Photon beam softening coefficients evaluation for a 6 MeV photon beam for an aluminum slab: Monte Carlo study using BEAMnrc Code,DOSXYZnrc Code,and BEAMDP code

Determination and understanding the photon beam attenuation by the photon beam modifier and the radiation beam softening for clinical use is more important part of material study for the beam modifier enhancements and the linac improvements. A Monte Carlo model was used to simulate 6 MeV photon beams from a Varian Clinac 2100 accelerator with the flattening filter and the later was replaced by the aluminum slab with variable thickness. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3 mm for distance to agreement. The purpose was to investigate aluminum material attenuation and beam softening coefficients as a function of the inserted aluminum slab thickness and of off-axis distance. The attenuation and beam softening coefficients were not identical for the same off-axis distance and they varied as a function of aluminum slab thickness. The results of our study were shown that the beam softening coefficients were varied with thickness beam modifier material used for beam softening and the off-axis distance inside the irradiation field. Thereafter, the softening coefficient a ₁ have a maximum of 2.5 × 10^–1 cm^–1 for the aluminum slab thickness of 1 mm, 1.4 × 10^–1 cm^–1 for the aluminum slab thickness of 1.5 mm and 4.47 × 10^–2 cm^–1 for the aluminum slab thickness of 2 mm. The maximum of the second softening coefficient a ₂ was 1.02 × 10^–2 cm^–2 for the aluminum slab thickness of 1 mm, was 1.92 × 10^–2 cm^–2 for the aluminum slab thickness of 1.5 mm and was 1.93 × 10^–2 cm^–2 for the aluminum slab thickness of 2 mm. Our study can be a basic investigation of photon beam softening material that will be used in the future linac configuration and also in the photon beam modifiers.     

Spatially-resolved spectroscopic diagnosing of aluminum wire array Z-pinch plasmas on QiangGuang-I facility

Spatially-resolved crystal spectrometers with a high spectral resolution are developed to diagnose K-shell x-ray radiation from Z-pinch plasmas. These diagnostic apparatuses are successfully applied to aluminum wire array Z-pinch experiments on QiangGuang-I facility, a driver with a pulsed current up to about 1.5 MA in 80 ns. Time-integrated experimental results show that the K-shell x-ray emission lines of aluminum Z-pinch plasmas are dominated by line emissions from helium-like ionisation state. Bright spots that might have higher electron temperature or density are produced randomly in location and size along the z-axis during implosions. According to the experimental data, the electron temperature and the ion density are estimated to be between 250 eV and 310 eV, and between 7.0×10¹⁹cm^-3 and 4.0×10¹⁹ cm^-3 respectively, while the ion temperature is inferred to be about 10.2 keV, which is much higher than the electron temperature.     

Spectroscopic diagnostics of carbon and aluminum laser-produced plasmas

VUV emission spectra of plasmas produced by focusing laser radiation with intensity of 10¹⁰–10¹¹ W/cm² on carbon and aluminum targets were studied. Using the partial local thermodynamic equilibrium model for an electron density exceeding 10¹⁷ cm^?3, the spectroscopic diagnostics and the analysis of ion composition of plasmas were carried out. The electron temperatures determined for carbon and aluminum plasmas from the ratio of intensities of ionic lines were found to be 8±3 eV and 11±4 eV, respectively. Stark broadening of aluminum lines was measured and parameters of electron broadening were determined. Using the spatially resolved measurement of Stark line broadening, the spatial density distribution and the law of electron gas expansion were found. The electron gas in the hot region of size 5 mm with an average density of (5±2) 10₁₇cm ^?3 experienced one-dimensional expansion according to the law 1/z ^1.1 with increasing distance z from the target.     

Comparison of atomic potentials and bremsstrahlung Gaunt factors in dense aluminum plasmas

We determine and compare high temperature high density atomic potentials for dense aluminum plasmas. We then evaluate bremsstrahlung Gaunt factors from these potentials utilizing various methods. The potentials considered are obtained from density functional theory, from the hypernetted chain/Poisson model and from the Thomas-Fermi model. The bremsstrahlung spectra obtained for these three potentials, with the partial wave expansion method and for incident electrons of about 1 keV, are in qualitative agreement. We indicate in which circumstances and with what precision bremsstrahlung Gaunt factors can also be estimated from much simpler potentials, such as the Debye or ion sphere model, and from much simpler calculations of the spectrum, such as the Born-Elwert approximation or a simple classical mechanics approach. The aluminum plasmas considered have temperatures of 0.5-1 keV and electron densities of 10²⁵, 10²⁴, 10²³cm^-3.     

Calorimetric study of laser-irradiated thin foil targets

Hydrodynmaic efficiency of laser-irradiated thin aluminum and gold-coated aluminum targets was experimentally determined using a specially designed cone calorimeter. Velocity of the accelerated target and ablation pressure were also estimated from the experimental data. The laser irradiance range used in the experiments was between 10¹² and 10¹³ watts/cm². Experiments indicate that the fall in the hydrodynamic efficiency due to gold coating on aluminum target is about 12% at an irradiance of 8 × 10¹² W/cm².     

