LiF Rods Research Articles

Epsilon near zero based phenomena in metamaterials

We present and analyze unique phenomena of enhanced transmission through systems of subwavelength dielectric cylinders embedded in an epsilon near zero host. Our analysis shows that these phenomena are due to Mie-resonance modes arisen in the dielectric cylinders. Subwavelength waveguides and lenses are proposed based on coupling of these modes between neighboring cylinders. Finally, the proposed phenomena and their possible applications are numerically demonstrated in the THz regime in a realistic polaritonic material of LiF rods in KCl.

Self-organization approach for THz polaritonic metamaterials

In this paper we discuss the fabrication and the electromagnetic (EM) characterization of anisotropic eutectic metamaterials, consisting of cylindrical polaritonic LiF rods embedded in either KCl or NaCl polaritonic host. The fabrication was performed using the eutectics directional solidification self-organization approach. For the EM characterization the specular reflectance at far infrared, between 3 THz and 11 THz, was measured and also calculated by numerically solving Maxwell equations, obtaining good agreement between experimental and calculated spectra. Applying an effective medium approach to describe the response of our samples, we predicted a range of frequencies in which most of our systems behave as homogeneous anisotropic media with a hyperbolic dispersion relation, opening thus possibilities for using them in negative refractive index and imaging applications at THz range.

In vivo urethral dose measurements: a method to verify high dose rate prostate treatments.

Radiation doses delivered in high dose rate (HDR) brachytherapy are susceptible to many inaccuracies and errors, including imaging, planning and delivery. Consequently, the dose delivered to the patient may deviate substantially from the treatment plan. We investigated the feasibility of using TLD measurements in the urethra to estimate the discrepancy in treatments for prostate cancer. The dose response of the 1 mm diam, 6 mm long LiF rods that we used for the in vivo measurements was calibrated with the 192Ir HDR source, as well as a 60Co teletherapy unit. A train of 20 rods contained in a sterile plastic tube was inserted into the urethral (Foley) catheter for the duration of a treatment fraction, and the measured doses were compared to the treatment plan. Initial results from a total of seven treatments in four patients show good agreement between theory and experiment. Analysis of any one treatment showed agreement within 11.7% +/- 6.2% for the highest dose encountered in the central prostatic urethra, and within 10.4% +/- 4.4% for the mean dose. Taking the average over all seven treatments shows agreement within 1.7% for the maximum urethral dose, and within 1.5% for the mean urethral dose. Based on these initial findings it seems that planned prostate doses can be accurately reproduced in the clinic.

Temperature Dependence of the Emittance of LiF and LiNbO3 in the Near-Infrared Spectra

This study investigates the temperature dependence of the normal emittance in 3–5, 5, 10.6 and 8–12 µm spectral range for LiNbO3 and LiF rods with both crystal and polycrystalline structures. According to the results, the normal emittance in the near-infrared spectra increases with increasing wavelength. The normal emittances at 10.6 and 8–12 µm are not only invariant with temperature, but also exceed 0.9. When the temperature increases, the normal emittances at 3–5 and 5 µm for the LiF single-crystal rod continuously increase regardless of the phase change from the solid state to the liquid state. Before the temperature reaches the Curie point, the normal emittances at 3–5 and 5 µm for the LiNbO3 single-crystal rod increase with increasing temperature. The normal emittances at 3–5 and 5 µm then decrease as the temperature further increases. A significant decrease in the normal emittance at 3–5 and 5 µm occurs when the emission is only from the melt. In addition, the magnitudes of the normal emittance at 3–5 and 5 µm are enhanced by the scattering effect due to the porous structure of the solid.

Thermal Radiative Emission of LiF and LiNbO 3 in the Near-Infrared Spectra

The thermal radiation for the wavelengths of 3–5, 5, 10.6 and 8–12 µ m emitted from heating rods is measured in this study by a two-dimensional thermal imaging radiometer. The LiNbO3 and LiF rods with both single crystal and polycrystalline structures are measured. The volumetric emission is enhanced by the scattering effect induced by the polycrystalline rod's porous structure. When the phase is changed from solid to liquid, the sharp decrease of the emission in the 3–5 µ m spectrum has been observed for the polycrystalline rods due to the disappearance of the scattering effect of the porous structure. Indeed, the emission in the 3–5 µ m spectrum does not significantly decrease for a LiF single-crystal rod. However, it still exists for a LiNbO3 single-crystal rod except that the difference in the emission levels decreases. As the phase change from solid to liquid occurs, the emission of LiF and LiNbO3 in the 10.6 µ m spectrum continuously increases regardless of whether the structure is a single crystal or polycrystalline.

