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Radioactive ^{129}Sb, which can be treated as a proton plus semimagic ^{128}Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, B(E2), for the 2^{+}⊗πg_{7/2} multiplet members and candidate πd_{5/2} state were measured. The results indicate that the total electric quadrupole strength of ^{129}Sb is a factor of 1.39(11) larger than the ^{128}Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wave functions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

International audience; Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM) is an experiment for studying the origin, acceleration, and propagation mechanisms of high-energy cosmic rays. The ISS-CREAM instrument was launched on the 14th of August 2017 to the ISS aboard the SpaceX-12 Dragon spacecraft. The Top and Bottom Counting Detectors (TCD/BCD) are parts of the ISS-CREAM instrument and designed for studying electron and gamma-ray physics. The TCD/BCD each consist of an array of 20 × 20 photodiodes on a plastic scintillator. The TCD/BCD can separate electrons from protons by using the difference between the shapes of electromagnetic and hadronic showers in the high energy region. The Boosted Decision Tree (BDT) method, which is a deep learning method, is used in this separation study. We will present results of the electron/proton separation study and rejection power in various energy ranges.

International audience; Cosmic Ray Energetics and Mass for the International Space Station (ISS-CREAM) is designed to directly measure the energy spectra of high-energy cosmic rays, encompassing proton to iron nuclei, over the energy range from 1012 to 1015 eV [1]. The capability to measure an extended energy range enables us to probe the origin and acceleration mechanisms of cosmic rays. The ISS-CREAM instrument is configured with the balloon-borne CREAM calorimeter (CAL) for energy measurements and four layers of a finely segmented Silicon Charge Detector (SCD) for charge measurements. In addition, two new compact detectors have been developed for electron/proton separation: Top and Bottom scintillator-based counting detectors (TCD/BCD) and a boronated scintillator detector (BSD). Simulations use the GEANT3 package [2] with the FLUKA hadronic model [3]. An isotropic event generator was developed for the ISS-CREAM geometry with particles incident from the upper hemisphere. We will present simulation results regarding ISS-CREAM performance, including trigger rates, energy resolution, energy response, tracking resolution, charge efficiency, etc.

Half of the elements heavier than iron are believed to be produced through the rapid neutron-capture process ($r$ process). The astrophysical environment(s) where the $r$ process occurs remains an open question, even after recent observations of neutron-star mergers and the associated kilonova. Features in the abundance pattern of $r$-process ashes may provide critical insight for distinguishing contributions from different possible sites, including neutron-star mergers and core-collapse supernovae. In particular, the largely unknown neutron-capture reaction rates on neutron-rich unstable nuclei near $^{132}\mathrm{Sn}$ could have a significant impact on the final $r$-process abundances. To better determine these neutron-capture rates, the $(d,p)$ reaction has been measured in inverse kinematics using radioactive ion beams of $^{126}\mathrm{Sn}$ and $^{128}\mathrm{Sn}$ and a stable beam of $^{124}\mathrm{Sn}$ interacting with a ${({\mathrm{CD}}_{2})}_{n}$ target. An array of position-sensitive silicon strip detectors, including the Super Oak Ridge Rutgers University Barrel Array, was used to detect light reaction products. In addition to the present measurements, previous measurements of $^{130,132}\mathrm{Sn}(d,p)$ were reanalyzed using state-of-the-art reaction theory to extract a consistent set of spectroscopic factors for $(d,p)$ reactions on even tin nuclei between the heaviest stable isotope $^{124}\mathrm{Sn}$ and doubly magic $^{132}\mathrm{Sn}$. The spectroscopic information was used to calculate direct-semidirect $(n,\ensuremath{\gamma})$ cross sections, which will serve as important input for $r$-process abundance calculations.

The exotic Borromean nucleus $^{20}$Mg with $N$ = 8, located at the proton drip-line provides a unique testing ground for nuclear forces and the evolution of shell structure in the neutron-deficient region. We report on the first observation of proton unbound resonances together with bound states in $^{20}$Mg from the $^{20}$Mg($d$,$d'$) reaction performed at TRIUMF. Phenomenological shell-model calculations offer a reasonable description. However, our experimental results present a challenge for current first-principles nuclear structure approaches and point to the need for improved chiral forces and {\it ab initio} calculations. Furthermore, the differential cross section of the first excited state is compared with distorted-wave Born approximation calculations to deduce a neutron quadrupole deformation parameter of $\beta_n$=0.46$\pm$0.21. This provides the first indication of a possible weakening of the $N$ = 8 shell closure at the proton drip-line.

