Session KD: Mini-Symposium on GRETINA Results II

The Effect of Hole Mobility on GRETINA Basis Quality

The GRETINA array makes use of the concepts of signal decomposition, which fits the observed waveform from each crystal segment with a linear combination of pre-calculated basis signals to localize the interaction of gamma-rays within the detector volume, and then $\gamma$-ray tracking to reconstruct the $\gamma$-ray scattering sequence. The capability of reconstructing the position of the interaction with resolution at the level of a few millimeters is a fundamental requirement for reliable tracking and is important to understand and quantify the limiting factors of position resolution. Improvements in signal decomposition can be realized by better modeling the crystals. Specifically, we need a better understanding of drift velocity anisotropy, which causes considerable differences in pulse shape rise time depending on the position of the spatial charge carrier creation. In this presentation we will discuss the sensitivity of the predicted pulse shapes when varying hole mobility parameters and the influence of these changes on the inferred interaction points.   

$^{26}$Al,$^{30}$P(d,n) transfer reaction studies of key hydrogen burning resonances relevant for cosmic $\gamma$-ray emission and heavy element production in novae

$^{26}$Al(d,n)$^{27}$Si and $^{30}$P(d,n)$^{31}$S transfer reactions have been studied in inverse kinematics to study key astrophysical resonances in $^{27}$Si and $^{31}$S. These are relevant for abundance calculations of the cosmic $\gamma$-ray emitter $^{26}$Al, and for the abundances of heavy elements (e.g. silicon), highly dependent on the $^{30}$P(p,$\gamma$)$^{31}$S reaction, observed in novae ejecta. A primary beam of $^{36}$Ar (150 MeV/A) impinging on a Be target produced around 30 MeV/u beams of $^{26}$Al and $^{30}$P which bombarded a 10~mg~cm$^{-2}$-thick CD$_2$ target (CH$_2$ for background). The $^{27}$Si/$^{31}$S ions were analyzed by the S800 spectrometer and identified by energy loss and time-of-flight measurements. $\gamma$ rays from the decays of excited states in $^{27}$Si/$^{31}$S were detected in coincidence with the recoiling $^{27}$Si/$^{31}$S ions using GRETINA. By measuring the number of coincident events, and correcting for the angular distributions of the $\gamma$ rays, this provides an angle integrated measurement of the $(d,n)$ cross-sections, and a measure of the proton partial widths for the key astrophysical resonances in $^{27}$Si and $^{31}$S.   

Measurement of astrophysically important excitation energies of $^{58}$Zn with GRETINA

The level structure of proton-rich $^{58}$Zn has been measured with the next-generation $\gamma$-ray tracking array GRETINA in conjunction with the large-acceptance spectrometer S800 at the National Superconducting Cyclotron Laboratory at MSU. $^{58}$Zn is expected to play an important role in the rapid proton capture process (rp process) during Type I X-ray bursts. $^{58}$Zn is located in the vicinity of doubly-magic $^{56}$Ni, which is a waiting point for further processing in the rp process. The reaction $^{57}$Cu(p,$\gamma$)$^{58}$Zn determines the effective lifetime of $^{56}$Ni since the electron-capture lifetime of $^{56}$Ni is larger than 1000~s and $^{56}$Ni is in (p,$\gamma$)~-~($\gamma$,p) equilibrium with $^{57}$Cu at typical rp-process temperatures. Proton capture on $^{57}$Cu is the only open break-out reaction channel within typical burst timescales. So far, the $^{57}$Cu(p,$\gamma$) rate has large uncertainties due to the unknown level structure of $^{58}$Zn. This presentation will focus on the details of the study and present the extracted level scheme of $^{58}$Zn. Moreover, the astrophysical implications will be discussed.   