Accuracy in clinical electron beam dose planning using pencilbeam algorithms

The accuracy of two different pencil beam models for electron beam dose planning is shown by comparisons with measured dose distributions. The investigation is restricted to two-dimensional geometries. In one model the multiple scattering of the electrons is considered by the small angle Gaussian approximation of Fermi and Eyges. In the other, the generalized Gaussian model, the large angle single scattering events are also considered. The parallel slab approximation is used in both models. The comparisons have been made for two different anatomical phantoms. One phantom was constructed to simulate a transversal cross-section through the chest wall and lung in radiotherapy of breast cancer. The other phantom was made to simulate the head at the level of the nose. The measurements in these phantoms were made with thermoluminescence dosimetry, LiF rods. The accuracy of the oblique incidence case was investigated in a homogenous water phantom. The results show that for oblique incidence and geometries where the semi-infinite slab approximation is reasonably good, the generalized Gaussian model is more accurate. Within and behind low density cavities, in the phantom, the Gaussian model often gives a better agreement to experimental data. This is shown to be due to mutually balancing errors from the semi-infinite slab approximation and the Gaussian approximation.

Supralinearity of radiothermoluminescent dosimetry

The authors studied how supralinearity influences individual "deep trap" terms (or levels) of a glow curve that represents the luminous flux emitted by heated LiF rods as a function of energy required for heating. It is assumed that a Gaussian function can be used for glow curve analysis, developing measured glow curves to represent individual "deep trap" terms. Analysis reveals that supralinearity occurs much earlier and more markedly with high temperature peaks than with low ones. This has a bearing on practice.

Effect of independent and coupled vibrations of dislocations on low-temperature thermal conductivity in alkali halides

Recent experimental evidence has established that the scattering of phonons by dislocations at low temperatures in [110] LiF rods is primarily a dynamic rather than a static process, but calculations of the effects of vibrating dislocations have failed to provide quantitative agreement with experimental observations of the magnitudes and temperature dependence of the effect. We have performed a calculation which includes many factors previously neglected and show that the effects of resonance angle scattering, phonon focusing, and the resolved shear stress factor have an important influence on the effect of vibrating dislocations on thermal conductivity. However, the detailed calculation shows that in the case of [110] LiF rods the inclusion of these previously neglected factors still does not resolve the discrepancies observed and it is concluded that independently vibrating dislocations are not the important phonon scatterers above 0.1 K in LiF. It is shown, however, that the experimental data may be fit by assuming reasonable densities and distributions of "optically" vibrating dislocation dipoles.

Measurement of stopping power for electron beams with LiF dosimeters

An Estimate of collision stopping power ratio for electrons of initial energy 30 MeV and effective energy (20 MeV) has been carried out by LiF dosimetry, for the elements C, Al, Cu, Sn and Pb relative to C. LiF rods 1 mm dia. by 6 mm length were used. The stopping power ratios obtained from these LiF dosimetric studies agreed with theoretical values uses ( Berger et al., 1964) (1) within ±2%, i.e. within the combined experimental and theoretical errors on the compared values. These results confirm the reliability of LiF for dosimetry by cavity theory with absorbers of widely different atomic numbers for high energy electron beam.

A Study of the Cavity Displacement Effect in "Thimble Chambers" Using LiF Dosimeters

A study of the cavity displacement effect has been carried out using LiF rods of diameter 1 mm and length 6 mm. These LiF dosimeters were introduced into different sizes of cylindrical air cavities of length 30 mm each and diameters 30, 20, and 10 mm, respectively. A wide range of radiation energy was covered. A cavity displacement factor of 0.84 ± 0.08 expressing the distance of the effective measuring depth point from the center of the cavity as a fraction of the radius was obtained for X rays and γ rays. Plots showing the variation of the ratios of the cavity dose to the point cavity dose with cavity size and radiation energy are presented. A table giving the above-mentioned ratio in percentage for a 5 mm cavity diameter at a depth of 5 cm in a polystyrene phantom is also included. Further investigations at 39 MeV electron energy and for other radiation qualities are planned.

Energy dependence with respect to absorbed dose in water of lithium fluoride rods in high energy electron beams

Indicates that the sensitivity of LiF rods expressed as thermoluminescent response per rad in water, is dependent on both energy and geometrical dimensions of the LiF dosimeter. The sensitivity per rad in water of extruded LiF rods varies approximately by 25% for electron beam energies ranging from 1 to 20 MeV. A model was developed which predicted the change of sensitivity per rad in water of the LiF rods and the calculated and measured responses were compared.

In vivo dosimetry with extruded lithium fluoride rods.

In external radiation-beam therapy it is difficult to calculate the dose to a point when the treatment beam passes through air-containing tissue. Of two approaches used to correct for the effects of tissue heterogeneity, one determines the water-equivalent thickness that the x-ray beam passes through; the other measures the dose to particular points in a patient directly. The dose delivered to the esophagus during radiation therapy of the upper chest was measured by inserting a catheter containing LiF rods into the esophagus, and the measured dose agreed with the dose calculated using the determined water-equivalent thickness of the patient.