Collective quadrupole behavior in $^{106}_{46}$Pd: deficit of E2 strength of the three-phonon levels

The low-lying excited states in $^{106}$Pd exhibit a structure that resembles a 3-phonon quituplet, thus making $^{106}$Pd an excellent candidate for a ``good quadrupole vibrator.'' To examine this possibility, excited states in $^{106}$Pd were investigated using the (n,n$^{\prime}\gamma$) reaction at the University of Kentucky. Level lifetimes, spins, transition multipolarities, and multipole mixing ratios were determined. The feeding to the proposed two-phonon triplet of states $J^{\pi}$(E$_{x}$,keV) = 4$^{+}$(1229), 2$^{+}$(1128) and 0$^{+}$(1134) was studied for states up to $\sim$3~MeV, and observed $E$2 decay strength sums were \textless\ 50\% of that expected for low-energy quadrupole vibrational collective behavior. This deficiency of strength cannot be explained by considering the fragmentation of the three-phonon states. {\it This material is based upon work supported by the U.S. NSF under Grant No. PHY-0956310.}