Search

The total fusion excitation function for 10 Be+ 40 Ar has been measured over the center-of-momentum (c.m.) energy range from 12 to 24 MeV using a time-projection chamber (TPC). The main purpose of this experiment, which was carried out in a single run of duration 90 h using a ≈100 particle per second (pps) 10 Be beam, was to demonstrate the capability of an active-target TPC to determine fusion excitation functions for extremely weak radioactive ion beams. Cross sections as low as 12 mb were measured with acceptable (50%) statistical accuracy. It also proved to be possible to separate events in which charged particles were emitted from the fusion residue from those in which only neutrons were evaporated. The method permits simultaneous measurement of incomplete fusion, break-up, scattering, and transfer reactions, and therefore fully exploits the opportunities presented by the very exotic beams that will be available from the new generation of radioactive beam facilities.

Helical spectrometers, such as the HELIOS device at Argonne National Laboratory, have proven to be a powerful tool for the study of nuclear spectroscopy (Lighthall, 2010). However, due to the expense in procuring a large bohr solenoid, relatively few of these devices exist. However, at the University of Notre Dame, TwinSol , is composed of two such magnets. The availability of intense light-ion beams from the Notre Dame Nuclear Science Laboratory (NSL) in combination with a large-bore superconducting solenoids provides an excellent opportunity to develop a new solenoid spectrometer at the University of Notre Dame, one optimized for measurements in normal kinematics.

We have surveyed a series of elastic scattering measurements from the literature involving tightly bound (9Li, 10Be, 12C, 16O, 19F), weakly bound (6Li, 7Li, 8Li, 8He, 9Be, 17F), and exotic (6He and 11Li) nuclei on a 208Pb target at energies close to the Coulomb barrier. The corresponding data were then converted and analyzed in terms of the interaction distance, where critical-interaction and strong-absorption distances were extracted. Larger values were obtained for the exotic 6He and 11Li nuclei, as compared with both weakly bound and tightly bound projectiles. The influence of long-range nuclear interactions and Coulomb dipole polarizability are discussed. A correlation between the critical interaction distance and binding energy for a given configuration is observed.

Reaccelerated radioactive beams near the Coulomb barrier, which are starting to be available from the ReA3 accelerator at NSCL and in next future at FRIB, will open up new opportunities for the study of nuclear structure near the drip lines. Efficient measurement techniques must be developed to compensate for the limited intensity of the most exotic beams. The Active-Target Time Projection Chamber (AT-TPC) constructed at MSU solves this problem by providing the increased luminosity of a thick target while maintaining a good energy resolution by tracking the reaction vertex over an essentially 4 π solid angle. The AT-TPC and similar detectors allow us to take full advantage of the radioactive ion beams at present and future nuclear physics facilities to explore the frontier of rare isotopes.

Deuterated scintillators have been used as a tool for neutron spectroscopy without Neutron Time-of-Flight (n-ToF) for more than 30 years. This article will provide a brief historical overview of the technique and current uses of deuterated scintillators in the UM-DSA and DESCANT arrays. Pulse-shape discrimination and spectrum unfolding with the maximum-likelihood expectation maximization algorithm will be discussed. Experimental unfolding and cross section results from measurements of (d,n), ( 3 He,n) and (α,n) reactions are shown.

A cosmic-ray veto shield for the University of Notre Dame neutron wall has been constructed in order to carry out low-rate neutron time-of-flight (TOF) measurements. The veto detector employs plastic scintillator embedded with wavelength-shifting fibers and is read out by a versatile coupling to standard photomultiplier tubes (PMTs). This veto shield is effective at reducing the muon background by about one order of magnitude and therefore increases the sensitivity for experiments with low neutron yields. The 26Mg(3He,n)28Si reaction was used as a testbed for the veto.

