Effective Number Density Research Articles

High-dielectric-constant Ta2O5/n-GaN metal-oxide-semiconductor structure

High-dielectric-constant Ta2O5 has been grown on the n-GaN epifilm by rf magnetron sputtering. Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics. Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×104 cm/s as an upper limit using a modified dead-layer model. A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta2O5 as the gate metal. Capacitance-versus-voltage characteristics have been measured. The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 7.0×1016 cm−3. The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 4.1×1012 cm−2. Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta2O5, and matches closely with calculated values. Hysteresis is observed and ascribed to positive mobile charges derived as 2.1×1012 cm−2. The capacitance dependence on the frequency and the leakage current are discussed.

A simple method for CT-Scanner calibration against effective photon energy

An original method for CT-scanner calibration against effective photon energy is presented. It is based on optimization of the CT-scanner output characteristic (the dependence of the measured linear attenuation coefficient on the calculated value). A significant gain in accuracy is achieved with respect to the calibration techniques which are conventionally used in commercial computed tomography. The method can be recommended for use in dual-energy computed tomography. As an example of its application, measurements of the effective atomic number and electron density of organic plastics are described. It is shown that the method provides acceptable results even for an increased noise level in CT images.

Determination of the dose distribution of therapeutic electrons at interfaces

The variation of dose close to tissue/inhomogeneity or tissue/air interfaces was investigated in slab and anthropomorphic phantoms for electron beams between 4 and 20 MeV. Effective source‐surface distance (ESSD), beam obliquity, backscatter from internal interfaces, and dose to the sidewalls of air cavities were investigated using an Attix ion chamber, different thicknesses of thermoluminescent dosimeters, and EGS4 and MCNP Monte Carlo simulations. Surface dose was found to vary due to the SSD and beam obliquity. Relative surface dose decreased as the SSD increased due to the effect of low energy scattering from the cone and ESSD determined at the surface differed from the ESSD measured at The difference was found to be most significant for the combination of high energy beams and small field sizes. Surface dose increased with angle for obliquely incident beams from approximately 30° up to a maximum, which was found at approximately 80°, and then decreased rapidly. This dose maximum angle increased with increasing electron beam energy for both surface and near surface depths. Dose variations at internal tissue/inhomogeneity interfaces are due to disequilibrium of electron scatter. The backscatter of electrons (EBF) from inhomogeneities was found to be dependent on the effective atomic number and relative electron density of the scatterer. Where dose upstream of the scatterer was reduced due to a lack of backscattered electrons. EBF was dependent on the mean energy of the beam incident on the interface, but was independent of the shape of the energy spectrum. The energy of backscattered electrons at the interface was generally low, but increased with increasing atomic number of the inhomogeneity and was related to the upstream extent of backscattered electrons. Dose variation along the side wall of a cavity was dependent on the angular distribution of the beam entering the cavity and dimensions of the cavity.

Characterization of polycrystalline TlBr films for radiographic detectors

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. Instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TlBr are compared to other materials, and possible solutions are discussed.

ミクロ炉物理学

The author wants to propose Reactor as a new challenge of Reactor Physics. The Microscopic Reactor Physics is a research field to understand the overall nuclear reactor system by considering the space-dependent microscopic properties such as effective cross sections, number densities, power distribution, and temperature distribution within fuel rods. In this paper some examples of the Microscopic Reactor Physics are illustrated to extend the theory and experiment of Reactor Physics, and future contribution to related research field such as fuel integrity and reactor safety.

Experimental demonstration of the frequency dependence of the effective scatterer number density

In previous papers, a data reduction method for determining a frequency-dependent ‘‘effective scatterer number density’’ in ultrasound was reported [Chen et al., J. Acoust. Soc. Am. 95, 77–95 (1994); 96, 2556–2563 (1994)]. The effective scatterer number density is the actual number density of scatterers multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of the scatterers. Evaluation of this technique based on measurements applied to a phantom with a distribution of scatterer diameters is reported here. This condition yields an effective scatterer number density that depends on the dominant scatterer concentration at the analysis frequency. Measured scatterer number densities at 3.5, 5, and 8 MHz were in agreement with theoretical computations of this parameter for the phantom.

