VK Centres Research Articles

Surface exciton emission of MgO crystals

MgO crystals have been exposed to vacuum ultraviolet (VUV) radiation from a synchrotron, with energies up to 9 eV, and the emitted light, at wavelengths above 200 nm, was observed. It is concluded that bulk excitons, play an important role in the diffusion of energy inside MgO crystals, resulting in 5.85 eV (212 nm) emission from the MgO terraces of large (0.2–2 µm) MgO : F crystals. In the case of aliovalent impurity doping, then the bulk exciton energy is also transferred to the Vk centres and 5.3 eV (235 nm) light is emitted. Both fluorine and silicon doping appear to promote UV surface emission, acting similarly to an ns2 ion inside MgO, while strong scandium doping is killing the surface emission completely. The 212 nm surface UV emission and the 235 nm bulk UV emission can be excited only at the bandgap edge. Broadband visible light, centred around 400 nm, is also emitted. Contrary to the UV emission, this is not generated when excited at the bandgap edge; instead, we find that it is only excited at sub-bandgap energies, with a maximum at the 5C surface excitation energy of 5.71 eV (217 nm) for the MgO terraces.

Temporal signal stability of KCl:Eu2+ storage phosphor dosimeters

Current KCl:Eu(2+) prototype dosimeters require a wait time of 12 h between irradiation and dosimetric readout. Although irradiating the dosimeters in the evening and reading on the following day works well in the clinical schedule, reducing the wait time to few hours is desirable. The purposes of this work are to determine the origin of the unstable charge-storage centers and to determine if these centers respond to optical or thermal excitation prior to dosimetric readout. Pellet-style KCl:Eu(2+) dosimeters were fabricated in-house for this study. A 6 MV photon beam was used to irradiate the dosimeters. After x ray irradiation, dosimeters were subjected to external excitation with near-infrared (NIR) light, ultraviolet (UV) light, or thermal treatment. Photostimulated luminescence (PSL) signal's temporal stability was subsequently measured at room temperature over a few hours using a laboratory PSL readout system. The dosimeters were also placed in a cryostat to measure the temperature dependence of the temporal stability down to 10 K. Strong F-band was present in the PSL stimulation spectrum, indicating that F-centers were the electron-storage centers in KCl:Eu(2+) where an electron was stored at a chlorine anion vacancy. Due to deep energy-depth (2.2 eV), F-centers were probably not responsible for the fast fading in the first a few hours post x ray irradiation. In addition, weak NIR bands were present. However, there was no change in PSL stabilization rate with intense NIR excitation, suggesting that the NIR bands played no role in the PSL fading. At temperatures lower than 77 K there was almost no signal fading with time. Noticeable PSL was observed for undoped KCl samples at room temperature, suggesting that Cl(2) (-) V(k) centers served as hole-storage centers for both undoped and doped KCl where a hole was trapped by a chlorine molecular ion. V(k) centers were stable at low temperature and became mobile at room temperature, probably causing the observed PSL fading with time. On the other hand, V(k) center could be stabilized by Eu(2+) activator or oxygen in the lattice, leading to the stable component in the PSL. A thermal process at elevated temperatures (60 °C or higher) was able to significantly accelerate the migration process resulting in a fast stabilization of PSL. However, this could not be accomplished using intense UV excitation. Thermal treatment enables KCl:Eu(2+) prototypes to be ready for readout in 1 h without the need of applying a large time-dependent correction factor. However, this cannot be achieved using optical preexcitation.

Trapping of Holes and Excitons in Scintillators: CsI and ${\rm LaX}_{3}$ ( ${\rm X}={\rm Cl}$, Br)

We present computational results for trapping of polarons and excitons in undoped CsI, LaCl3 and LaBr3 , using plane-wave-pseudopotential density functional theory with Hartree-Fock exact exchange. The optimized VK center and STE in CsI is a distortion of two iodine atoms from the lattice to form an interstitial bound I2- molecule, consistent with previous theoretical and experimental results. In both LaCl3 and LaBr3, the relaxed STE configuration involves only one displaced halide ion, and does not form an X2-. The calculated luminescence energy for the STE in LaCl3 and LaBr3 is 4.1 and 3.7 eV, respectively.