The production of the p–nuclei is one of the unsolved puzzles in nuclear astrophysics. A possible mechanism is the nucleosynthesis in the neutrino–driven winds of core–collapse supernovae (νp–process), but it carries uncertainties, mostly in the supernova dynamics and the nuclear physics input. The pp-chain breakout reaction 7Be(α,γ)11C, which occurs prior to the supernova explosion, was identified as an important link which can influence the nuclear flow of the z/p-process and the final abundances of the p-nuclei. Nevertheless, its reaction rate is poorly known over the relevant energy range (T=1.5-3 GK). To improve the 7Be(α,γ)11C rate for vp-process nucleosynthesis temperatures, the first measurement of the strengths of two important resonances with unknown strength was recently performed at TRIUMF. A radioactive 7Be beam (t1/2 = 53.24 d) beam and the DRAGON recoil separator were used. The experimental details and preliminary results for the resonance strengths will be discussed.

International audience; The Cosmic Ray Energetics And Mass (CREAM) instrument on the International Space Station (ISS) is an experiment to study origin, propagation, acceleration and elemental composition of cosmic rays. The Top Counting Detector (TCD) and Bottom Counting Detector (BCD) are parts of the detector suite of the ISS-CREAM experiment and are designed to separate electrons and protons for studying electron and gamma-ray physics. In addition, the TCD/BCD provide a redundant trigger to that of the calorimeter and a low energy trigger to the ISS-CREAM instrument. After launching, the TCD/BCD trigger was found to be working well. Also, the TCD/BCD have been stable and their hit positions were confirmed to be well matched with other detectors on board. We present the performance and status of the TCD/BCD in flight.

International audience; The Cosmic Ray Energetics And Mass experiment for the International Space Station (ISS-CREAM) is a space-borne mission designed for the precision measurement of the energy and elemental composition of cosmic rays. The Silicon Charge Detector (SCD), placed at the top of the ISS-CREAM payload, consists of 4 layers. Each layer has 2688 silicon pixels and associated electronics arranged in such a fashion that its active detection area of 78.2 × 73.6 cm 2 is free of dead area. The foremost goal of the SCD is to efficiently and precisely measure the charge of cosmic rays passing through it. The 4-layer configuration was chosen to achieve the best precision in measuring the charge of cosmic rays within the constraints on the mass, volume and power allotted to it. The amount of material used for its support structure was minimized as well to reduce the chance of interactions of the cosmic ray within the structure. Given the placement of the SCD, its 4-layer configuration and the minimal amount of material in the cosmic-ray trajectory, the SCD is designed to measure the charge of cosmic rays ranging from protons to iron nuclei with excellent detection efficiency and charge resolution. We present the design and fabrication of the SCD as well as its performance during space environment tests which it underwent successfully. We also present its performance in charge measurement using heavy ions in a beam test at CERN, the European Organization for Nuclear Research.

The Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM) instrument is a next-generation experiment for the direct detection and study of cosmic-ray nuclei and electrons. With a long exposure in low Earth orbit, the experiment will determine the particle fluxes and spectral details of cosmic-ray nuclei from hydrogen to iron, over an energy range of about 1 0 12 eV to > 1 0 15 eV, and of cosmic-ray electrons over an energy range of about 5 × 1 0 10 eV to > 1 0 13 eV. The instrument was deployed to the ISS in August 2017 on the SpaceX CRS-12 mission. We review the design, implementation and performance of one of the ISS-CREAM detector systems: a boron loaded scintillation detector used in discriminating electron-induced events from the much more abundant cosmic-ray nuclei.

International audience; The Cosmic Ray Energetics and Mass experiment at the International Space Station (ISS-CREAM) is developed for studying the origin, acceleration and propagation mechanism of high energy cosmic rays. The Top and Bottom Counting Detectors (TCD/BCD), sub-detectors of the ISS-CREAM instrument, are developed for electron/ γ -ray physics. The TCD/BCD help distinguish electrons from protons by comparing the hit and shower width distributions for electrons and protons. The e/p separation capability of the TCD/BCD is studied by using the GEANT3 simulation package, and optimal parameters for the e/p separation are obtained.