TwinSol , a pair of coupled, superconducting solenoids, was one of the first devices capable of producing beams of radioactive nuclei at energies near the Coulomb barrier. A primary beam from University of Notre Dame (UND) tandem accelerator is used to bombard a primary target producing a secondary beam in flight. TwinSol is used to gather, separate, and focus the recoils. Since it was commissioned at the UND in 1997, at least 58 publications have reported data from its use and there have been hundreds of collaborators from many different countries that use this device. Currently, plans are in place at the UND to provide several upgrades to TwinSol , including a multi-cell gas production target and the possible addition of a third solenoid. Upgrades currently in progress will be discussed along with future plans.

Organic liquid scintillation detectors have shown promise as neutron detectors for characterizing special nuclear materials in various arms-control applications. Deuterated liquids, such as EJ315, are attractive for spectroscopy applications because the backward-peaked n–d scattering results in enhanced coupling between incident neutron energy and resulting pulse height. This will likely allow better spectrum unfolding. In this work, we present a measured neutron response matrix for a three-by-two-in. cylindrical EJ315 detector, and compare to the hydrogen-based liquid EJ309 in an otherwise identical assembly. We also present measured light-output relations for both detectors. A continuous-spectrum neutron source, obtained with the bombardment of 11B with 5.5-MeV deuterons at the tandem Van de Graaff accelerator at the University of Notre Dame, was used for the measurement. Detected neutrons were sorted via time of flight into 100-keV energy groups, over the energy range from 0.5 to 15 MeV. The resulting response matrix represents a detailed characterization of pulse-height response to neutrons over that energy range. To the authors' knowledge, a measured response matrix and light-output relation for EJ315 have not been reported, and published characterizations of the similar NE230 are of less detail over this energy range. Matrix-condition analysis indicates an advantage for EJ315 over EJ309 in the number of independent parameters available in inversion procedures.

Development and evaluation of a large-volume multi-element digital-signal-processing (DSP)-based deuterated (C6D6) liquid scintillator array for the study of reactions involving neutrons without the need for measurement of neutron time-of-flight (ToF) is described. In-beam testing was conducted at the University of Notre Dame (UND) 10 MV FN tandem Van de Graaff accelerator using a deuteron beam at Ed=16.0 MeV. The energy response and detector efficiency of 100 mm dia. ×; 150 mm long cylindrical deuterated liquid scintillator (Eljen 315) detectors was determined in the range En=4.8 to 8.7 MeV and En=6.8 to 21.2 MeV, respectively. In addition, we determined that optimized digital pulse-shape discrimination (DPSD) in liquid scintillator detectors using enhanced liquid (Eljen 315MOD) and other methods can yield improved recoil-particle tagging. As we demonstrate, this is particularly important in deuterated scintillators at neutron energies above 15 MeV. This is especially advantageous for study of nuclear reactions involving radioactive beams, since these often have large, positive reaction Q values.

The Prototype AT-TPC, a detector based on time projection chamber (TPC) technology was built at the National Superconducting Cyclotron Laboratory, Michigan State University. The chamber gas, called the active target, of the Prototype AT-TPC is utilized simultaneously as a reaction target and a tracking medium of charged particles for measuring low-energy nuclear reactions. In pursuit of luminosity, efficiency and resolution for reaction studies at a new generation of radioactive isotope facilities, the Prototype AT-TPC provides one of the largest active volumes in the world measuring 25 cm in diameter and 50 cm long along the beam axis, millimeter-precision tracking capability, and a high electric field of 1 kV/cm/atm for fast electron drift, which limits the detector dead time. Commissioning of the detector using standard alpha sources and radioactive 6 He beams has demonstrated the detector's long-term stability as well as its performance, which allowed good reconstruction of reaction kinematics.

Deuterated liquid scintillators are promising from the perspective of n / γ separation and neutron energy measurement without time-of-flight. In order to turn these into precision instruments for nuclear physics, however, the absolute efficiency must be known. The efficiency is typically a function of energy and is strongly dependent on the electronics thresholds used with the photomultiplier detectors. This study involves the extraction of this efficiency using the d(d,n)3He reaction (Ed=16.0 MeV) and n-3He coincidence events. The detection and identification of the ions are very efficient and allows one to project pure monoenergetic neutron spectra all the way down to the electronics threshold. Timing information was also recorded in these measurements to provide further constraints and a consistency check.