Deformation luminescence produced during application and release of pressure on to gamma-irradiated CaF2:RE crystals

Calcium fluoride CaF2 is an interesting host lattice for rare earth (RE) activators. CaF2 crystals doped with different concentrations of Dy, Ce, Er and Gd have been grown by the Bridgman technique and their deformation luminescence (DL) induced by room temperature gamma irradiation has been recorded. When a uniaxial pressure is applied on to gamma-irradiated CaF2:RE crystals, initially the DL intensity increases with time, attains a maximum value and then it decreases with time. Although the DL intensity produced during the release of pressure is less, its rise and decay behaviours are similar to that obtained during the application of pressure. The DL intensity depends on dopant, concentration of dopant, irradiation doses and mass of the load or applied pressure. It is suggested that the moving dislocation produced during deformation of crystals capture holes from hole trapped centres (like perturbed Vk centre) and the subsequent radiative recombination of the dislocation holes with electrons give rise to DL.

Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF singlecrystals

The emission spectra and the excitation spectra of various emissions have been measured inLiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence ofself-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single excitingphoton of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondaryexciton. The tunnel phosphorescence has been detected after the irradiation of LiF byan electron beam or x-rays at 6 K, and several peaks of thermally stimulatedluminescence (TSL) at 12–170 K appeared at the heating of the sample. It wasconfirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes(VK centres). TheTSL peak at ∼160 K is ascribed to the thermal ionization ofF′ centres. The TSL at 20–30 K and 50–65 K is caused by the diffusion of interstitialfluorine ions (I centres) or H interstitials, respectively. The TSL peak at∼13 K, the most intense after electron or x-irradiation, cannot be detected after LiFirradiation by VUV radiation, selectively forming excitons or e–h pairs. Thecreation of a spatially correlated anion exciton and an e–h pair is needed forthe appearance of this peak: an exciton decays into an F–H pair, a hole forms aVK and an electron transformsH into I (an F–I–VK group is formed) or an F centre into a two-electronF′ centre(an F′-H -VK group). The analysis of the elementary components of the 9–16 K TSLshowed that a phonon-induced radiative tunnel recombination ofF′–VK (5.6 eV),F–H (∼3 eV)and F–VK (3.4 eV) occurs within these groups.

Scintillation mechanisms in Ce3+ doped halide scintillators

AbstractLast couple of years witnessed the development of various new Ce3+ doped halide scintillators (LaCl3, LaBr3, LuI3, Cs2LiYCl6, Cs2LiYBr6) that possess excellent gamma ray or thermal neutron detection properties. The scintillation pulse in LaBr3:Ce3+ is 20 times faster than in the most commonly used scintillator NaI:Tl. This, combined with a more then two times better energy resolution and higher gamma ray stopping power, makes it ideally suited for many different applications. In this work the scintillation properties and mechanisms of Ce3+ doped inorganic halide (Cl, Br, I) compounds are reviewed; especially the role of Vk centers and self trapped excitons (STEs) in the energy transfer from the ionization track to Ce3+ is treated. Aspects of the lifetime and thermal stability of the self trapped exciton, the migration speed of Vk centers and STEs, spectral overlap between STE emission and Ce3+ absorption, and the influence of the type of anion (Cl, Br, I) are addressed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations of the electron paramagnetic resonance g factors for the trigonal Er3+−VK centers in KMF3 (M=Mg and Zn)

The electron paramagnetic resonance g factors gǁ and g⊥ for the trigonal Er3+−VK centers (where VK denotes K+ vacancy) in KMF3 (M=Mg and Zn) are theoretically investigated from the perturbation formulas of the g factors for a 4f11 ion in trigonal symmetry. In these formulas, the contributions to the g factors from the second-order perturbation terms and the admixture of various energy levels are taken into account. In the above trigonal centers, the impurity Er3+ resides in the octahedral M2+ site of both crystals, associated with one VK in the C3-axis due to charge compensation. By analyzing their g factors, we find that the impurity Er3+ in both centers would move towards the VK by an amount ΔZ along the C3-axis due to electrostatic attraction. The calculated g factors for the Er3+ centers based on the above displacements ΔZ show good agreement with the observed values. The validity of the results is discussed.

Temperature-induced variation of the emission band intensities of an SrAl12O19:Pr phosphor

The spectral characteristics of the X-ray-excited luminescence from an SrAl12O19:Pr phosphor featuring cascade photon emission were studied in a range of temperatures from 90 to 500 K. It is demonstrated that an important role in the energy transfer to the emission centers in SrAl12O19:Pr is played by competitive processes of the formation of Vk centers and the trapping of holes by Pr3+ ions.

Ionic, electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF2 BaF2 and LiBaF3 crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF3 indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by Vk center detrapping and hole-diffusion controlled tunnel recombination within pairs like {Dn e−…Vk }.

