LiF Single Crystals Research Articles

Fluorescent emission spectra of lithium fluoride with use of synchrotron radiation.

The soft x-ray emission spectra of single-crystal LiF excited with synchrotron radiation are presented. The radiative decay of the lithium 1s core exciton is observed. Radiative transitions between the valence band and the core hole are also observed in undamaged samples. Spectra obtained with broadband synchrotron-light excitation are shown to be dominated by diffuse scattering from damaged sample surfaces. Excitation with photon energies between 100 and 150 eV generates true emission spectra. An important result is that electrons excited into the conduction band are frequently trapped in an exciton state before recombining with the core hole.

The influence of temperature on the deformation and fracture of an lif single crystal subjected to friction and wear

Sliding friction experiments were conducted with an LiF single crystal in contact with a steel ball at temperatures of 25–300 °C in air. The influence of break-in conditions and the initial structural state on the temperature dependence of microhardness as well as friction and wear characteristics were investigated. A comparison of the obtained results with the data in an LiF stress—temperature map has been carried out. The region of normal friction is associated with dislocation glide (plasticity region on the map) and the region of galling and seizure is associated with dislocation creep (power creep region on the map). The results indicate that the temperature of transition to seizure and galling is comparable with the temperature of transition to high temperature creep. Creep transition temperatures estimated for a series of metals showed fine agreement with the temperature of adhesive interaction.

Mobile dislocations and internal stresses in the latent hardening of lithium fluoride crystals

Abstract The density of mobile dislocations in gamma-irradiated LiF single crystals has been determined by measuring the potential difference developed between the side surfaces of a specimen while compressing it between tilted plates. It has been found that the density of mobile dislocations in the latent deformation is about half that in the primary deformation and the density in the soft deformation is about twice that in the latent deformation. Stress-relaxation tests have been performed on unirradiated as well as irradiated LiF single crystals during primary, latent and soft deformations. It is found that 90% of latent hardening is due to an increase in internal stresses and only 10% is due to an increase in effective stresses. Similarly, when the slip directions are reversed, softening is mainly due to the lower internal stresses.

Ion beam modification of sodium precipitates embedded in LiF

LiF single crystals have been implanted with 5 × 10 16 Na + cm −2 at 300 K and annealed at 600 K. Sodium precipitates are then observed by their optical absorption band at 520 nm due to plasma oscillations of electrons in metallic sodium. The evolutions of these sodium clusters under bombardment with neon ions, xenon ions and sodium ions have been studied at different temperatures. Ion beam effects characterized by in situ optical absorption are strongly dependent on the mobilities of intrinsic defects created in LiF.

Energy Measurements of Thermally Stimulated Exoelectrons from LiF Single Crystals

Abstract In the course of energy measurements of thermally stimulated exoelectrons from LiF single crystals irradiated with X rays, the integral energy spectrum is found to be broad and to deviate from an exponential distribution, and the onset of saturation current is not around zero volts. However, with an increasing period of illumination with UV radiation just after X ray irradiation, the onset of saturation current goes toward zero volts and the energy spectrum can be approximated by an exponential distribution. The mean energy is then evaluated to be approximately 0.3 eV. Experimental spectra are compared with those of Monte Carlo calculations based on a model with an overthermal exponential distribution, taking into account electron-phonon interactions and satisfactory agreement is obtained.

Energy Measurements of Thermally Stimulated Exoelectrons from LiF Single Crystals

Energy Measurements of Thermally Stimulated Exoelectrons from LiF Single Crystals

Application of TSEE Characteristics to High Energy Radiation Dosimetry Around an Electron Linear Accelerator

The response of the exoelectron dosemeter to the absorbed dose has been investigated with the LiF sample irradiated with high energy electrons from a linear accelerator and ? rays from a 60Co source. The energy absorbed in the thin surface layer, which can be related to the origins of exoelectron emission, is, in general, smaller than the energy liberated there by primary radiation. In this paper the surface dose is calculated by the Monte Carlo Code EGS4. It is pointed out that the air layer in front of the sample also plays an important role by supplying secondary electrons to the surface region of the sample. The emission density of exoelectrons from a LiF single crystal for unit absorbed dose is found to be 5 x 104 electrons.cm-2.Gy-1, and nearly constant independent of the low LET radiation type.

