THE CALIBRATION AND IMPLEMENTATION OF CR-39 NUCLEAR TRACK DETECTOR MATERIAL AS A FAST NEUTRON MICRODOSIMETER

A detailed microdosimetric characterization of the M. D. Anderson 42 MeV (p,Be) fast neutron beam was performed using the techniques of microdosimetry and a 1/2 inch diameter Rossi proportional counter. These measurements were performed at 5, 15, and 30 cm depths on the central axis, 3 cm inside, and 3 cm outside the field edge for 10 $\times$ 10 and 20 $\times$ 20 cm field sizes. Spectra were also measured at 5 and 15 cm depth on central axis for a 6 $\times$ 6 cm field size. Continuous slowing down approximation calculations were performed to model the nuclear processes that occur in the fast neutron beam. Irradiation of the CR-39 was performed using a tandem electrostatic accelerator for protons of 10, 6, and 3 MeV and alpha particles of 15, 10, and 7 MeV incident energy on target at angles of incidence from 0 to 85 degrees. The critical angle as well as track etch rate and normal incidence diameter versus linear energy transfer (LET) were obtained from these measurements. The bulk etch rate was also calculated from these measurements. Dose response of the material was studied, and the angular distribution of charged particles created by the fast neutron beam was measured with CR-39. The efficiency of CR-39 was calculated versus that of the Rossi chamber, and an algorithm was devised for derivation of LET spectra from the major and minor axis dimensions of the observed tracks. The CR-39 was irradiated in the same positions as the Rossi chamber, and the derived spectra were compared directly. ^

Beamlet based direct aperture optimization for MERT using a photon MLC

PURPOSE A beamlet based direct aperture optimization (DAO) for modulated electron radiotherapy (MERT) using photon multileaf collimator (pMLC) shaped electron fields is developed and investigated. METHODS The Swiss Monte Carlo Plan (SMCP) allows the calculation of dose distributions for pMLC shaped electron beams. SMCP is interfaced with the Eclipse TPS (Varian Medical Systems, Palo Alto, CA) which can thus be included into the inverse treatment planning process for MERT. This process starts with the import of a CT-scan into Eclipse, the contouring of the target and the organs at risk (OARs), and the choice of the initial electron beam directions. For each electron beam, the number of apertures, their energy, and initial shape are defined. Furthermore, the DAO requires dose-volume constraints for the structures contoured. In order to carry out the DAO efficiently, the initial electron beams are divided into a grid of beamlets. For each of those, the dose distribution is precalculated using a modified electron beam model, resulting in a dose list for each beamlet and energy. Then the DAO is carried out, leading to a set of optimal apertures and corresponding weights. These optimal apertures are now converted into pMLC shaped segments and the dose calculation for each segment is performed. For these dose distributions, a weight optimization process is launched in order to minimize the differences between the dose distribution using the optimal apertures and the pMLC segments. Finally, a deliverable dose distribution for the MERT plan is obtained and loaded back into Eclipse for evaluation. For an idealized water phantom geometry, a MERT treatment plan is created and compared to the plan obtained using a previously developed forward planning strategy. Further, MERT treatment plans for three clinical situations (breast, chest wall, and parotid metastasis of a squamous cell skin carcinoma) are created using the developed inverse planning strategy. The MERT plans are compared to clinical standard treatment plans using photon beams and the differences between the optimal and the deliverable dose distributions are determined. RESULTS For the idealized water phantom geometry, the inversely optimized MERT plan is able to obtain the same PTV coverage, but with an improved OAR sparing compared to the forwardly optimized plan. Regarding the right-sided breast case, the MERT plan is able to reduce the lung volume receiving more than 30% of the prescribed dose and the mean lung dose compared to the standard plan. However, the standard plan leads to a better homogeneity within the CTV. The results for the left-sided thorax wall are similar but also the dose to the heart is reduced comparing MERT to the standard treatment plan. For the parotid case, MERT leads to lower doses for almost all OARs but to a less homogeneous dose distribution for the PTV when compared to a standard plan. For all cases, the weight optimization successfully minimized the differences between the optimal and the deliverable dose distribution. CONCLUSIONS A beamlet based DAO using multiple beam angles is implemented and successfully tested for an idealized water phantom geometry and clinical situations.