Improved time-of-flight (ToF) capability has been added to the University of Michigan (UM)–University of Notre Dame (UND) TwinSol low-energy dual 6T solenoid-based radioactive nuclear-beam (RNB) facility at the UND FN tandem Van de Graaff accelerator. An extended low-background beam line has been combined with a newly-constructed ca.1 meter diameter ISO-250 based scattering chamber capable of sub-ns ToF of RNBs. These together with the intrinsic, low ToF spread in the TwinSol ion-optical system yields improved ToF information for low-energy RNB nuclear reaction studies. Using either the intrinsic accelerator bunched and pulsed-selected beams with RF timing, or with the addition of an MCP+foil timing system (or similar) detector at the dual-solenoid cross-over region allows for reduction of the energy spread inherent in these low-energy RNB reactions. As an example, using the ToF information reduces the FWHM energy spread in a high-intensity 28 MeV 8Li beam from about 1 MeV to

A modified version of Gafchromic ® EBT radiochromic medical imaging film, which is light insensitive and requires no special development or processing, is shown to be useful for imaging low-energy (few MeV) ion beams and radioactive sources. It appears especially well suited for use with low-intensity short-lived (radioactive) secondary beams (RNB). The film can assist in optimizing the collimation needed to minimize unwanted background ions, and to accurately determine the ion-optical alignment of RNB production and transport systems. It allows for effective imaging of beam profiles and relative intensities throughout the beam-transport system and especially at locations not readily accessible to electronic imaging detectors. The special advantages of using the film for RNBs, which often are spatially extended and contaminated with unwanted ions, are demonstrated.

Methods for reducing the energy spread in low-energy short-lived secondary radioactive nuclear beams produced in a solenoid-based system are examined. In particular, the use of a tapered energy-loss absorber placed off-focus is studied and a first set of in-beam tests reported. In addition, realistic ion-optical ray-tracing time-of-flight calculations for energy-spread compensation are presented for both short-(3 m) and long-(6 m) flight path options. The advantages and disadvantages of the various techniques are discussed.

We describe the design and operation of a detector system for measuring all-photon decays of mesons photoproduced in a tagged photon beam with energies between 4.3 and 5.4 GeV and a flux of 5×107 tagged photons per second. Photons from meson decays were detected with a lead-glass calorimeter with an energy resolution of 11% at 1 GeV. Various veto and trigger components were also present. Final states with as many as six photons were successfully detected and reconstructed.

The study of low-energy reactions with radioactive-ion beams has been greatly enhanced by the recent use of active-target detectors, which have high efficiency and low thresholds to detect low-energy charged-particle decays. Both of these features have been used in experiments with the Prototype Active-Target Time-Projection Chamber to study α -cluster structure in unstable nuclei and 3-body charged-particle decays after implantation. Predicted α -cluster structures in 14 C were probed using resonant α scattering and the nature of the 3- α breakup of the 0 2 + Hoyle state in 12 C after the beta decay of 12 N and 12 B was studied. These experiments used in-flight radioactive-ion beams that were produced using the dual superconducting solenoid magnets TwinSol at the University of Notre Dame. Preliminary results from these experiments as well as the development of future radioactive beams to be used in conjunction with the PAT-TPC are presented.

A large, segmented neutron detector array, the “neutron wall”, has been constructed for use with the TwinSol radioactive nuclear beam facility at the University of Notre Dame. At present, the array consists of 16 position-sensitive plastic scintillator bars. The position and energy resolution of the array, and its neutron-detection efficiency are described.

