Alkali Iodide Crystals Research Articles

Role of alkalis on the incorporation of iodine in simple borosilicate glasses

To understand the influence of alkali elements on the incorporation mechanisms and incorporation limit of iodine in borosilicate glasses used for nuclear waste immobilization, series of model glasses with different alkali contents (22 or 35 mol% Na2O, or 22 mol% of a mixture of alkalis: Na substituted by Li, K and Cs) were loaded with iodine (from 1 000 to 10 000 ppm at. under the form of iodide) at 1100 °C. When the incorporation limit of iodine was reached, alkali iodide crystals were observed (e.g., NaI). For oversaturated samples containing two alkalis, iodide crystals were analyzed and it was found that crystals are enriched in the heavier one of the two relative to bulk composition. Crystal-free glasses were studied by Electron Probe Micro-Analysis to measure the incorporation limit of iodine. A complex variation of incorporation limit was found. Initial substitution of Na by other alkalis (<50%) leads to a decrease in I incorporation limit, this decrease being smallest for Li and largest for Cs. For substitution >50%, only the potassium-bearing system could be studied. In this case, the incorporation limit was non-linear and passed through a minimum for a substitution of ∼75%. Glass structure was determined by X-Ray absorption spectroscopy and Nuclear Magnetic Resonance, providing evidence for local I environments rich in alkalis. It is concluded that high alkali content and high concentrations of Na2O are particularly favorable for iodine incorporation.

Dipole and Deformation Dipole Polarization of Ions in Rock-Salt Structure Crystals

The comparative study is done on the dipole and deformation dipole polarization. With the local density approximation, the values of the dipole polarizability and the deformation dipole tensor of the ions in the rock-salt structure alkali iodide crystals are calculated. The Szigeti effective charge is obtained from the deformation dipole tensor. The crystalline potential is treated by the spherical solid model. The response to the perturbation is calculated by the self-consistent field potential. The calculated results are compared with those in other alkali halide crystals. The ease of the ionic migration is discussed by using the deformation dipole tensor.

Thermal Bleaching ofVBands at Room Temperature in Alkali Iodide Crystals Containing Excess Iodine

Thermal stability of V centers has been investigated at room temperature (RT) for additively colored alkali-iodide crystals by means of optical absorption measurements. V 2 and V 3 bands due to V centers in the crystal were gradually bleached by annealing at temperatures near RT. The bleaching rate of the bands is proportional to the square of the V -center concentration. The activation energies for bleaching of the V bands in KI and RbI crystals are estimated from the bleaching rates to be 0.85±0.04 eV and 0.82±0.03 eV, respectively. The V 2 bands in the crystals containing divalent cations bleach only weakly at RT. The effect of impurities, such as divalent cations, on the thermal stability of the V centers will be discussed.

Radiation effects of An ArF excimer laser in alkali iodide crystals

Radiation effects of an ArF excimer laser (193 nm) have been studied for two kinds of samples of alkali iodides. One is the usual bulk crystals and the other the thin crystals grown in a small gap of two quartz plates. After irradiation, we have found a remarkable striped pattern in both samples by a microscope. The pattern is well interpreted in terms of the light intensity distribution of the Fresnel diffraction induced by a knife edge. The microscopic mechanism of the appearance of the pattern is discussed on the basis of the Raman Scattering spectra of both samples.

Relaxation of electron-hole pairs and scintillation mechanism in alkali halide crystals

The mechanism of the ionizing radiation energy transfer from host lattice to luminescence centers is discussed taking into account the results of the recent experimental investigations of electron-hole pair relaxation in alkali iodide crystals. The high scintillation yield in CsI-Tl, CsI-Na and partially in Nal-Tl crystals is explained by the process of the motion and capture of a one-center self-trapped exciton by impurity ions.

