Proton-excited luminescence of silicates: Experimental results and lunar implications

Spectral energy distributions and excitation efficiencies as functions of mineral composition, irradiation time, proton energy, proton flux, sample geometry, and sample temperature have been determined from laboratory measurements of silicate luminescence by low-energy proton excitation. The maximum of the luminescence spectrums of many silicates under continuous excitation by 5-kev protons is in the red when the mineral is first excited; the luminescence intensity decreases, and the maximum shifts from red to blue with time. This phenomenon is here described as a red flash. After the initial red flash, a second red flash, subsequent intensity decay, and blue shift can be produced by a sudden increase of incident ion energy. In both cases the red flash has a principal decay period of about 15 to 20 minutes. The intensity of both the flash and aged luminescence response is directly proportional to incident ion energy and flux. Small particle size for powders, high roughness for solid surfaces, and low mineral temperature favor high luminescence intensity. These data qualitatively agree with the concept of solar-ion excitation as the origin of reported transient lunar reddening near Aristarchus and Kepler. However, these measurements show that the energy efficiency for proton excitation of silicate luminescence is between 10−4 and 10−6; this range is several orders of magnitude lower than values previously reported. In view of the known solar-ion flux, the low efficiencies indicate that luminescence on the sunlit lunar surface resulting from solar-ion excitation is far below the sensitivity of earth-based detection apparatus and that other energy sources or other explanations for the origin of lunar color flashes must be sought.