A 3-D CT assisted Monte Carlo evaluation of intracavitary brachytherapy implants

Intracavitary brachytherapy (ICB) combined with external beam irradiation for treatment of cervical cancer is highly successful in achieving local control. The M.D. Anderson Cancer Center employs Fletcher Suit Delclos (FSD) applicators. FSD applicators contain shields to limit dose to critical structures. Dosimetric evaluation of ICB implants is limited to assessing dose at reference points. These points serve as surrogates for treatment intensity and critical structure dose. Several studies have mentioned that the ICRU38 reference points inadequately characterize the dose distribution. Also, the ovoid shields are rarely considered in dosimetry. ^ The goal of this dissertation was to ascertain the influence of the ovoid shields on patient dose distributions. Monte Carlo dosimetry (MCD) was applied to patient computed tomography(CT) scans. These data were analyzed to determine the effect of the shields on dose to standard reference points and the bladder and rectum. The hypothesis of this work is that the ICRU38 bladder and rectal points computed conventionally are not clinically acceptable surrogates for the maximum dose points as determined by MCD. ^ MCD was applied to the tandem and ovoids. The FSD ovoids and tandem were modeled in a single input file that allowed dose to be calculated for any patient. Dose difference surface histograms(DDSH) were computed for the bladder and rectum. Reference point doses were compared between shielded and unshielded ovoids, and a commercial treatment planning system. ^ The results of this work showed the tandem tip screw caused a 33% reduction in dose. The ovoid shields reduced the dose by a maximum of 48.9%. DDSHs revealed on average 5% of the bladder surface area was spared 53 cGy and 5% of the rectal surface area was spared 195 cGy. The ovoid shields on average reduced the dose by 18% for the bladder point and 25% for the rectal point. The Student's t-test revealed the ICRU38 bladder and rectal points do not predict the maximum dose for these organs. ^ It is concluded that modeling the tandem and ovoid internal structures is necessary for accurate dose calculations, the bladder shielding segments may not be necessary, and that the ICRU38 bladder point is irrelevant. ^

Geochemistry of ash layers from the Bonin Forearc

Many ash-rich layers, varying from a few millimeters to several centimeters thick, were identified in the sedimentary sequences penetrated during Ocean Drilling Program Leg 125 at Sites 782, 784, and 786, located about 400 to 500 km south of Tokyo in the Bonin forearc. The total age range of the ash layers is from Eocene to Pleistocene, although not all sites cover this full span. The ashes consist of vitric, microlite-bearing, and crystal-rich components; the glassy shards are typically highly vesicular, with elongate, flattened bubbles. The dominant crystalline phases are orthopyroxene, clinopyroxene, and plagioclase. The major-element compositions of individual vitric shards collected from selected layers of Holes 782A, 784A, and 786A were determined by electron microprobe analyses; particular care was taken to ensure that the analytical results were not compromised by electron beam damage to the glasses. Compositions range from basalt through andesite and dacite to rhyolite and generally belong to a tholeiitic, low-K suite. There is no indication of any regular secular change during the evolution of the Bonin arc from tholeiitic through calc-alkalic to alkali compositions with time. In Holes 782A and 784A, some high-K rhyolite compositions of late Miocene and Pleistocene age are present. A clear chemical distinction has existed since arc inception between the source(s) of these ashes and the upper mantle source(s) tapped during construction of the igneous basement that formed the forearc.

