Discovery of the α-emitting isotope Pa.

Synthesizing isotopes located far away from the line of β-stability is the core research topic in nuclear physics. However, it remains a challenge due to their tiny production cross sections and short half-lives. Here, we report on the observation of a very neutron-deficient isotope Pa produced via the fusion-evaporation reaction Lu(Ca, 5n)Pa at a newly constructed China Accelerator Facility for Superheavy Elements. The measured α-particle energy of E = 8284(15) keV and half-life of ms of Pa allow us to extend the α-decay systematics and test the predictive power of theoretical models for heavy nuclei near the proton drip line. Based on its unhindered α-decay character, the spin and parity of Pa is proposed to be (3), supported by the large-scale shell model and cranked shell model calculations. This isotope is discovered with substantial statics within  ∼ 3 days using intensive 2 pμA beam, demonstrating the tremendous capability of the facility for the study of heavy and superheavy nuclei.