Reinvestigation of the Direct Two-proton Decay of the Long-lived Isomer 94Agm [0.4 s, 6.7 MeV, (21+)] Page: 1 of 10
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Reinvestigation of the Direct Two-proton Decay of the Long-lived Isomer
94Ag' [0.4 s, 6.7 MeV, (21+)]
J. Cerny, 12 D.M. Moltz,1 D.W. Lee,2 K. Perajarvi,3 B.R. Barquest,' L.E. Grossman,' W.
Jeong,' and C.C. Jewett'
Department of Chemistry, University of California, Berkeley, California 94720
2Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California
3STUK-Radiation and Nuclear Safety Authority, P.O.Box 14, Helsinki, FIN-00881,
Abstract: An attempt to confirm the reported direct one-proton and two-proton decays of
the (21+) isomer at 6.7(5) MeV in 94Ag has been made. The 0.39(4) s half-life of the
isomer permitted use of a helium-jet system to transport reaction products from the 40Ca
+ naNi reaction at 197 MeV to a low-background area; 24 gas AE-(Si) E detector
telescopes were used to identify emitted protons down to 0.4 MeV. No evidence was
obtained for two-proton radioactivity with a summed energy of 1.9(1) MeV and a
branching ratio of 0.5(3)%. Two groups of one-proton radioactivity from this isomer had
also been reported; our data confirm the lower energy group at 0.79(3) MeV with its
branching ratio of 1.9(5)%.
PACS numbers: 23.50.+z, 21.10.-k, 21.60.Cs, 27.60.+j
Sometimes proton/neutron coupling preferences near doubly closed nuclear shells cause
nuclear states to be caught in "spin-traps" - isomeric states of high angular momentum
in which rapid gamma ray decay is forbidden - so that other, slower decay processes such
as beta-decay or direct low-energy proton emission can then compete. In fact, proton
radioactivity  was discovered in 1970 as a 1.5% branch in the dominant beta-decay of
a spin-trap isomer in 53Co (0.25s, 19/2-). In a series of experiments by the online mass
separator group at GSI in Darmstadt [2-9], the odd Z = odd N nuclide 94Ag has been
shown to have such a spin-trap - a long-lived (21+) state at 6.7(5) MeV excitation with a
half-life of 0.39(4) s which decays primarily by beta-decay and by beta-delayed proton
emission. In the more recent publications, Mukha and collaborators have reported
additional decay modes for this 94Ag isomer: direct proton emission in 2005  and -
most surprisingly - direct two-proton emission in 2006 . Observing both of these
latter types of decay from the same nuclear state is unique. Further, the quantum-
mechanical conditions necessary to explain this two-proton decay from an odd -Z parent
state require a highly deformed prolate--(cigar)-shaped isomer; the two protons must
then be emitted simultaneously with a relatively large angular momentum "either from
the same or from opposite ends of the 'cigar.'" Though follow-up measurements
related to a more accurate excitation energy of the 94Ag isomer and to the gamma decay
scheme in the 92Rh daughter have been reported (and will be discussed later), no
1 Though single-proton decay from an odd Z nuclide can be expected under suitable
conditions, two-proton decay is expected to occur from even Z, very proton-rich nuclides
(and as such has been recently observed in 45Fe, e.g., see ).
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Cerny, J.; Moltz, D. M.; Lee, D. W.; Perajarvi, K.; Barquest, B. R.; Grossman, L. E. et al. Reinvestigation of the Direct Two-proton Decay of the Long-lived Isomer 94Agm [0.4 s, 6.7 MeV, (21+)], article, March 5, 2009; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc834418/m1/1/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.