International audience; The Cosmic Ray Energetics And Mass experiment for the International Space Station (ISS-CREAM) is a space-borne mission designed for the precision measurement of the energy and elemental composition of cosmic rays. It is scheduled to be launched and installed on the ISS in August 2017. The Silicon Charge Detector (SCD), placed at the top of the ISS-CREAM payload, consists of 4 layers. Each layer has 2688 silicon pixels and associated electronics arranged in such a fashion that its active detection area of 78 x 74 cm2 is free of any dead area. The foremost goal of the SCD is to efficiently and precisely measure the charge of cosmic rays passing through it. The 4-layer configuration was chosen to achieve the best precision in measuring the charge of cosmic rays within the constraints on the mass, volume and power allotted to it. The amount of material used for its support structure was minimized as well to reduce the chance of interactions of the cosmic ray within the structure. Given the placement of the SCD, its 4-layer configuration and the minimal amount of material in the cosmic-ray trajectory, the SCD is designed to measure the charge of cosmic rays ranging from protons to iron nuclei with excellent detection efficiency and charge resolution. We present the design and fabrication of the SCD. It successfully underwent space environment tests including vibration and thermal-vacuum qualification. We present the performance of the SCD during these tests, as well as its charge-measurement performance on the ground using cosmic muons and heavy ions in CERN beam tests.

International audience; The Cosmic-Ray Energetics And Mass (CREAM) instrument for the International Space Station (ISS) is a detector for studying the origin, acceleration and propagation mechanism of high-energy cosmic rays. The ISS-CREAM instrument is scheduled to launch in 2017 to the ISS. The Top and Bottom Counting Detectors (TCD/BCD) are designed for studying electron and gamma-ray physics. The TCD/BCD are composed of a plastic scintillator and an array of photodiodes The active detection areas of the TCD/BCD are 500 $\times$ 500 mm$^2$ and 600 $\times$ 600 mm$^2$, respectively. The TCD/BCD were completed in 2015 and passed the environmental tests for safety in a space environment. After finishing these tests, the TCD/BCD were integrated with the payload. The TCD is located between the carbon target of the ISS-CREAM instrument and the calorimeter, and the BCD is located below the calorimeter. The TCD/BCD can distinguish between electrons and protons by using the different shapes between electromagnetic and hadronic showers in the high-energy region. We study the TCD/BCD performance in various energy ranges by using GEANT3 simulation data. Here, we present the status of the electron and proton separation study with the TCD/BCD simulation.

International audience; The Cosmic Ray Energetics And Mass for the International Space Station (ISS-CREAM) instrument is designed and built to measure elemental spectra of cosmic-ray nuclei (1 ≤ Z ≤ 26) and electrons. It will measure energy of incident cosmic rays from 1011 to 10^15 eV with a tungsten/scintillator sampling calorimeter and densified carbon target with an interaction length of ~ 1 λL. A finely segmented, four-layer silicon charge detector will identify the elemental composition with a resolution of ~ 0.15e. The instrument is triggered by selectable, independent, and combined algorithms from the calorimeter and a scintillator-based counting detector on the top and bottom of the calorimeter. The counting detectors also provide separation of protons and electrons using differences in the shower shapes. A boronated scintillator detector provides additional e/p separation by looking at late scintillation light produced by a particle interacting in the calorimeter system. ISS-CREAM underwent vibrational, electromagnetic, thermal/vacuum, and telemetry systems tests at various NASA f acilities to qualify for rocket transportation and space operations. All testing and integration were completed and ISS-CREAM was delivered to NASA. It is now flight ready and waiting for launch on SpaceX-12 in 2017. ISS-CREAM integration, environmental qualification, and instrument performance will be presented.

Of particular interest in astrophysics is the $^{34}\mathrm{S}(p,\ensuremath{\gamma})^{35}\mathrm{Cl}$ reaction, which serves as a stepping stone in thermonuclear runaway reaction chains during a nova explosion. Though the isotopes involved are all stable, the reaction rate of this significant step is not well known, due to a lack of experimental spectroscopic information on states within the Gamow window above the proton separation threshold of $^{35}\mathrm{Cl}$. Measurements of level spins and parities provide input for the calculation of resonance strengths, which ultimately determine the astrophysical reaction rate of the $^{34}\mathrm{S}(p,\ensuremath{\gamma})^{35}\mathrm{Cl}$ proton capture reaction. By performing the $^{37}\mathrm{Cl}(p,t)^{35}\mathrm{Cl}$ reaction in normal kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory, we have conducted a study of the region of astrophysical interest in $^{35}\mathrm{Cl}$, and have made the first-ever constraint on the spin and parity assignment for a level at $6677\ifmmode\pm\else\textpm\fi{}15$ keV $({E}_{r}=306$ keV), inside the Gamow window for novae.

