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21The design, realization and operation of a prototype or "demonstrator" version of an active target and time projection chamber (ACTAR TPC) for experiments in nuclear physics is presented in detail. The heart of the detection system features a micromegas gas amplifier coupled to a high-density pixelated pad plane with square pad sizes of 2×2 mm 2 . The detector has been thoroughly tested with several different gas mixtures over a wide range of pressures and using a variety of sources of ionizing radiation including laser light, an α-particle source and heavy-ion beams of 24 Mg and 58 Ni accelerated to energies of 4.0 MeV/u. Results from these tests and characterization of the detector response over a wide range of operating conditions will be described. These developments have served as the basis for the design of a larger detection system that is presently under construction.1

The ACtive TARget and Time Projection Chamber (ACTAR TPC) is a novel gas-filled detector that has recently been constructed at GANIL. This versatile detector is a gaseous thick target that allows the tracking of charged particles in three dimensions and provides a precise reaction energy reconstruction from the vertex position. A commissioning experiment using resonant scattering of a 3.2 MeV/nucleon 18 O beam on an isobutane gas (proton) target was performed. The beam and the heavy scattered ions were stopped in the gas volume, while the light recoil left the active volume and were stopped in auxiliary silicon detectors. A dedicated tracking algorithm was applied to determine the angle of emission and the length of the trajectory of the ions, to reconstruct the reaction kinematics used to built the excitation functions of the 1 H( 18 O, 18 O) 1 H and 1 H( 18 O, 15 N) 4 He reactions. In this article, we describe the design of the detector and the data analysis, that resulted in center of mass reaction energy resolutions of 38(4) keV FWHM and 54(9) keV FWHM for the proton and alpha channels, respectively.

The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical r-process in producing nuclei heavier than A ∼ 190. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region, neutron excitations in 207 Hg have been probed using the neutron-adding (d,p) reaction in inverse kinematics. The radioactive beam of 206 Hg was delivered to the new ISOLDE Solenoidal Spectrometer at an energy above the Coulomb barrier. The spectroscopy of 207 Hg marks a first step in improving our understanding of the relevant structural properties of nuclei involved in a key part of the path of the r-process.The nucleus 207 Hg lies in the almost completely unexplored region of the nuclear chart below proton number 82 and just above neutron number 126, both "magic" numbers representing closed shells in the nuclear shell model [1]. The doubly-magic nucleus 208 Pb is the cornerstone of this region, a benchmark nucleus in our understanding of the single-particle foundation of nuclear structure. This region, highlighted on the nuclear chart in Fig. 1, is unique in that its single-particle structure remains unexplored.The nucleosynthesis of heavy elements via the rapid neutron-capture (r-) process path [2] crosses this region, as shown in Fig. 1. The robustness of the N = 126 neutron shell closure plays a crucial role in the nucleosynthesis of the actinides [3][4][5][6][7]. The recent observation of a neutron star merger has provided a new focus of interest [8,9], suggesting a possible astrophysical environment for r-process nucleosynthesis [10-13].Approaching the r-process path along the N = 126 isotonic chain from Pb, the binding energies (the degree to which neutrons are bound by the mean-field potential created by the decreasing number of all other nucleons) decrease, eventually crossing zero binding and becoming unbound. Near closed shells, the level density is low, so the usual statistical assumptions of many resonances participating in neutron capture is not valid, and specific nuclear-structure properties become important. Knowledge of ground-state binding energies of nuclei with N = 126 + n is important in defining the waiting point caused by the N = 126 closure, the bottleneck which is responsible for the third peak in solar system elemental abundances at nuclear mass A ∼ 195 [14]. The binding energies are critical to how the r-process evolves. The energies of ground and excited states have significant consequences for the rate at which direct s-, p-, (and possibly d-) wave neutron-capture (n,γ) reactions proceed [15][16][17]. This was discussed recently in the context of the N = 82 shell closure in Ref. [18].As zero binding is approached, the energies of s orbitals increase less rapidly than those of states with higher angular momenta [19]. This behavior has been studied for light nuclei [20,21] and, in the vicinity o...

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