Speaker
Description
In a series of experiments conducted over the past few years, we have explored questions of collectivity and nuclear shape in iron isotopes on the neutron-rich side of the valley of stability.
A Coulomb excitation experiment at the ALTO facility aimed to determine a large set of E2 matrix elements to address two key questions: Why is the measured B(E2; 4+1 → 2+1 ) value significantly higher than predicted by otherwise reliable calculations (e.g., [1])? What is the "shape" of 58Fe? Our motivation was also partly driven by the observation in Figure 1 that the second 2+ state lies below the first 4+ state in 58Fe, hinting at possible triaxial deformation. In the experiment a beam of 58Fe was acceleratedto 220 MeV and directed at a 2 mg/cm2 self-supporting 208Pb target. Recoiling nuclei were detected at backward angles in the laboratory frame using the HIL-Warsaw DSSSD detector, in coincidence with γ rays measured by the NuBall2 γ-ray spectrometer. The extracted γ-ray intensities are currently being analyzed using the GOSIA code.
Carpenter et al. [2] proposed a scenario for shape coexistence in iron isotopes, based on the moments of inertia of yrast bands and the identification of two similar structures—one of which decreases in energy
with increasing neutron number. In Figure 1 it is shown how the energy of the 0+2 state is decreasing in energy with the neutron number. This suggests that an excited 0+ state in 60Fe may be the deformed state that becomes the ground state in the second island of inversion. To explore this hypothesis, we performed an experiment at INFN Legnaro to measure the lifetimes of excited 0+ states in 60Fe. An 18O beam, accelerated to 40 MeV, bombarded a 58Fe target mounted in a plunger device. Recoiling 60Fe ions were stopped in a gold foil, while 16O ions were detected using the Spider silicon detector in coincidence with γ rays recorded by AGATA.
Figure 1: Energy-level systematics in iron isotopes showing the lowering of the 02 state. Also visbile is the low 2+2 in 58Fe, giving a hint of triaxial deformation.
In this presentation, we will discuss the first results of these experiments and provide an update on
the status of the analysis.
References
[1] M. Klintefjord et al, Measurement of lifetimes in 62,64Fe,61,63 Co, and 59Mn, Phys. Rev. C 95 (2017)
024312. doi:10.1103/PhysRevC.95.024312.
URL http://link.aps.org/doi/10.1103/PhysRevC.95.024312
[2] M. P. Carpenter, R. V. F. Janssens and S. Zhu, Shape coexistence in neutron-rich nuclei near n = 40,
Phys. Rev. C 87 (2013) 041305. doi:10.1103/PhysRevC.87.041305.
URL http://link.aps.org/doi/10.1103/PhysRevC.87.041305