Alpha decay properties of new protactinium isotopes

Abstract

Evaporation residues produced in the reactions 40Ar + 181Ta and 40Ar + 184W were separated from the primary beam by the velocity filter SHIP and detected by a Δ E-E counter telescope. The technique of delayed coincidences was applied to individually identify the reaction products implanted into a Si surface barrier detector by their subsequent α-decays. The previously unknown nuclei 215Pa and 218Pa were identified by their known daughter decays. 215Pa was found to decay with E α = 8.085±0.015MeV and t 1/2 = 14± 2 3ms. For 218Pa two α-lines were found at 9.535±0.015MeV and 9.614±0.020MeV with a corresponding half-life of 120 +40 -20μs. The half-life of the 8.33 MeV α-decay of 217Pa. was determined to be 4.9 +0.6 -0.4 ms. Furthermore a new isomer was found in 217Pa which decays with E = 10.16±0.02MeV and t 1/2 = l.6 +1.0 -0.5ms. The decay of this isomer is tentatively interpreted as being hindered by an angular momentum of about 10ħ of the emitted α-particle. The ground state α-decay energies of these very proton-rich isotopes agree well with systematics but deviate considerably from theoretical mass predictions.