High precision γ spectroscopy of 69,71Zn from (n, γ) reactions using EXILL

P. Bączyk1,a, M. Czerwiński1, A. Korgul1, T. Rząca-Urban1, W. Urban1, A. Blanc2, M. Jentschel2, P. Mutti2, U. Köster2, T. Soldner2, G. de France3, G. Simpson4, and C.A. Ur5 1Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warszawa, Poland 2Institut Laue-Langevin, 6, rue Jules Horowitz, 38042 Grenoble Cedex 9, France 3GANIL, Bd. Becquerel, BP 55027, 14076 CAEN Cedex 05, France 4School of Engineering, University of the West of Scotland, Paisley PA1 2BE, Scotland 5INFN Sezione di Padova, Via F. Marzolo 8, 35131 Padova, Italy


Introduction
The neutron-rich nuclei in the vicinity of Z = 28 and N = 40, 50 shell closures are important playground for probing nuclear models.The systematic behaviour of the energies of the 2 + excited states in this region is presented in Fig. 1.An interesting regularity, called isotonic symmetry or valence proton symmetry [1], is seen as the 2 + energies of certain pairs of isotones have similar values.These are: Se and Fe (two protons/holes in the f orbital), Kr and Cr (four protons/holes in the f orbital), Ge and Zn isotones (protons in the p 3/2 orbital).It is also seen that the occupation of the p 3/2 orbital is already sufficient to eradicate the N=40 shell closure, which is present only in Ni isotopes.Furthermore, in a recent study [2] an onset of deformation has been found at N > 36 in Fe isotopes, corresponding to two proton holes in the Z = 28 shell.
The Shell Model is able to describe nuclear excitations not only in closed-shell nuclei and their immediate neighbours, but also in nuclei with larger number of valence nucleons, reproducing also collective effects there [3][4][5][6].The reliability of such studies critically depends on a precise information concerning the structure of nuclei of interest, like the identification of all low-lying excited levels, their spins, parities and decay properties.It is well established that the neutron capture reactions serve as an excellent tool for such type of "complete" spectroscopy.With the development of very efficient arrays of γ spectrometers, the measurements of γ radiation following the neutron capture reactions offer now new, rich and complete information on nuclear structure [7].
Our recent measurements of γ radiation following slow-neutron capture on 68 Zn and 70 Zn nuclei, performed a e-mail: pawel.baczyk@fuw.edu.pl with the very efficient germanium array EXILL at ILL Grenoble provided rich, new information on 69 Zn and 71 Zn isotopes (for the latter nucleus it was the first measurement with slow neutrons).One can study here interesting effects like the onset of collectivity in the vicinity of N = 40 shell closure or the population of high-spin isomers.Starting from a capture level with much lower spin, they are fed through the so-called intermediate (doorway, gateway) states, which have important astrophysical and practical applications (medical radioisotopes, pumping of γ lasers).

Results
The first step of the data analysis was to calibrate the EX-ILL array.A very precise energy calibration was done basing on 28 Al lines.A single, second order polynomial was fitted to the data points in the entire 0-9 MeV range.The accuracy is of the order of 20 eV and the systematic error is about 15 eV (this estimate is based on the comparison of calculated and tabulated value of neutron binding energy for 28 Al).Furthermore, the efficiency calibration in a range from 30 to 8000 keV was performed.Using these precise calibrations and high statistics coincidence data (10 10 triggerless single (γ, time) events, 10 9 γγ coincidence events and 2•10 8 γγγ coincidences) we have obtained significantly extended and more precise information on 69 Zn and 71 Zn nuclei.A partial level scheme of 69 Zn is shown in Fig. 2. Angular correlations and polarization measurements resulted in unique spin and parity assignments.The 872.5-keV level has spin and parity 5/2 + due to observation of the 5609.7-keVfeeding -primary transition.It decays solely to the 9/2 + isomer and is fed from the 1178.5 In our spectra we could identify at least 90 primary transitions in 69 Zn (to be compared with 13 primary transitions reported in the literature).Using such cascades, and the precise energy calibration we have determined neutron binding energies of 68,69,71 Zn isotopes observed in our data with much higher accuracy than reported before, as shown in Table 1.
With these accurate S n values, excitation energies of the 9/2 + isomers in 69 Zn and 71 Zn were determined very precisely, as shown in Table 2.The energy of 438.62 (5) keV for 69 Zn agrees well with the literature value of 438.638(18) keV, justifying this method.

DOI: 10
.1051/ C Owned by the authors, published by EDP Sciences, 2015

Figure 3 .
Figure 3. Systematics of 5/2 + states in odd-A Zn isotopes.This work has been supported by the Polish National Science Centre under the contract DEC-2013/09/B/ST2/03485.The authors thank services of the ILL, LPSC and GANIL for supporting the EXILL campaign.The EXOGAM collaboration and the INFN Legnaro are acknowledged for the loan of Ge detectors.
-keV, newly observed level, showing features of a doorway state for the isomer.Figure 2. A fragment of 69 Zn level scheme.

Table 1 .
Comparison of neutron separation energies S n reported in this work with literature values [8].

Table 2 .
Comparison of energy of 9/2 + isomeric state reported in this work with literature values.Energies are given in keV.+ states in odd-A Zn isotopes, compared in Fig.3to 2 + energies in the respective Zn cores suggest collective character of the 5/2 + excitations, built on top of the 9/2 + isomers.The halflife of the 5/2 + level in71Zn was measured in this work to be about 40 ns.This preliminary value translates to B(E2) of about 19 W.u., which is similar to the collectivity observed in the core68,70Zn nuclei.