at INFN-LNS, detailing the measurement protocol used. Direct comparison with the accepted value in the literature shows a discrepancy of more than three sigma. We propose this new value as a reference, discussing previous experiments.

In this contribution, an analysis of an experiment with the 30 and 52 MeV 7Li beam and the 7LiF and 6LiF targets is given, emphasizing population of cluster states by triton transfer to 7Li. Results concerning both reactions are given and discussed. Aside from the dominant α + d structure in 6Li, a rather large overlap with 3He+t cluster configuration populates cluster states in 10Be by triton pick-up. A comparision with t pick-up from 7Li, resulting with detected α instead of 3He, with a considerably higher Q-value is given. A special attention was paid to analysing and compensating non-uniformity of thicknes of standard 50 μm thin silicon detectors used in the measurement. Rather than measuring their thickness independantly, a method was derived to calculate and compensate the thickness variation directly from the measured data.

Double Sided Silicon Strip Detectors (DSSSD) are highly segmented detectors that are widely used in nuclear physics especially in radioactive beam experiments where, due to the low beam intensities, one needs to cover large solid angles with high granularity. A study of the response of DSSSDs, using 7Li and 16O beams at different energies is presented. In order to characterize the detector behavior for events corresponding to particles entering the detector in the interstrip gap both for ohmic and junction sides, signals of positive and negative polarities were acquired at the same time. Different procedures for the selection of full energy events and for the determination of the corresponding efficiencies are shown and discussed.

The radiative capture reaction 12C(α,γ)16O plays an important role in helium burning in massive stars and their subsequent evolution [1]. However, despite various experimental studies, the cross section of this reaction at stellar energies remains highly uncertain. The extrapolation down to stellar energy (Ecm∼300 keV) of the measured cross sections at higher energies is made difficult by the overlap of various contributions of which some are badly known such as that of the 2+ (Ex=6.92 MeV) and 1- (Ex=7.12 MeV) sub-threshold states of 16O. Hence, to further investigate the contribution of these two-subthreshold resonances to the 12C(α,γ)16O cross section, a new determination of their a-reduced widths and so their a- spectroscopic-factors was performed using 12C(7Li,t)16O transfer reaction measurements at two incident energies and a detailed DWBA analysis of the data [2]. The measured and calculated differential cross sections are presented as well as the obtained spectroscopic factors and the a- reduced widths as well as the assymptotic normalization constants (ANC) for the 2+ and 1- subthreshold states. Finally, the results obtained from the R-matrix calculations of the 12C(α,γ)16O cross section using our obtained a-reduced widths for the two sub-threshold resonances are presented and discussed., SCOPUS: cp.j, info:eu-repo/semantics/published

Due to their high selectivity, transfer and sequential decay reactions are powerful tools for studies of both single particle (nucleon) and cluster states in light nuclei. Their use is particularly simple for investigations of α -particle clustering (because α -particle has J π =0 + , which simplifies spin and parity assignments to observed cluster states), but they are also easily applicable to other types of clustering. Recent results on clustering in neutron-rich isotopes of beryllium, boron and carbon obtained measuring the 10 B+ 10 B reactions (at 50 and 72 MeV) are presented. The highly efficient and segmented detector systems used, built from 4 Double Sided Silicon Strip Detectors (DSSSD) allowed detection of double and multiple coincidences and, in that way, studies of states populated in transfer reactions, as well as their sequential decay.

The Trojan Horse Method has been applied to study the 7 Li ( p , α ) 4 He and 6 ( Li ( d , α ) 4 He reactions through the 7 Li(d, αα)n and 6 Li( 6 Li, αα) 4 He three body processes, respectively. The electron screening potential deduced from these experiments is much larger than the adiabatic approximation prediction for both cases; the systematic discrepancy between data and theoretical predictions is thus confirmed. Astrophysical implications of these measurements are also discussed.

We have performed precision elastic backscattering measurements for the weakly bound nuclei 6,7Li on the medium and heavy mass targets 58Ni, 116,120Sn and 208Pb at sub- and near-barrier energies (E c.m. /V bar = 0.6 to 1.3). Excitation functions of elastic scattering cross-sections have been measured at ±160° and ±170° and the corresponding ratios to Rutherford scattering and relevant barrier distributions have been extracted. These measurements complement recent work on a 28Si target, for probing systematically the potential at sub- and near-barrier energies and relevant reaction mechanisms, while indicate a possible breakdown of the dispersion relation.