PENETRATION DEPTHS OF 830nm DIODE LASER IRRADIATION IN THE HEAD AND NECK ASSESSED USING A RADIOGRAPHIC PHANTOM MODEL AND WAVELENGTH-SPECIFIC IMAGING FILM

One of the major claims laid against low reactive-level laser therapy (LLLT) is its purported lack of penetration in biologic tissues, particularly inhomogeneous turbid soft tissue and bone. In order to assess the actual penetration depth of 830 nm laser energy in the head and neck region, a commercially-available radiographic ‘phantom’ model was used (Rand Phantom, Rand Laboratories, U.S.A.). This model consists of dried human skeletal bones (skull and cervical vertebrae) cast inside a proprietary urethane compound having the same effective atomic number and optical density as soft tissue, thus simulating a mix of soft tissue types with randomly distributed fat at radiographic wavelengths and exposure doses. The phantom is sliced horizontally at 2.5 cm intervals allowing the insertion of X-ray sensitive film for training of radiographers or assessment of penetration of X-rays in the head and neck. In this study the authors used wavelength specific imaging film for 830 nm radiation (Konika Medical Laser Imaging Film, LP820H). The phantom is designed to replicate absorption of tissue at X-ray wavelengths. X-ray photons have a very high frequency (≅ 3 x 1018 Hz), approximately three orders of magnitude higher than near infrared (IR) photons at 830 nm (frequency ≅ 3 x 1013 Hz). It was therefore theorized that the penetration of near IR laser energy would be minimized by the high optical density of the phantom’s ‘soft tissue’ thereby providing an accurate penetration model. In a preliminary experiment, penetration of 830 nm diode laser energy (OhLase-3D1, Proli Japan, GaAlAs, continuous wave, 60 mW, 1∼20 sec) was assessed in a skin model (MIX-DP, Saisei Medical, Japan) at thicknesses from 2 mm to 15 mm. The optical density of the ‘skin’ slices was assessed using densinometry (X-RITE Co., Inc., USA), and then the laser penetration was assessed for each skin model thickness with the medical imaging film placed under the ‘skin’, developed, and the esposure areas plotted graphically. It was found that thepenetration of 830 nm laser energy decreased with the increase in optical density, although in a nonlinear fashion, but that extending the exposure time increased the penetration. In the subsequent experiment the laser was irradiated at various points of the phantom head and neck, maintaining the angle of the probe to the head at about 15°, with 830 nm sensitive imaging film inserted between the slices. Contrary to the results of the previous experiment, at exposure times of 180 sec and 300 sec, penetration of laser energy at 830 nm into the head and neck model as assessed by exposure patterns seen in the 830 nm-sensitive film, was in the range of several centimeters rather than millimetres. It is thought that the presence of bone possibly has an echo effect on incident laser energy at 830 nm thereby giving greater penetration than in the skin alone model. It is also possible that the bone structure may also help to focus the laser energy. The implications for applications of LLLT are discussed.

Wavelength dependence of two-beam coupling gain coefficients of BaTiO3:Ce crystals

Two-beam coupling gain coefficients of transmission and reflection gratings in BaTiO3:Ce crystals versus wavelength between 570 and 680 nm have been measured. The former decreases more quickly than predicted by theory. The latter was found to vary significantly with wavelength, while theory shows that it should vary little. In order to explain the experimental results we assumed that the effective carrier number density Neff depends on wavelength. By calculating Neff from the measured results we found that Neff is approximately a linearly decreasing function of wavelength. The slopes and intercepts for the linear functions are given for the two crystals studied.

A method for determination of frequency-dependent effective scatterer number density.

Previous researchers have shown that for low scatterer concentrations, the scatterer number density in an ultrasound pulse-echo experiment can be obtained from ratios of the fourth to the second moment of the backscattered signal. In this paper a new method is presented for determining an effective scatterer number density, which is the actual number density multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of all scatterers. The method of data reduction goes beyond the work of previous authors in that, in addition to accounting for the possibility that different sets of scatterers may dominate the echo signal at different frequencies, it also explicitly retains both the temporal nature of the data acquisition and the properties of the ultrasound field in the data reduction. Tests of the method in phantoms yielded good agreement between measured values of the effective scatterer number density and number densities calculated using first principles.