Ionic conductivity of pure and doped Li3N

The ionic conductivity of Li₃N crystals doped with various metal ions (magnesium, copper and aluminum) or hydrogen has been investigated. The metal ions have a negative effect on the conductivity whereas hydrogen increases it. The intrinsic Li⁺ ionic conductivity of pure Li₃N is (2·-4)×10^-4Ω^-1cm^-1 at room temperature with an activation energy of 0.26?0.27 eV. Doping with hydrogen to a maximum level of 0.5?1.0 atom% results in a conductivity of 6×10^-3Ω^-1cm^-1 and an activation energy which has been lowered to 0.20 eV. A model is proposed for the action of hydrogen whereby the Li-N bonds next to an NH^2- group are weakened thereby facilatating the creation of Li⁺ Frenkel defects and the vacancy migration. Hydrogen-doped Li₃N is termed an enhanced intrinsic conductor.     

Multiphonon absorption in InP

IR absorption measurements have been made using a high resolution Fourier-transform spectrometer on crystals of InP in the region of (400–4000)cm^-1 at temperature between 11–300k. Very weak, new absorption bands, their absorption coefficients are of ∼10^-1cm^-1, were observed at 996cm^-1, 965cm^-1, 932cm^-1, 838cm^-1, 776cm^-1, 742cm^-1, 718cm^-1, 590cm^-1, 558cm^-1 and 538cm^-1. The frequencies and temperature dependence of these weak bands indicate that they arise from three-phonon processes and appropriate assignments are given. We observed using IR absorption technique for the first time the boron contamination in LEC(B₂O₃)-CZ InP crystal. The content of boron contamination has been proved to be of ∼10¹⁶cm^-3.     

Far-infrared laser oscillation in p-Ge

Time-resolved measurements, together with spectroscopic study with a grating monochromator, are made on far-infrared stimulated emission from p-Ge in crossed electric and magnetic fields. Gain saturation is confirmed, for the first time, to occur to establish laser oscillation. Small-signal gain per unit length deduced from the time constant of light amplification is 7.4 × 10^-3 cm^-1 and 2.7 × 10^-2 cm^-2, respectively, for samples with N_A − N_D ≅ 4.5 × 10¹³cm⁻³ and 1.7 × 10¹⁴ cm^-3. Output power detected at distance 28 cm from the sample is of order 1–10 W, but higher power over 100W is suggested for the total output from the sample.     

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10¹⁵-10¹⁶ cm^-3) and heavily (10¹⁸-10¹⁹ cm^-3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10²⁰ cm^-3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.     

Pressure broadening of the oxygen λ5577 Å line by rare gas

We report on measurements of collision broadening of the electric quadrupole transition λ5577.35 Å in oxygen by argon, krypton and xenon. The broadening constant (linewidth added per unit atom density) was found to be: (4.3 ± 2.3) × 10^-21 cm^-1 atom^-1 cm³ for argon (reported earlier), (8.0 ± 1.7) × 10^-21 cm^-1 atom^-1 cm³ for krypton, and (10.0 ± 2.0) × 10^-21 cm^-1 atom^-1 cm³ for xenon. We discuss some experimental problems associated with these measurements.     

Luminescence studies on swift heavy ion irradiated nanocrystalline aluminum oxide

Pellets of nanocrystalline aluminum oxide synthesized by a combustion technique are irradiated with 120 MeV Au⁹⁺ ions for fluence in the range 5×10¹¹-1×10¹³ ions cm⁻². Two photoluminescence (PL) emissions, a prominent one with peak at ∼525 nm and a shoulder at ∼465 nm are observed in heat treated and Au⁹⁺ ion irradiated aluminum oxide. The 525 nm emission is attributed to F₂²⁺-centers. The PL intensity at 525 nm is found to increase with increase in ion fluence up to 1×10¹² ions cm⁻² and decreases beyond this fluence. Thermoluminescence (TL) of heat-treated and swift heavy ion (SHI) irradiated aluminum oxide gives a strong and broad TL glow with peak at ∼610 K along with a weak shoulder at 500 K. The TL intensity is found to increase with Au⁹⁺ ion fluence up to 1×10¹³ ions cm⁻² and decreases beyond this fluence.     

Polarization of fluorescence of mercurochrom

Mercurochrom is a dibromo mercury substituted derivative of fluorescein and is highly fluorescent. The polarization of fluorescence of this dye is measured for concentrations varying from 10^-6 g/cm³ to 10^-2 g/cm³ and the possibility of using the polarization spectrum for identifying different forms of molecular aggregation is explored.     

