K-PI=0(-PI=4(+) 2-PHONON GAMMA-VIBRATIONAL STATES IN ER-166() AND K)

The (n,n'gamma) reaction has been used to search for two-phonon gamma- vibrational states in Er-166. Levels at 1943 and 2028 keV have been ob served with collective transitions to the gamma band. The latter, whic h has B(E2;2028 --> 2(gamma)(+)) = 7.4 +/- 2.5 W.u. (Weisskopf unit) i s interpreted as the I,K-pi = 4,4(+) two-gamma-phonon state. The forme r, for which the data indicate spin 0, has B(E2; 1943 --> 2(gamma)(+)) = 21 +/- 6 W.u. and is interpreted as the I, K-pi = 0, 0(+) two-gamma -phonon state. This is the first observation of the K-pi = O+ two-gamm a-phonon state in a well-deformed nucleus, and its identification plac es stringent limits on the nuclear models.