Thermoluminescence spectra from sulphates, fluorides and garnets doped with rare earth ions.
Luminescence measurements have been applied to three different structures
namely, sulphate, fluorides and YAG. In all cases the RE doping suppresses the intrinsic
emission and results in intense luminescence characteristic of the RE dopant.
Additionally, in double doped samples, or contaminated ones, the TL data show that
each dopant defines a glow peak, which is displaced in temperature relative to the
others. Examples of this were discussed for CaS04:Ce,Mn; YAG:Nd,Tb,Cr,Mn;
BaF2:Ho,Ce and BaF2:Tm,Ce. The data are discussed in terms of an energy transfer
model between different parts of extended defect complexes which encompass the RE
ion and the lattice defects.
Calcium sulphate doped with Dy define a TL peak near 200°C suitable for
radiation measurements, but when co-doped with Ag the TL peak move to higher
temperatures with minor effects on the peak sensitivity. In Ce,Mn double doped
samples, the peak temperatures differ by -7°C between the Ce and Mn sites.
The TL glow curves from alkaline earth fluorides are complex and contain several
overlapping peaks. Curve fitting show that the peak maxima below room temperature
are insensitive to the RE dopant. Additionally the host material has a modest effect on
the peak positions. Above room temperature each dopant provides a TL curve specific
to the added RE ion and do not show common peaks. Concentration has many effects on
the resultant glow curve, and even at the lowest concentration used here (0.01%) there is
evidence of cluster formation. Samples with high RE content show low values of the
frequency factor consistent with the energy transfer model in that the emission from
RE-RE cluster dominates over the emission from direct charge recombination within the
defect complex. The effect of concentration and the TL mechanism operating below
room temperature are also discussed.
Luminescence signals from the near surface of YAG:Nd (via CL) were contrasted
with those from the bulk material via RL. Results indicate that the outer few micron
layers differ significantly in luminescence response from the bulk crystal. The
differences were ascribed to result from solvents that enter the YAG lattice during the
growth stage or subsequently from cleaning treatments via the dislocations caused by
cutting and polishing. Additionally, the growth stage may include gases from the
residual air in the growth furnace trapped into the YAG lattice. In each case there is a
discontinuity in luminescence intensity and/or emission wavelengths at temperatures
which mach the phase transitions of the contaminants. At the transition temperature
there will be a sudden pressure change and this will induce surface expansion or bulk
compression. The differences between the two cases were detected by the alternatives of
CL and RL excitation, where the Nd or Er lines have moved in opposite directions. The
detection of such low concentrations of solvents/trapped gases by luminescence is
extremely difficult due to experimental limitations. Hence their role in luminescence
generation is normally ignored.