In this paper details of the experimental procedure and data analysis of the collision of 11Be+64Zn around the Coulomb barrier are described and discussed in the framework of different theoretical approaches. In a previous work [ A. Di Pietro et al. Phys. Rev. Lett. 105 022701 (2010)], the elastic scattering angular distribution of the collisions 9, 10Be+64Zn as well as the angular distribution for the quasielastic scattering and transfer/breakup cross sections for the 11Be+64Zn reaction were briefly reported. The suppression of the quasielastic angular distribution in the Coulomb-nuclear interference angular region observed in the collision of the 11Be halo nucleus with respect to the other two beryllium isotopes was interpreted as being caused by a long-range absorption owing to the long decay length of the 11Be wave function. In this paper, new continuum-discretized coupled-channel calculations of the 11Be+64Zn reaction are reported in the attempt to interpret the effect of coupling with the breakup channels on the measured cross sections. The calculations show that the observed suppression of the Coulomb-nuclear interference peak is caused by a combined effect of Coulomb and nuclear couplings to the breakup channels.

The ${}^{12}$C($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{16}$O reaction plays a crucial role in stellar evolution. The rate of this reaction determines directly the ${}^{12}$C-to-${}^{16}$O abundance ratio at the end of the helium burning phase of stars and consequently has a big effect on the subsequent nucleosynthesis and even on the evolution of massive stars. However, despite many experimental studies, the low-energy cross section of ${}^{12}$C($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{16}$O remains uncertain. The extrapolation of the measured cross sections to stellar energies ($E\ensuremath{\sim}300$ keV) is made particularly difficult by the presence of the 2${}^{+}$ (${E}_{x}=6.92$ MeV) and 1${}^{\ensuremath{-}}$ (${E}_{x}=7.12$ MeV)subthreshold states of ${}^{16}$O. To further investigate the contribution of these two subthreshold resonances to the ${}^{12}$C($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{16}$O cross section, we determine their $\ensuremath{\alpha}$-reduced widths via a measurement of the transfer reaction ${}^{12}$C(${}^{7}$Li, $t$)${}^{16}$O at two incident energies, 28 and 34 MeV. The uncertainties on the determined $\ensuremath{\alpha}$-spectroscopic factors and the $\ensuremath{\alpha}$-reduced widths were reduced thanks to a detailed distorted-wave Born approximation analysis of the transfer angular distributions measured at the two incident energies. The $R$-matrix calculations of ${}^{12}$C($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{16}$O cross section using our obtained $\ensuremath{\alpha}$-reduced widths for the 2${}^{+}$ and 1${}^{\ensuremath{-}}$ subthreshold resonances lead to an $E1$ $S$ factor at 300 keV of $100\ifmmode\pm\else\textpm\fi{}28$ keV b, which is consistent with values obtained in most of the direct and indirect measurements as well as the NACRE collaboration compilation while the result for the $E2$ component ${S}_{E2}\phantom{\rule{0.28em}{0ex}}(300\phantom{\rule{4.pt}{0ex}}\text{keV})=50\ifmmode\pm\else\textpm\fi{}19$ keV b disagrees with the NACRE adopted value.

Focus of this article is put on a method of compensating for non-uniformity of 50μm thin silicon detectors, rather than measurement results. The same high sensitivity of particle identification on thin-detector thickness enables to accurately model thickness variation using the measurement data, rather than measuring the thickness separately.

One of the major problems in nuclear astrophysics concerns the estimation of electron screening effects on nuclear reaction rates. We have proposed investigating the electron screening effects of nuclear systems which decay via α-particles, using fragmented beams at relativistic energies produced at the present GSI FRS-ESR facility. By looking at the modification of the half-lives and Q -value of highly charged emitters, we expect to estimate the electron screening energies. © 2012 The Royal Swedish Academy of Sciences.