We demonstrate an approach to measure the total fusion cross-section for beams of low-intensity light nuclei. Fusion residues resulting from the fusion of 20,16 O+ 12 C are directly measured and distinguished from unreacted beam particles on the basis of their energy and time-of-flight. The time-of-flight is measured between a microchannel plate (MCP) detector, which serves as a start, and a segmented silicon detector, which provides a stop. The two main difficulties associated with the initial implementation of this approach are charge trapping in the silicon detector and slit scattering in the MCP detector. Both these obstacles have been characterized and overcome. To reduce atomic slit scattering in the measurement we have eliminated wires from the beam path by developing a gridless MCP detector. The total fusion cross-section for 16 O+ 12 C in the energy range E CM =8.0–12.0 MeV has been measured in agreement with established literature values (down to the 100 mb level).

The time dependence of the relative light emission of Eljen Technology EJ-200 polyvinyltoluene-based plastic scintillator was measured between 1 and 10 μ s after the passage of a particle shower, a singly charged particle (atmospheric muon), and with a UV LED exciting the fluor. This was compared in magnitude to the integrated response for the prompt light (within 500 ns of excitation). A model with a time-dependent yield consisting of three exponentially decaying components (fast, medium, and slow) was developed to fit the data. Note that the exact time structure of early ( 1 μ s ) light emission was not measured for individual components, only for all three components together This model assumes all three components share the same rise time. The decay time constants of the fast, medium and slow components are, respectively, 7.8 ns, 490 ns, and 2370 ns. The relative total normalized yields for each component are: fast 95.8%, medium 2.2%, and slow 2.0%.

International audience; The Boron And Carbon Cosmic rays in the Upper Stratosphere (BACCUS) balloon-borne exper-iment flew for 30 days over Antarctica in December 2016. It is the successor of the CREAMballoon program in Antarctica which recorded a total cumulative exposure of 161 days. BAC-CUS is primarily aimed to measure cosmic-ray boron and carbon fluxes at the highest energiesreachable with a balloon or satellite experiment, in order to provide essential information for abetter understanding of cosmic-ray propagation in the Galaxy. The payload is made of multipleparticle physics detectors which measure the charge up to Z=26 and energy of incident particlesfrom a few hundred GeV to a few PeV. The newly designed Transition Radiation Detector (TRD)measures signals that are a function of the charge and Lorentz factor. In April 2016, BACCUSwas taken to CERN in its flight configuration to characterize its detectors’ response to beams ofelectrons and pions. The performance of the TRD using beam test data are reported in this paper.

In this study, the presence of neutron transfer channels with positive Q values can enhance sub-barrier fusion cross sections. Recent measurements of the fusion excitation functions for 58Ni+132,124Sn found that the fusion enhancement due to the influence of neutron transfer is smaller than that in 40Ca +132,124Sn although the Q values for multineutron transfer are comparable. The purpose of this study is to investigate the differences observed between the fusion of Sn + Ni and Sn + Ca. Methods: Fusion excitation functions for 46,50Ti +124Sn have been measured at energies near the Coulomb barrier. As a result, a comparison of the barrier distributions for 46Ti+124Sn and 40Ca+124Sn shows that the 40Ca+124Sn system has a barrier strength resulting from the coupling to the very collective octupole state in 40Ca at an energy significantly lower than the uncoupled barrier. In conclusion, the large sub-barrier fusion enhancement in 40Ca induced reactions is attributed to both couplings to neutron transfer and inelastic excitation, with the octupole vibration of 40Ca playing a major role.

The use of neutron-rich radioactive isotope beams for synthesizing heavy elements is a subject of continuing interest. Current experiments utilize low intensity beams to study reaction mechanisms for fusion induced by neutron-rich radioactive nuclei. Enhanced fusion cross sections were observed near and below the Coulomb barrier. A lowering of the barrier height was observed for heavy mass compound systems. Future experiments will benefit from improving detectors and increasing beam intensities.

