Direct mass measurements of A=80 isobars

Direct mass measurements of A=80 isobars were performed using the second cyclotron of SARA (Système Accélérateur Rh?ne-Alpes) of the ISN (Institut des Sciences Nucléaires) in Grenoble as a time-of-flight mass spectrometer. Using the 300 MeV ⁵⁸Ni +²⁷Al reaction, the isobars ⁸⁰Rb, ⁸⁰Sr, ⁸⁰Y, and ⁸⁰Zr (only one event!) were measured. For ⁸⁰Y we found a mass value of 79.934320(180) u, corresponding to a decay energy Q (⁸⁰Y →⁸⁰Sr) of (9120 ± 170) keV. Comparing this value to the former experiments of Lister et al. and Della Negra et al. results in a difference of about 2.2 MeV. However, it does agree with the recommended value of Audi and Wapstra, which is derived from systematic trends. For ⁸⁰Zr the single event was identified with a probability of 0.999975 and a mass value of 79.940400(1600) u could be deduced from our experiment. Received: 22 Juli 1997 / Revised version: 3 February 1998