Application of TSEE Characteristics to High Energy Radiation Dosimetry Around an Electron Linear Accelerator

Abstract The response of the exoelectron dosemeter to the absorbed dose has been investigated with the LiF sample irradiated with high energy electrons from a linear accelerator and ? rays from a 60Co source. The energy absorbed in the thin surface layer, which can be related to the origins of exoelectron emission, is, in general, smaller than the energy liberated there by primary radiation. In this paper the surface dose is calculated by the Monte Carlo Code EGS4. It is pointed out that the air layer in front of the sample also plays an important role by supplying secondary electrons to the surface region of the sample. The emission density of exoelectrons from a LiF single crystal for unit absorbed dose is found to be 5 x 104 electrons.cm-2.Gy-1, and nearly constant independent of the low LET radiation type.

Use of Mohr's Salt for High Level Gamma Dosimetry (Up to 108 Gy)

Fricke's dosemeter, which has been used for many years as a transfer dosemeter for doses up to 400 Gy employs Mohrs salt Fe(SO4)2(NH4)26H2O in an ultra-pure solution. The originality of the method proposed is that the salt is irradiated in the solid state. Spectrophotometrc measurements are then made on the absorption peak (345 nm) of a solution of this salt in 0.1 N hydrochloric acid. The solution is more and more diluted as the dose increases in order to remain always under conditions where the Beer-Lambert law is applicable. Two series of experiments are described. In the first series, a 60Co source was used to irradiate small plastic containers filled with powdered Mohr salt. In the second series, 3 mm diameter glass tubes filled with powder were irradiated using a fuel element source (fuel elements from the OSIRIS reactor) which enabled very high integrated doses to be reached. The decaying fission products of this latter irradiator required regular determinations of the corresponding dose rates during irradiation. Alanine ESR dosimetry was used for this purpose. The method proposed is simple, cheap and accurate. A comparison with single crystal LiF measurements is made.

Use of Mohr's Salt for High Level Gamma Dosimetry (Up to 108 Gy)

Abstract Fricke's dosemeter, which has been used for many years as a transfer dosemeter for doses up to 400 Gy employs Mohrs salt Fe(SO4)2(NH4)26H2O in an ultra-pure solution. The originality of the method proposed is that the salt is irradiated in the solid state. Spectrophotometrc measurements are then made on the absorption peak (345 nm) of a solution of this salt in 0.1 N hydrochloric acid. The solution is more and more diluted as the dose increases in order to remain always under conditions where the Beer-Lambert law is applicable. Two series of experiments are described. In the first series, a 60Co source was used to irradiate small plastic containers filled with powdered Mohr salt. In the second series, 3 mm diameter glass tubes filled with powder were irradiated using a fuel element source (fuel elements from the OSIRIS reactor) which enabled very high integrated doses to be reached. The decaying fission products of this latter irradiator required regular determinations of the corresponding dose rates during irradiation. Alanine ESR dosimetry was used for this purpose. The method proposed is simple, cheap and accurate. A comparison with single crystal LiF measurements is made.

Effect of High AC Field and X-Ray Irradiation on the Dielectric Properties of Quenched LiF Single Crystals