Angular distributions of the Coulomb breakup of radioactive 17F were measured by impinging a 10 MeV/nucleon beam on 208Pb and on 58Ni to study the dynamic polarization effects. The breakup products, oxygen and a proton, were detected in coincidence. First-order perturbation theory significantly overpredicts the breakup cross section for the 208Pb target. Dynamical calculations with a dynamic polarization as the leading order correction were performed. The calculations reproduce the data for 17F on 58Ni but overpredict the breakup of 17F on 208Pb by a factor of two at forward angles.

Beams of 132,134 Sn and 134 Te were used to bombard a 64 Ni target and measure fusion excitation functions near the Coulomb barrier. Fusion induced by 132,134 Sn and 134 Te below the barrier is enhanced as compared to stable Sn and Te beams. The enhancement in 132 Sn and 134 Te is essentially due to the larger radii which lower the Coulomb barriers. However, the enhancement in 134 Sn induced fusion cannot be accounted for by the change in nuclear size.

A complete set of electromagnetic moments, B(E2;0+1 2+1), Q(2+1), and g(2+1), have been measured from Coulomb excitation of semi-magic 112,114,116,118,120,122,124Sn (Z = 50) on natural carbon and titanium targets. The magnitude of the B(E2) values, measured to a precision of ~4%, disagree with a recent lifetime study [Phys. Lett. B 695, 110 (2011)] that employed the Doppler- shift attenuation method. The B(E2) values show an overall enhancement compared with recent theoretical calculations and a clear asymmetry about midshell, contrary to naive expectations. A new static electric quadrupole moment, Q(2+1), has been measured for 114Sn. The static quadrupole moments are generally consistent with zero but reveal an enhancement near midshell; this had not been previously observed. The magnetic dipole moments are consistent with previous measurements and show a near monotonic decrease in value with neutron number. The current theory calculations fail to reproduce the electromagnetic moments of the tin isotopes. The role of 2p-2h and 4p-4h intruders, which are lowest in energy at mid shell and outside of current model spaces, needs to be investigated in the future.

The Galactic 1.809-MeV $\ensuremath{\gamma}$-ray signature from the $\ensuremath{\beta}$ decay of $^{26g}\mathrm{Al}$ is a dominant target of $\ensuremath{\gamma}$-ray astronomy, of which a significant component is understood to originate from massive stars. The $^{26g}\mathrm{Al}(p,\ensuremath{\gamma})^{27}\mathrm{Si}$ reaction is a major destruction pathway for $^{26g}\mathrm{Al}$ at stellar temperatures, but the reaction rate is poorly constrained due to uncertainties in the strengths of low-lying resonances in $^{27}\mathrm{Si}$. The $^{26g}\mathrm{Al}(d,p)^{27}\mathrm{Al}$ reaction has been employed in inverse kinematics to determine the spectroscopic factors, and hence resonance strengths, of proton resonances in $^{27}\mathrm{Si}$ via mirror symmetry. The strength of the 127-keV resonance is found to be a factor of 4 higher than the previously adopted upper limit, and the upper limit for the 68-keV resonance has been reduced by an order of magnitude, considerably constraining the $^{26g}\mathrm{Al}$ destruction rate at stellar temperatures.

Initial measurements are presented of the (d,p) reactions on neutron-rich N = 50 isotones along the r-process path of nucleosynthesis with radioactive ion beams of 82Ge and 84Se. Prospects for measurements with unstable 130,132Sn beams are discussed.

A setup for measuring cross-sections of evaporation residues produced in the collision of two heavy ions is described. The system, which detects all reaction products in a narrow angular range around the beam direction, is designed to work best with low-intensity beams ( ⩽ 10 5 ions / s ) and can be highly efficient for reactions studied in inverse kinematic conditions, i.e., heavy-mass beam on lighter-mass targets. The system as presented here has been optimized to study evaporation residues from reactions induced by radioactive ion beams with charges near Z ∼ 50 and energies near 4 MeV/nucleon. Continuous sampling of the beam velocity, energy loss and position allow for the measurement of accurate evaporation residue cross-sections also in situations where the beam contains a mixture nuclides.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation and single-nucleon transfer. The B(E2;0+ → 2+) values for first 2+ excited states of even-even neutron-rich 132-136Te and 126-130Sn have been measured by Coulomb excitation in inverse kinematics. Neutron transfer onto a 134Te beam from 9Be and 13C targets, to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the νi13/2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

Single-neutron transfer reactions have been measured on two N = 50 isotones at the Holifield Radioactive Ion Beam Facility (HRIBF). The single-particle-like states of 83Ge and 85Se have been populated using radioactive ion beams of 82Ge and 84Se and the (d, p) reaction in inverse kinematics. The properties of the lowest-lying states —including excitation energies, orbital angular momenta, and spectroscopic factors— have been determined for these N = 51 nuclei.