Spectroscopy of neutron rich exotic isotopes via their isobaric analog states (IAS) in less exotic nuclei is discussed. Several different experimental techniques, which can be applied to search for IAS of exotic isotopes, are described. Successful application of these techniques to the studies of heavy helium isotopes 7 He and 9 He led to the observation of unknown IAS in 7 Li and 9 Li. Spectroscopic information for these states were obtained, and implication of these findings to the structure of 7,9 He is considered.

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.

A double superconducting solenoid system is being installed at the Pelletron Laboratory of the University of S ao Paulo, Brazil. This system allows the production of secondary beams of light exotic nuclei like 8Li, 6He and others. The first results using this facility are presented.

The heavy helium isotopes 7,9 He were studied via their isobaric analog states (IAS) in 7,9 Li. The IAS were populated via resonance reactions of protons with radioactive beams of 6,8 He. The isospin-conserving neutron decay of T=3/2 resonances in 7 Li and proton decay of T=5/2 resonances in 9 Li were measured. New spectroscopic information on these states were obtained, and compared with the properties of levels in 7,9 He.

Resonances in 6Be and 5Li have been investigated by deuteron transfer reactions with radioactive ion beams, 2H(8B,α)6Be, 2H(7Be,α)5Li, respectively. There is an indication of a possible resonance at excitation energy of 9.6 MeV in 6Be, just below the 3He+3He threshold. The structure of the 6Li nucleus has been also investigated by the neutron transfer reaction 3He(7Li,α)6Li. In 6Li, a new resonance is observed at about 12.4 MeV in excitation energy and the presence of a resonance at 15.2 MeV is confirmed. The data are preliminary and a detailed analysis is in progress. The instalation of a Radioactive Ion Beam facility in Brazil (RIBRAS) is also reported.

We describe recent results using the fully implemented version of the twin superconducting solenoid magnet low-energy radioactive nuclear beam (RNB) apparatus “TwinSol”. In particular a number of recent experiments involve implantation and subsequent measurements using short-lived nuclear beams such as 8 Li, 6 He, 7 Be and 8 B. Several are described. Many of these exploit special features of the twin-solenoid design. Recent improvements include a major upgrade (to 10 MV) of the University of Notre Dame Van de Graaff accelerator and the addition of a low-background beam line extension for RNB-induced gamma-ray measurements.

Neutron capture of short-lived isotopes can be important within several astrophysical environments. The laboratory investigation of these reactions can be difficult since these isotopes cannot form a target. We used radioactive beams and measured the inverse reaction instead. Particularly we used 15 C and 9 Li beams to measure the 14 C(n , γ) 15 C , and 8 Li ( n , γ) 9 Li reactions. We determined the reaction rates within the astrophysically interesting temperature range. Our data for the 14 C capture rises quickly up to 8 μb and then it decreases by a factor of two at 1 MeV. We investigated neutron capture by 8 Li which is one possible leak of the bridging reaction 8 Li (α, n ) 11 B of the A=8 gap. We found that this leak is lower than the theoretical expectations.

A gamma-ray spectroscopy program has been established at the University of Notre Dame using a low-energy radioactive beam of 6 He for fusion–evaporation reactions. There are many experimental difficulties to overcome in the design of any such facility, most notably the large neutron flux associated with producing the secondary beam. Solutions to the problems are discussed and illustrated with the reaction 6 He + 63 Cu .

The fusion of 6He with a 209Bi target displays a large enhancement at energies near to and below the Coulomb barrier. Recently, a 4He group of remarkable intensity, which dominates the total reaction cross-section, has also been observed in the near-barrier interaction of the same system. It is argued that this transfer/breakup channel acts as a doorway state to fusion.

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5–20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), 13C(d,n), 27Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.

Elastic scattering angular distributions of the radioactive weakly bound $^{7}\mathrm{Be}$ on $^{27}\mathrm{Al}$ have been measured at energies near the Coulomb barrier. The interaction optical potential is investigated by using different potentials. Total reaction cross sections have also been obtained.

