Three-phonon excitations in 124Sn

The low-lying levels of 124Sn have been explored using the (n,n′γ) reaction to identify multiphonon quadrupole excitations. Yields of γ rays as a function of incident neutron energy, γ-γ coincidences, and γ-ray angular distributions have been measured. From the excitation functions and coincidence measurements, the level and decay schemes were determined with good confidence in the γ-ray placements. The angular distributions provide multipolarity information on the decay γ rays which lead to spin assignments for levels and also enable the determination of lifetimes or lifetime limits from Doppler shifts. A close-lying triplet of levels, representing the two-phonon multiplet, occurs near an additional 4+ level which mixes with the 4+ two-phonon excitation. A three-phonon quintet of levels has been found, including one new level, which decays almost exclusively to the two-phonon triplet. No other levels in the three-phonon energy region decay other than weakly to the two-phonon excitations; only one other level has observable decays to the two-phonon triplet.