The use of point defects in digital radiography

Colour centres in alkali halides such as F centres and H or VK centres have been studied intensely for decades but have hardly found any applications. However, in activator-doped halides their generation mechanisms upon irradiation with X-rays, their room temperature stability and photostimulated recombination processes are the basis of important applications in digital radiography, which has recently become commercially available. An overview is given of the present understanding of the point defect mechanisms in the commercially used X-ray storage phosphor BaFBr:Eu2+ and its shortcomings. Further developments in BaFBr-based X-ray storage phosphors are discussed. Promising cubic X-ray storage phosphor materials with high figures of merit and potentially higher spatial resolution, such as RbBr:Ga+, CsBr:Ga+, RbBr:Eu2+ and CsBr:Eu2+ have recently been studied in detail and an overview of the generation and recombination mechanisms of the electron and hole trap centres active in the information storage and read-out processes is presented. Finally, first results are shown of the use of glass ceramics as potential new storage material in particular for high-energy X-rays.

F centre production in CsI and CsI–Tl crystals under Kr ion irradiation at 15 K

We present results of simultaneous in situ luminescence and optical absorption studies in scintillator CsI and CsI–Tl crystals, exposed to very dense electronic excitations induced by 86Kr ions (8.63 MeV/amu). Irradiation at 15 K leads to the formation of the prominent F absorption band. In addition, several other features of the broad absorption between exciton and F bands were ascribed to an anion vacancy, α centre (240 nm), self-trapped hole, Vk centre (410 nm) and interstitials, H centres (560 nm). We have found that low doping of thallium (∼1017 cm−3) causes the F centre formation to proceed more rapidly than in pure crystal. On the other hand, we were not able to create any amount of F centres in heavily doped CsI–Tl. We have shown that point defects created by heavy ions manifested themselves in luminescence ageing.

X-ray excited luminescence of Sr 1− xEu xF 2+ x solid solutions

X-ray excited luminescence of SrF2 revealed a sharp and a broad emission band at 231.5 and 316.5 nm respectively while the presence of small concentrations of Eu+3 in SrF2 showed emission bands at 301 and 417 nm. Increase in europium concentration resulted in sharp bands towards red region around 593, 617, 693 and 702 nm and suppressed the broad bands at 301 and 417 nm. The emission bands at 231.5, 316.5 nm in SrF2 and 275, 301 nm in Sr1-XEuXF2+X are attributed to host emission and ionization of VK centers due to fluorine ions while the others are characteristic emission bands of europium. An energy transfer from the host as well as VK centers emission may be responsible for exciting europium and resulting sharp emissions towards red region.

On the reproducibility of the glow curve of single crystal and commercial LiF

Heterogeneous interstitial nucleation was observed as a result of the exposure to high dose rate and repeated irradiations of single crystals of LiF doped with Mg and Ti. A comparison under the same conditions with the irradiation of commercial LiF dosimeters was done. The temperature region for which the observations were made applies to the region where VK centers are mobile. Therefore, the recombination of some defects produced by gamma radiation are primarily actual for the behavior around room temperature. Glow curves were obtained from irradiated LiF. The main disadvantages found are almost the same as those presented for commercial LiF. Also, in both cases we found differences in the thermoluminiscence response after storage was observed. The study was also focused to answering the question of how many times a dosimeter can be re-used after a severe irradiation.

Hole self-trapping and recombination in LiBaF3

We investigated the electron paramagnetic resonance (EPR), recombination afterglow and thermostimulated luminescence (TSL) of the X-irradiated LiBaF3 crystals. After X-irradiation at 80 K, an EPR of the self-trapped hole centre VK (F− 2) oriented along the [110] axis is identified. X-irradiation at temperatures below 200 K results in a creation of a long-term temperature-independent afterglow—tunnelling luminescence (TL), with main emission bands at 300, 370 and 430 nm. The short wavelength TL bands are associated with the tunnelling recombination of the electron centre with the VK centre, with thermal stability estimated to be about 130 K.

Detection of electron paramagnetic resonance of radiation-induced defects in BaFBr via recombination luminescence

We investigated the recombination processes in the X-ray storage phosphor BaFBr: Eu2+ via the detection of the microwave-induced changes in the recombination luminescence in a high magnetic field. Resonances of the F(Br−) centre, of the VK(Br2 −) centre, of the Eu2+ ground state and of a hitherto unknown hole centre were observed. The luminescence band of the F(Br−)-VK(Br2 −) recombination peaks at about 400 nm, that of the F(Br−)-new hole centre recombination at about 350 nm.

