Search for Electron-Capture Delayed Fission in the New Isotope Md244

Abstract

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 þ0.19 −0.09 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 þ0.6 −0.3 ms and 5 þ3 −2 ms were observed. The 0.9 þ0.6 −0.3 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 þ3 −2 ms could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.

Some Remarks on the Discovery of Md244

“…The authors reported two α energies of E α ¼ 8640 AE 20, 8680 AE 20 keV, and a half-life of T 1=2 ¼ 0.35 þ0.23 −0.18 s and also a spontaneous fission activity of T 1=2 ¼ 0.90 þ0.23 −0.16 ms. This fission activity with T 1=2 ¼ 0.9 þ0.6 −0.3 ms was also observed by Khuyagbaatar et al [2]. The fission activity was attributed to the ground state decay of 245 Md, and the α activity to an isomeric state 245m Md [3].…”

“…Evidently the chains P3, P4, and P6 agree with the data reported for 245 Md in Refs. [2,3]. The energy of the daughter in P5 is higher than the values reported for 241 Es in Ref.…”

“…This event was attributed to 245 Md in Ref. [2] [6], with 245 Fm being the product of the p3n channel. The cross-section ratio σðp3nÞ=σð4nÞ ≈ 0.25 may appear unusually high, but it has to be considered that one approaches the proton drip line, and proton binding energies are already low.…”

“…The orange hatched area marks the range of α energies where the events attributed to 244 Md in Ref. [2] were observed. [3]; these data are taken from the experiment analysis logbook [5].…”

“…4 −0.1 s (4 events). In a publication by Khuyagbaatar et al, identification of 244 Md was reported using the reaction 197 Auð 50 Ti; 3nÞ 244 Md [2]. The experiment was performed at two bombarding energies of 239.8 and 231.5 MeV (center of target), corresponding to excitation energies of E Ã ¼ 32.7 and E Ã ¼ 26.2 MeV.…”