A new device (TwinSol) installed at the Nuclear Structure Laboratory at the University of Notre Dame implements a pair of large-bore 6 T superconducting solenoids for producing relatively pure, high-intensity beams of exotic light nuclei at low-energies (10–80 MeV). Typical beams include 8 Li and 6 He (T 1/2

We propose a method to produce usable amounts (1–100/s) of low-energy, very neutron-, proton-rich or highly-polarized radioactive nuclear beams (RNBs) via a two-stop production method which utilizes a high-intensity secondary RNB with a second production target. For example: 7 Li+ 9 Be→ 8 Li+ 9 Be→ 9 Li. The latter ( 9 Li) is very difficult to produce at low energies (a few MeV/u) as it is several nucleons away from the line of stability and no efficient one-step mechanism exists. In contrast, high-cross-section transfer reactions such as 9 Be( 7 Li, 8 Li) combined with a high-efficiency ion-optical collection devices can produce 8 Li and other near-stability RNBs ( 8 B, 7 Be, 6 He,…) at very good intensities, viz. 10 4 –10 8 /s. Thus, it appears feasible to use a second, high cross-section reaction – perhaps now with a radioactive or polarized second production target, to produce and exotic, low-energy tertiary RNBs, including polarized RNBs. These and other aspects are described together with some initial tests using the new UM-UND TwinSol RNB apparatus (see M. Lee, this Proceedings).

The use of a high-efficiency BaF spectrometer for in-beam γ-tagged γ-ray spectroscopy of low-energy radioactive nuclear beams (RNBs) is examined in this study. Our results demonstrate that although BaF detectors have poor energy resolution, they still can (with proper tagging to eliminate background) be a highly efficient tool for in-beam γ-ray spectroscopy of RNBs.

Total reaction cross sections, σ R , on Si were measured near 40 A MeV for the proton-halo candidate 12 N and the two-proton-halo candidate 17 Ne, and were compared with σ R for other light proton-rich nuclei. The A -dependence shows enhanced σ R 's for 12 N and 17 Ne, relative to their neighbors, but the effect is smaller than for 8 B which has been argued to have a proton halo. In general, nuclei with loosely bound last protons ( S p ⩽ 1.5 MeV) have significantly larger σ R 's than their neighbors. Cross sections for charge-removal from 12 N and 17 Ne also were obtained.

We have made the first attempt at determining the ${}^{8}$Li$(n,\ensuremath{\gamma})$${}^{9}$Li cross section at astrophysical energies. This reaction competes with the ${}^{8}$Li$(\ensuremath{\alpha}{,n)}^{11}$B reaction in the inhomogeneous big bang model and with ${}^{8}$Li $\ensuremath{\beta}$ decay in the $r$ process. It may affect the primordial abundance and the stellar production of Ag12 nuclei. Using a radioactive beam of ${}^{9}$Li and the Coulomb dissociation method we attempted to measure the cross section of the inverse reaction ${}^{9}$Li$(\ensuremath{\gamma}{,n)}^{8}$Li. We report only an upper limit because we were unable to determine and subtract the nuclear dissociation. Through the detailed balance theorem, this lead to an upper limit for the cross section of interest, ${}^{8}$Li$(n,\ensuremath{\gamma})$${}^{9}$Li. The limit is 19$\ifmmode\pm\else\textpm\fi{}$8 b for ${E}_{n}$ = 0--500 keV or, when expressed as a reaction rate, 7200 cm${}^{3}$ s${}^{\ensuremath{-}1}$ mole${}^{\ensuremath{-}1}$.