Dielectric constant (K), loss (tan δ) — and hence conductivity σ — of LiF single crystals which are subjected to a combination of treatments like quenching, high ac field treatment and later X-ray irradiation, are studied in the frequency region 102 to 107 Hz and in the temperature range 30 to 350 °C. These treatments are found to increase the room temperature K and tan δ-values appreciably particularly in the low-frequency region. These increases seem to be connected with space charge polarisation due to significant increase in the concentration of charged lattice defects (like vacancies) in the samples. Temperature variation of K and tan δ at different frequencies for these samples seem to support this conclusion. The activation energy value for conduction in the intrinsic region (300 to 350 °C), calculated from log σ versus 1/T is 0.98 eV for as-cleaved LiF; this value is found to decrease in the variously treated LiF samples. The results of the present work indicate that LiF samples which are quenched, ac field treated, and later X-ray irradiated, contain the largest concentration of lattice defects. An attempt is made to understand the results. Dielektrizitätskonstante (K), Verlustfaktor (tan δ) — und damit die Leitfähigkeit δ — von LiF-Einkristallen werden nach einer Kombination von Behandlungen, wie Abschrecken, Belastung mit hohem Wechselfeld und nachfolgender Röntgenbestrahlung, im Frequenzbereich von 102 bis 107 Hz und Temperaturbereich von 30 bis 350 °C untersucht. Es wird gefunden, daß diese Behandlungen die Werte von K und tan δ bei Zimmertemperatur beträchtlich erhöhen, insbesondere im Bereich niedriger Frequenzen. Diese Zunahme scheint mit einer Raumladungspolarisation infolge einer beträchtlichen Zunahme der Konzentration geladener Gitterdefekte (ähnlich Leerstellen) in der Probe verknüpft zu sein. Die Temperaturvariation von K und tan δ bei vereschiedenen Frequenzen scheint diesen Schluß für diese Proben zu bestätigen. Die Aktivierungsenergie für die Leitfähigkeit im Intrinsicbereich (300 bis 350 °C) der aus dem Verlauf log σ über 1/T berechnet wird beträgt 0,98 eV für frischgespaltenes LiF; es wird gefunden, daß dieser Wert in den unterschiedlich behandelten LiF-Proben abnimmt. Die Ergebnisse der Untersuchung zeigen, daß LiF-Proben, die abgeschreckt werden, einem Wechselfeld ausgesetzt und nachfolgend röntgenbestrahlt werden, die höchste Konzentration von Gitterdefekten aufweisen. Es wird versucht, die Ergebnisse zu verstehen.

Optical absorption and thermoluminescence of LiF single crystals irradiated with X rays in strong electric fields

Etude de l'influence du champ (ac ou dc) sur l'absorption des centres F et M et sur la thermoluminescence

Kapitza conductance of crystals cleaved under He II

The Kapitza conductance of LiF and KBr single crystals has been measured immediately after cleaving under superfluid helium. Heating techniques were used to evaluate the conductance and it approached the phonon radiation limit for these materials. Cleaving LiF under He ll reduced the Kapitza conductance compared to an aged, cleaved surface by only a few percent. Despite the very good cleaved surface obtained with LiF crystals, the results suggest that sufficient microfracturing of the surface occurs upon cleaving to significantly increase the Kapitza conductance over the acoustic mismatch theory values. These results are in contradiction to results obtained by phonon reflection techniques.

Mechanical Behaviour of LiF Single Crystals in Compression and Tension at Temperatures from −196°C to 800°C

AbstractThe work hardening characteristics of LiF single crystals in compression and tension has been investigated as a function of temperature. It was shown that in temperature range from 200 °C to 600 °C four stages of hardening were observed and the main part of plastic flow proceeds in stage IV. The parabolic hardening was not observed. The flow stresses and hardening rates in all stages in compression are higher than those in tension. As a temperature increase this difference decreases. The hardening rates, stresses which correspond to the beginning of the linear hardening stages and the duration of those stages decrease as a temperature increase. At temperatures above 600 °C the yield drop occured. Taking into account a good agreement between the temperature dependences of the critical resolved shear stress and τIII the assumption was made that τIII corresponds to resolved shear stress in a secondary glide system.

Annealing of deffects in γ-irradiated LiF studied by positron annihilation

Positron lifetime spectra and Doppler broadened spectra have been measured for LiF single crystals gamma-irradiated at room temperature. Isothermal and isochronal annealings of the samples have been performed in a temperature range of 20–600°C.

