Search

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.

A method whereby mass differences can be measured using the 25-MV tandem accelerator at the Holifield radioactive ion beam facility (HRIBF) is presented. The technique is unconventional in that mass differences will be measured as position differences at the image of the energy-analyzing magnet following the accelerator and will be made at energies where independent isobaric determination will be possible by a ΔE measurement. The technique is appropriate where the unknown mass is the most neutron-rich isobaric constituent of the beam, such as for neutron-rich fission fragments.

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.

Trialkylphosphine Oxide (TRPO), which was utilized as the extractant of the TRPO process, is degraded by γ irradiation, and the radiolytic products produced lead to retention of heavy metal ions. The radiolytic products, especially the polymeric ones (or high molecular weight products, HMP), were studied by electrospray ionization mass spectrometry (ESI‐MS). The ESI‐MS profiles of the various TRPO systems were correlated with the retention of plutonium of the irradiated various systems, and the molecular weights of the HMP were determined. In addition, the structure of the HMP was further studied with tandem mass spectrometry and FT‐IR detection, and their other properties were researched. Based on this information, the structure of the HMP was speculated.

The tin chain of isotopes provides a unique region in which to investigate the evolution of single-particle structure, spreading from N = 50 at 100Sn, through 10 stable isotopes and the N = 82 shell closure at 132Sn out into the r-process path. Direct reactions performed on radioactive ion beams are sensitive spectroscopic tools for studying exotic nuclei. Here we present one experiment knocking out neutrons from tin isotopes that are already neutron deficient and two reactions that add a neutron to neutron-rich 130Sn. Both techniques rely on selective particle identification and the measurement of γ rays in coincidence with charged ions. We present the goals of the two experiments and the particle identification for the channels of interest. The final results will be presented in future publications.

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.

Negatively charged atomic ions have been cooled in gas-filled rf-quadrupoles from injection energies between 10 and 150 eV to final energies below 0.5 eV. By choosing the atoms or molecules of the buffer gas to have low masses as compared to the masses of the ions to be cooled, the energy transfer in ion–atom or ion–molecule collisions is minimized. For small enough initial ion energies thus only a small percentage of the ions is neutralized in the collision processes. For Cl− and I− ions cooled in a He buffer gas we have achieved absolute transmission efficiencies of more than 50%.

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

Extraction and retention of plutonium by irradiated 30% trialkylphosphine oxide (TRPO) in kerosene were investigated. Factors that influence the retention of plutonium are discussed. The experimental results show that when 30% TRPO–kerosene–HNO3 is irradiated above a dose of 2 × 106 Gy, it produces a few Pu-complexing radiolytic degradation products. Radiation dose was the primary factor affecting retention of plutonium in the organic phase. At a radiation dose of 5 × 106 Gy, about 40% of the plutonium extracted by the solvent was left in the organic phase even after five back-extraction contacts with 0.6 mol/L oxalic acid. The reasons for plutonium retention were investigated. The products causing the retention were the polymeric ones with high molecular weight, and these were recovered by vacuum distillation of the irradiated 30% TRPO–kerosene. The phosphonic and phosphinic acids produced by irradiation of TRPO had no effect on the retention of plutonium.

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.

A fast and efficient method to study (p,p) and (p,α) resonances with radioactive beams in inverse kinematics is described. It is based on the use of thick targets and large area double-sided silicon strip detectors (DSSDs) to detect the recoiling light-charged particles and to determine precisely their scattering angle. The first nuclear physics experiments with the technique have been performed recently at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge with stable beams of 17 O and radioactive beams of 17 F . The high-quality resonance measurements obtained demonstrate the capabilities of the technique. Pure 17 F beams from HRIBF were produced by fully stripping the ions and separating the interfering and more abundant 17 O ions by the beam transport system. The removal of interfering isobars is one of the various common challenges to both accelerator mass spectrometry (AMS) and radioactive ion beam (RIB) production. Experiments done with RIBs will benefit from the use of the most efficient techniques for production, isobar separation, transport and detection.