Fabrication of sub-100 nm IDT SAW devices on insulating, semiconducting and conductive substrates

This work describes the electron-beam (e-beam) lithography process developed to manufacture nano interdigital transducers (IDTs) to be used in high frequency (GHz) surface acoustic wave (SAW) applications. The combination of electron-beam (e-beam) lithography and lift-off process is shown to be effective in fabricating well-defined IDT finger patterns with a line width below 100 nm with a good yield. Working with insulating piezoelectric substrates brings about e-beam deflection. It is also shown how a very thin organic anti-static layer works well in avoiding this charge accumulation during e-beam lithography on the resist layer. However, the use of this anti-static layer is not required with the insulating piezoelectric layer laying on a semiconducting substrate such as highly doped silicon. The effect of the e-beam dose on a number of different layers (of insulating, insulating on semiconducting, semiconducting, and conductive natures) is provided. Among other advantages, the use of reduced e-beam doses increases the manufacturing time. The principal aim of this work is to explain the interrelation among e-beam dose, substrate nature and IDT structure. An extensive study of the e-beam lithography of long IDT-fingers is provided, in a wide variety of electrode widths, electrode numbers and electrode pitches. It is worthy to highlight that this work shows the influence of the e-beam dose on five substrates of different conductive nature

Swift heavy ion damage to sodium chloride: synergy between excitation and thermal spikes

Systematic data on the effect of irradiation with swift ions (Zn at 735 MeV and Xe at 929 MeV) on NaCl single crystals have been analysed in terms of a synergetic two-spike approach (thermal and excitation spikes). The coupling of the two spikes, simultaneously generated by the irradiation, contributes to the operation of a non-radiative exciton decay model as proposed for purely ionization damage. Using this scheme, we have accounted for the π-emission yield of self-trapped excitons and its temperature dependence under ion-beam irradiation. Moreover, the initial production rates of F-centre growth have also been reasonably simulated for irradiation at low temperatures ( < 100 K), where colour centre annealing and aggregation can be neglected.

Ferromagnetic resonance and magnetooptic study of submicron epitaxial Fe(001) stripes

We present a combined magnetooptic and ferromagnetic resonance study of a series of arrays of single-crystalline Fe stripes fabricated by electron beam lithography on epitaxial Au(001)/Fe(001)/MgO(001) films grown by pulsed laser deposition. The analysis of the films revealed a clear fourfold magnetocrystalline anisotropy, with no significant presence of other anisotropy sources. The use of a large series of arrays, with stripe widths between 140 and 1000 nm and separation between them of either 200 nm or 500 nm, allowed studying their magnetization processes and resonance modes as well as the effects of the dipolar interactions on both. The magnetization processes of the stripes were interpreted in terms of a macrospin approximation, with a good agreement between experiments and calculations and negligible influence of the dipolar interactions. The ferromagnetic resonance spectra evidenced two types of resonances linked to bulk oscillation modes, essentially insensitive to the dipolar interactions, and a third one associated with edge-localized oscillations, whose resonance field is strongly dependent on the dipolar interactions. The ability to produce a high quality, controlled series of stripes provided a good opportunity to achieve an agreement between the experiments and calculations, carried out by taking into account just the Fe intrinsic properties and the morphology of the arrays, thus evidencing the relatively small role of other extrinsic factors.

Floating bare tether as upper atmosphere probe

Use of a conductive bare tape electrically floating in low Earth orbit as an effective electron beam source to produce artificial auroral effects, free of problems that mard tandard beams, is considered. Ambient ions impacting the tape with keV energies over most of its length liberate secondary electrons that race down the magnetic field, excite neutrals in the E layer, and result in auroral emissions. The tether would operate with both a power supply and a plasma contactor off at nighttime; power and contactor would be on at daytime for reboost. Tomographic analysis of auroral emissions from the footprint of the beam, as observed from the spacecraft, can provide density profiles of dominant neutral species in the E layer. A characteristic tether system, at altitude 300 km and moderate orbital inclination, would involve an aluminum tape with a length of 20 km, a width of 15 mm, and a thickness of 0.2 mm for a full-system mass around 1200 kg, with two thirds going into the power subsystem.

