Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208 Pb shows that at very forward angles J π = 1 − states are strongly populated via Coulomb excitation. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV, i.e., in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

No abstract

We perform the first direct mass measurements of neutron-rich calcium isotopes beyond neutron number 34 at the RIKEN Radioactive Isotope Beam Factory by using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{55-57}Ca are determined for the first time to be -18650(160), -13510(250), and -7370(990)  keV, respectively. We examine the emergence of neutron magicity at N=34 based on the new atomic masses. The new masses provide experimental evidence for the appearance of a sizable energy gap between the neutron 2p_{1/2} and 1f_{5/2} orbitals in ^{54}Ca, comparable to the gap between the neutron 2p_{3/2} and 2p_{1/2} orbitals in ^{52}Ca. For the ^{56}Ca nucleus, an open-shell property in neutrons is suggested.

A 76 Ge( 3 He,t) 76 As charge-exchange experiment at an incident energy of 420 MeV has been performed with an energy resolution of 30 keV. The Gamow-Teller GT − strength distribution in 76 As, which is the intermediate nucleus in the double-beta (ββ) decay of 76 Ge, has been extracted. An unusually strong fragmentation of the GT − strength is observed even at low excitation energies of E x 5 MeV. By combining the data with those for GT + transitions from a recent 76 Se(d, 2 He) 76 As measurement, the nuclear matrix element for the 76 Ge 2νββ decay has been evaluated. A lack of correlation among the GT transition strengths feeding the same levels from the two different directions is observed. The impact on the 76 Ge 2νββ decay nuclear matrix element is discussed. PHYSICAL REVIEW C 86, 014304 (2012) 76 Ge 76 As 76 Se 2 − 1 + 1 + 1 + 0 + 0 + β − β − 44.425(1) keV 86.787(1) keV 120.258(1) keV Q β − β − = 2039.00(5) keV Q β − = 2962 keV Q EC = 923 keV J π =1 +