Probing the nuclear structure of fermium by laser spectroscopy
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Abstract
The nuclear structure of heavy nuclei with proton numbers Z>100 has been subject to various experimental and theoretical studies aimed at uncovering their fundamental properties. The mere existence of such heavy elements is based on the nuclear stabilization induced by shell effects, which counteract spontaneous fission due to the strong Coulomb repulsion, and determine the limits of nuclear existence. Moreover, shell effects can influence various nuclear properties, including the shape and size of heavy nuclei. Laser spectroscopy serves as a powerful tool to unveil such phenomena. By probing the atomic structure and the hyperfine sub-structure, various nuclear ground-state properties can be accessed, as the nuclear mean-square charge radius, spin and nuclear moments.
The available insight on nuclear ground-state properties around the N=152 shell gap, in the region of the deformed heavy actinides, remains scarce. However, the limited information on the atomic level structures, which are strongly impacted by relativistic effects, and the low production yields and short half-lives hamper experimental studies with common laser spectroscopy methods. In addition, model predictions of atomic and nuclear properties to guide experimental studies are challenged by the complex many-body problem and the dense level scheme structures.
The RADRIS method, tailored to laser spectroscopy investigations of the heaviest actinide elements, was employed to access the element fermium (Z=100) for the studies presented in this work. With recent developments of indirect production schemes, exotic fermium isotopes were accessed via the decay of directly produced nobelium mother isotopes from fusion-evaporation reactions.
The technical advancements made within this work towards the study of longer-lived and shorter-lived isotopes, in combination with an enhancement in the overall efficiency, widened the scope of RADRIS to more previously inaccessible fermium isotopes. With these new developments, on-line laser spectroscopy studies in fermium were performed down to minute production rates for some isotopes. In addition, studies of longer-lived isotopes in up to pg-sample sizes from reactor-breeding were performed via highly sensitive hot-cavity laser spectroscopy at the RISIKO separator.
In summary, a chain of eight isotopes ranging from accelerator-produced 245Fm to the reactor-bred 257Fm was investigated by utilizing various production schemes and applying different on-line and off-line advanced laser spectroscopy techniques. Isotope shifts of an atomic transition were measured to extract changes in the nuclear mean-square charge radius across the N=152 shell gap. The data was compared to predictions of various nuclear models relying on energy density functionals. A good agreement was found between the smooth trend in the experimental results and the calculations, and between the models themselves. This implies that bulk properties have a prevailing influence, while the impact of shell effects on the nuclear charge radius observable appears to diminish. The findings made in this work will help to refine nuclear models, allowing for more accurate predictions within the range of the heavy and eventually super heavy elements. The achieved methodological improvements opened new avenues for experimental studies of more exotic isotopes in the region of the heaviest actinide elements. This, in turn, may provide initial insights into the atomic structure, particularly where information about atomic levels is currently unavailable, thus allowing for further nuclear structure investigations to enhance our understanding of these exotic nuclei.