In this work is reported a study on the efficiency for full energy reconstruction of double sided silicon strip detectors. In order to investigate the effect of the electrode segmentation on the detector response signals, of both polarities for each side of the detectors was acquired at the same time. The efficiency for full energy reconstruction was studied as a function of the detector bias and the incident particle energy.

6 pags., 3 figs. -- Rutherford Centennial Conference on Nuclear Physics 8–12 August 2011, Manchester, UK, Elastic scattering and direct reactions have been studied for the collisions induced by the three Beryllium isotopes 9,10,11Be, on a medium mass 64Zn target at energies near the Coulomb barrier. The elastic-scattering angular distribution of the 11Be halo nucleus shows a deviation from the classical Fresnel type diffraction behavior in the Coulomb-nuclear interference peak angular region. The deduced total reaction cross-sections for the 11Be collision is more than a factor of two than the ones measured in the collisions induced by 9;10Be. Moreover, for 11Be a large contribution to the total reaction cross-section due to transfer and break-up processes has been observed.

Fusion cross sections for the 6Li+120Sn and 7Li+119Sn systems have been measured. We aim to search for possible effects due to the different neutron transfer Q-values, by comparing the fusion cross sections for the two systems below the barrier. This experiment is the first step of a wider systematic aiming to study the above problems in collisions induced by stable and unstable Li isotopes on tin all forming the same compound nucleus.

Fusion and transfer $+$ breakup channels have been studied in the collision induced by the two-neutron-halo ${}^{6}$He on a ${}^{64}$Zn target at energies from below to above the Coulomb barrier. For comparison, the reaction induced by the stable isotope ${}^{4}$He on the same target has been studied. The fusion cross section has been measured by using an activation technique, detecting off-line the delayed x-ray activity following the electron capture decay of the evaporation residues. New measurements of the ${}^{4}$He $+$ ${}^{64}$Zn fusion cross section at sub-barrier energies have been performed in order to cover the same energy range of the ${}^{6}$He $+$ ${}^{64}$Zn fusion cross section and to compare the excitation functions for the two systems down to the lowest energy data point measured for ${}^{6}$He. From the new comparison a sub-barrier fusion enhancement has been observed in the ${}^{6}$He case with respect to the ${}^{4}$He one whereas no effect on the ${}^{6}$He fusion cross section has been seen at energies above the barrier. It has been concluded that such enhancement seems to be due to the diffuse halo structure properties of the ${}^{6}$He nucleus. Moreover, the reactions induced by ${}^{6}$He have shown a strong yield of $\ensuremath{\alpha}$ particles coming from direct processes.

$\ensuremath{\gamma}$-ray production cross sections have been measured in proton irradiations of N, Ne, and Si and $\ensuremath{\alpha}$-particle irradiations of N and Ne. In the same experiment we extracted also line shapes for strong $\ensuremath{\gamma}$-ray lines of $^{16}\mathrm{O}$ produced in proton and $\ensuremath{\alpha}$-particle irradiations of O. For the measurements gas targets were used for N, O, and Ne and a thick foil for Si. All targets were of natural isotopic composition. Beams in the energy range up to 26 MeV for protons and 39 MeV for $\ensuremath{\alpha}$ particles were delivered by the Institut de Physique Nucl\'eaire--Orsay tandem accelerator. The $\ensuremath{\gamma}$ rays were detected with four high-purity Ge detectors in the angular range ${30}^{\ifmmode^\circ\else\textdegree\fi{}}$ to ${135}^{\ifmmode^\circ\else\textdegree\fi{}}$. We extracted 36 cross-section excitation functions for proton reactions and 14 for $\ensuremath{\alpha}$-particle reactions. For the majority of the excitation functions no other data exist to our knowledge. Where comparison with existing data was possible, usually a very good agreement was found. It is shown that these data are very interesting for constraining nuclear reaction models. In particular, the agreement of cross section calculations in the nuclear reaction code talys with the measured data could be improved by adjusting the coupling schemes of collective levels in the target nuclei $^{14}\mathrm{N}$, $^{20,22}\mathrm{Ne}$, and $^{28}\mathrm{Si}$. The importance of these results for the modeling of nuclear $\ensuremath{\gamma}$-ray line emission in astrophysical sites is discussed.

