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Decay properties of 68,69,70Mn: Probing collectivity up to N=44 in Fe isotopic chain

Abstract : The β decays View the MathML sourceMn68→Fe68, View the MathML sourceMn69→Fe69 and View the MathML sourceMn70→Fe70 have been measured at the RIBF facility at RIKEN using the EURICA γ spectrometer combined with an active stopper consisting of a stack of Si detectors. The nuclei were produced as fission fragments from a beam of 238U at a bombarding energy of 345 MeV/nucleon impinging on a Be target and selected using the BigRIPS separator. Half-lives and β-delayed neutron emission probabilities have been extracted for these decays, together with first experimental information on excited states populated in 69,70Fe. The data indicate a continuously increasing deformation for Fe isotopes up to A=70A=70. This is interpreted, as for Cr isotopes, in terms of the interplay between the quadrupole correlations of the ν1d5/2ν1d5/2 and ν0g9/2ν0g9/2 orbitals and the monopole component of the π0f7/2–ν0f5/2π0f7/2–ν0f5/2 interaction. Magic numbers, originating from large energy gaps in the shell structure of the single-particle states, constitute one of the fundamental features governing nuclear structure. The access to new exotic species, where the number of protons (Z) and neutrons (N) is highly asymmetric, showed the onset of new magic numbers and an evolution of shell gaps driven by the enhanced role of pairing interactions and of tensor terms [1] and [2]. These interesting findings, obtained using radioactive beams and confirmed by the many developments in shell model calculations, are based on a very extensive experimental and theoretical work. Presently they are motivating additional experimental and theoretical investigations addressing these important issues on shell structure in nuclei further away from stability. Indeed, the evolution of shell structure depends strongly on the occupations of proton and neutron orbitals near the Fermi surface and on their mutual interactions, and, therefore, it can only be identified via systematic studies of nuclear properties in different isotopic and isotonic chains. In this context, one region in the chart of nuclides attracting particular attention is that around 78Ni, a key region to study the path toward the N=50N=50 shell closure and its implications on the astrophysical r-process [3], [4] and [5] for nucleosynthesis. In particular, the study of the evolution of excited states for the Cr, Fe, Zn, Ge isotopes provides a stringent test to shell model calculations leading to N=50N=50. The existence of the sub-shell closure at N=40N=40 was proposed since a relatively large gap separates the pf shell from the neutron g9/2g9/2 single-particle state. This picture was initially confirmed by the measurement of a large energy of the first 2+2+ state in 68Ni and, more recently, of its B(E2) value: the level energy is much higher than the neighbouring even–even isotopes [6], while the B(E2) is indeed the smallest in the Nickel chain [7]. Langanke and collaborators [8] argued, however, by comparing several approaches, that the small experimental B(E2) value could not be a conclusive argument in favour of a shell closure, since the missing strength lies in excited states above 4 MeV. Recent mass measurements support this picture, implying a relatively small shell gap [9]. The study of nuclei around 68Ni, in particular removing protons from the 0f7/20f7/2 orbital, confirmed the picture of the vanishing of the shell closure at N=40N=40: the observed rapid drop in the energies of the 2+2+ states in Fe (Z=26Z=26) [10], [11], [12], [13] and [14] and Cr (Z=24Z=24) [15] and [16] isotopic chains points to an increased collectivity in these nuclei, which is expected to reach its maximum at N=40N=40 and N=38N=38 in Fe and Cr, respectively [17]. It has also been suggested that the ground states of 62–68Fe are dominated by spherical configurations, at variance with the ground states of 60–64Cr, which are associated to deformed shapes [18]. A very recent intermediate Coulomb excitation experiment [19] extended the measurement of the View the MathML sourceB(E2:01+→21+) which were known only up to 66Fe [13] and [14] and gave the first result at N=40N=40 in the Cr chain. The large B(E2) value found for 68Fe suggests an increased collectivity in this nucleus as compared to the lighter isotopes. This large body of experimental works suggest that shell-model calculations have to include the neutron 1d5/21d5/2 orbital in order to reproduce the large quadrupole collectivity found in these isotopic chains. These computational-heavy