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A program to investigate the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix by studying super-allowed mixed mirror β decays has been initiated at the TwinSol facility at Notre Dame. These mixed Fermi/Gamow-Teller (F-GT) decays, occurring between T=1/2 isospin doublets in mirror nuclei, provide a complimentary check on the data from super-allowed pure Fermi decays from 0+to 0+ states. The first part of the program, involving the measurement of the lifetimes of the relevant nuclei to the required accuracy of one part in 103 or better, has nearly been completed. However, the additional complication introduced by F-GT mixing requires the use of an ion trap to measure the mixing ratio ρ with similar accuracy. The lifetime measurements, as well as progress in installing an ion trap at TwinSol, will be discussed. In addition, since the ion trap will require a dedicated beam line for its operation, an opportunity presented itself to greatly improve the performance of TwinSol for reaction studies with exotic nuclei. This took the form of an added dipole switching magnet coupled to a third solenoid to form the new TriSol facility currently under construction. The expected properties of TriSol, and its application to reaction studies of interest for nuclear astrophysics, will also be discussed.

Unexpected and challenging experimental results, at below barrier energies for weakly bound nuclei, are briefly reviewed in this article. The emphasis will be on our recent breakup results for 8B+208Pb at deep sub-barrier energies, indicating a dominance of direct mechanisms at this low energy regime. We will also present, a preliminary analysis of the 4He and 3He - particle production events for the 7Be + 208Pb reaction. These data were collected at the same experiment and at similar deep sub-barrier energies, exhibiting large yields compatible with cluster transfer processes. To confirm these results a new experiment to be performed at TriSol is planned, as soon as the upgrading of this facility will be completed.

Four classes of measurements that are important for achieving a complete understanding of the reactions of exotic nuclei are identified and classified according to their degree of difficulty. Previously reported data for near- and sub-barrier fusion of 8 B+58 Ni are critically reviewed. The influence of breakup protons on the evaporation proton measurements for this system is shown to be small at all energies except for the lowest one measured, and corrections are made for this process. Model dependencies in the deduced fusion cross sections are assessed using three different evaporation codes. Data sets for targets of 58 Ni and 28 Si are shown to be consistent with each other, and with fusion enhancement up to energies that are greater than the Coulomb barrier Vb (Ec.m. ≲ Vb + 1.5×ћω ). An important difference with the behavior of neutron-halo systems is thereby confirmed. Possible fusion suppression beyond this energy is suggested by the 28 Si data. A proposal to measure the 8 B+40 Ar fusion cross section using an active-target time projection chamber is discussed.

Reactions in inverse kinematics close to the Coulomb barrier offer unique opportunities to study exotic nuclei, but they are plagued by the difficulty to efficiently and precisely measure the characteristics of the emerging particles. The Active Target Time Projection Chamber (AT-TPC) offers an elegant solution to this dilemma. In this device, the detector gas of the time projection chamber is at the same time the target in which nuclear reactions take place. The use of this new paradigm offers several advantages over conventional inert target methods, the most significant being the ability to increase the luminosity of experiments without loss of resolution. The AT-TPC and some results obtained on resonant α scattering to explore the clustering properties of neutron-rich nuclei are presented, as well as fusion cross section results using a 10 Be radioactive beam. In addition, the first re-accelerated radioactive beam experiment using the fully commissioned ReA3 linac was conducted recently at the NSCL with the AT-TPC, where proton resonant scattering of a 4.6 MeV/u 46 Ar beam was used to measure the neutron single-particle strength in 47 Ar.

Sub-barrier fusion cross sections (σfus ) for the 6 Li + 58 Ni system, obtained from the respective evaporation protons, are examined in the present work. With respect to expectations of a simple one-dimensional barrier penetration model, a large enhancement of the data is observed. Good consistency with equivalent data reported previously for similar systems is found. A comparison with total reaction cross sections (σR ), deduced from elastic scattering measurements reported previously, indicates that σfus is close to σR within the measured energy range. To estimate the contribution of complete fusion (CF), an optical model analysis of the elastic scattering data is performed where CF is identified with the absorption in a short range volume potential. A surface polarization potential is added to the bare nuclear potential to simulate the effect of peripheral reactions. The results obtained indicate that other mechanisms different from CF may be dominant, especially in the lower energy region.

Calculations of breakup and direct proton transfer for the 8 B+ 58 Ni system at energies around the Coulomb barrier ( E B,lab =22.95 MeV) were performed by the continuum-discretized coupled channels (CDCC) method and the coupled-reaction-channels (CRC) method, respectively. For the 7 Be+ 58 Ni interaction, we used a semimicroscopic optical model potential (OMP) that combines microscopic calculations of the mean-field double folding potential and a phenomenological construction of the dynamical polarization potential (DPP). The 7 Be angular distribution at E lab =25.75 MeV from the 8 B breakup on 58 Ni was calculated and the spectroscopic factor for 8 B → 7 Be+ p vertex, S expt = 1.10 ± 0.05, was deduced. The astrophysical S 17 (0) factor was calculated equal to 20.7 ±1.1 eV•b, being in good agreement with the previously reported values.

We have performed continuum-discretized coupled channels (CDCC) calculations of the breakup of 8 B on 58 Ni in the energy interval 10 - 26 MeV in the c.m. system. Elastic scattering angular distributions for the 8 B + 58 Ni system at five different energies around the Coulomb barrier were studied by using the CDCC model and taking into account a coupling between elastic and breakup channels. The energy-dependent sets found for the OPs parameters reproduce well the elastic scattering data and the fusion and reaction cross sections. Finally, a comparison of the reduced total reaction cross sections for the 8 B + 58 Ni system with the 6 He + 208 Pb and 6 He + 209 Bi systems is presented.

A program of study of the reaction mechanisms of exotic “halo” nuclei at energies near the Coulomb barrier is described. The unique opportunities presented by the ReA3 reaccelerated beam facility at the NSCL, coupled with an active-target time projection chamber (AT-TPC), will be fully exploited in this program. An approximately half-scale “prototype” AT-TPC has been constructed and is currently undergoing tests. This device has recently been installed on the TwinSol [1] beam line at the University of Notre Dame, and preliminary results of reaction studies using a low-energy 6 He beam are reported.

We have developed a multi-element deuterated liquid scintillator array for the study of reactions involving neutrons utilizing high-speed digital signal processing. A systematic study of (d,n) reactions at Ed = 16 MeV on 9 Be, 11 B, 13 C, 14 N, 15 N, and 19 F has been conducted from 10 o to 160 o (lab). In addition to previously unmeasured back-angle cross sections, this data can complement existing and future ( 3 He,d) measurements as an analog to (p,γ) for astrophysics applications.