International audience; Elastic scattering and direct reactions have been studied for the collisions induced by the three Beryllium isotopes 9,10,11Be, on a medium mass 64Zn target at energies around the Coulomb barrier. The elastic scattering angular distributions, measured for the three systems at the same center of mass energy, were analyzed within the Optical Model and reaction cross-sections were deduced from optical model calculations. For the 11Be induced reaction the transfer/break-up angular distribution was also extracted.

The structure of the weakly bound nuclei is expected to influence the fusion mechanism at energies around and below the Coulomb barrier. In fact direct channels may be favoured with respect to fusion by the low binding energies, while Coupling of the break-up channel can be responsible for a fusion cross-section enhancement. In this context the {sup 6}Li+{sup 64}Zn collision has been studied at several energies around the Coulomb barrier. The fusion cross section was measured by using an activation technique where the radioactive evaporation residues produced in the reaction were identified by the X-ray emission which follows their electron capture decay. The elastic scattering angular distributions were analyzed within the Optical Model and total reaction cross-sections were deduced from optical model calculations. (authors)

New fusion data for the {sup 4}He+{sup 64}Zn system at sub-barrier energies are measured to cover the same energy region of previous measurements for {sup 6}He+{sup 64}Zn. Aim of the experiment was to compare the fusion excitation functions for the two system to investigate on the effects of the {sup 6}He neutron-halo structure on the fusion reaction mechanism at energies around the Coulomb barrier. The fusion cross section was measured by using an activation technique. Comparing the two systems, we observe an enhancement of the fusion cross section in the reaction induced by {sup 6}He, at and below the Coulomb barrier.

Elastic‐scattering angular distributions for the 6Li+64Zn system were measured at energies from below to above the Coulomb barrier. The experimental data were analyzed within the optical model to study the energy dependence of the interaction potential. The results suggest the presence of the breakup threshold anomaly. Preliminary results on the fusion cross‐sections for the same system are also reported.

Comunicación presentada al: "Frontiers in nuclear structure, astrophysics, and reactions: Finustar 3" celebrado del 23 al 27 de agosto del 2010 en Grecia.-- et al., In this contribution, results of experiments performed with the three Beryllium isotopes 9,10,11Be on a medium mass 64Zn target, at a center of mass energy of ≈1.4 the Coulomb barrier, will be discussed. Elastic scattering angular distributions have been measured for the 9,10Be reactions. In the 11Be case the quasielastic scattering angular distribution was obtained. In the halo nucleus case, the angular distribution exhibit a non-Fresnel-type pattern with a strong damping of the Coulomb-nuclear interference peak. Moreover, it is found that the total reaction cross-section for the halo nucleus induced collision is more than double the ones extracted in the collisions induced by the non-halo Beryllium isotopes. A large contribution to the total-reaction cross-section in the 11Be case could be attributed to transfer and/or break-up events.

6 páginas, 4 figuras.-- et al., The collision induced by the three Beryllium isotopes, 9,10,11Be, on 64Zn target were investigated at Ec.m. ¿ 1.4 the Coulomb barrier. Elastic scattering angular distributions were measured for the 9,10Be collisions whereas, in the 11Be case the quasielastic scattering angular distribution was obtained. A strong damping of the quasielastic cross-section was observed in the 11Be case, especially in the angular range around the Coulomb-nuclear interference peak. In this latter case a large total-reaction cross-section is found, more than a factor of two larger than the ones extracted in the reactions induced by the non-halo Beryllium isotopes. A large contribution to the total-reaction cross-section in the 11Be case could be attributed to transfer and/or break-up events.

The 15O(α,γ)19Ne reaction is generally considered as a potential breakout reaction from the hot CNO cycle. Under nova conditions, the reaction rate is dominated by a single sub-Coulomb resonance at Ec.m.=504 keV which corresponds to a 19Ne excitation energy of ER=4.033 MeV. Results from a d(18Ne,19Ne)p experiment show that states of astrophysical interest are populated in this reaction. An upper limit for the α-branching ratio of the 4.033 MeV state was identified and branching ratios for other states in 19Ne were found to be in good agreement with stable beam based results.

