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Excited states were investigated in 21 F and 25 Na using the 9 Be( 14 C,pnγ) reaction at 30, 35, and 45 MeV and the 9 Be( 18 O,pnγ) reaction at 35 MeV. Protons were detected and identified in an E-∆E telescope at 0 o in coincidence with one or more γ radiations in the FSU Compton-suppressed Ge detector array. Many new levels and electromagnetic decays were observed, especially among the higher spin states. Angular distributions and mean lifetimes were measured wherever possible in both nuclei. The energy levels of the positive-parity states in the two nuclei agree rather well with shell model calculations using both the USDA and WBP interactions up to the highest spins observed of 13/2h. Both a weak coupling approximation and shell model calculations using the WBP interaction generally reproduce the negative-parity states in 21 F. The shell model calculations reproduce relatively well the measured M1 and E2 transitions in both nuclei, but overpredict the parity-changing E1 transitions in 21 F, the only nucleus in which negative-parity states were observed in the present experiment.

The first lifetime measurement used to study the magnetic response of halo nuclei is presented. The lifetime of the first excited state of the one-neutron halo nucleus 19 C has been measured by two complementary Doppler-shift techniques with the GRETINA array. The B(M1; 3/2 + →1/2 + g.s. ) strength of 3.21(25)×10 −3 µ 2 N determined for this decay represents a strongly hindered M1 transition among light nuclei. Shell model calculations predict a strong hindrance due to the near-degeneracy of the s 1/2 and d 5/2 orbitals among neutron-rich carbon isotopes, while tensor corrections and loosely bound effects are necessary to account for the remaining strength.PACS numbers: 21.10. Tg, 21.60.Cs, 23.20.Lv, 27.20.+n The electromagnetic response of atomic nuclei plays a central role in characterizing the static and dynamic nuclear properties in terms of spatial, spin and isospin degrees of freedom. The giant resonance is one famous example, exhibiting a significant strength from coherent collective motion between protons and neutrons [1]. Depending on the excitation energy region of nuclei, the electromagnetic transition strength can provide essential information to deduce internal configurations of nuclei, quantify collectivity and deformation, and constrain the nuclear equation of state.At the limit of nuclear stability, exotic structures can emerge due to the rearrangement of shell-model orbitals [2,3]. When the s-wave strength appears close to the threshold, quantum tunneling of valence neutrons leads to extended wave functions known as halos [4,5]. In this case, a new degree of freedom in collective modes is naïvely expected from a relative motion between the core and halo neutron, inducing so-called soft collective motions [6,7]. Non-resonant dipole excitations in light nuclei and pygmy dipole modes in medium and heavy nuclei have been extensively studied through Coulomb excitation with rare isotope beams, revealing a sizable electric dipole (E1) strength in the low-energy region [8]. However, the magnetic response of halo nuclei is not well understood, mainly due to difficulties in selectively inducing the magnetic excitation in intermediate-energy nuclear reactions [9]. Currently, only static magnetic properties have been studied for the one-neutron halo nucleus 11 Be through the β-NMR measurement of the magnetic moment [10] and hyperfine splitting measurement to deduce the magnetization radius [11]. Regarding the dynamic response, a hindered magnetic dipole (M1) strength has been observed for the 1/2, where a possible halo structure in the excited 1/2 + state is discussed.The present paper reports the first study on the dynamic magnetic response of the neutron halo nucleus 19 C. In a simplistic model of the halo, an s 1/2 neutron is coupled to a 0 + core, causing the low-energy M1 response to vanish due to the absence of a spinflip partner for the s 1/2 orbital. However, the realistic picture is more complex in 19 C, because non-negligible core-excitation components have recently been suggested by an inclusi...

The 9 Be( 14 C, αγ) reaction at E Lab =30 and 35 MeV was used to study excited states of 19 O. The Florida State University (FSU) γ detector array was used to detect γ radiation in coincidence with charged particles detected and identified with a silicon ∆E-E particle telescope. Gamma decays have been observed for the first time from six states ranging from 368 to 2147 keV above the neutron separation energy (S n =3962 keV) in 19 O. The γ decaying states are interspersed among states previously observed to decay by neutron emission. The ability of electromagnetic decay to compete successfully with neutron decay is explained in terms of neutron angular momentum barriers and small spectroscopic factors implying higher spin and complex structure for these intruder states. These results illustrate the need for complementary experimental approaches to best illuminate the complete nuclear structure. 1The decay mode(s) of a nuclear quantum state is (are) one of its most important properties after energy. Usually nuclear decay follows the hierarchy of the fundamental forces of nature.Decay by emission of one or more particles mediated by the strong nuclear interaction is normally the dominant mode for those states unbound to particle emission. Bound excited states usually decay by emission of electromagnetic radiation to the ground state. The ground state, in turn, decays much more slowly by β decay mediated by the weak nuclear interaction until the lowest energy neutron-to-proton ratio has been reached. Of course, there are exceptions: lower energy charged particle decay from unbound states is inhibited by the Coulomb barrier, and large spin change or low emission energy can inhibit γ decay so much that β decay occurs first. Even decay by neutrons which face no Coulomb barrier can be inhibited by large spin change, as is well known in high-spin spectroscopy of medium and heavy nuclei. However, based on the simple picture of barrier penetrability, neutron decay is usually assumed to dominate over radiative decay when angular momentum barriers are not very high. But Fermi's Golden Rule [1] states that the decay rate is the product of the coupling strength, the density of final states, and the matrix element between the wavefunction of the parent state and the daughter state (usually called spectroscopic factor (S)). What is often overlooked is that this latter factor due to the nuclear structure may be more instrumental than the angular momentum barrier in limiting neutron decay rates below those of electromagnetic decay. Examples are states with complex structure often involving intruder configurations.The role of factors beyond barrier penetrability is highlighted by observations reported in this paper of the decays of unbound states in 19 O populated in the 9 Be( 14 C,αγ) reaction which favors higher-spin states, which are typically more complex and may involve intruder configurations. The importance of 19 O with a closed major proton shell and an exactly halffilled neutron d 5/2 orbital has been recognized wi...