The recently commissioned Recoil Mass Spectrometer (RMS) at the Holifield Radioactive Ion Beam Facility (HRIBF) is described. Consisting of a momentum separator followed by an E-D-E Rochester-type mass spectrometer, the RMS is the centerpiece of the nuclear structure endstation at the HRIBF. Designed to transport ions with rigidities near K = 100, the RMS has acceptances of +/- 10% in energy and +/- 4.9% in mass-to-charge ratio. Recent experimental results are used to illustrate the detection capabilities of the RMS, which is compatible with many detectors and devices. (C) 2000 Elsevier Science B.V. All rights reserved.

A focal plane detector has been constructed for detecting fusion residues in a gas-filled Enge split-pole spectrograph. It provides position, timing and energy for particle identification. With a timing channel plate detector in the target chamber, the particle time-of-flight can be measured. This detector system is particularly suitable for studying fusion–evaporation reactions in inverse kinematics.

The low intensity of the RIBS presently available at the first generation production facilities (10(5)-10(6) pps) and the necessity to reconstruct the event kinematics in RIB measurements require detection systems having both a large solid-angle coverage and a high granularity. The EXODET (EXOtic DETector) apparatus has been accomplished to respond to these requirements and the first experiment has been successfully performed studying the F-17 scattering on Pb-208 at 90.4 MeV.

The collision dynamics of 75-keV ${\mathrm{C}}_{60}^{\ensuremath{-}}$ with ${\mathrm{H}}_{2}$ gas has been studied. Coincidence measurements demonstrate that fragments such as ${\mathrm{C}}_{4}$ and ${\mathrm{C}}_{8}$ as well as the commonly assumed ${\mathrm{C}}_{2}$ species are fragmentation partners to heavy fragments. Odd- $n$ as well as even- $n$ light fragments are coincident with even- $n$ heavy fragments, indicating sequential decay following the initial binary fragmentation. Periodicity in yield distributions suggests that rings as well as chains can be extracted from ${\mathrm{C}}_{60}$ and still leave a fullerene structure.

Fusion and quasielastic excitation functions were measured for {sup 40}Ca+{sup 46,48,50}Ti at energies close to the Coulomb barrier. The interest in these systems lies in the opposite target-mass dependence of surface vibration couplings and neutron-transfer couplings. The cross sections were measured at small energy steps and with high precision, so that the structure in the excitation functions (barrier distributions) could be revealed through a differentiation process. The subbarrier fusion data show a larger enhancement with increasing target mass number, which clearly indicates the importance of transfer couplings. The agreement between the fusion and quasielastic barrier distributions is quite good. Simplified coupled-channel calculations were performed to reproduce the fusion data. A very good agreement is obtained by coupling to low lying excited states (2{sup +} and 3{sup {minus}}) and to the most positive neutron-transfer Q value (two-neutron projectile pickup channel). {copyright} {ital 1998} {ital The American Physical Society}

A new pixel-structure detector array with a large solid angle coverage has been used for the first time to study the elastic scattering of exotic F-17 nuclei from a Pb-208 target at 90.4 MeV. The experimental data have been analyzed in the framework of the optical model potential and the real and imaginary strong absorption radii have been evaluated. These quantities have been compared with those obtained for the system F-19+Pb-208 at the same energy in the center of mass frame. The F-17+Pb-208 reaction cross section is more similar to those of the systems O-16,O-17+Pb-208 rather than to the one of the system F-19+Pb-208 at similar energies: this indicates that in the energy range around the Coulomb barrier the breakup channel is still weak. The exclusive breakup cross section F-17-->O-16+p has been measured for the first time at energy below the Coulomb barrier.

A new detector apparatus has been designed and developed to be used in experiments performed with radioactive ion beams. It consists of 16 highly segmented silicon strip detectors arranged in two‐layer telescopes and subtending a large solid angle (about 70% of 4π sr). An innovative readout system for the position information that uses highly integrated electronics (ASIC chips) has been implemented. A first successful experiment has been performed at the Argonne National Laboratory (USA) to study the 17F scattering by a 208Pb target at 90.4 MeV of incident energy. The 17F angular distribution has been analyzed and the optical model potential best‐fit parameters determined. The same analysis performed on 17F data taken at higher incident energy, in completely different experimental conditions, gives consistent results. The comparison with experiments performed with stable beams (19F, 16O, 17O) indicates a behavior for the 17F more similar to that of the Oxygen isotopes than to the 19F one. Despite of the short data collection time, also the cross section for the 17F →16O + p break‐up process has been estimated.