Self-trapped excitons in pure and Na- and Tl-doped caesium halides and the recombination luminescence

On the basis of a method used earlier to predict the off-centre relaxed triplet self-trapped excitons in alkali halides, similar systems in caesium halides are studied. This work confirms the expected off-centre relaxation along the [100] cubic axes with increasing magnitude in the order CsI, CsBr and CsCl. The calculated emission energies are in reasonable agreement with observed values. The well-known 2.95 eV emission band of CsI:Na has been studied as a tunnelling recombination between a close pair consisting of a VK centre and a Na atom. For a number of close-pair geometries the emission energies are close to 3 eV. The strong emission bands of CsI:Tl at 2.25 eV and 2.55 eV have been interpreted as arising from tunnelling recombination of close pairs each consisting of a Tl0 and a VK centre. The calculated emission energies and polarizations are discussed in conjunction with the experimental data.

On the thermoluminescence of LiF from 83 to 320 K

Thermoluminescence was studied in LiF (TLD-100) single crystals from 83 to 320 K. In this temperature range, the glow curve presents six peaks at 138, 153, 193, 260, 283, and 300 K and a seventh peak at approximately 240 K. Based on annealing response, linearity, phototransference, and optical bleaching experiments, the glow peak at 138 K was related to VK hole centers, while the remaining glow peaks were associated with electron traps, although a clear relation between the highest temperature glow peak, at 300 K, and electron centers could not be established with certainty. The seven glow peaks emit at three wavelengths, namely, 270, 300, and 420 nm. The emission band at 270 nm is more intense for the glow peaks at 138 and 153 K. For the remaining glow peaks, the emission bands at 300 and 420 nm are the most important ones. The emission band at 270 nm was related to the recombination of VK holes at recombination centers other than the ones normally associated with the emission band at 420 nm. The emission band at 300 nm was related to the recombination of electrons and VK centers. It was verified that the glow peaks at 138, 153, 193, and 260 K obey first order kinetics.

Hole-electron mechanism of F-H pair generation in RbCl crystals with impurity electron traps

Production of F, Cl 3 − , Ag0, and Tl0 centers in RbCl:Ag and RbCl:Tl crystals by photons having energies ranging from 5 to 10 eV has been studied at 295 and 180 K. It is shown that creation of near-impurity excitations is accompanied by formation of F centers localized in the vicinity of Ag+ and Tl+ ions. F centers are produced in direct optical generation of self-trapped excitons. In addition to the well-known mechanism of F-H pair production in nonradiative recombination of electrons with self-trapped holes, a hole-electron process has been revealed for the first time to operate in RbCl:Ag having deep electron traps. By this mechanism, F-H pairs appear in the following sequence of stages: thermally stimulated unfreezing of hopping diffusion of self-trapped holes (V K centers), tunneling electron transfer from Ag0 to the approaching V K centers, and subsequent nonradiative decay of triplet self-trapped excitons near Ag+ ions.

Electron paramagnetic resonance of doped and undoped BaFBr detected by recombination luminescence

The recombination processes in the X-ray storage phosphor BaFBr:Eu2+ was investigated by measuring the microwave-induced changes in the recombination luminescence in a (high) magnetic field. Besides the resonances of the F(Br−) centre, of the VK(Br−2) centre and of the Eu2+ ground state we observed a line of a hitherto unknown hole centre. The maximum of the luminescence band in which the VK(Br−2) centre could be detected was about 400nm. The resonance of the unknown hole centre belongs to a luminescence band peaking at about 350nm.

Photo- and thermally stimulated luminescence and defects in UV-irradiated CsI:Tl and CsI:Pb crystals

It has been shown that the photoionization of impurity centres followed by fast migration of the optically released nonrelaxed holes and their subsequent self-trapping occurs on excitation of CsI:Tl and CsI:Pb crystals in the impurity-induced absorption bands. From comparison of the characteristics of VK centres optically created at 4.2K in CsI:Tl and CsI:Pb crystals it has been suggested that the thermally stimulated luminescence peaks observed in CsI crystals at 60 and 90K arise from closely associated and the separated {VK-electron centre} pairs, respectively, and that 0°-diffusion of VK centres can occur when the closely associated pairs are present in the CsI crystals. A scintillation mechanism has been proposed for doped CsI crystals which explains the time and temperature dependences of the scintillations as well as the high efficiency of CsI:Tl and CsI:Na scintillators.