Fission following the fusion of {sup 6}He with a {sup 209}Bi target has been studied near the Coulomb barrier, at excitation energies of 32 and 34 MeV in the compound system. These new experimental data are in disagreement with previous work which reported an anomalously large fusion-fission yield, when compared with {sup 4}He-induced fission of {sup 209}Bi at similar excitation energies. In fact, the {sup 6}He-induced fusion-fission yield appears to be smaller than that for {sup 4}He, in qualitative agreement with conventional statistical model calculations. {copyright} {ital 1998} {ital The American Physical Society}

We present new calculations and experimental measurements of the quasielastic cross section angular distribution for ${}^{8}$He scattering from ${}^{12}$C at 60 MeV/nucleon. ${}^{8}$He is treated as a five-body $\ensuremath{\alpha}+4n$ system and the six-body ${}^{8}$He+target scattering calculations make use of the eikonal few-body method and the cluster orbital shell model approximation for the ${}^{8}$He wave function. The qualitative features of the new data are successfully described without parameter variation. The sensitivity of the calculations to correlations in the ${}^{8}$He wave function is assessed.

The neutron spectroscopy of the $\ensuremath{\beta}$-delayed neutron decay of ${}^{14}$Be has been investigated. A strong neutron group, which accounts for much of the $\ensuremath{\beta}$-decay yield and could explain the discrepancy in total neutron branching ratio found in a previous experiment, was observed. We found that the total width of the final state for this transition is $60\ifmmode\pm\else\textpm\fi{}5\ifmmode\pm\else\textpm\fi{}6$ keV, and the measured half-life of the ${}^{14}$Be $\ensuremath{\beta}$ decay was $4.29\ifmmode\pm\else\textpm\fi{}0.18$ ms. The limits on the branching ratio and log$\mathrm{ft}$ value determined for this group are compared with the earlier work and the results of a shell-model calculation.

A radioactive beam of 26 MeV 8 B has been utilized to measure the Coulomb dissociation of 8 B on 58 Ni. At these subcoulomb barrier energies, the method provides an accurate means to understand the role of E2 photons in the breakup reaction. The results are critical for the determination of the S 17 factor of the inverse reaction — the astrophysically relevant radiative proton capture of 7 Be - using the method of Coulomb excitation.

Subcoulomb breakup of 8 B on a 58 Ni target has been studied, at an incident energy of 26 MeV. These data provide a measure of the E2 component of the breakup cross section as a function of the astrophysical factor S 17 for radiative capture of protons on 7 Be . Implications for the solar neutrino problem are discussed.

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.

Proton pair correlations relevant for the neutrinoless double-$\ensuremath{\beta}$ decay of ${}^{76}$Ge have been probed via the ${}^{74,76}$Ge(${}^{3}$He,$n$) reactions at 16 MeV. No evidence for pairing vibrations in either nucleus is observed at sensitivity limits of $\ensuremath{\sim}$6$%$ and $\ensuremath{\sim}$19$%$ of the ground-state strength in ${}^{76}$Se and ${}^{78}$Se, respectively. These results are relevant for the understanding of matrix elements for neutrinoless double-$\ensuremath{\beta}$ decay. The lack of pairing vibrations is consistent with a simple BCS structure for the ground states of ${}^{76}$Ge and ${}^{76}$Se, assumed in quasiparticle random phase approximation (QRPA) models of the process.

The structure of ${}^{10}$Be was studied via resonant $\ensuremath{\alpha}$-particle scattering of a neutron-rich ${}^{6}$He beam. A time projection chamber, PAT-TPC, was operated in an active-target mode to provide a gaseous ${}^{4}$He target and trace the beam and reaction products traversing its active tracking volume. This significantly lowered the detection threshold of reaction products at low energies. Elastic scattering, inelastic scattering to the ${}^{6}$He 2${}^{+}$ state, and the ${}^{6}$He($\ensuremath{\alpha}$, 2$n$)${}^{8}$Be reaction were measured below an energy of 6 MeV in the center-of-mass frame. Continuous spectra of excitation functions and angular distributions were obtained from unambiguously-identified recoiling $\ensuremath{\alpha}$ particles for the elastic and inelastic channels. While a resonance of the 4${}^{+}$ state at 10.15 MeV in ${}^{10}$Be previously reported was confirmed, no other resonances were identified in the elastic channel over the measured energy region. The results are in line with antisymmetric molecular dynamics calculations that predict the limits of $\ensuremath{\alpha}$ clustering in high-spin states due to a spin-orbit force.