Single-neutron states in (133)Sn and (209)Pb, which are analogous to single-electron states outside of closed atomic shells in alkali metals, were populated by the ((9)Be, (8)Be) one-neutron transfer reaction in inverse kinematics using particle-γ coincidence spectroscopy. In addition, the s(1/2) single-neutron hole-state candidate in (131)Sn was populated by ((9)Be, (10)Be). Doubly closed-shell (132)Sn (radioactive) and (208)Pb (stable) beams were used at sub-Coulomb barrier energies of 3 MeV per nucleon. Level energies, γ-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 3p(1/2) candidate in (133)Sn, new absolute cross sections for the 1h(9/2) candidate in (133)Sn and 3s(1/2) candidate in (131)Sn, and new lifetimes for excited states in (133)Sn and (209)Pb. This is the first report on excited-state lifetimes of (133)Sn, which allow for a unique test of the nuclear shell model and (132)Sn double-shell closure.

Light charged particles from ${}^{18}{\mathrm{O}+}^{100}\mathrm{Mo}$ reactions at 200 MeV bombarding energy have been measured in singles and in coincidence with high-energy \ensuremath{\gamma} rays $(g~10\mathrm{MeV})$ produced primarily in the decay of the giant-dipole resonance. Particle spectra are analyzed in terms of a moving source fit to decompose preequilibrium and evaporative components. Preequilibrium particle multiplicities and energies indicate a reduction in the compound nucleus excitation energy of approximately 20% and a loss of mass of approximately three mass units relative to complete fusion. The energy lost to preequilibrium emission is shown to affect significantly the strength, width, and centroid energy of the giant-dipole resonance deduced from fits to high-energy \ensuremath{\gamma}-ray emission spectra. Evaporation residue cross sections were measured for the same reaction from 100 to 217 MeV in order to determine the fusionlike event cross section.

Energy-integrated one- and two-proton transfer cross sections and fusion excitation functions have been measured for and at energies near the Coulomb barrier. As in previous results with projectiles, anomalous dependence of the transfer probability on distance of closest approach is observed at energies only slightly above the barrier. Measured fusion cross sections show sub-barrier enhancement similar to that measured in other systems. Coupled-channel calculations including transfer form factors extracted from the measurements come closer to the data in the measurements and produce the full enhancement for .

This paper discusses the role played by transfer reactions on the sub-barrier fusion enhancement. A semiclassical formalism is used to derive the transfer form factors, that are used in coupled-channel calculations. It is shown that transfer reactions that take place at small distances may be an important doorway to fusion. The relation between this formalism and the long-range absorptive fusion potential is also discussed. Results of calculations for the , and systems are presented.

Light-charged particles were measured in coincidence with evaporation residues for {sup 12}C+{sup 144}Sm, {sup 35}Cl+{sup 121}Sb, and {sup 60}Ni+{sup 96}Zr reactions which populate the compound nucleus {sup 156}Er at the same excitation energy. The particle spectral shape for {sup 12}C+{sup 144}Sm is harder than the other two reactions. A one-body dissipation model predicts that the fusion times for the {sup 35}Cl- and {sup 60}Ni-induced reactions are similar and are about twice as long as that of the {sup 12}C-induced reaction. Statistical model calculations considering particle evaporation during compound nucleus formation successfully reproduce the particle spectra for all the reactions. {copyright} {ital 1997} {ital The American Physical Society}