calculations predict an onset of deformation for N=40N=40 in the Fe chain and at N=38N=38 in the Cr one. Chromium isotopes are expected to show maximum deformations in this region, being the proton 0f7/20f7/2 orbital half filled [17], [20] and [21]. The work presented in this letter is intended to extend the experimental knowledge of the isotopic chain of Fe in the neutron rich side by new measurements that have provided the half-lives and β-delayed neutron emission probabilities of their β-decaying parents 68,69,70Mn, and the excitation energies of the first excited states up to N=44N=44. These data are compared with the experimental systematics for Ni, Fe, Zn, Ge, and with recent shell model calculations to deduce the orbitals involved in these new excitations and collective effects, supporting the picture of an increasing deformation while adding neutrons. The experiment here discussed was performed at RIKEN as part of the EURICA campaign at the Radioactive-Isotope Beam Factory (RIBF) facility. The nuclear species were produced by means of in-flight fission of a 238U beam at a bombarding energy of 345 MeV/nucleon. The experiment collected data for an equivalent time of 3 days with an average primary beam intensity of 10 pnA. The resulting fragments were separated in the Big-RIPS separator, by the use of degraders at the intermediate dispersive foci [27]. The cocktail beam was transported through the ZeroDegree spectrometer down to the final focal plane where it was then slowed down in an Al degrader to ensure the implantation of the species of interest in the 5 silicon detectors of the WAS3ABi array [28]. The total count rate at the final focal plane was limited to 100 pps to ensure correct ion-β correlations. The Si array was surrounded by the EURICA spectrometer consisting of 12 EUROBALL HPGe cluster detectors [29]. Eighteen small volume LaBr3(Ce) scintillator detectors were also employed for fast-timing measurements [30]. The yields for the mother nuclei, after implantation, were: 6700 68Mn ions, 4300 69Mn ions and 400 70Mn ions. Once the different isotopes were identified, they were associated to their subsequent β decay by imposing spatial and temporal correlations: we requested the β event to be registered in the pixel where the implantation occurred, or in the closest neighbours, and, in addition, a time correlation window was imposed. All β events satisfying these conditions were associated with an implant. Typical β efficiency for 68,69,70Mn was estimated to be around 60%. Since the half-lives of the Mn isotopes of interest are similar to those of their Fe and Co successors [22], a fit to the Bateman equations [31], including the activity of daughter and grand-daughter nuclei, was performed for each species. Half-lives of decay successors were fixed to their literature values [22] if known. The β-delayed neutron emission branch is expected to contribute significantly to the total decay rate of the neutron-rich Mn isotopes [23]. Since β -delayed neutron emission probabilities (PnPn) are not measured in 68–70Mn, they have been deduced as free parameters in the fit. When the statistics were sufficiently high, the half-lives were cross-checked with values obtained gating on de-exciting γ-ray transitions of the daughter nuclei, by fitting the decay spectrum with an exponential decay curve. The evaluated half-lives for the discussed decays are the following: View the MathML sourceMn68→Fe68T1/2=38.3±3.6 msT1/2=38.3±3.6 ms, and T1/2=35.2±2 msT1/2=35.2±2 ms gating on γ transitions (521+865+1250+1514 keV521+865+1250+1514 keV); View the MathML sourceMn69→Fe69T1/2=24.1±2.6 msT1/2=24.1±2.6 ms, and T1/2=25.8±2.8 msT1/2=25.8±2.8 ms with gates on the following γ rays: 135, 325, 355, 521, 1207 keV. In the case of View the MathML sourceMn70→Fe70 the half-life could only be extracted from a fit of the decay curve, resulting in T1/2=19.9±1.7 msT1/2=19.9±1.7 ms. Error bars include both statistical and systematic errors, calculated from the χ-square minimization described earlier. The measured half-lives are reported in the bottom panel of Fig. 1: filled circles represent the values extracted from a fit to the β-decay spectrum, while filled triangles are obtained gating on de-exciting γ transitions in the daughter nuclei.
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G. Benzoni, A.I. Morales, H. Watanabe, S. Nishimura, L. Coraggio, et al.. Decay properties of 68,69,70Mn: Probing collectivity up to N=44 in Fe isotopic chain. Physics Letters B, Elsevier, 2015, 751, pp.107-112. ⟨10.1016/j.physletb.2015.10.025⟩. ⟨in2p3-01257762⟩



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