We have performed the first direct measurement of the ^{38}K(p,γ)^{39}Ca reaction using a beam of radioactive ^{38}K. A proposed ℓ=0 resonance in the ^{38}K+p system has been identified at 679(2) keV with an associated strength of 120_{-30}^{+50}  meV. Upper limits of 1.16 (3.5) and 8.6 (26) meV at the 68% (95%) confidence level were also established for two further expected ℓ=0 resonances at 386 and 515 keV, respectively. The present results have reduced uncertainties in the ^{38}K(p,γ)^{39}Ca reaction rate at temperatures of 0.4 GK by more than 2 orders of magnitude and indicate that Ar and Ca may be ejected in observable quantities by oxygen-neon novae. However, based on the newly evaluated rate, the ^{38}K(p,γ)^{39}Ca path is unlikely to be responsible for the production of Ar and Ca in significantly enhanced quantities relative to solar abundances.

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 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 βn=0.46±0.21. This provides the first indication of a possible weakening of the N = 8 shell closure at the proton drip-line. PACS numbers: 24.50+g, 25.45.De, 25.60.-t, 25.70.EfThe evolution of shell structure over the nuclear landscape is a manifestation of strong interactions in the complex nuclear many-body system. Properties of nuclei at the neutron and proton drip-lines provide new arenas to investigate the effects of large proton-neutron asymmetry and understand the persistence of mirror symmetry. Shell structure evolution in neutron-rich and proton-rich nuclei [1][2][3][4][5][6][7][8][9][10][11][12] are leading to new insights into nuclear forces, including the role of three-body forces [13,14]. The region around the N = 8 shell closure draws particular interest, since this shell gap disappears at the neutron drip-line and leads to the formation of a twoneutron halo in the Borromean nucleus 11 Li. The small two-neutron separation energy (S 2n = 360 keV) of 11 Li results in the excited states of 11 Li being unbound. Less is known about the structure of nuclei at the proton drip- * Present address: KVI-CART, University of Groningen, 9747 AA Groningen, The Netherlands line. The N = 8 isotone at the proton drip-line, 20 Mg, is also a Borromean system whose two-proton separation energy is S 2p = 2.337(27) MeV [15]. There is no experimental information on resonances above the proton threshold in 20 Mg. In this work we present the first observation of a resonance in 20 Mg through deuteron inelastic scattering. This measurement provides new insight into shell evolution as well as tests of ab initio predictions. The resonance(s) in 20 Mg could also contribute to a potential breakout path from the hot CNO cycle via two-proton capture on the waiting point nucleus 18 Ne in Type-I x-ray bursts [16].Microscopic cluster model [17] predictions for the 2 + state agree with the experiment [18,19]. A 4 + state is predicted at 3 MeV, this state is predicted around 3.8 MeV, using a beyond the mean field approach [20]. Predictions based on an angular momentum projected generator coordinate framework [21], find the 2 + 1 and the 4 + 1 states to be higher in energy around, 3.5 MeV and 7.8 MeV, respectively. Ab initio calculations in a manyb...

Levels up to energies of 4 MeV have been studied with the 91Zr(3He, d)92Nb reaction with an incident particle energy of 25.5 MeV. The product deuterons were analyzed by a split-pole spectrometer. Angular distributions were obtained for many of the transitions and compared with DWBA calculations. The assigned transfer l values and spectroscopic factors were used to assign shell model structures to these levels.