Identification of new mendelevium and einsteinium isotopes in bombardments of209Bi with40Ar

Some Remarks on the Discovery of ^{244}Md

In two recent papers by Pore et al. and Khuyagbaatar et al., discovery of the new isotope ^{244}Md was reported. The decay data, however, are conflicting. While Pore et al. report two isomeric states decaying by α emission with E_{α}(1)=8.66(2)  MeV, T_{1/2}(1)=0.4_{-0.1}^{+0.4}  s and E_{α}(2)=8.31(2)  MeV, T_{1/2}(2)≈6   s, Khuyagbaatar et al. [Phys. Rev. Lett. 125, 142504 (2020).PRLTAO0031-900710.1103/PhysRevLett.125.142504] report only a single transition with a broad energy distribution of E_{α}=(8.73-8.86)  MeV and T_{1/2}=0.30_{-0.09}^{+0.19}  s. The data published in Pore et al. are very similar to those published for ^{245m}Md [E_{α}=8.64(2), 8.68(2) MeV, T_{1/2}=0.35_{-0.16}^{+0.23}  s [V. Ninov, F. P. Heßberger, S. Hofmann, H. Folger, G. Münzenberg, P. Armbruster, A. V. Yeremin, A. G. Popeko, M. Leino, and S. Saro, Z. Phys. A 356, 11 (1996).ZPAHEX0939-792210.1007/s002180050141] ]. Therefore, we compare the data presented for ^{244}Md in Pore et al. with those reported for ^{245}Md in Ninov et al. and also in Khuyagbaatar et al. We conclude that the data presented in Pore et al. shall be attributed to ^{245}Md with small contributions (one event each) from ^{245}Fm and probably ^{246}Md.

Search for Electron-Capture Delayed Fission in the New Isotope ^{244}Md

The electron-capture decay followed by a prompt fission process was searched for in the hitherto unknown most neutron-deficient Md isotope with mass number 244. Alpha decay with α-particle energies of 8.73-8.86 MeV and with a half-life of 0.30_{-0.09}^{+0.19}  s was assigned to ^{244}Md. No fission event with a similar half-life potentially originating from spontaneous fissioning of the short-lived electron-capture decay daughter ^{244}Fm was observed, which results in an upper limit of 0.14 for the electron-capture branching of ^{244}Md. Two groups of fission events with half-lives of 0.9_{-0.3}^{+0.6}  ms and 5_{-2}^{+3}  ms were observed. The 0.9_{-0.3}^{+0.6}  ms activity was assigned to originate from the decay of ^{245}Md. The origin of eight fission events resulting in a half-life of 5_{-2}^{+3}  ms could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.