Thermoluminescence investigation of donor (Ce${}^{3+}$, Pr${}^{3+}$, Tb${}^{3+}$) acceptor (Eu${}^{3+}$, Yb${}^{3+}$) pairs in Y${}_3$Al${}_5$O${}_{12}$

The thermoluminescence properties of Y${}_3$Al${}_5$O${}_{12}$ (YAG) doped with an electron donor lanthanide (Ce${}^{3+}$, Pr${}^{3+}$, or Tb${}^{3+}$) together with an electron acceptor lanthanide (Yb${}^{3+}$ or Eu${}^{3+}$) were studied with the aim to locate the energy levels of each divalent and each trivalent lanthanide within the band gap. The activation energies needed to liberate electrons trapped by Yb${}^{3+}$ or Eu${}^{3+}$ were determined from thermoluminescence heating rate plots. With an automated thermoluminescence excitation spectroscopy technique, information on the energy level location of the electron donors Ce${}^{3+}$, Pr${}^{3+}$, and Tb${}^{3+}$ was obtained. By combining all data with an empirical model, a scheme containing all the divalent and trivalent lanthanide energy levels in YAG has been constructed.

Figure 1

Normalized TL glow curves of (a) YAG: Ce${}^{3+}$, (b) YAG: Pr${}^{3+}$, (c) YAG: Tb${}^{3+}$, (d) YAG: Ce${}^{3+}$, Yb${}^{3+}$, (e) YAG: Pr${}^{3+}$, Yb${}^{3+}$, (f) YAG: Tb${}^{3+}$, Yb${}^{3+}$, (g) YAG: Ce${}^{3+}$, Eu${}^{3+}$, (h) YAG: Pr${}^{3+}$, Eu${}^{3+}$, and (i) YAG: Tb${}^{3+}$, Eu${}^{3+}$ after 600-mGy β irradiation (solid line) and after excitation with photons (red/dark gray dotted lines) with the following wavelengths: in (d) and (g), λ $=$ 340 nm, and in (f) and (i), λ $=$ 228 nm. The samples with Pr${}^{3+}$ do not show a glow peak after excitation with λ $=$ 240 nm. The heating rate β $=$ 2 ${}^{\circ }$C/s.

Figure 2

$\lambda T$-contour plots of (a) YAG: Ce${}^{3+}$, (b) YAG: Pr${}^{3+}$, (c) YAG: Tb${}^{3+}$, (d) YAG: Ce${}^{3+}$, Yb${}^{3+}$, (e) YAG: Pr${}^{3+}$, Yb${}^{3+}$, (f) YAG: Tb${}^{3+}$, Yb${}^{3+}$, (g) YAG: Ce${}^{3+}$, Eu${}^{3+}$, (h) YAG: Pr${}^{3+}$, Eu${}^{3+}$, and (i) YAG: Tb${}^{3+}$, Eu${}^{3+}$ after exposure to irradiation of 2.5 kGy from a ${}^{60}$Co source. The heating rate is β $=$ 2 ${}^{\circ }$C/s.

Figure 3

TL emission spectra of (a) YAG: Ce${}^{3+}$, Eu${}^{3+}$, (b) YAG: Pr${}^{3+}$, Eu${}^{3+}$ (the dotted line is shown with a factor of 50 enlarged intensity), and (c) YAG: Tb${}^{3+}$, Eu${}^{3+}$ (the dotted line is shown with a factor of 20 enlarged intensity) at 335 ${}^{\circ }$C.

Figure 4

Heating rate plots of (a) YAG: Ce${}^{3+}$, Yb${}^{3+}$, (b) YAG: Pr${}^{3+}$, Yb${}^{3+}$, (c) YAG: Tb${}^{3+}$, Yb${}^{3+}$, (d) YAG: Ce${}^{3+}$, Eu${}^{3+}$, (e) YAG: Pr${}^{3+}$, Eu${}^{3+}$, and (f) YAG: Tb${}^{3+}$, Eu${}^{3+}$ after β irradiation.

Figure 5

TL excitation spectra of (a) YAG: Tb${}^{3+}$, Yb${}^{3+}$, (b) YAG: Ce${}^{3+}$, Yb${}^{3+}$, and (c) YAG: Pr${}^{3+}$, Yb${}^{3+}$ at room temperature. The arrows indicate the positions of the absent 5$d$ bands.

Figure 6

Energy level scheme of the lanthanides in YAG.