Neutron-Proton Coupling and the Lifetime of the First Excited State in $^{16}$C

Nuclei near the valley of $\beta $-stability have strongly correlated proton and neutron spatial distributions. This need not be the case for nuclei with a large excess of one nucleon type and the search for new phenomena and structure effects due to the ``decoupling'' of neutrons and protons is of great interest in nuclear structure physics. Cited examples of decoupled behavior include neutron-halo nuclei with measurably different proton and neutron radial distributions, and low-energy dipole modes such as ``pygmy'' resonances where, simplistically, a core of equal numbers of protons and neutrons oscillates against the excess neutron ``skin'''. Recently, another example was suggested to occur in $^{16}$C where the measurement of an anomalously quenched B(E2;2$^{+}\to ^{ }$0$^{+})$ value of 0.63 e$^{2}$fm$^{4 }$ combined with a large nuclear deformation led to the suggestion that the $^{16}$C valence neutrons were decoupled from its near-spherical proton core (N.Imai et al., PRL 92 (2004) 062501; Z.Elekes et al., PLB 586 (2004) 34; H.J.Ong et al., PRC 73 (2006) 024610). In this talk I will discuss a new lifetime measurement for the first-excited 2$^{+}$ state in $^{16}$C carried out at the LBNL 88-Inch Cyclotron using the Recoil Distance Method and $^{9}$Be($^{9}$Be,2p) fusion-evaporation reaction. The mean lifetime was found to be 11.7(20) ps corresponding to a B(E2) of 4.15(73) e$^{2}$fm$^{4}$, consistent with other even-even closed shell nuclei and neighboring systematics. Our result does not support the interpretation of decoupled protons and neutrons in $^{16}$C. The revised value provides an important benchmark for theory. Time permitting I will present results on the neutron-rich nucleus $^{30}$Ne produced in a 2p knockout reaction performed at the NSCL using the S800 spectrometer and SeGA gamma-ray detector. The measured (quenched) 2p knockout cross-section, when compared to theory, suggests a significant difference in the neutron intruder content between $^{32}$Mg and $^{30}$Ne, contrary to current shell models.