An introduction of Yb ions to Y-3(Al,Ga)(5)O-12:Tb phosphors is found to be effective in preventing the slow buildup of luminescence during electron-beam irradiation. It was also discovered that discoloration and efficiency degradation in this class of phosphors can be reduced by doping of Yb or Eu ions. Thermoluminescence measurements show that trap states formed by the doping compete with intrinsic lattice defects in capturing free electrons, resulting in the decreased number of the electrons trapped in the color centers. This doping effect is related to the low charge-transfer state energy of the trivalent rare earth ions described above.