Evaporation protons from the fusion of the ${}^{3}\mathrm{He}{+}^{58}$Ni system were measured at three energies near but above the Coulomb barrier. The respective elastic scattering angular distribution was also determined for one of these energies. The fusion data, after being properly reduced to eliminate size and charge effects, are compared to the recently measured fusion excitation function for the proton-halo system ${}^{8}\mathrm{B}{+}^{58}$Ni. As a reference, fusion data for the ${}^{16}\mathrm{O}{+}^{58}$Ni system are also presented. With respect to this reference, the fusion cross sections for the proton-rich systems show an enhancement likely related to static effects. An excitation function for the respective total-reaction cross section was extracted from the measured elastic data along with additional data from the literature. Surprisingly, this excitation function follows the trend expected for weakly bound systems.

Total reaction cross sections, ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$, of 20--60A MeV $^{4,6,8}\mathrm{He}$, $^{6--9,11}\mathrm{Li}$, and $^{10}\mathrm{Be}$ were measured by injecting magnetically separated, focused, monoenergetic, identified secondary beams of those projectiles into a Si detector telescope and measuring their energy-deposition spectra. These ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$'s, accurate to about 3%, were compared with predictions of optical, strong absorption, and microscopic models. The latter gave the best overall fit to the data, providing long-tailed matter densities were assumed. The best available optical potentials generally overpredicted the data by about 10%. Strong absorption calculations, in which the isospin-dependent term is quite important, were often unsuccessful, especially for projectiles with large neutron excess. Two-neutron removal cross sections were measured for $^{6}\mathrm{He}$ and $^{11}\mathrm{Li}$; the $^{11}\mathrm{Li}$ data were slightly overpredicted by a microscopic model which includes correlation effects for the $^{11}\mathrm{Li}$ valence neutrons. Both 2n and 4n removal from $^{8}\mathrm{He}$ were observed, in about a 2:1 ratio. Subtraction analysis of the data indicates that $^{4}\mathrm{He}$ is a good core within $^{6}\mathrm{He}$ and $^{8}\mathrm{He}$, as is $^{9}\mathrm{Li}$ within $^{11}\mathrm{Li}$. \textcopyright{} 1996 The American Physical Society.

The quasielastic scattering of {sup 9}Li on a {sup 12}C target has been measured at an incident energy of 540 MeV. The new experimental data are used to extract an effective interaction for {sup 9}Li + {sup 12}C scattering. The uncertainty in this interaction was previously a major obstacle to extracting information on the structure of {sup 11}Li from existing {sup 11}Li + {sup 12}C quasielastic-scattering data.

The sub-Coulomb breakup of {sup 8}B on a {sup 58}Ni target is studied at an incident energy of 26 MeV. These data provide a measure of the {ital E}2 component of the breakup cross section as a function of the astrophysical factor S{sub 17} for radiative capture of protons on {sup 7}Be. Implications for the solar neutrino problem are discussed. {copyright} {ital 1996 The American Physical Society.}

Projectiles of 11 Li were photodisintegrated into 9 Li + n + n by absorption of a virtual photon in a Pb target. The momenta of the three particles in the final state were measured, and the dipole strength function was determined up to about 1 MeV above the disintegration threshold. On the average, the 9 Li residue had a greater velocity than the neutrons. Interpreted as a post-breakup acceleration of 9 Li in the coulomb field of the Pb nucleus, the lifetime of the excited 11 Li was found to be too short for the disintegration process to have gone through a resonant state such as the soft dipole resonance. Because the complete kinematics was measured, we could view the events in the rest frame of 11 Li. In that frame the distribution of angle between the two neutrons, when the response of the neutron detection system was taken account of, was isotropic. Hence, we have no evidence for a dineutron in 11 Li. Two recently-constucted neutron walls will be used in future experiments.