Collisions induced by {sup 9,10,11}Be on a {sup 64}Zn target at the same c.m. energy were studied. For the first time, strong effects of the {sup 11}Be halo structure on elastic-scattering and reaction mechanisms at energies near the Coulomb barrier are evidenced experimentally. The elastic-scattering cross section of the {sup 11}Be halo nucleus shows unusual behavior in the Coulomb-nuclear interference peak angular region. The extracted total-reaction cross section for the {sup 11}Be collision is more than double the ones measured in the collisions induced by {sup 9,10}Be. It is shown that such a strong enhancement of the total-reaction cross section with {sup 11}Be is due to transfer and breakup processes.

In this contribution, results concerning different reaction channels for the collisions induced by the three Be isotopes, 9,10,11 Be , on a 64 Zn target at energies around the Coulomb barrier will be presented. The experiments with the radioactive 10,11 Be beams were performed at REX-ISOLDE (CERN) whereas the experiment with the stable weakly bound 9 Be beam was performed at LNS Catania. Elastic scattering angular distributions have been measured for the three systems 9,10,11 Be + 64 Zn at the same center of mass energy. The angular distributions were analyzed with optical potentials and reaction cross sections were obtained from optical model calculations, performed with the code PTOLEMY. For the 11 Be + 64 Zn reaction, the break-up angular distribution was also measured.

An experimental technique is described that is able to measure reaction cross-sections at energies around the Coulomb barrier by using low intensity beams and a Si detector as an active target. Set-up optimization was carefully investigated in terms of collimation, detector efficiency and pile-up rejection. The method has been tested by measuring the total reaction cross-section sigma(R)(E) for the (7)Li + (28)Si system in the energy range of E(lab) = 12-16 MeV. The deduced excitation function sigma(R)(E) agrees with the data obtained in a previous experiment. The presented technique can also be applied in order to determine total reaction cross-sections for low intensity radioactive beams at energies around the Coulomb barrier. (C) 2009 Elsevier B.V. All rights reserved. Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment

The 12C(α,γ)16O reaction plays an important role in helium burning in massive stars and their evolution. However, despite many experimental studies, the low‐energy cross section of 12C(α,γ)16O still highly uncertain. The extrapolation of the measured cross sections to stellar energies (E∼300 keV) is made difficult by the presence of the two sub‐threshold states at 6.92 and 7.12 MeV of 16O. In order to further investigate the contribution of these two‐subthreshold resonances to the 12C(α,γ)16O cross section, we performed a new determination of the α ‐reduced widths via a measurement of the transfer reaction 12C(7Li,t)16O at two incident energies, 34 and 28 MeV. The measured and calculated differential cross sections will be presented as well as the obtained spectroscopic factors and the α ‐reduced widths.

Elastic-scattering angular distributions for the $^{6}\mathrm{Li}+^{64}\mathrm{Zn}$ system were measured at eight beam energies from below to above the Coulomb barrier, $12.0\ensuremath{\leqslant}{E}_{\mathrm{lab}}\ensuremath{\leqslant}22.0$ MeV. The experimental data were analyzed within the optical model to study the energy dependence of the interaction potential and to obtain total reaction cross sections. The results obtained using several optical model potentials show similar behavior. The energy dependence of the strengths of the real and imaginary potentials suggests the presence of the breakup threshold anomaly. It is shown that the extracted energy dependence of the interaction potential at sub-barrier energies is very sensitive to possible experimental errors.

Six proton-emitting states in $^{19}\mathrm{Ne}$ were studied through the inelastic scattering reaction $\mathrm{H}(^{19}\mathrm{Ne},p)^{19}\mathrm{Ne}^{*}(p)^{18}\mathrm{F}$. Their energies and widths were derived from the protons detected at zero degree, while proton-proton angular correlations between the detector at zero degree and a segmented annular detector were used to determine their spin value. In addition to the known states, a new broad $J=\frac{1}{2}$ resonance has been evidenced at ${E}_{x}\ensuremath{\approx}7.9\text{ }\text{ }\mathrm{MeV}$, $\ensuremath{\approx}1.45\text{ }\text{ }\mathrm{MeV}$ above the proton emission threshold. By introducing this resonance, the $^{18}\mathrm{F}(p,\ensuremath{\alpha})^{15}\mathrm{O}$ destruction rate in novae is significantly enhanced. This reduces the chance to observe the cosmic $\ensuremath{\gamma}$-ray emission of $^{18}\mathrm{F}$ from novae in space telescopes.

