M. Maghrabi

Rare-earth-size effects on thermoluminescence and second-harmonic generation

The substitution of rare-earth ions into insulating host crystals introduces lattice strains and, for non-trivalent sites, a need for charge compensation. Such effects alter the site symmetry and this is reflected in properties such as the wavelength, linewidth, lifetime and relative intensity of the rare-earth transitions. Equally clear, but less well documented, is the influence on second-harmonic generation (even from cubic crystal lattices). For example, in bismuth germanate, second-harmonic generation efficiency varies by factors of more than 100 as a result of different rare-earth dopant ions. The ions are variously incorporated as substitutional ions, pairs, clusters, or even as precipitates of new phases, but the detailed modelling is often speculative. This article summarizes some recent studies which explore the role of rare-earth ions in thermoluminescence and second-harmonic generation. There are numerous differences in glow peak temperature, for nominally the same defect sites, which are thought to indicate charge trapping and recombination within coupled defect sites, or within a large complex. Size and cluster effects can be modified by heat treatments. This review considers the similarity and trends seen between numerous host lattices which are doped with rare-earth ions. For thermoluminescence there are trends in the variation in glow peak temperature with ion size, with movements of 20 to 50 K. Examples are seen in many hosts with extreme effects being suggested for zircon, with peak shifts of 200 K (probably from precipitate phases).

Luminescence Detection of Phase Transitions

Luminescence efficiency, emission spectra and excited state lifetimes are all factors influenced by crystalline phase, pressure and temperature. Discontinuous changes in these luminescence parameters occur during phase transitions of insulating materials whilst simultaneously exciting and heating, or cooling, the samples. Radioluminescence with X-rays emphasises bulk crystal effects, whereas cathodoluminescence can be used to probe changes near to the surface. This review includes examples of new transitions, hysteresis and the value of the method for rapid surveys. Phase changes also occur for impurities and absorbates, when they exist as nanoparticles, these modify the host luminescence and reflect on crystal growth and surface treatments. Impurity effects are apparent even at the ppm level. Examples and the future potential of the method are discussed.

Optical detection of phase transitions in potassium niobate

Abstract Luminescence data from potassium niobate crystals are reported which show intensity and/or wavelength variations on heating, or cooling, through the phase transition temperatures. The luminescence signals can clearly identify the structural changes between the various crystalline phases. The transition temperatures differ between heating and cooling. They were recorded near 247 and 491 K (heating) or 220 and 478 K (cooling). The hysteresis indicates the presence of metastable material, (e.g. a supercooled structure). The data resolve the previously cited differences in transition temperatures for KNbO3 from different laboratories. The luminescence signals show further details in the variations of spectra, intensity and transition discontinuities of the luminescence which are related to material quality between samples, even from a single supplier. The luminescence data underline the sensitivity of these crystals to structural damage from electron, X-ray or thermal treatments and offer the opportunity to assess crystalline quality prior to device fabrication.

Investigations on the activity concentrations of 238U, 226RA, 228RA, 210PB and 40K in Jordan phosphogypsum and fertilizers.

The activity concentrations of naturally occurring radionuclides ((238)U, (226)Ra, (228)Ra, (210)Pb and (40)K) in Jordanian phosphate ore, fertilizer material and phosphogypsum piles were investigated. The results show the partitioning of radionuclides in fertilizer products and phosphogypsum piles. The outcome of this study will enrich the Jordanian radiological map database, and will be useful for an estimation of the radiological impact of this industrial complex on the immediate environment. The activity concentration of (210)Pb was found to vary from 95 +/- 8 to 129 +/- 8 Bq kg(-1) with a mean value of 111 +/- 14 Bq kg(-1) in fertilizer samples, and from 364 +/- 8 to 428 +/- 10 Bq kg(-1) with a mean value of 391 +/- 30 Bq kg(-1) in phosphogypsum samples; while in phosphate wet rock samples, it was found to vary between 621 +/- 9 and 637 +/- 10 Bq kg(-1), with a mean value of 628 +/- 7 Bq kg(-1). The activity concentration of (226)Ra in fertilizer samples (between 31 +/- 4 and 42 +/- 5 Bq kg(-1) with a mean value of 37 +/- 6 Bq kg(-1)) was found to be much smaller than the activity concentration of (226)Ra in phosphogypsum samples (between 302 +/- 8 and 442 +/- 8 Bq kg(-1) with a mean value of 376 +/- 62 Bq kg(-1)). In contrast, the activity concentration of (238)U in fertilizer samples (between 1011 +/- 13 and 1061 +/- 14 Bq kg(-1) with a mean value of 1033 +/- 22 Bq kg(-1)) was found to be much higher than the activity concentration of (238)U in phosphogypsum samples (between 14 +/- 5 and 37 +/- 7 Bq kg(-1) with a mean value of 22 +/- 11 Bq kg(-1)). This indicates that (210)Pb and (226)Ra show similar behaviour, and are concentrated in phosphogypsum piles. In addition, both isotopes enhanced the activity concentration in phosphogypsum piles, while (238)U enhanced the activity concentration in the fertilizer. Due to the radioactivity released from the phosphate rock processing plants into the environment, the highest collective dose commitment for the lungs was found to be 1.02 person nGy t(-1). Lung tissue also shows the highest effect due the presence of (226)Ra in the radioactive cloud (0.087 person nGy t(-1)).

