Three-phonon excitations in 124Sn

The low-lying levels of Sn-124 have been explored using the (n, n'y) reaction to identify multiphonon quadrupole excitations. Yields of gamma rays as a function of incident neutron energy, gamma-gamma coincidences, and gamma-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 gamma-ray placements. The angular distributions provide multipolarity information on the decay gamma 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. (C) 2004 Elsevier B.V. All rights reserved.