Some observations regarding the effects of background rare-earth impurities on the thermoluminescence and optical absorption of CaF2:Mn

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Abstract
At low Mn concentrations the optical absorption spectra of irradiated CaF2:Mn may be strongly influenced by the presence of trace rare-earth (RE) impurities. The RE absorption centres are predominantly due to divalent RE ions, either isolated or associated with NN F centres, known as photochromic (PC) centres, and ionised PC centres (i.e. an RE2+ ion with a NN anion vacancy). Some of these centres have a low thermal stability and anneal between room temperature and 200 degrees C. During the annealing of the centres noted above, it is observed that the Mn-related absorption peaks increase in intensity. After this increase, the spectrum is then characteristic of Mn-doped crystals. The temperatures over which the Mn-related absorption features disappear increase as the Mn content increases. During this anneal, intense thermoluminescence (TL) is observed. Accordingly, the temperature at which the TL is observed to be most intense shifts to higher values as the Mn content is raised. In samples intentionally doped with Ce (in addition to Mn) the authors find that the Ce 'quenches' very effectively the Mn-related absorption. If the Ce content is high enough, only Ce-related features can be seen in the absorption spectrum. The TL too is dominated by the Ce defects. They conclude that both the sensitivity and the thermal stability of the TL from CaF2:Mn are significantly affected by the presence of trace quantities (at the 10 p.p.m. level) of rare-earth ions. These data may be relevant to reported batch-to-batch irregularities in the TL properties of this material.