Luminescence spectra of CaSO4 with Ce, Dy, Mn and Ag codopants

Thermoluminescence (TL) and radio-thermoluminescence spectral analysis techniques have been applied to doped calcium sulphate samples designed for radiation measurements at elevated temperatures. CaSO4 :Dy, when co-doped with Ag, provides a TL dosimetric peak near 350 °C which is useful for radiation measurements at high temperatures. Dopants of Ce, Mn and Dy variously move the peak temperature from 400 °C to 200 °C. Each dopant ion gives a characteristic emission spectra, which for CaSO4 :Ce, Mn samples indicate that there is a systematic temperature difference of ~7 °C between the glow peaks from the Ce and Mn sites. The CaSO4 :Dy samples show a discontinuity in the emission wavelength from the Dy ions near T = 200 °C and a decrease in the radioluminescence fluorescence in the same temperature region. In each case it is proposed that the dopants form part of large, complex defects, instead of isolated trapping and recombination centres. The data offer further evidence for a localized phase transition of the defect complex at 200 °C. Low-temperature data, from 20 K, show similar differences in the peak temperature from the various dopants and additionally indicate reproducible discontinuities in the wavelength positions and intensities, for all samples, at T = 230 K. This again suggests structural phase adjustments of the defect sites.

Influence of trapped impurities on luminescence from MgO:Cr

Luminescence measurements during heating of MgO and MgO:Cr reveal a consistent set of temperature related discontinuities in intensity, accompanied by changes in emission wavelengths. Whilst the luminescence signals are derived from the host material the intensity and wavelength discontinuities are caused by impurities. It is proposed they are caused by phase changes of nanoparticle size impurity inclusions incorporated during growth, and/or from later surface treatments. The evidence suggests there are inclusions of impurities such as neon, oxygen, nitrogen and possibly chlorine. Additionally the influence of solvents, cleaning materials and water vapour alters the luminescence from the surface. Surface and bulk signals are separable by comparisons of cathodoluminescence and radioluminescence. In order to show impurity phase transition effects the nanoparticle inclusions must contain several hundred atoms.

Influence of phase transitions of ice on near-surface cathodoluminescence

Luminescence signals respond to phase transitions both of the host material and of impurities on the nanoparticle-size scale. During cathodoluminescence spectral measurements of many insulators, strong intensity changes (up to 100 times) have been recorded at ~170 K, which are here ascribed to the phase transitions of water that has diffused into the near-surface regions, or is trapped within the bulk material in nanoparticle quantities. The intensity step correlates with the ice transition between the cubic and hexagonal phases. In many materials there are also weaker features near 230 K which match the low-pressure ice-to-vapour transition. Some signals are apparent in radioluminescence data when the water is within the bulk material. The impurity phase changes can modify the emission spectra of the host. Examples are given for several insulators (Nd:YAG, zircon, MgO:Cr, PbWO4, strontium barium niobate) and a superconductor. The data have implications for quantitative luminescence analyses and underline the significant and influential presence of water contaminants. In many surface layers, such as surface optical waveguides or those of superconductors, the ice may significantly influence the behaviour of the host material.

Thermoluminescence spectra of rare earth doped Ca, Sr and Ba fluorides

The low temperature TL spectra of undoped alkaline earth fluorides consistently show a continuous broad band emission around 280-300 nm. The origin of this emission is related to the relaxation of the self-trapped exciton (STE) in the form of an F-H pair. In doped samples, the broad emission is quenched in favour of emissions from the rare earth (RE) impurity sites. The degree of quenching varies between the REs. The spectral measurements showed that the temperature of the glow peaks below room temperature is to a large extent independent of the RE impurity, although some effects are related to the concentration. Additionally, the host material has minimal effect on the glow peak temperatures, Tmax. The scale of the observed differences in Tmax between the three hosts is similar to the differences in annealing temperatures of the Vk and H centres in these hosts. Above room temperature, the glow peaks are specific to the added RE ions and do not show common peaks.

Ion beam induced charge and cathodoluminescence imaging of response uniformity of CVD diamond radiation detectors

The uniformity of response of CVD diamond radiation detectors produced from high quality diamond film, with crystallite dimensions of >100 μm, has been studied using ion beam induced charge imaging. A micron-resolution scanning alpha particle beam was used to produce maps of pulse height response across the device. The detectors were fabricated with a single-sided coplanar electrode geometry to maximise their sensitivity to the surface region of the diamond film where the diamond crystallites are highly ordered. High resolution ion beam induced charge images of single crystallites were acquired that demonstrate variations in intra-crystallite charge transport and the termination of charge transport at the crystallite boundaries. Cathodoluminescence imaging of the same crystallites shows an inverse correlation between the density of radiative centres and regions of good charge transport.

Structural and impurity phase transitions of LiNaSO4:RE probed using cathodo-thermoluminescence

Spectrally resolved cathodo-thermoluminescence spectra of rare earth (RE) doped LiNaSO4 measured from 20 to 673 K reveal several anomalies in the RE emission lines and intensities. The low (20–300 K) temperature data show a discontinuous change in intensity at ~170 K that is either a marked intensity enhancement or a drop truncating the entire spectrum. Such an effect on the host luminescence has previously been assigned to a transition between cubic and hexagonal polymorphs of ice nanoparticle inclusions. Similar, but less profound anomalies are seen above room temperature (300–673 K) where the changes take the form of either a discontinuity in intensity at ~480 K or reduced intensity in the range 430–530 K. There are changes in the relative intensities of different emission lines of the same dopant in this temperature range. Such high temperature variations are ascribed to structural phase changes within the LiNaSO4 crystals. The behaviours may result from Li-poor surfaces or twin boundaries behaving like Na2SO4. This phase change is suggested in the open literature for LiNaSO4 but not yet fully documented, perhaps because the effects span a wide range of temperatures or due to experimental features inherent in most luminescence facilities.