Search for the isovector monopole resonance via the $^{28}$Si($^{10}$Be,$^{10}$B+$\gamma$)$^{28}$Al reaction

The isovector giant monopole resonance (IVGMR) is a fundamental mode of collective oscillation in which the neutron and proton fluids in a nucleus radially expand and contract in an out-of-phase manner. Observation of the IVGMR has been difficult due to the lack of a probe that will excite only its non-spin-flip ($\Delta S=0$) transitions. The IVGMR's spin-transfer ($\Delta S=1$) counterpart, the isovector spin giant monopole resonance, is much more strongly excited at bombarding energies higher than 60 MeV/$u$. By way of the ($^{10}$Be,$^{10}$B+$\gamma$) charge-exchange reaction, the selectivity for the excitation of the IVGMR can be gained. In this probe, the superallowed Fermi transition $^{10}$Be($0^{+}$,g.s.)$\rightarrow$$^{10}$B($0_{1}^{+}$,1.74 MeV,$T=1$) allows a nearly pure isolation of the $\Delta S=0$ component by detecting the 1022 keV gamma rays from the deexcitation of the $^{10}$B. We measured the double differential cross sections for the $^{28}$Si($^{10}$Be,$^{10}$B+$\gamma$) reaction at 100 MeV/$u$ using the large acceptance S800 Spectrometer at the National Superconducting Cyclotron Laboratory with the GRETINA array detecting the gamma rays emitted from the $^{10}$B ejectile. In this presentation, we will report preliminary reults of the IVGMR in $^{28}$Al.   

Measurement of the Gamow-Teller strength distributions via the (t,$^{3}$He$\gamma$) reactions on $^{45}$Sc and $^{46}$Ti

Electron captures (EC) of \textit{pf}-shell nuclei play an important role in pre-supernova stellar evolution and crustal heating of neutron stars. Astrophysical models show clear sensitivity to the details of the Gamow-Teller (GT) strength distributions, which have been extensively studied by means of charge-exchange reactions. In the present work, we measured the GT strength distributions in some of the lightest \textit{pf}-shell nuclei, ${}^{45}\mathrm{Ca}$ and ${}^{46}\mathrm{Sc}$, via the $(t,{}^{3}\mathrm{He}\gamma)$ reaction on stable ${}^{45}\mathrm{Sc}$ and ${}^{46}\mathrm{Ti}$ stationary targets at $E _t = 115 \, \mathrm{MeV}/\mathrm{nucleon}$ using the GRETINA array and the S800 spectrometer at the NSCL. Coincidence measurement with deexcitation $\gamma$ rays from the residual nuclei allowed us to study the detailed structure of low-lying GT strength, which is of particular importance for astrophysical applications. In this presentation, we will report preliminary results of the experiment and compare them to theoretical calculations.   

Differential recoil-distance lifetime measurement of the 2$_{1}^{+}$ state in $^{74}$Rb

The structure of $^{74}$Rb has received much attention because of its unique location in the nuclear chart. It lies in a region of rapid increase of E2 strength along the N$=$Z line between $^{72}$Kr and $^{76}$Sr, and coexistence between prolate and oblate shapes is known to be present in neighboring $^{74}$Kr. These observations make the understanding of collectivity in $^{74}$Rb highly desirable, but in contrast to neighboring nuclei, little is known about the nature of $^{74}$Rb beyond its level scheme. To address this problem, we have performed an experiment to measure the lifetime of the 2$_{1}^{+}$ state in $^{74}$Rb using a previously undemonstrated differential plunger technique. This was accomplished using the new NSCL TRIPLEX plunger in combination with GRETINA, taking advantage of the latter system's capabilities to achieve sufficient resolution for the new technique. Results will be presented and implications for the structure of $^{74}$Rb will be discussed.   

Spectroscopy of Mirror Nuclei in the Upper-fp Shell Using GRETINA

Isospin symmetry breaking in nuclei of the upper-fp shell has been investigated in a recent experiment at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). A $^{78}$Kr primary beam (with energy 150 MeV.A) was fragmented on a Be target to produce a cocktail of secondary beams, including $^{66}$As, $^{65}$Ge, and $^{64}$Ga, which were selected with the A1900 separator. These secondary beams were in turn fragmented at the target position of S800 magnetic spectrograph, which was used to identify neutron-deficient reaction products including $^{62,63}$Ga, $^{63}$Ge, and $^{65}$As. Gamma-rays emitted from excited states in these nuclei were detected with GRETINA, which comprises 28 highly-segmented, tapered hexagonal, close-packed HPGe and is a first generation gamma-ray tracking array. Many new states and transitions have been identified in these nuclei of the upper-fp shell, including a candidate for the T=1 2$^+$ state in $^{62}$Ga. Preliminary results from the analysis of this data will be presented and the implications for our understanding of isospin--symmetry breaking effects will be discussed.