The Transient Absorption Due to Self-Trapped Excitons Localized at Iodine Dimers in Alkali Halide Crystals

The transient absorption spectra of a self-trapped exciton (STE) localized at iodine dimers (I 2 2- * ) in iodine-doped alkali chloride and bromide crystals (AX:I; A=Na, K, Rb; X=Cl, Br) have been investigated. The absorption bands due to electron-transitions appear below 1 eV, and the bands due to hole-transitions around 1.5 and 3.0 eV. The absorption bands due to the electron-transitions are located in much lower energy region than those of intrinsic STEs in nominally pure alkali chloride or bromide crystals. The bands due to the hole-transitions are very similar to those of V k (I 2 - ) centers in alkali iodide crystals. These results suggest that the STE localized at iodine dimers is in the “on-center” configuration and its electronic state is diffused in comparison with that of “off-center” type STE.

Relaxation Processes of Free Excitons Accompanied by Triplet Luminescence in Alkali Iodide Crystals

Luminescence of free excitons (FEs) and self-trapped excitons (STEs) in alkali iodide crystals are investigated under excitation in the n =1 exciton absorption region using a narrow-band UV laser. Experimental results support the identification that the prominent peaks of the FE luminescence spectra of KI and RbI are triplet emission and that of NaI is singlet emission. It is clarified that most singlet FEs in KI and RbI are scattered to the triplet state during the relaxation in the n =1 exciton band. The accumulation of FEs on the triplet state induces the decrease in the intensity of singlet STE luminescence. On the other hand, the scattering rate of the singlet FE to the triplet FE state in NaI is found to be small. The relation between scattering rate of FE and the magnitude of the exchange energy relative to the LO phonon energy is discussed.

Creation of molecular defects in alkali iodide crystals by irradiation with fast heavy ions

We show for the first time that irradiation at room temperature of alkali iodide crystals by fast heavy ions ( 40Ar 14+ ions of 5.53 MeV/A at a fluence of 10 13 ions/cm 2) leads to the formation of V-type defects. By means of UV visible optical absorption and resonant Raman scattering measurements, we have identified I − 3, I − 5 and (I 2) n, in Kl and only I − 3 and I − 5 in RbI irradiated under similar conditions. These preliminary results are consistent with previous investigations obtained in iodide crystals after exposure to X or γ rays at room temperature.

Structure of excitation spectra of luminescence induced by photo-generated excitons in alkali iodide crystals

The quantum yield of the luminescence induced by free excitons in alkali iodide crystals is found to depend on crystal purity, temperature and exciting light energy; the yield is large for the samples containing a large amount of impurity or for the case of a large penetration depth of the exciting light, while the yield is small for pure samples and for small penetration depths. The photo-generated excitons suffer nonradiative decay when they reach the crystal surface in the diffusion process. In pure samples, the structure of the quantum yield spectra reflects the penetration depth of the incident light which is related to the absorption coefficient due to exciton generation in alkali iodide crystals.

Nature of the state of stress produced by xenon and some alkali iodides when used as pressure media.

The ruby ${R}_{1}$ and ${R}_{2}$ line splitting was studied as a function of pressure to 55 GPa in xenon and 70 GPa in three alkali iodides. The ruby ${R}_{1}$ and ${R}_{2}$ line splitting within the xenon medium was found to remain almost constant up to 55 GPa, while in the case of alkali iodide crystals the splitting was found to increase with increasing pressure and amounted to twice as much at 70 GPa as at 0 GPa. This shift in the splitting has been related to the nonhydrostatic component of the stress and provides a method for measuring the compressive yield stress.

Dispersion Property of Thin Alkali Iodide Crystals Observed with Interference Method

The reflection and transmission spectra have been measured for thin NaI, KI, RbI and CsI single crystals with 0.1∼0.3 µm thickness at 11 K. Pronounced structures due to the Fabry-Perot interference are observed in the vicinity of the exciton resonance, showing a strong dispersion of the refractive index. By fitting the obtained results with the theoretical dispersion formula we determine the background dielectric constant, the transverse exciton energy and the longitudinal-transverse (L-T) splitting for each iodide. The present values of L-T splitting are compared with the ones determined from different experiments or calculated independently from the oscillator strength of excitons. On the basis of these experimental values, the validity of the polariton concept in alkali iodide crystals is discussed.

