Long-term fading of LiF:Mg, Cu, P and LiF:Mg, Ti thermoluminescence detectors with standard and modified activator composition

Within a space experiment MATROSHKA, a human phantom with over 3000 TLDs inside, was exposed outside of the International Space Station during the period of 1.5 year to determine radiation organ doses that would be received by astronauts during the extra vehicular activities on the Earth orbit. Due to the long exposure period and unusual temperature conditions it was necessary to consider the loss of signal of TL detectors in time (fading). One-year fading, determined in standard conditions (t=20∘C) for lithium fluoride detectors varied between 5%±1.4%5%±1.4% (1 SD) for MTS-7 (LiF:Mg7,Ti), 24%±1.7%24%±1.7% (1 SD) for MTT-7 (LiF:Mg7,Ti) and 9%±2.5%9%±2.5% (1 SD) for MCP-7 (LiF:Mg7, Cu, P). To reduce fading different post-exposure annealing were investigated (temperatures from 100 to 150∘C and time from 10 to 50 min). The optimum condition was observed for 120∘C/30min which decreases the fading below 5%. This annealing, intended to remove the peak 3 in the LiF TL glow curves, was applied to all TL detectors just by IFJ Krakow in MATROSHKA experiment.