Evidence for264108, the heaviest known even-even isotope

Abstract

We have observed the decay of one atom of the doubly even isotope 108. We observed α-decay with a halflife, of (76 −36 +364 ) Μs. The full α-decay energy could not be measured, as the α escaped the detector. With the knowledge of the halflife, an estimate on the α-decay energy can be made. From the formula of Rasmussen9 and a reduced α-width of 1, a value which has been measured for260106 and also for the doubly even isotones of N=156, we obtain Q=(11.0 −0.3 +0.1 MeV. This value is higher than given by Moeller10 (10.35 MeV), and agrees with that of Uran and Zeldes11 (10.9 MeV). A hindrance factor wα of 0.5 increases the Qα-value by 0.14 MeV. The mass excess of264108 follows as (120.0±0.3) MeV. The groundstate shell effect amounts to (−5.0±0.3) MeV using the macroscopic mass values of Ref. 10. The formation cross section of (3.2 −2.6 +6.1 ) pb overlaps with the value of ∼ 5 pb from Dubna for the 6 ms sf-activity mentioned initially but is considerably lower than an extrapolation from the reaction58Fe (208Pb,n)265108 which gave 10 pb with the lower limit of 4 pb.