Special Astrophysics Seminar

Direct mass measurements of trans-fermium nuclei with MRTOF-MS: Toward identification of superheavy elements via mass spectroscopy

Yuta Ito

Riken Research Institute

Precision mass measurements of unstable nuclei, providing direct measure of the nuclear binding energy, are invaluable for nuclear structure study and have potential for particle identification of atomic nuclide by the precision mass value.

For trans-fermium nuclei, of importance for understanding the shell evolution in heavy nuclear system to inspect mass models toward so-called "island of stability" proposed by G.T. Seaborg in 1960s and the unique identification during new elements search, the mass measurements require fast measurement time even for such a heavy mass nuclei and high efficiency to tolerate extremely low production yields.

Direct mass measurements of trans-fermium nuclei were, so far, performed for only 6 nuclei of nobelium and lawrencium with the Penning trap mass spectrometer SHIPTRAP.

Recently we implemented a multi-reflection time-of-flight mass spectrograph (MRTOF-MS) located after a cryogenic helium gas catcher and a multiple ion trap system coupled with the gas-filled recoil ion separator GARIS-II and performed direct mass measurements of wide range of actinide isotopes and mendelevium isotopes for the first time.

Using 48Ca beam on natTl target, we produced 249-251Md by fusion-evaporation reaction and successfully measured those masses including new masses of 249-250 Md with sub-ppm mass precision. They were extracted as doubly charged atomic ions from the gas cell as well as other actinides such as nobelium and fermium. Combined with known alpha decay Q-value of 249-250Md, we could newly determine masses of isotopes on the decay chain from berkelium to meitnerium.