International audience; The reactions 9,10,11Be+64Zn have been studied at energies around the Coulomb barrier in order to better understand the role played by the halo features on the reaction dynamics. Preliminary results on such studies will be reported in this paper.

An open problem in Nuclear Astrophysics concerns the understanding of electron‐screening effects on nuclear reaction rates at stellar energies. In this framework, we have proposed to investigate the influence of the electron cloud on α‐decay by measuring Q‐values and α‐decay half‐lives of fully stripped, H‐like and He‐like ions. These kinds of measurements have been feasible just recently for highly‐charged radioactive nuclides by fragmentation of 238U at relativistic energies at the FRS‐ESR facility at GSI. In this way it is possible to produce, efficiently separate and store highly‐charged α‐emitters. Candidates for the proposed investigation were carefully selected and will be studied by using the Schottky Mass Spectroscopy technique. In order to establish a solid reference data set, lifetimes and Qα‐value measurements of the corresponding neutrals have been performed directly at the FRS, by implanting the separated ions into an active Silicon stopper.

The $^{13}\mathrm{C}$($\ensuremath{\alpha},n$)$^{16}\mathrm{O}$ reaction is considered the main neutron source for the s process in low mass asymptotic giant branch (AGB) stars. In the Gamow peak, the cross section sensitively depends on the $1/{2}^{+}$ subthreshold state of $^{17}\mathrm{O}$ (${E}_{x}=6.356$ MeV). In this work, we determined the astrophysical $S$ factor through an evaluation of the \ensuremath{\alpha} spectroscopic factor and the corresponding asymptotic normalization factor (ANC) of the 6.356 MeV state using the transfer reaction $^{13}\mathrm{C}$($^{7}\mathrm{Li}$,$t$)$^{17}\mathrm{O}$ at two different incident energies. Our result confirms that the contribution of the $1/{2}^{+}$ state is dominant at astrophysical energies. Our reaction rate at $T=0.09$ GK is slightly lower than the value adopted in the Nuclear Astrophysics Compilation of REaction rates (NACRE), but two times larger than the one obtained in a recent ANC measurement.

The reaction 13C(α,n)16O is considered as the main neutron source for s‐process in low‐mass asymptotic giant branch stars. At low energies of astrophysical interest, the contribution of the subthreshold state 6.356 MeV of 17O to the 13C(α,n)16O cross section should be taken into account. However, the results of previous studies of this contribution lead to different conclusions.Hence, we investigated the effect of this resonance on the astrophysical S‐factor through a new precise measurement of the alpha spectroscopic factor, Sα, of the 6.356 MeV state using the transfer reaction 13C(7Li,t)17O at two different incident energies. The measured angular distributions and the obtained spectroscopic factors will be presented as well as their impact on C(α,n)16O cross section and reaction rate.

The ground state of particle‐unbound nuclei is observed as a resonance. According to the Heisenberg's uncertainty principle, the shorter is the lifetime of the state the broader is the resonance. The Breit‐Wigner formalism describes perfectly the shape of the resonance when energy‐dependent partial‐widths are used. In the low energy tail of a ground state resonance near the particle‐emission threshold, the partial‐width for the emission of a charged particle through the Coulomb barrier is drastically reduced. The fact that the proton decay is strongly hindered seems to be equivalent to the assertion that the decay of the proton is strongly delayed. In this interpretation and as an example, possible consequences are discussed in astrophysical context. Two reactions 15O(p,β+)16O and 15O(p,γ)(β+)16O are proposed as new pathways to bypass the 15O waiting point. Cross‐sections were calculated after precise measurements of energies and widths of the proton‐unbound 16F low lying states, obtained using the H(15O,...

The feasibility of inclusive neutron production measurements in reactions induced by low-intensity radioactive beams using a 4π thermalization counter is studied. The time response of the detector is investigated experimentally by a technique that results in an enhanced sensitivity to weak components with long capture times. Complementary Monte Carlo simulations are presented. The capture time response is found to be independent on the neutron energy above 0.1 MeV. The capability of the capture time information in the unambiguous identification of neutron signals correlated to the projectile arrival on the target even in the presence of an intense background contamination is shown. As an application case, the 8 Li( 4 He,n) 11 B reaction at the Big-Bang temperature is commented.

