Lithium borate discs for skin dose measurements: Application to total body superficial electron beam therapy

Abstract

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li2B4O7) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag+ ions substituting for Li+ ions. They also have Ag+ ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag+ ions that replace Li+ ions and electrons are trapped at the interstitial Ag+ ions, i.e., the radiation forms Ag2+ (4d9) ions and Ag0 (4d105s1) atoms. These Ag2+ and Ag0 centers have characteristic EPR spectra. The Ag0 centers also have a broad optical absorption band peaking near 370 nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400 nm light produces an intense OSL response when emission is monitored near 270 nm. Electrons optically released from the Ag0 centers recombine with holes trapped at Ag2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370 nm peak of the absorption band of the Ag0 electron traps). Oxygen vacancies are also present in the Ag-doped Li2B4O7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described.