Light charged particle decay from the {sup 156}Er compound nucleus, populated by {sup 12}C+{sup 144}Sm and {sup 60}Ni+{sup 96}Zr at the same excitation energy, were measured in coincidence with the evaporation residues. The high energy slope of charged particle spectra for the {sup 60}Ni-induced reaction is steeper than for the {sup 12}C-induced reaction. Model calculations including particle evaporation during fusion result in good agreement with the data. This suggests that the difference in the charged particle spectra between the two entrance channels is due to a longer formation time in the {sup 60}Ni-induced reaction. {copyright} {ital 1997} {ital The American Physical Society}

The $g$ factor and $B(E2)$ of the first excited ${2}^{+}$ state have been measured following Coulomb excitation of the neutron-rich semimagic nuclide ${}^{134}$Te (two protons outside ${}^{132}$Sn) produced as a radioactive beam. The precision achieved matches related $g$-factor measurements on stable beams and distinguishes between alternative models. The $B(E2)$ measurement exposes quadrupole strength in the ${2}_{1}^{+}$ state beyond that predicted by current large-basis shell-model calculations. This additional quadrupole strength can be attributed to coupling between the two valence protons and excitations of the ${}^{132}$Sn core. However, the wave functions of the low-excitation positive-parity states in ${}^{134}$Te up to ${6}_{1}^{+}$ remain dominated by the $\ensuremath{\pi}{({g}_{7/2})}^{2}$ configuration.

The extraction of detailed nuclear structure information from transfer reactions requires reliable, well-normalized data as well as optical potentials and a theoretical framework demonstrated to work well in the relevant mass and beam energy ranges. It is rare that the theoretical ingredients can be tested well for exotic nuclei owing to the paucity of data. The halo nucleus Be-11 has been examined through the 10Be(d,p) reaction in inverse kinematics at equivalent deuteron energies of 12,15,18, and 21.4 MeV. Elastic scattering of Be-10 on protons was used to select optical potentials for the analysis of the transfer data. Additionally, data from the elastic and inelastic scattering of Be-10 on deuterons was used to fit optical potentials at the four measured energies. Transfers to the two bound states and the first resonance in Be-11 were analyzed using the Finite Range ADiabatic Wave Approximation (FR-ADWA). Consistent values of the spectroscopic factor of both the ground and first excited states were extracted from the four measurements, with average values of 0.71(5) and 0.62(4) respectively. The calculations for transfer to the first resonance were found to be sensitive to the size of the energy bin used and therefore could not be used to extract a spectroscopic factor., 16 Pages, 10 figures

The fusion cross sections of radioactive ${}^{134}$Te + ${}^{40}$Ca were measured at energies above and below the Coulomb barrier. The evaporation residues produced in the reaction were detected in a zero-degree ionization chamber providing high efficiency for inverse kinematics. Both coupled-channel calculations and comparison with similar Sn + Ca systems indicate an increased sub-barrier fusion probability that is correlated with the presence of positive $Q$-value neutron transfer channels. In comparison, the measured fusion excitation functions of ${}^{130}$Te + ${}^{58,64}$Ni, which have positive $Q$-value neutron transfer channels, were accurately reproduced by coupled-channel calculations including only inelastic excitations. The results demonstrate that the coupling of transfer channels can lead to enhanced sub-barrier fusion but this is not directly correlated with positive $Q$-value neutron transfer channels in all cases.

Fission fragment anisotropies have been measured to energies extending well above the barrier for three entrance channels leading to the same compound nucleus {sup 248}Cf. For the {sup 12}C + {sup 236}U and {sup 16}O + {sup 232}Th systems, lying on either side of the Businaro-Gallone critical asymmetry, the mean square angular momentum is matched at an excitation energy of 62 MeV. The anisotropies, although different at lower energies, are in agreement within experimental errors at this excitation energy and thus show no entrance channel dependence. {copyright} {ital 1996 The American Physical Society.}

The (d, p) reaction was measured with the radioactive ion beams of 126Sn and 128Sn in inverse kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory, utilizing the Super ORRUBA silicon detector array. Angular distributions of reaction protons were measured for several states in 127Sn and 129Sn to determine angular momentum transfers and deduce spectroscopic factors. Such information is critical for calculating direct (n,γ) cross sections for the r-process as well as for constraining shell model parameters in the A≈130 region. Combined with previous experiments on 130Sn and 132Sn, these results will provide a complete set of (d, p) reaction data on even tin isotopes between stable 124Sn and doubly-magic 132Sn.

