Admittance spectroscopy and deep-level transient spectroscopy measurements have been performed on n+-on-p Hg1-xCdxTe (x=0.595) planar photodiodes grown by improved zone melt method. After 1.0 Mrads gamma irradiation, a new trap center is observed at 0.19 eV above the valence band, while the well-known 0.15 eV trap level disappears. The trap densities for these two levels are almost the same. We attribute this phenomenon to the gamma irradiation, which produces a compound defect correlated with Hg vacancy. © 1998 American Institute of Physics.