Influences of iron doping on the photorefractive properties of KTa1−xNbxO3 crystals

The influences of iron doping on the photorefractive properties of KTa1−xNaxO3 (KTN) crystals were studied. The two-wave-mixing gain coefficients and the grating formation rates were measured in undoped and Fe-doped KTN samples. Photorefractive parameters, such as the effective carrier number density, were determined for these samples. It was shown that the gain coefficient increases significantly with the doped Fe content, but the grating formation rate decreases with it, and that all the influences of iron doping originate mainly from the variation of the effective carrier number density.

A Multispectral Transmittance Method for Simultaneous Determination of Size Distribution and Optical Dispersion Parameters of in situ Soot.

An analytical procedure is proposed to simultaneously determine the size distribution and optical dispersion parameters of soot from multispectral transmittance data for a luminous flame. Blokh's distribution and a gamma distribution are assumed as particle size laws, but only Blokh's distribution is successfully utilized throughout the study. The dispersion model of three bound and one free electrons developed by the author is adopted to describe the complex refractive index of soot. Of the dispersion parameters, only the effective free electron number density is treated as an adjustable parameter for each soot and the remaining parameters are fixed at adequate values. The proposed method is applied to Lee-Yu-Tien's data of spectral extinction coefficients of small-pool diffusion flames of six fuels. It was found that the most probable radius of in situ soot particles is less than 0.1 μm for luminous flames examined here and that the relationship between the effective free electron number density and the fuel H/C ratio is well represented by an expression derived from our previous formula.

Determination of effective atomic number and electron density of some fatty acids by gamma-ray attenuation

Total photon mass attenuation coefficients of 7 fatty acids (lauric acid, palmitic acid, stearic acid, oleic acid, myristic acid, castor oil, mustard oil) have been measured in the energy range 356–1116 keV. The experimental results are analysed in terms of total cross sections, effective atomic numbers and electron densities.

New shielding materials for photons and neutrons produced by radiotherapy units

Electron accelerators are widely applied in medicine and industry. In order to estimate the shielding needed for the treatment-rooms equipped with such machines, it is necessary to know the attenuation coefficient of high-energy X-rays in several shielding materials. The attenuation coefficient of X-rays of energies between 4 and 10 MeV in concrete has already been established. Concrete is the most frequently used shielding material because of its low cost. Sometimes, it is necessary, because of geometrical constraints, to use steel or lead in order to reduce the thickness of the shielding. For those materials the attenuation factors of X-rays, for the primary beam, have already been estimated. However, the use of iron increases the cost of shielding, while the use of lead is often precluded by its prohibitive cost. In such cases the author proposes the use of new shielding materials which combine high effective atomic number and high density which provides reduced shielding thickness at low cost because the proposed materials are industrial by-products.

100. 実効原子番号、電子密度による CT 画像の定量解析 (2) : ステロイド投与による骨組成変化(CT-3 計測)

今回の検討から、ISKDCステロイド剤投与方式における1クール目と2クール目の骨動態の相違を示唆する事が出来、これはステロイドオステオポローシス発生機序解明に役立つとともに、薬剤投与の予後予測に有益な指標となりうると思われる。

100. 実効原子番号、電子密度による CT 画像の定量解析 (2) : ステロイド投与による骨組成変化(第 46 回総会学術大会会員研究発表予稿)

100. 実効原子番号、電子密度による CT 画像の定量解析 (2) : ステロイド投与による骨組成変化(第 46 回総会学術大会会員研究発表予稿)

A consistent definition of the number density of pseudo-isotopes

A definition of the effective number density of lumped isotope chains is proposed and its physical interpretation discussed. As the first integral of the system of depletion equations it is claimed to be the most consistent argument in terms of which the pseudo-isotopes effective group cross-sections should be tabulated.