We describe an experiment optimized to determine the transition probabilities for excitation of the first excited 2+ state in 132Sn. The large excitation energy (4.04 MeV) and consequent small excitation cross-section, together with the modest beam intensity available makes this a challenging experiment. The preliminary result is B(E2; 0+ → 2+) = 0.11 ± 0.03e 2 b 2. The high efficiency and generalized nature of the setup enabled us to also measure the first 2+ state in the two-neutron nucleus 134Sn. We have determined a value of B(E2; 0+ → 2+) = 0.029 ± 0.005e 2 b 2 which shows no sign of the asymmetry with respect to the N = 82 shell closure exhibited by the Te isotopes.

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ ) 19 Ne reactions in astrophysical environments depend on the properties of 19 Ne levels above the 18 F + p threshold. There are at least 8 levels in the mirror nucleus 19 F for which analogs have not been observed in 19 Ne in the excitation energy range E x = 6.4 – 7.6 MeV . These levels may significantly enhance the 18 F + p reaction rates, and thus we have made a search for these levels by measuring the 1 H( 18 F, p ) 18 F excitation function over the energy range E c . m . = 0.3 - 1.3 MeV . We have identified and measured the properties of a newly observed level at E x = 7.420 ± 0.014 MeV , which is most likely the mirror to the J π = 7 2 + 19 F level at 7.56 MeV. We have additionally found a significant discrepancy with a recent compilation for the properties of a 19 Ne state at E x = 7.5 MeV and set upper limits on the proton widths of missing levels.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2+ excited states of even-even neutron-rich 132–136Te and 126–134Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the ν i 13 / 2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

We have measured the B(E;0+ → 2+) for the first excited 2+ states in the double-closed shell nucleus 132Sn and the two-neutron nucleus 134Sn. The results, based on a preliminary analysis are shown in Fig. 1 along with measurements on the stable Sn isotopes, and earlier results on 126,128,130Sn [1] . The experimental setup developed for the 132,134Sn measurements was also employed in a successful measurement of B(E;0+ → 2+) for the closed-neutron-shell nucleus 82Ge.

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ) 19 Ne reactions at T = 0.1 – 0.4 GK are necessary to understand the emission of positron-annihilation gamma rays expected from novae. Observations of such gamma rays by space-based telescopes would provide direct tests of nova models. These rates, however, are uncertain because of the unknown level structure of 19 Ne above the 18 F+ p threshold. Missing levels in 19 Ne were searched for by stopping a 24-MeV 18 F beam in a thick polypropylene target, and scattered protons were detected in a silicon-strip detector. The 18 F( p , p ) 18 F excitation function has been measured from E c.m. = 0.3 - 1.3 MeV covering most energies of astrophysical interest, and a significant discrepancy was observed with a recent compilation of 19 Ne resonance parameters.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of exciting measurements around the 132 Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2 + excited states of neutron-rich 132,134,136 Te and 126,128,130,132,134 Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134 Te beam, from 9 Be and 13 C targets to populate single-particle states in 135 Te, has also been studied. Results, and plans for future experiments with these neutron-rich beams, are presented.

Evaporation residue cross sections have been measured with neutron-rich radioactive 132 Sn beams on 64 Ni in the vicinity of the Coulomb barrier. The average beam intensity was 2 × 10 4 particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. The measurement is compared to coupled-channel calculations taking into account inelastic excitation and nucleon transfer.

The study of neutron single-particle strengths in neutron-rich nuclei is of interest for nuclear structure and nuclear astrophysics. The distribution of single-particle strengths constrains the effective Hamiltonian and pairing interactions and determines neutron interaction rates that are crucial for understanding the synthesis of heavy nuclei in supernovae via the rapid neutron capture process. Particularly important are the neutron single-particle levels in nuclei near closed neutron shells. Radioactive ion beams from the Holifield Radioactive Ion Beam Facility have been used to study (d,p) reactions in inverse kinematics in order to probe neutron single-particle states in exotic nuclei. The results of a measurement with a 82 Ge beam will be presented.

