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Intermediate-energy Coulomb excitation measurements are performed on the N ! 40 neutron-rich nuclei 66;68 Fe and 64 Cr. The reduced transition matrix elements providing a direct measure of the quadrupole collectivity BðE2; 2 þ 1 ! 0 þ 1 Þ are determined for the first time in 68 Fe 42 and 64 Cr 40 and confirm a previous recoil distance method lifetime measurement in 66 Fe 40 . The results are compared to state-ofthe-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed. DOI: 10.1103/PhysRevLett.110.242701 PACS numbers: 25.70.De, 27.50.+e For many decades the nuclear shell structure originally proposed by Mayer [1] and Jensen and coworkers [2], where energy gaps are predicted at specific nucleon numbers, was a paradigm of nuclear physics, as it was consistent with the experimental findings at or near the valley of beta stability. However, with the possibility of producing more exotic nuclei, the traditional magic numbers have been observed to be weakened or to disappear while new subshell gaps have emerged. In particular, the role of the proton-neutron tensor interaction has been recognized as driving changes in the shell structure [3]. Alterations to the effective single-particle orbital gaps can lead to enhanced particle-hole excitations, which are supported by deformation and pairing effects, and may give rise to new regions of well-developed nuclear deformation.A region of recent interest is that of the neutron-rich isotopes near N ¼ 40, below the 28 Ni isotopes. In many ways structurally similar to the ''island of inversion '' nuclei near N ¼ 20 [4], the Fe and Cr isotopes in this region have been experimentally observed to exhibit increasingly collective behavior, rather than the near-magic behavior naively expected assuming a robust N ¼ 40 subshell gap. In a schematic way, the development of collectivity moving from 28 Ni to 26 Fe and 24 Cr is understood as a result of a narrowing of the N ¼ 40 subshell closure and the enhancement of quadrupole collectivity through promotion of neutron pairs across the subshell gap. With the removal of protons from the 1f 7=2 orbital, the attractive tensor and central parts of the p-n interaction between 1f 7=2 proton holes and neutrons in the 1g 9=2 and 2d 5=2 orbits pull these neutron single-particle levels down in energy. At the same time, the repulsive tensor ð1f 7=2 Þ À1 À 1f 5=2 interaction dominates over the central attractive p-n interaction and drives the neutron 1f 5=2 orbital up, effectively quenching the N ¼ 40 gap. Looking at it another way, adding 12 neutrons to 48 Ca produces a gapless 60 Ca; as protons are added in the 1f 7=2 orbit, the repulsive interaction between the 1f 7=2 protons and the 1g 9=2 and 2d 5=2 neutrons and the strongly attractive 1f 7=2 -1f 5=2 interaction opens the N ¼ 40 gap up to its value in 68 Ni. The disappearance of the N ¼ 40 gap towards 60 Ca supports ...

A new experimental technique is presented using proton-γ-γ correlations from (94)Mo(d,p)(95)Mo reactions which allows for the model-independent extraction of the photon strength function at various excitation energies using primary γ-ray decay from the quasicontinuum to individual low-lying levels. Detected particle energies provide the entrance excitation energies into the residual nucleus while γ-ray transitions from low-lying levels specify the discrete states being fed. Results strongly support the existence of the previously reported low-energy enhancement in the photon strength function.

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about the assembled fuel either by imaging the photons emitted by the hot central plasma, or by active probing of the dense shell by a separate high energy short pulse flash. The planned use of these targets and diagnostics to assess and optimize the assembly of the fuel and how this relates to the predicted performance of DT targets is described. It is found that a good predictor of DT target performance is the THD measurable parameter, Experimental Ignition Threshold Factor, ITFX ∼ Y × dsf 2.3, where Y is the measured neutron yield between 13 and 15 MeV, and dsf is the down scattered neutron fraction defined as the ratio of neutrons between 10 and 12 MeV and those between 13 and 15 MeV.

The lifetime of the 2_+(1) state in 16C has been measured with the recoil distance method using the 9Be(9Be,2p) fusion-evaporation reaction at a beam energy of 40 MeV. The mean lifetime was measured to be 11.7(20) ps corresponding to a B(E2;2_+(1)-->0+) value of 4.15(73)e_2 fm_4 [1.73(30) W.u.], consistent with other even-even closed shell nuclei. Our result does not support an interpretation for "decoupled" valence neutrons.