Towards the limits of stability – new decay study of the lightest mendeleviums

 

 

The exploration of neutron-deficient isotopes in the vicinity of the Z = 100 deformed shell gap, offers valuable insight into the nuclear structure and the boundaries of stability for nuclei with extreme neutron-to-proton ratios. To investigate the limits of stability and also the effects of the single-particle states on the decay modes of these nuclei, the neutron-deficient isotopes of mendelevium (244,245Md) were the subject of study in two recent experiments at GSI[1] and Lawrence Berkeley National Laboratory (LBNL)[2,3]. The results of the two experiments initiated a debate[4] on the mass assignment of these observed isotopes of mendelevium.

The α-decay energies of the reported 244Md events in the experiment at Berkeley were assigned to the neighboring isotope 245Md in a contemporaneous as well as an earlier experiment at GSI[5]. To verify the published results from the recent Md experiments at LBNL and GSI, a new  experiment was conducted in May-June, 2024 at the Fragment Mass Analyzer (FMA)[6] located at the Argonne Tandem Linear Accelerator System (ATLAS) facility of Argonne National Laboratory (ANL). In this experiment, instead of the two-step procedure applied at Berkeley[2,3], the mass (A) and α-decay energies (Eα) of the evaporation residues (ERs) were measured simultaneously. This was achieved using the mass-separation capability of FMA in conjunction with the focal plane decay station consisting of silicon detectors arranged in a box configuration surrounded by five germanium clover detectors.

 

The focal-plane detector system of the FMA was tested and calibrated using the reaction 174Yb(40Ar,4-5n)210,209Ra, providing both mass calibration and verification of the detector electronics. With the beam energy tuned to 185 MeV, targeting the 2n evaporation channel of the 209Bi(40Ar,2n)247Md reaction, we observed 11 correlated recoil–alpha events consistent with 247Md. Subsequently, the beam was re-tuned to 210 MeV to populate the 4n channel, to produce 245Md. In the analysis we identified α-decay events consistent with the expected decay properties of 245Md[1]. The detailed experimental results and decay spectroscopy data will be presented in this contribution.

[1] J. Khuyagbaatar et al., Phys. Rev. Lett. 125, 142504 (2020)
[2] J. L. Pore et al., Phys. Rev. Lett. 124, 252502 (2020)
[3] J. M. Gates and J. L. Pore, Eur. Phys. J. A 58, 1 (2022)
[4] F. P. Heßberger et al., Phys. Rev. Lett. 126, 182501 (2021)
[5] V. Ninov et al., Zeitschrift für Physik A Hadrons and Nuclei 356, 11 (1996)
[6] C. N. Davids et al., Nucl. Instr. and Meth. B 70 (1992)