We have studied the elastic scattering of the exotic 17 F nuclei from 208 Pb at Coulomb barrier energies. For the measurement we used a new detector array with pixel structure and a large solid angle. Optical potential analysis shows that 17 F has an imaginary potential much less deep than 19 F. Information on the exclusive 17 F breakup cross section has been deduced too.

Evaporation residue cross sections have been measured using neutron-rich radioactive 132Sn beams incident on a 64Ni target in the vicinity of the Coulomb barrier. This is the first experiment using accelerated 132Sn beams to study nuclear reaction mechanisms. The average beam intensity was 2x 104 particles per second and the smallest cross section measured was less than 5 mb. A large sub-barrier fusion enhancement was observed compared to evaporation residue cross sections for 64Ni on stable even Sn isotopes. The enhancement cannot be accounted for by a simple barrier shift due to the change in nuclear sizes. Coupled-channel calculations including inelastic excitation underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of 132Sn and 64Ni.

The width of the giant dipole resonance (GDR) built on excited states was determined from a measurement of γ -decays in coincidence with 17 O particles scattered inelastically from 120 Sn. The bombarding energy was 80 MeV/u. A width of 4±1 MeV, consistent with the width of the GDR built on the ground state, was found at a temperature T =1 MeV. This result is in disagreement with adiabatic thermal shape fluctuation calculations, indicating an overestimation of the influence of thermal shape fluctuations at low temperature.

Radioactive {sup 17}F beams were produced at the Holifield Radioactive Ion Beam Facility (HRIBF) using the Isotope Separator On-Line (ISOL) technique. Two of the experiments using accelerated {sup 17}F beams to study reaction mechanisms are presented: the simultaneous emission of two protons from a resonance in {sup 18}Ne and the breakup of {sup 17}F by {sup 208}Pb.

Two 1− levels at 5.35(10) and 6.85(5) MeV were observed for the first time in 20O. The strong direct excitation and subsequent γ-ray decay of these states in virtual photon scattering at 100 MeV/nucleon, along with B(Eλ) predictions for 20O states in this energy region, established their dipole character. The extracted B(E1)↑ values of ≃0.062(16) e 2 fm 2 and ≃0.035(9) e 2 fm 2 for the 5.35 and 6.85 MeV states, respectively, are significantly larger than shell model calculations predict. Such large dipole strengths are not observed for low-lying 1− states in 18O, indicating a shift of dipole strength towards lower energies as one approaches the neutron drip-line.

High-precision reduced electric-quadrupole transition probabilities $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ have been measured from single-step Coulomb excitation of semi-magic $^{58,60,62,64}\mathrm{Ni}$ ($Z=28$) beams at 1.8 MeV per nucleon on a natural carbon target. The energy loss of the nickel beams through the carbon target were directly measured with a zero-degree Bragg detector and the absolute $B(E2)$ values were normalized by Rutherford scattering. The $B(E2)$ values disagree with recent lifetime studies that employed the Doppler-shift attenuation method. The present high-precision $B(E2)$ values reveal an asymmetry about $^{62}\mathrm{Ni}$, midshell between $N=28$ and 40, with larger values towards $^{56}\mathrm{Ni}$ ($Z=N=28$). The experimental $B(E2)$ values are compared with shell-model calculations in the full $pf$ model space and the results indicate a soft $^{56}\mathrm{Ni}$ core.

$\ensuremath{\gamma}$-ray transitions in ${}^{134}\text{Te}$, ${}^{135}\text{Te}$, and ${}^{136}\text{Te}$ were measured from ${}^{9}\text{Be}$-induced reactions with a radioactive ${}^{132}\text{Sn}$ beam at a sub-Coulomb barrier energy of 3 MeV per nucleon using particle-$\ensuremath{\gamma}$ coincidence spectroscopy. The transitions were selected by gating on alpha-like particles in a CsI detector following a combination of $({}^{9}\text{Be},\ensuremath{\alpha}1n)$, $({}^{9}\text{Be},\ensuremath{\alpha}2n)$, and $({}^{9}\text{Be},\ensuremath{\alpha}3n)$ incomplete fusion-evaporation reactions. Distorted-wave Born approximation calculations suggest little to no contribution from the $({}^{9}\text{Be},{}^{7}\text{He})$, $({}^{9}\text{Be},{}^{6}\text{He})$, and $({}^{9}\text{Be},{}^{5}\text{He})$ direct reactions.$\ensuremath{\gamma}$-ray transitions from previously known ${2}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ and ${4}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet members in ${}^{135}\text{Te}$ are observed. A new $\ensuremath{\gamma}$ ray is observed, assigned to the third-excited state in ${}^{135}\text{Te}$, and new ${2}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet members are suggested. In addition, spin assignments are made by using recent one-neutron transfer data. The updated experimental data for ${}^{135}\text{Te}$ are compared to shell-model calculations for a relatively complete set of states up to the yrast ${15/2}^{\ensuremath{-}},{4}^{+}\ensuremath{\bigotimes}\ensuremath{\nu}\phantom{\rule{0.16em}{0ex}}2{f}_{7/2}$ multiplet member at 1505 keV.

