Session JD: Mini-Symposium on GRETINA Results I

Single-particle structure of silicon isotopes approaching $^{42}$Si

$^{42}$Si would seem to be a candidate for a magic nucleus with a filled N=28 neutron shell and a Z=14 proton subshell closure, however, experimental evidence has shown it to be collective [1]. It has been proposed that this breakdown is due to the monopole effect of the tensor force at large isospin, and that both proton excitations across Z=14 and neutron excitations across N=28 contribute to the increased collectivity of $^{42}$Si [1,2]. To experimentally investigate the relative contributions of protons and neutrons to the collectivity, we have performed one-proton and one-neutron knockout reactions on $^{36,38,40}$Si using the GRETINA array and S800 spectrometer at the NSCL. We will present preliminary results in comparison to large-scale shell model calculations.\\[4pt] [1] B. Bastin et al, Phys. Rev. Lett. 99, 022503 (2007)\\[0pt] [2] T. Otsuka et al, Phys. Rev. Lett. 95, 232502 (2005)   

Isomeric character of the 4$^{+}_{1}$ state in $^{44}$S: Mechanisms of breaking of the N$=$28 shell

The N$=$28 nucleus $^{44}$S exhibits a rich structure of excitations which illustrates different mechanisms of breaking the N$=$28 shell. A Coulomb excitation measurement [1] and an implantation-decay experiment [2] established the coexistence of 2p2h-deformed and 0p0h-spherical configurations. A two-proton knockout reaction [3] indicated a 4$^{+}$ state which shell model calculations suggest is likely isomeric, prolate-deformed and formed from a 1p1h configuration. A recent two-proton knockout experiment measured the lifetime of this 4$^{+}$ state using the recoil distance method and the GRETINA array. Results for the lifetime of the 4$^{+}$ state will be presented and its implication for the mechanisms of breaking the N$=$28 shell will be discussed.\\[4pt] [1] T. Glasmacher \textit{et al.}, Phys.Lett. \textbf{B 395} (1997), 163 \newline [2] C. Force \textit{et al.}, Phys.Rev.Lett. \textbf{105}, 102501 (2010) \newline [3] D. Santiago-Gonzalez \textit{et al.} Phys.Rev. \textbf{C 83}, 061305 R (2012)   

Studies of $N \approx 40$ Ni isotopes via neutron-knockout (nKO) and deep-inelastic (DI) reactions

V. BADER, T. BAUGHER, D. BAZIN, J.S. BERRYMAN, B.A. BROWN, C. LANGER, N. LARSON, S.N. LIDDICK, E. LUNDERBERG, S. NOJI, C. PROKOP, S.R. STROBERG, S. SUCHYTA, D. WEISSHAAR, S. WILLIAMS, NSCL/MSU, M. ALBERS, M. ALCORTA, P.F. BERTONE, M.P. CARPENTER, J. CHEN, C.R. HOFFMAN, F.G. KONDEV, T. LAURITSEN, A.M. ROGERS, D. SEWERYNIAK, S. ZHU, ANL, C.M. CAMPBELL, LBNL, H.M. DAVID, D.T. DOHERTY, U. of Edinburgh/ANL, A. KORICHI, CSNSM-IN2P3/ANL, C.J. LISTER, U. of Mass.-Lowell, K. WIMMER, Central Mich. U. --- Excited states in $^{68}$Ni were populated in 2nKO reactions at NSCL. Prompt $\gamma$ rays were detected with the GRETINA array located in front of the S800 separator. A hodoscope at the S800 focal plane captured the $^{68}$Ni ions, where isomeric decays could be correlated with prompt $\gamma$ rays. Decay of the first excited state, a $0^+$ isomer, was observed, confirming that its energy substantially differs from the literature value. Comparing the decay patterns of excited states with shell-model calculations provides insight into their underlying structure. Data from $^{70}$Zn + $^{208}$Pb DI reactions studied with Gammasphere provide results consistent with the 2nKO. Single-particle strengths are also under investigation in the odd-$A$ Ni isotopes via 1nKO reactions.   

Single-particle structure of neutron-rich N=40 isotopes

Spectroscopic studies of neutron-rich nuclei around N=40 have been performed at NSCL utilizing the S800 spectrometer and the GRETINA gamma detector array. The study focused on the one-neutron removal reactions from $^{68}$Ni and $^{64,66}$Fe. An experimental challenge in this region of the nuclear chart is the occurrence of low-lying isomeric states resulting from the neutron $\nu g_{9/2}$ intruder orbital. This experiment employed a new technique of combined prompt and delayed gamma-spectroscopy using a CsI(Na) array at the end of the focal plane detector box of the S800 spectrograph to tag the population of an isomeric state. This approach allowed to quantify the occupancies of the intruder neutron $\nu g_{9/2}$ and $\nu d_{5/2}$ orbitals in $^{68}$Ni and $^{64,66}$Fe.   

