Development of the helium-jet fed on-line mass separator RAMA and its application to studies of T/sub z/ = -2 nuclei

Abstract

The study of nuclei far from beta stability is hampered greatly when the nuclide of interest decays in a manner identical to that of a nuclide produced in greater yield in the same bombardment. One solution to this problem is through the use of an on-line mass separator. Development of the Berkelely helium-jet fed on-line mass separator RAMA is discussed as applied to studies of the A = 4n, T/sub z/ = -2 nuclides. RAMA (Recoil Atom Mass Analyzer) has typical efficiencies of 0.1% for approx. 75 elements with melting points less than or equal to 2000/sup 0/C. This efficiency permits decay studies to be readily performed on nuclei with production cross sections greater than or equal to 500 ..mu..b for ..gamma..-ray spectroscopy and greater than or equal to 1 ..mu..b for discrete energy charged particle spectroscopy. The mass range on the normalized RAMA focal plane is +- 10%. The quoted efficiency is for a mass resolution of M/..delta..M approx. 300. RAMA has been used to observe two members of the A = 4n, T/sub z/ = -2 series of beta-delayed proton emitters, /sup 20/Mg and /sup 24/Si. Observation of beta-delayed protons from a mass-separated sample of /sup 20/Mg(t/sub 1/2/ approx. 95 ms) establishes the mass-excess of the lowest T = 2 (0/sup +/) state in /sup 20/Na (13.42 +- .05 MeV), thereby completing the mass twenty isospin quintet. A similar measurement of the decay of /sup 24/Si (t/sub 1/2/ approx. 100 ms) establishes the mass-excess of the lowest T = 2 (0/sup +/) state in /sup 24/Al (5.903 +- 0.009 MeV). The mass 24 isospin quintet is incomplete because the mass of /sup 24/Si remains unknown. In both cases, excellent agreement is obtained using only the quadratic form of the isobaric multiplet mass equation (IMME). 79 references.