Absolute cross-section measurements in XQQ instruments - The NBS round robin

Triple-quadrupole (QQQ) tandem mass spectrometry (MS/MS) is used for the analysis of multicomponent mixtures [1]. The analysis makes use of the collisionally activated dissociation (CAD) of ions. A ion may be a molecular radical cation, a protonated molecule, or a progeny fragment ion (daughter, granddaughter, etc., produced by the CAD of a larger precursor parent ion). A ion selected by the first quadrupole (Ql) interacts with a gas within the second quadrupole (Q2). Q2 channels undissociated ions and progeny fragment ions into the third quadrupole (Q3) for mass analysis. The instrument thus produces a CAD spectrum of each initially selected ion. But XQQ instruments (QQQ, BEQQ, etc.) are complex ion-optical devices [2-B]. So the choice of parameter settings and/or of instrument design can provide a distorted view of the molecular dynamics of the CAD process (e.g., if there are scattering losses due to poor ion containment within Q2, fringing fields between Q2/Q3, etc. [2-8]). So one observes instrument-dependent CAD spectra. The key MS/MS parameters are: 1) the target thickness=(actual path length traversed by the ion in its complex oscillatory trajectory through the gas target) X (effective number density of the CAD gas); 2) the type of gas (influences the extent of energy transfer); 3) the center-of-mass interaction energy, Ecm; 4) the energy level of the analyzing quadrupole Q3 relative to that of Q2; 5) the Mathieu parameter q2 (rf voltage of Q2) and restrictive interquadrupole apertures of diameter < 1.4 ra; and 6) differences in mass-dependent conversion gain of ion detectors.

Monte-Carlo simulations of light transmission in dispersions of paramagnetic particles

Calculations were made of pair correlation functions in dispersions of interacting paramagnetic particles, as a function of applied magnetic field. Calculations were also made of light transmission through the colloid, which included a geometric shadowing effect and also took account of interference effects between the particles by effective extinction cross-section dependent on the microscopic structure. This, coupled with an effective number density determined by the geometric shadowing effects were used to calculate light transmission curves. These show minima and maxima consistent with experimental data. These data were incompatible with a simple model based on geometrical shadowing alone, which gave a monotonic increase of transmission with applied magnetic field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Validation of absolute target thickness calibrations in a QQQ instrument by measuring absolute total cross-sections of Ne + (Ne, Ne) Ne +

Total cross-sections, σ( E), for the symmetric (resonant) charge transfer reaction Ne + + Ne → Ne + Ne + were measured in our triple-quadrupole (QQQ) tandem mass spectrometer for collision energies E coll ⋍ 5–60 eV (LAB coordinates). This is the range of energies typically used for collision-induced dissociation (CID) in QQQ instruments. We measured identical σ( E) from both the rate of reactant ion decay and the rate of product ion formation. This validates that our QQQ instrument is kinetically well behaved. Our σ( E), for E ⋍ 5–60 eV and P ⋍ 0.04–1 mtorr are in excellent agreement with the Rapp-Francis theory as corrected by Dewangan. Our kinetic method provides a means whereby the absolute target thickness [(actual path length traversed by the ion) × (effective number density of the CID target)] of gas targets can be accurately calibrated in situ for collision energies in the range of ca. 5–60 eV (LAB). Knowledge of target thicknesses is essential for the development of generic CID spectral libraries generated in different types of QQQ instruments [i.e. collision chamber and molecular beam targets]. An interlaboratory round robin test protocol is being formulated to assess the utility of our method for standardization of generic operating parameters. Letters of inquiry from prospective participants may be sent to NBS.

EXACT TREATMENT OF THE DUAL-ENERGY METHOD IN CT USING POLYENERGETIC X-RAY SPECTRA

The theoretical background of the dual-energy method in CT is discussed in detail, especially the meaning of a measured linear attenuation coefficient of a polyenergetic X-ray spectrum in the presence of beam hardening. Using exact values for the incident spectra the iteration procedure shown here allows one to obtain the true values of the effective atomic number and electron density of an absorber material. The applicability of the method in CT is illustrated by computer simulation of the image reconstruction for a homogeneous and an inhomogeneous phantom.