Recent calculations suggest that the rate of neutron capture by 130Sn has a significant impact on late-time nucleosynthesis in the r-process. Direct capture into low-lying bound states is expected to be significant in neutron capture near the N=82 closed shell, so r- process reaction rates may be strongly impacted by the properties of neutron single particle states in this region. In order to investigate these properties, the (d, p) reaction has been studied in inverse kinematics using a 630 MeV beam of 130Sn (4.8 MeV/u) and a (CD2)n target. An array of Si strip detectors, including SIDAR and an early implementation of the ORRUBA, was used to detect reaction products. Results for the 130Sn(d, p)131Sn reaction are found to be very similar to those from the previously reported 132Sn(d, p)133Sn reaction. Direct-semidirect (n, ) cross section calculations, based for the first time on experimental data, are presented. The uncertainties in these cross sections are thus reduced by orders of magnitude from previous estimates.

New information is reported on single-neutron states above the doubly closed-shell nucleus 132Sn. A radioactive ion beam of 134Te(N = 82) at 565 MeV and a stable ion beam of 136Xe(N = 82) at 560 MeV were used to study single-neutron states in the N = 83 nuclei 135Te and 137Xe, respectively, by (13C, 12C ) and (9Be, Be ) direct reactions in inverse kinematics. Particle- and particle- - coincidence measurements using HPGe and CsI arrays allowed determination of decay paths, high-precision level energies, multipolarities of transitions, and relative cross sections. One-neutron transfer with heavy ions is employed to gain selectivity to both low- and high-spin single-neutron states above the N = 82 shell closure. Results are presented for the 13/2+ states in the N = 83 nuclei 135Te and 137Xe at 2108.8(9) keV and 1752.6(3) keV, respectively, and for the 3 collective octupole state observed at 3749(5) keV in 134Te(N = 82) inelastic scattering, all previously unknown. While the 13/2+ state (or 1i13/2 centroid) in 133Sn(Z = 50, N = 83) remains unknown, the present results provide the best empirical measure of its energy available to date.

Recent calculations suggest that the rate of neutron capture by (130)Sn has a significant impact on late-time nucleosynthesis in the r process. Direct capture into low-lying bound states is expected to be significant in neutron capture near the N=82 closed shell, so r-process reaction rates may be strongly impacted by the properties of neutron single particle states in this region. In order to investigate these properties, the (d,p) reaction has been studied in inverse kinematics using a 630 MeV beam of (130)Sn (4.8 MeV/u) and a (CD(2))(n) target. An array of Si strip detectors, including the Silicon Detector Array and an early implementation of the Oak Ridge Rutgers University Barrel Array, was used to detect reaction products. Results for the (130)Sn(d, p)(131)Sn reaction are found to be very similar to those from the previously reported (132)Sn(d, p)(133)Sn reaction. Direct-semidirect (n,γ) cross section calculations, based for the first time on experimental data, are presented. The uncertainties in these cross sections are thus reduced by orders of magnitude from previous estimates.

Fusion excitation functions for the reactions of ${}^{124,132}$Sn with ${}^{40,48}$Ca were measured at energies near and below the Coulomb barrier. Weak sub-barrier fusion enhancement accounted for by coupling to the first 2${}^{+}$ and 3${}^{\ensuremath{-}}$ states in the target and projectile was observed for ${}^{124,132}\text{Sn}+{}^{48}\text{Ca}$. However, the ${}^{124,132}\text{Sn}+{}^{40}\text{Ca}$ fusion cross sections are very strongly enhanced below the barrier and this difference survives even after correcting for trivial size effects. Although the enhancement appears to be related to the existence of large positive $Q$ values for neutron transfer reactions, it is not proportional to the magnitudes of those $Q$ values, which are much larger for ${}^{132}\mathrm{Sn}+{}^{40}\mathrm{Ca}$ than for ${}^{124}\mathrm{Sn}+{}^{40}\mathrm{Ca}$.