InP的多声子红外吸收

利用高分辨的傅里叶光谱仪,在400—4000cm^-1和11—300K范围内,观察到一些其吸收系数为10^-1cm^-1的新弱吸收峰,频率位置为996,965,932,838,806,776,742,718,590,558,538cm^-1。从这些弱吸收峰的频率位置和温度依赖关系,可以认为它们是由于三声子的晶格吸收过程所引起的。对此,我们做了适当的指认。此外,我们首次利用红外吸收法证实,LEC(B₂O₃)-CZInP单晶存在B玷污,其玷污量大约是10¹⁶cm^-3。 关键词：     

µ−spin rotation study of the temperature-dependent relaxation rate of acceptor centers in silicon

Temperature-dependent remanent polarization of negative muons in a silicon crystal doped with phosphorus (3.2 × 10¹², 2.3 × 10¹⁵, and 4.5 × 10¹⁸ cm^?3) and aluminum (2 × 10¹⁴ and 2.4 × 10¹⁸ cm^?3) was examined. Measurements were made over the temperature range 4–300 K in a magnetic field of 2000 G perpendicular to the muon spin. Temperature dependence of the relaxation rate was determined for the magnetic moment of a shallow Al acceptor center in a nondeformed silicon sample, and the hyperfine interaction constant was estimated for the interaction between the magnetic moments of muon and electron shell of the muonic mAl atom in silicon.     

硅的低温电学性质

本文在20°—300°K研究了室温载流子浓度2×10¹²—1×10²⁰cm^-3含硼或磷(砷)Si的电学性质。对一些p-Si样品用弱场横向磁阻法及杂质激活能法进行了补偿度的测定,并进行了比较。从霍尔系数与温度关系的分析指出,对于较纯样品,硼受主能级的电离能为0.045eV,磷施主能级为0.045eV,在载流子浓度为10¹⁸—10¹⁹cm^-3时发现了费米简并,对载流子浓度为2×10¹⁷—1×10¹⁸cm^-3的p-Si及5×10¹⁷—4×10¹⁸cm^-3的n-Si观察到了杂质电导行为。从霍尔系数与电导率计算了非本征的霍尔迁移率。在100°—300°K间,晶格散射迁移率μ满足关系式AT^-a,其中A=2.1×10⁹,α=2.7(对空穴);或A=1.2×10⁸,α=2.0(对电子)。另外,根据我们的材料(载流子浓度在5×10¹¹—5×10²⁰cm^-3间),分别建立了一条电阻率与载流子浓度及电阻率与迁移率的关系曲线,以提供制备材料时参考之用。     

Stark broadening tables for He I λ4922Å

The Stark broadening of He I λ4922Å and its forbidden components by both ions and electrons is calculated using a theory that includes ion dynamic effects. Tables are presented for temperatures from 5000°K to 40,000°K covering the density range 10¹³ cm^-3 to 10¹⁶ cm^-3 for both helium and hydrogen ionic perturbers.     

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phonon--plasmon coupled (LOPC) modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2×10¹⁸ cm^-3 and 8×10¹⁸ cm^-3 with a carrier mobility of 30--55 cm²/(V·s) for n-type 4H-SiC substrates and 1×10¹⁶--3×10¹⁶ cm^-3 with mobility of 290--490 cm²/(V·s) for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×10¹⁶ cm^-3 with mobility of 380 cm²/(V·s). It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.     

Study of hydrogenated AlN as an anti‐reflective coating and for the effective surface passivation of silicon

Stacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual‐layer stack. Deposited on 1 Ω cm p‐type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 10¹² cm^–2 and a very low interface defect density below 5 × 10¹⁰ eV^–1 cm^–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti‐reflective coatings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the role of prepulses during solid target heating by picosecond laser pulses

X-ray emission spectra of the plasma created at the surface of magnesium, aluminum, copper, and zinc targets heated by 1-ps laser pulses with a peak power density of up to 10¹⁶ W/cm² were measured. The effect of a picosecond prepulse on the spectra was studied for various power densities and intensity contrasts of the main laser pulse. It is established that the emission spectra of laser plasmas are weakly affected by a change from 10⁵ to 10⁷ in the main pulse contrast relative to the first prepulse. Variations in the parameters of emission from aluminum and magnesium plasmas were calculated using relative intensities and widths of the resonance lines of H-and He-like ions and their two-electron satellite peaks.     

Characterization of deep level defects in sublimation grown p-type 6H-SiC epilayers by deep level transient spectroscopy

In this study deep level transient spectroscopy has been performed on boron–nitrogen co-doped 6H-SiC epilayers exhibiting p-type conductivity with free carrier concentration (N_A–N_D)∼3×10¹⁷ cm⁻³. We observed a hole H₁ majority carrier and an electron E₁ minority carrier traps in the device having activation energies E_v+0.24 eV, E_c −0.41 eV, respectively. The capture cross-section and trap concentration of H₁ and E₁ levels were found to be (5×10⁻¹⁹ cm², 2×10¹⁵ cm⁻³) and (1.6×10⁻¹⁶ cm², 3×10¹⁵ cm⁻³), respectively. Owing to the background involvement of aluminum in growth reactor and comparison of the obtained data with the literature, the H₁ defect was identified as aluminum acceptor. A reasonable justification has been given to correlate the E₁ defect to a nitrogen donor.