Optical absorption spectra of s2 impurity ions in aqueous halide ion glasses

The optical absorption bands of aqueous 0·05 M Sn 2+ in 7 M LiI at 77°K appear at 361, 352, 325, 310, 300, 292 and 262 nm. They are considered to be the A 1, A 2, B, C 1, C 2, C 3 and D′ bands since the positions and relative intensities lie within the range of those bands for Sn 2+-doped alkali iodide crystals. Upon warming the glass there is an uncorrelated increase in the B band and a decrease in the A 1 band. In 3·6, 4·1, 4·6 and 5·1 M CaCl 2 glasses with 5×10 −3 M Sn 2+ the A 1 and A 2 bands show uncorrelated increases with increasing concentration of Cl −. Comparable observations are reported for Pb 2+-doped glasses of lithium halides and CaI 2. In general the spectra of the Sn 2+- and Pb 2+-doped glasses correlate well with those of the corresponding crystal systems. The effect of temperature and halide-ion concentration are attributed to shifts in chemical equilibria among the well-known halo complexes, MX n 2− n MX n−1 1− n +X −, each having a characteristic, absorption and emission. Absorptions may be attributed to M 2+( 3 P 1 ← 1 S 0) and M 2+( 1 P 1 ← 1 S 0) in the complex, shifted by partially covalent bonding of n halide ions.

Impact Ionization in Alkali Iodide Crystals

The electroluminescence near the metal-insulator contacts is examined in four alkali iodide crystals. The instantaneous light emission is studied as a function of the electric field and of the temperature. The results obtained are in agreement with a recent avalanche-multiplication theory. The observed effect supports the existence of impact-ionization processes in alkali halide crystals at low temperatures and the formation of potential barriers at the metal-insulator interface.

Paramagnetic Resonance Spectrum of O− Trapped in KCl, RbCl, and KBr

The impurity ion O− has been introduced into KCl, RbCl, and KBr and detected at 4°K by EPR spectroscopy. The center has orthorhombic symmetry. A quantitative understanding of the g factor and of the O17 hyperfine structure was obtained with the aid of equations derived in a recent analysis of the an-alogous S− impurity. The hyperfine splitting was described by the atomic parameters ψ2(0), 〈r−3〉l, and 〈r−3〉s. The present results indicate that in an earlier investigation of O− trapped in alkali iodide crystals erroneous values were obtained for these parameters.

Paramagnetic Resonance Spectra of O− Trapped in Alkali Iodide Crystals

The impurity ion O− has been introduced into alkali iodide crystals and detected at 4°K by EPR spectroscopy. A quantitative understanding of the g factor and 17O hyperfine interaction was obtained with the aid of equations derived in a recent analysis of the analogous S− impurity.

Scintillation Response of Aikali Iodides to Alpeha Particles and Protons

The relative pulse height for all alkali iodide crystals, with and without activation, in response to alpha particles and protons was measured as a function of the incident particles energy at both liquid nitrogen and room temperatures. In all cases the response curve is linear for protons of an energy range from 1 to 10 Mev, linear for alpha particles of high energies, but clearly nonlinear generally below 15 Mev. The response curve of activated crystals at liquid nitrogen temperature is generally flatter than that at room temperature. Unactivated crystals have a unique response only at liquid nitrogen temperature. For all crystals the proton scintillation efficiency curve lies slightly below the linear portion of the alpha particles curve. A sharp change in dL/dE is observed in the region of 130 < dE/dx < 160 kev/mg-cm-2, indicating a threshold for a process which has a decreased probability of exciting luminescence.

On the V-centers in Alkali Iodide Crystals

The properties of V-center were investigated in the alkali iodide crystals (KI and NaI) containing excess iodine, In KI, the absorption spectrum was similar to that obtained by Mollwo, but for details, there were some difference. It was pointed out that the V-band in KI can be understood as superposition of three bell-shaped bands I , II , and III . Of these, both band I and II can be safely assigned to V 2 -band. As for band III , however, explanation is not possible at present though it seems to have fairly close relation to the other two bands. Some proposals were made in regard to the mechanism of the coloring with the pointed electrode and the explanation of the doublet structure of V 2 -band in KI.