VERIFICACIÓN DE LAS HIPÓTESIS ESPECÍFICAS

The emissions observed for IL, CL and RL (Figs. 4.19 and 4.20) can be ascribed to different REE based on comparison with the synthetic samples. However, some emissions like those ~313, 548 nm and the IR emissions can be ascribed to

FIGURE 4.27:Intensity of PL versus dopant level for Dy3+ (blue; EX/EM 352/577 nm), Eu3+ (red; EX/EM 394/576 nm) and Sm3+ (green; EX/EM 405/598 nm) in synthetic fluorapatite.

Pagel 2001; Gorobets & Rogojine 2002; Kempe & Götze 2002). The two broad emissions ~360 and ~450 nm have been ascribed to Ce3+_{and Eu}2+_{, respectively, and}

generally the spectra are similar to spectra in these publications. However, the presence of intrinsic defects cannot be ruled out.

An interesting result is the difference in relative intensity between the ~360 and ~450 nm emission in RL and CL, where the former dominates in CL but the latter in RL. One explanation for this difference is that in CL only the surface of the sample is excited whereas in RL the whole sample is excited. In accordance with Fig. 1.2 the 360 nm emission will not be significantly absorbed by the sample in CL, whereas in RL more of the 360 nm emission relative to the 450 nm emission will be absorbed by the sample. Hence, the 450 nm emission will appear more intense in RL than in CL for a similar chemistry. Another possible explanation is connected with the ability of x- rays to change the oxidation state of some REE3+ → REE2+ (e.g. Lakshmanan & Tomita 1999; Park et al. 2006; Peng & Hong 2007; and references therein). If x-rays during RL reduce Eu3+ → Eu2+, it would explain the more intense RL of the 450 nm emission than CL. However, if Eu3+ is being reduced, a reduction of Sm3+ → Sm2+ may also be expected, but no observed differences between the CL and RL suggest the presence of Sm2+.

The PL emissions are generally weak (Figs. 4.22 and 4.24) and excitation of five different samples with 405 nm only shows emission relating to Sm3+ _indicating

that little energy from Sm3+ is transferred to other activators. The excitation spectra for different samples reveal typical excitation bands for Sm3+ (Fig. 4.23). However, a broad excitation band ~310 nm indicates the presence of a structurally-related excitation band. Excitation using the broad UV excitation band reveals emissions related to several REE for DUR, where a broad emission ~565 nm is observed for MOI. Broad emission in apatite bands around 565 nm have previously been ascribed to Mn2+ (e.g. Barbarand & Pagel 2001; Kempe & Götze 2002; Waychunas 2002), which is further supported by the fact that MOI has the highest Mn content (Table

Chapter 4 dating young sediments. The method is based on the formation of traps within the band-gap, caused by radiation damage from the surrounding rocks/sediments. The number of defects relates to the integrated radiation dose of the material, which is in turn dependent on the age of the material,i.e.high luminescence intensity equals old sediments. The number of traps present in the band-gap can be estimated by the number of decay curves, or lifetimes of the emissions. The modelling of at least two components with different life-times in ILI (Section 4.3.7.2) indicates the presence of at least two traps in the band gap. However, the apatite samples have been exposed to sunlight, i.e. any OSL signal would have been bleached prior to my luminescence studies.

It is more likely that during continuous UV irradiation the sample is being electronically modified, either in form of exciton or electron-hole recombination. Another possibility is that an electron is changing state and creates an effect similar to the colour change of hackmanite. The observed reduction in intensity during PL could explain why PL of apatite often is weak, whereas high-energy excitation like CL is strong. The high-energy continuously populates the traps or creates the special electron states, whereas PL only has sufficient energy to change it back or create the recombination. However, intriguing as this effect is, it will be very time-consuming to fully characterise and is therefore beyond the scope of this thesis.

4.5 Conclusions

REE luminescence is not associated with valence electrons and therefore considered to be fully or only weakly linked with the host structure. However, the Ho and Er data, in which PL is absent, but luminescence is present with high-energy excitation, show that the host structure plays an important role in the generation of REE luminescence. Furthermore, this study has shown that different REE respond differently to sample degeneration or defect formation during irradiation. For example, if Sm is more strongly coupled to the host than Dy, then the intensity of Sm emissions will be reduced more than Dy emissions by modifications to the lattice.

emissions from that REE are observed. This indicates that very little energy is transferred between the two activators. The synthetic sample only contained two different activators, but the same is observed for the natural samples where only Sm3+ emissions are observed after specific excitation of a Sm3+ absorption band. However, when the natural samples are excited with wavelengths in the broad UV excitation band, more emissions appear and the resulting spectrum resemble that acquired with high-energy excitations. Broad excitation bands are coupled to the structure, either as an anion charge transfer band (as observed for some single doped REE samples) or related to structural defects.

At low dopant levels both Eu and Sm show site-selective excitation and fine structures. At high dopant levels, the fine emission structures of the Dy doped sample disappear and for the Eu doped sample the site-selective excitation and the ability to define the number of sites based on the J = 0 transition ceases. The single-crystal analyses of natural apatites show that increasing REE content changes the distortion of the twoCa polyhedra and powder XRD shows how the crystallinity of the synthetic material does not change with increasing dopant level. Most likely, the changes in the distortion of the Ca polyhedra modify the site-symmetry, which is reflected in the hypersensitive electronic dipole transitions. Furthermore, the powder XRD of the Dy doped sample indicates that clusters are formed at high dopant levels, which also greatly affect the interpretation of the luminescence for estimating site population in non-symmetric equivalent sites.

In RL the broad emission ~450 nm is stronger than the ~360 nm emission, whereas in CL the latter dominates. If the 450 nm emission is ascribed to Eu2+ it is possible that during the RL experiment Eu3+ is being reduced to Eu2+ by x-rays. Alternatively, the difference can be explained by a combination of CL exciting only the surface region, whereas RL excites the entire sample. As fluorapatite absorbs UV then all of the 450 nm, emission generated deep inside the sample will pass through and reach the detector, whereas a large proportion of the UV (360 nm) generated

Chapter 4 The ideal dopant level for Dy3+_{, Sm}3+ _{and Eu}3+ _{also varies significantly from}

0.05 - 0.15 apfu for Dy3+ to 0.75 - 1.00 apfu for Eu3+. The fact that the ideal dopant level varies for different REE, combined with the REE dependent intensity difference through radiation damage, demonstrates that luminescence cannot be used to determine trace element concentration. Such compositional analysis may be possible for well-characterised samples after major testing of synthetic material with different dopant levels, and then most likely only within a narrow compositional area, but not for natural samples where several emission bands overlap. Luminescence is unlikely to compete routinely with other analytical methods with ppm-ppb limits of detection and micron-scale spatial resolution.

Multiple Luminescent Spectroscopy of the