Laser-Induced Optically Stimulated M Centre Luminescence in LiF

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Abstract
Highly purified LiF crystals have long been used as optical windows. Optical grade LiF is known for its excellent transmission from the deep ultraviolet through the infrared. Radiation dosimetry applications have so far been restricted to high level Mega-Rad gamma dosimetry using radiation-induced absorption peaks measured using a spectrophotometer. However, absorption measurements are a very insensitive way to measure these radiation damage centres. A more sensitive method of measuring radiation damage can be accomplished by measuring the M centre luminescence. At the Pacific Northwest Laboratory, excitation with a 442 nm He-Cd laser has significantly amplified the luminescence yield of the M centre. The He-Cd laser was the preferred excitation laser because absorption measurements in LiF indicate the peak of the M centre absorption occurs at 443 nm. Laser stimulation produces an excited state of the M centre, which undergoes a very strong Stokes' shift. The peak of the M centre emission spectrum occurs at 665 nm with a half-width of 0.36 eV. Since the excitation wavelength differs significantly from the emission wavelength, measurement of the deep red emission can be done simultaneously with the excitation. Therefore, readout of the LiF can be accomplished in a fraction of a second.

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