Orateur
Description
In the transfermium region (Z $\geq 100$), spontaneous fission (SF) is a prominent decay mode, often competing with alpha and beta decay. However, experimental measurements remain challenging for most super-heavy nuclei (SHN) due to their low production cross-section. Stability against SF is governed by the fission barrier, which is strongly influenced by shell effects [2]. Hence, studies of mendelevium isotopes, located one proton beyond the closed Z=100 shell, could provide important information on the effect of single particle orbitals on SF.
An experiment was performed at Argonne National Laboratory (ANL) to study the alpha-decay properties of the neutron-deficient $^{244,245}$Md, aiming to resolve discrepancies between two previous studies. These earlier experiments, carried out at the TASCA separator of GSI [3] and at the BGS separator of LBNL [4], measured the isotopes using fusion-evaporation reactions. However, they reported two different assignments for the alpha-decaying states of $^{244}$Md and $^{245}$Md.
In the ANL experiment, neutron-deficient Md isotopes were produced via the fusion-evaporation channels of the reaction $^{40}$Ar+$^{209}$Bi, and separated using the Fragment Mass Analyser (FMA) of the ATLAS facility [1]. The M/Q separation performed at the FMA, combined with the focal plane detection system, enables decay spectroscopy studies of exotic nuclei even at low production rates.
The results of alpha decay analysis have been reported elsewhere [5]. However, in addition to these, multiple SF events were observed tens of microseconds after recoil implantation. SF events were also observed in [3] during measurements of $^{244}$Md, but a definitive isotopic assignment was not made for these events. The present work will focus on the analysis of the fission events observed at ANL and on the investigation of their possible origin.
[1] C. N. Davis and J. D. Larson, NIM-B 40/41, 1224-1228 (1989)
[2] J. Khuyagbaatar, Eur. Phys. J. A 55, 134 (2019)
[3] J. Khuyagbaatar et al., Phys. Rev. Let. 125, 142504 (2020)
[4] J. L. Pore et al., Phys. Rev. Let. 124, 252502 (2020)
[5] S. Kumar et al., Acta Phys. Pol. B Proc. Suppl. 19, 1-A24 (2026)