Measurement of astrophysically important excitation energies of 58Zn with GRETINA

The level structure of neutron-deficient 58Zn has been extracted experimentally. This nucleus is important for the rapid proton-capture process. 58Zn was produced by using a (d,n)-type transfer reaction on 57Cu in inverse kinematics at beam energies of 75 MeV/u. Several γ-ray transitions have been identified. The experiment utilized the state-of-the-art GRETINA γ-ray energy tracking array in conjunction with the largeacceptance spectrometer S800 at NSCL. The excitation energies of the identified lowlying states in 58Zn are important for constraining the 57Cu(p,γ)58Zn reaction rate under X-ray burst conditions.


Introduction
Astrophysical Type I X-ray bursts take place in low-mass stellar binary systems consisting of a compact, dense neutron star, and a low-mass star. H/He-rich matter is accreted from the companion star onto the neutron star. The temperature and density of the accreted matter on the surface of the neutron star rises and eventually, at temperatures around T 9 = 0.2 (T 9 is measured in units of GigaKelvin), a thermonuclear runaway is ignited that powers the X-ray burst. Energy generation is mainly driven by a e-mail: langer@nscl.msu.edu fast proton and α-particle capture reactions in the so-called rapid proton-capture process (rp-process), whereas the much slower β + -decays determine the overall timescale of the process (10-100 s) and the final composition of the nuclear ashes, see e.g. [1][2][3].
The rp-process may extend up to Te under some circumstances. Along the reaction path, (p,γ) reaction rates on neutron-deficient nuclei need to be known to model the rp-process. At certain nuclei near the proton dripline, the so-called waiting points, the reaction flow that normally proceeds via rapid (p,γ) reactions is hampered due to the nuclear structure by photo disintegration. An important waiting point nucleus in the rp-process is the doubly-magic 56 Ni (N = Z = 28). The Q-value for proton capture is relatively low (Q = 0.695 MeV) and the electron-capture lifetime is approximately 22000 s under stellar conditions, thus exceeding typical burst times. This leads to the formation of a (p,γ)-(γ,p) equilibrium between 56 Ni and 57 Cu. Under these conditions, the reaction flow to heavier nuclei beyond 56 Ni depends on the 57 Cu(p,γ) reaction rate (see Fig. 1). So far, the reaction rate is almost entirely constrained by theory and it is dominated by the uncertainties in the excitation energies of low-lying states in 58 Zn. In Ref. [4], shell model calculations are used to obtain the astrophysical important part of the level structure of 58 Zn. The authors of Ref. [5] used updated shell-model calculations of the important states in 58 Zn to reevaluate the previously derived reaction rate. However, typical uncertainties of a few 100 keV in the calculated excitation energies for excited states in 58 Zn result in an uncertainty in the reaction rate of up to several orders of magnitude.

Experimental procedure
The experiment presented in this work aimed at measuring the excitation energies of low-lying states in 58 Zn with high accuracy in order to calculate an updated reaction rate for proton capture on 57 Cu. The measurement was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. Low-lying γ-decaying states in 58 Zn above the proton separation threshold (S p = 2279(50) keV [6]) were populated via the d( 57 Cu, 58 Zn)n proton transfer reaction. The radioactive 57 Cu beam was produced at the NSCL Coupled Cyclotron Facility by accelerating a stable 58 Ni beam to 160 MeV/u and impinging on a 752 mg/cm 2 9 Be target at the entrance of the A1900 fragment separator. Nucleon exchange and pickup reactions were used to produce radioactive 57 Cu (see similar experimental technique in [7]). A 300 mg/cm 2 Al wedge was placed in the intermediate plane of the A1900 for further purification of the 57 Cu beam, which was subsequently transported to the experimental setup, where the purity was around 20 %.
To induce the (d,n) proton transfer reaction the 57 Cu beam was directed onto a 225 mg/cm 2 CD 2 target. The target was placed in the center of the GRETINA array [8] which was used to detect inflight γ-rays emitted from the deexcitation of the populated states in 58 Zn. The 57 Cu beam energy was E beam ≈ 75 MeV/u at the secondary reaction target. The emerging neutron, produced in the d( 57 Cu, 58 Zn)n proton transfer reaction, was not observed. The 58 Zn recoil was identified in the focal plane of the S800 spectrometer by means of time-of-flight and energy loss. Angle and position of 58 Zn ions were also measured to reconstruct the kinematical properties of each event and to perform high-precision Doppler-correction of the γ-ray energies being emitted in the lab system.

Preliminary results
In this section, first preliminary results are presented. The left part of Fig. 2 shows the clear identification of 58 Zn events in the S800 focal plane, based on a time-of-flight measurement (between a fast plastic scintillator placed upstream of the target and the triggering detector of the same material in the focal plane) and an energy-loss measurement in an ion chamber (also situated in the focal plane). Calibration of the identification plot was achieved by assigning observed γ-ray transitions of isotopes in the vicinity of 58 Zn to the corresponding nuclei. As can be seen in Fig. 2 different other isotopes have also been identified.
Using the unique identification of the reaction products, a two-dimensional gate was applied to select the 58 Zn ions. Gamma-rays from 58 Zn can be identified through coincidence with identified 58 Zn ions in the S800 focal plane. The GRETINA γ-ray efficiency was around 5% for a γ-ray energy of 1 MeV. A γ-ray energy resolution of around 2% (FWHM) was obtained, mainly limited by the energy spread in the thick reaction target. Different peaks can be clearly identified (right part of Fig. 2). The strongest peak has an energy of E γ = 1356(5) keV and is assigned to belong to the decay of the first excited 2 + 1 state to the ground state in 58 Zn. In the mirror nucleus 58 Ni, the first 2 + 1 has an energy of 1454 keV, thus giving a shift of ∼100 keV. The shell-model calculation in [5] places this state at an energy of 1400 keV. Other states in 58 Zn will be identified using γ − γ coincidences and guidance from the known level structure in the stable and experimentally well-known isospin mirror nucleus 58 Ni. The astrophysical reaction rate for the 57 Cu(p,γ) reaction is mainly determined by the low-lying 2 + states in 58 Zn [5]. Since the theoretical predictions of the level energies exhibit large uncertainties, a tremendous reduction of uncertainty in the reaction rate is expected when using the experimentally extracted excitation energies with small errors.

Summary
The experimental technique and preliminary results of an experiment aiming at measuring the level structure of neutron-deficient 58 Zn have been presented. Several γ-ray transitions have been identified and a γ − γ coincidence analysis along with structure information from the isospin mirror partner 58 Ni is being performed to reconstruct the level structure. The experiment employed a new experimental approach by using a proton transfer reaction at relatively high beam energies combined with an angleintegrated measurement of the reaction products and the GRETINA array to extract astrophysically important excitation energies of low-lying states in 58 Zn.