The formation of gray tracks in KTP is initiated when a Nd:YAG laser produces above-band-gap photons which, in turn, create electron-hole pairs along the beam path through the crystal. These photons result from nonlinear processes (e.g., tripling of the fundamental beam, sum- frequency generation with a fundamental and a doubled beam, etc.). These processes will create 355-nm photons which, coincidentally, nearly match KTP's room-temperature band edge at 350 nm. Many of these electrons and holes produced by above-band-gap photons will recombine; however, a portion of them will be trapped at stabilizing entities such as vacancies or impurities and form 'stable' gray tracks. In the present work, x-rays are used to simulate the effects of an intense laser beam, and thus increase the total number of centers within the sample that are available for study with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. In flux-grown KTP crystals, holes are trapped at trivalent iron (Fe3+) ions, thus forming Fe4+ centers, and electrons are trapped on titanium ions having an adjacent oxygen vacancy, thus forming Ti3+-VO centers. At room temperature, the decay of these electron and hole traps has a half-life of approximately 2 hours. Optical absorption bands associated with these electron and hole traps give rise to the gray- track color.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.