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

• Received 22 May 2020
• Revised 10 July 2020
• Accepted 28 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.142504