Single-neutron states in $^{133}\mathrm{Sn}$ and $^{209}\mathrm{Pb}$, which are analogous to single-electron states outside of closed atomic shells in alkali metals, were populated by the ($^{9}\mathrm{Be}$, $^{8}\mathrm{Be}$) one-neutron transfer reaction in inverse kinematics using particle-$\ensuremath{\gamma}$ coincidence spectroscopy. In addition, the ${s}_{1/2}$ single-neutron hole-state candidate in $^{131}\mathrm{Sn}$ was populated by ($^{9}\mathrm{Be}$, $^{10}\mathrm{Be}$). Doubly closed-shell $^{132}\mathrm{Sn}$ (radioactive) and $^{208}\mathrm{Pb}$ (stable) beams were used at sub-Coulomb barrier energies of 3 MeV per nucleon. Level energies, $\ensuremath{\gamma}$-ray transitions, absolute cross sections, spectroscopic factors, asymptotic normalization coefficients, and excited-state lifetimes are reported and compared with shell-model expectations. The results include a new transition and precise level energy for the $3{p}_{1/2}$ candidate in $^{133}\mathrm{Sn}$, new absolute cross sections for the $1{h}_{9/2}$ candidate in $^{133}\mathrm{Sn}$ and $3{s}_{1/2}$ candidate in $^{131}\mathrm{Sn}$, and new lifetimes for excited states in $^{133}\mathrm{Sn}$ and $^{209}\mathrm{Pb}$. This is the first report on excited-state lifetimes of $^{133}\mathrm{Sn}$, which allow for a unique test of the nuclear shell model and $^{132}\mathrm{Sn}$ double-shell closure.

Excitation of the Isovector Giant Quadrupole Resonance in 208Pb by Coulomb Inelastic Scattering R. L. Varner, J. R. Beene, M. Chartier, ∗ J. F. Liang, D. Shapira, D. Bazin, B. Blank, ∗ B. Sherrill, M. Thoennessen, P. A. DeYoung, and G. F. Peaslee Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6368 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824

Radioactive ion beams of ${}^{17}\mathrm{F}$ were used to study several resonance states in ${}^{18}\mathrm{Ne}$. Clear evidence for simultaneous two-proton emission from the 6.15 MeV state $({J}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1}^{\ensuremath{-}})$ in ${}^{18}\mathrm{Ne}$ has been observed with the reaction ${}^{17}\mathrm{F}{+}^{1}\mathrm{H}$. Because of limited angular coverage, the data did not differentiate between the two possible mechanisms of simultaneous decay, diproton ${(}^{2}\mathrm{He})$ emission or direct three-body decay. The two-proton partial width was found to be $21\ifmmode\pm\else\textpm\fi{}3\mathrm{eV}$ assuming ${}^{2}\mathrm{He}$ emission and $57\ifmmode\pm\else\textpm\fi{}6\mathrm{eV}$ assuming three-body decay. The total width of the ${1}^{\ensuremath{-}}$ state was measured to be $50\ifmmode\pm\else\textpm\fi{}5\mathrm{keV}$. Several additional resonances that decay by single proton emission were also studied.

The breakup of radioactive 17 F into a proton and 16 O was measured by bombarding 208 Pb with 170 MeV 17 F. The angular correlations of the fragments and the energy distributions of the protons suggest that the dominant breakup mechanism is a direct process in which a proton is excited into the low energy continuum above the breakup threshold. The breakup cross section measured near the grazing angle is 6.6±0.7 mb/sr for analysis with postacceleration considered and 3.9±0.4 mb/sr without postacceleration. This cross section is small compared to the fusion cross section at energies near the Coulomb barrier, so the breakup can have little influence on the fusion process.