Persistent spectral hole burning for R' color centers in LiF crystals: Statics, dynamics, and external-field effects.

The persistent spectral hole-burning properties of R' color centers in pure and doped LiF single crystals at liquid-helium temperatures are described and analyzed. Using frequency-modulation spectroscopy and precision ratiometric transmission spectroscopy, a satellite hole structure has been observed which varies with position in the inhomogeneous line due to excited-state splittings. Transient effects in the optical absorption indicate that an intermediate state with millisecond lifetimes is involved in hole formation. The persistent holes have been observed to grow as the logarithm of time and the logarithm of burning intensity in certain regimes, and a phenomenological tunneling model has been used to explain the time and intensity dependence. The results suggest that R' centers interact via tunneling and local strains with a quasi-random distribution of nearby traps which may be other color centers in the host material. A linear Stark effect has been observed in agreement with earlier measurements. This work illustrates the power of hole-burning spectroscopy to uncover static and dynamic interactions normally hidden by inhomogeneous broadening.

Some features of indentation creep of LiF single crystals

The dependence of the size of the indentation and dislocation rosettes on loading time was investigated on the (001) plane of LiF single crystals. The measurements were performed in temperature range from room temperature to 170° C. The indentation time was varied from 0.2 to 103 sec. It was revealed that the change of the indentation size during creep was more significant than the change in dislocation ensemble tracks in the field of the concentrated load. It was shown that the dependence of the length of the dislocation rosette edge arms on loading time, when plotting in double logarithmic scale, was linear. This fact allowed the determination of parameterm, characterizing the dependence of the dislocation velocity on stress, using creep experiments. The values ofm proved to be in good agreement with the results obtained by different methods.

単結晶ＬｉＦの破壊じん性

The fracture toughness (KIc) of LiF single crystal was evaluated by the indentation microfracture technique (IMT) and the indentation strength technique (IST) at room temperature. The indentation crack on the (001) of ‹100› specimen resulted in the lowest KIc of about 0.5MNm-1.5 in both IMT and IST because the {110} is the preferred cleavage plane of this crystal. The Vickers hardness (HV) for the (001) was obtained to be about 0.9GPa. A fairly smooth planar was provided by the crack-propagation originated from the initial crack on the (001) of ‹110› specimen. However, cracks in the ‹110› (110) specimen became eventually angled at 45° to the original flaw. As the result, KIc≈0.7MNm-1.5, HV≈0.9GPa for the former, and KIc≈0.8MNm-1.5, HV≈1.2GPa for the latter. The indentation crack on both the (110) and (112) of ‹111› specimen didn't propagate along the extention of Vickers indentation diagonal and produced the rough surface after fracture. The KIc values for ‹111› (110) and ‹111› (112) were determined to be 1.5 and 1.8MNm-1.5, respectively, while HV≈1GPa. These results indicate that the KIc of single crystals is affected by the cleavability of crystals and the geometric configuration between the cleavage plane and the direction of crack propagation.

Optical Absorption and Thermoluminescence of AC Field-Treated LiF Single Crystals later Irradiated with X-Rays

Optical Absorption and Thermoluminescence of AC Field-Treated LiF Single Crystals later Irradiated with X-Rays

Laser stimulated thermoluminescence

Experimental and computational methods are presented for the complete characterization of the thermoluminescence response obtained from thermoluminescent phosphors upon exposure to localized Gaussian laser heating beams. A number of different phosphor configurations are described as examples. These include LiF:Mg,Ti (TLD-100, Harshaw Chemical Corporation) in form of chips, which are widely used in the dosimetry of ionizing radiation, and thin-layer dosimeters prepared either as self-supporting films or powder in a polyimide matrix, or on substrates of LiF single crystals or borosilicate glass. It is demonstrated that all relevant optical and thermal properties of the dosimeters can be determined by these methods and that, based on this knowledge, the expected thermoluminescence response of a given configuration can be simulated as a function of a number of experimental parameters.