The low-energy levels of /sup 19/Na were studied in the center-of-mass energy (E/sub CM/) range from 2.6 to 3.4 MeV, by using the elastic and inelastic scattering methods in inverse kinematics. The /sup 18/Ne+p scattering reactions, leading to the ground state and the first excited state of /sup 18/Ne, were investigated by using a 66 MeV radioactive /sup 18/Ne beam and a polyethylene (CH/sub 2/)/sub n/ target. The experimental methods and the set-up are described and preliminary results are presented.

The results concerning the study of 6 Li+ 120 Sn and 7 Li+ 119 Sn systems are presented. These two sistems are characterised by very similar structures of the interacting nuclei and by different Q-value for oneand two- neutron transfer. Our aim is to disentangle the possible effects due to the different n-transfer Q-values, at sub-barriers energies, by comparing the two fusion excitation function. In these experiments the fusion cross section has been measured by using a stack activation technique. No particular differences in the two fusion excitation functions have been observed. The influence of transfer channels on fusion cross-section has been object of investigations in the last years. In particular, the possible dependence of the fusion cross-section on the sign of the neutron transfer Q-value has been much debated in literature. The systematic approach used for the study of the Ca+Zr systems [1] provided relatively clear evidence of the relation between sub-barrier-cross section and the sign of the neutron transfer Q-value, in a model-independent way. According to experimental

We propose the sequential reaction process $^{15}$O($p$,$\gamma)(\beta^{+}$)$^{16}$O as a new pathway to bypass of the $^{15}$O waiting point. This exotic reaction is found to have a surprisingly high cross section, approximately 10$^{10}$ times higher than the $^{15}$O($p$,$\beta^{+}$)$^{16}$O. These cross sections were calculated after precise measurements of energies and widths of the proton-unbound $^{16}$F low lying states, obtained using the H($^{15}$O,p)$^{15}$O reaction. The large $(p,\gamma)(\beta^{+})$ cross section can be understood to arise from the more efficient feeding of the low energy wing of the ground state resonance by the gamma decay. The implications of the new reaction in novae explosions and X-ray bursts are discussed., Comment: submitted

The 6He + 6, 7Li and 6He + 12C scattering and reactions have been studied using an 18-MeV 6He beam. Experimental results for the elastic scattering on all three targets are in fair agreement with optical model predictions, using the potentials found in the analysis of the 6Li scattering on the same targets and at close beam energies. Several two-body exit channels show clear signatures of a direct reaction mechanism allowing extraction of spectroscopic information. The measured angular distribution for the 6He + 6Li → α + 8Li reaction indicated close similarity between the α + 2n configuration in 6He and the α + d configuration in 6Li. The obtained results for α -particle pickup from both 6Li and 7Li give large values of α spectroscopic factors for some 10Be states, indicating their well-developed α + 6He cluster structure. The exotic two-proton pickup reaction (6He, 8Be) was studied, as well as two-neutron and triton transfer reactions. Quasi-free scattering of 6He on deuteron and α particle in 6Li was also observed. The sequential decay reactions 6He + 6Li → 6He + α + d, 6He + 6Li → 2α + t + n, 6He + 7Li → 6He + α + t, and 6He + 12C → 10Be + 2α were clearly seen, and α clustering of some states in 6, 7Li, 8– 10Be, and 14C was thus studied. Several new spectroscopic results obtained for some 10Be states support the existence of a molecule-like rotational band in 10Be with a very large moment of inertia.

The 8Li+4He elastic scattering excitation function was measured by using the inverse kinematic thick target method. The 8Li beam was provided by the LNS radioactive facility EXCYT at an energy of 30 MeV and it was delivered into a large scattering chamber filled with 4He gas. The detection system was made by three silicon telescopes and one MCP. This last detector was used to measure the number of incident particles as well as the time of flight allowing for the discrimination between elastic and inelastic scattering. The setup and the experimental technique will be described in details and the results, as well as the comparison of the data with an R-matrix calculation, will be shown and discussed.