  New transitions in neutron rich $^{100}$Y have been identified in a
$^9$Be+$^{238}$U experiment with mass- and Z- gates to provide full fragment
identification. These transitions and high spin levels of $^{100}$Y have been
investigated by analyzing the high statistics $\gamma$-$\gamma$-$\gamma$ and
$\gamma$-$\gamma$-$\gamma$-$\gamma$ coincidence data from the spontaneous
fission of $^{252}$Cf at the Gammasphere detector array. Two new bands, 14 new
levels and 23 new transitions have been identified. The $K^{\pi}=4^+$ new band
decaying to an 1s isomeric state is assigned to be the high-$K$
Gallagher-Moszkowski (GM) partner of the known $K^{\pi}=1^+$ band, with the
$\pi 5/2[522] \otimes \nu 3/2[411]$ configuration. This 4$^+$ band is also
proposed to be the pseudo spin partner of the new $K^{\pi}=5^+$ band with a
5$^{+}$ $\pi 5/2[422] \otimes \nu 5/2[413]$ configuration, to form a $\pi
5/2[422] \otimes \nu [312$ $5/2,3/2]$ neutron pseudospin doublet. Constrained
triaxial covariant density functional theory and quantal particle rotor model
calculations have been applied to interpret the band structure and available
electromagnetic transition probabilities and are found in good agreement with
experimental values.