Direct lifetime measurement of excited states in $^{72}$Ni

The long isotopic chain of nickel contains three doubly-magic isotopes $^{48}$Ni, $^{56}$Ni and $^{78}$Ni. The nuclei close to doubly magic systems with small number of valence nucleons can help in the understanding of shell closure effects, isomerism, and single particle states. The recent B(E2; 2$^+ \to$ 0$^+$) measurements at GANIL, and the 2$^+$ inelastic scattering cross-section measurements at NSCL, reveal unexpected collectivity in $^{70}$Ni and $^{74}$Ni. We measured electromagnetic transition rates in $^{72}$Ni using the recoil distance technique coupled with the improved gamma-ray detection array GRETINA with the NSCL/Koln plunger at the NSCL. The excited 2$^+_1$ and 4$^+_1$ in $^{72}$Ni states were populated by a one-proton knockout reaction of a $^{73}$Cu secondary beam on beryllium target. Lifetimes were obtained by comparing the measured spectra to simulated ones based on an existing code which utilizes GEANT4 with the geometry of the present setup.   

Study of anomalous quadrupole collectivity in light Sn isotopes

The chain of semi-magic Sn nuclei, with many stable isotopes, has been a fertile ground for experimental and theoretical studies. Encompassing a major neutron shell from N$=$50 to 82, the properties and structure of these nuclei provided important data for the development of the pairing-plus-quadrupole model. Recent experimental information on B(E2) for 106,108,110,112Sn came as a surprise as it indicated a larger collectivity than the predicted parabolic trend of quadrupole collectivity. These data, instead, show an unexpectedly flat trend even as the number of valence particles is reduced from 12 to 6. To fully understand how collectivity is evolving in these isotopes, 108,110,112Sn have been studied using thick-target, inelastic proton scattering with GRETINA tagging inelastic scattering events by detecting gamma-rays from the prompt decay of states excited in the reaction. We will present the trend of 2$+$ excitation cross-sections, the deduced quadrupole deformation parameters, and observations of other low-lying collective states. Comparison of these (p,p') quadrupole deformation parameters with B(E2) data will provide new insights into the relative importance of proton and neutron contributions to collectivity in these nuclei.   

Inverse-kinematics proton scattering by $^{50,52}$Ca

The deformation lengths of low-lying collective states of $^{52,50}$Ca have been measured via inverse-kinematics proton scattering. A thick liquid hydrogen target was used along with the GRETINA array at the National Superconducting Cyclotron Laboratory. The measured deformation length of the first $2^+$ state of $^{50}$Ca has been combined with the existing lifetime measurement [1] to disentangle the relative proton and neutron contributions to the excitation. Preliminary results will be discussed. \\[4pt] [1] D. Montanari et al., Phys. Rev. C 85, 044301 (2012).   

Neutron Knockout to Probe 3N Forces in the Ca Isotopes

Recent calculations by Holt et al. [1] have suggested that the inclusion of 3N forces to describe the structure of neutron-rich Ca isotopes provides a more realistic description of the nuclear structure. Mass measurements have confirmed the importance of including 3N forces compared to NN-only interactions, but cannot discriminate between the predictions of phenomenological interactions and the NN+3N microscopic calculations. Neutron knockout along the $Z$=20 isotopes provides an opportunity to test the results of NN+3N calculations against phenomenological interactions. The calculations of Holt et al. predict a fragmentation of the 1$f_{7/2}$ neutron strength from the first 7/2- state in $^{49}$Ca into higher lying states, in contrast to the predictions of both GXPF1 and KB3G which concentrate the strength in the lowest lying 7/2- state. Differences are also observed in the summed $f_{7/2}$ strength to bound nuclear states in both $^{50}$Ca and $^{49}$Ca neutron knockout. We will report on a systematic study of one-neutron knockout along the Ca isotopes using GRETINA + S800. Comparison of experimental spectroscopic factors to calculations will provide insight into the role of 3N forces in describing the $Z$=20 isotopes.\\[4pt] [1] J. Holt et al., J. Phys. G 39, 085111 (2012).