Temperature performance of AlGaN/GaN MOS-HEMTs on Si substrates using Gd2O3 as gate dielectric

GaN based high electron mobility transistors have draw great attention due to its potential in high temperature, high power and high frequency applications [1, 2]. However, significant gate leakage current is still one of the issues which need to be solved to improve the performance and reliability of the devices [3]. Several research groups have contributed to solve this problem by using metal–oxide–semiconductor HEMTs (MOSHEMTs), with a thin dielectric layer, such as SiO2 [4], Al2O3 [5], HfO2 [6] and Gd2O3 [7] between the gate and the barrier layer on AlGaN/GaN heterostructures. Gd2O3 has shown low interfacial density of states(Dit) with GaN and a high dielectric constant and low electrical leakage currents [8], thus is considered as a promising candidate for the gate dielectrics on GaN. MOS-HEMTs using Gd2O3 grown by electron-beam heating [7] or molecular beam epitaxy (MBE) [8] on GaN or AlGan/GaN structure have been investigated, but further research is still needed in Gd2O3 based AlGaN/GaN MOSHEMTs.

Ti/Pd/Ag Contacts to n-Type GaAs for High Current Density Devices

The metallization stack Ti/Pd/Ag on n-type Si has been readily used in solar cells due to its low metal/semiconductor specific contact resistance, very high sheet conductance, bondability, long-term durability, and cost-effectiveness. In this study, the use of Ti/Pd/Ag metallization on n-type GaAs is examined, targeting electronic devices that need to handle high current densities and with grid-like contacts with limited surface coverage (i.e., solar cells, lasers, or light emitting diodes). Ti/Pd/Ag (50 nm/50 nm/1000 nm) metal layers were deposited on n-type GaAs by electron beam evaporation and the contact quality was assessed for different doping levels (from 1.3 × 1018 cm−3 to 1.6 × 1019 cm−3) and annealing temperatures (from 300°C to 750°C). The metal/semiconductor specific contact resistance, metal resistivity, and the morphology of the contacts were studied. The results show that samples doped in the range of 1018 cm−3 had Schottky-like I–V characteristics and only samples doped 1.6 × 1019 cm−3 exhibited ohmic behavior even before annealing. For the ohmic contacts, increasing annealing temperature causes a decrease in the specific contact resistance (ρ c,Ti/Pd/Ag ~ 5 × 10−4 Ω cm2). In regard to the metal resistivity, Ti/Pd/Ag metallization presents a very good metal conductivity for samples treated below 500°C (ρ M,Ti/Pd/Ag ~ 2.3 × 10−6 Ω cm); however, for samples treated at 750°C, metal resistivity is strongly degraded due to morphological degradation and contamination in the silver overlayer. As compared to the classic AuGe/Ni/Au metal system, the Ti/Pd/Ag system shows higher metal/semiconductor specific contact resistance and one order of magnitude lower metal resistivity.

Desenvolvimento e caracterização do filtro óptico de interferência variável para detectores de alta resolução espectral e biossensores.

O presente trabalho está fundamentado no desenvolvimento de uma metodologia e/ou uma tecnologia de obtenção e caracterização de filtros ópticos de interferência de banda passante variável [C.M. da Silva, 2010] e de banda de corte variáveis, constituídos por refletores dielétricos multicamadas de filmes finos intercalados por cavidades de Fabry-Perot não planares com espessuras linearmente variáveis, que apresentam a propriedade do deslocamento linear da transmitância máxima espectral em função da posição, isto é, um Filtro de Interferência Variável (FIV). Este método apresenta novas e abrangentes possibilidades de confecção de filtros ópticos de interferência variável: lineares ou em outras formas desejadas, de comprimento de onda de corte variável (passa baixa ou alta) e filtros de densidade neutra variável, através da deposição de metais, além de aplicações em uma promissora e nova área de pesquisa na deposição de filmes finos não uniformes. A etapa inicial deste desenvolvimento foi o estudo da teoria dos filtros ópticos dielétricos de interferência para projetar e construir um filtro óptico banda passante convencional de um comprimento de onda central com camadas homogêneas. A etapa seguinte, com base na teoria óptica dos filmes finos já estabelecida, foi desenvolver a extensão destes conhecimentos para determinar que a variação da espessura em um perfil inclinado e linear da cavidade entre os refletores de Bragg é o principal parâmetro para produzir o deslocamento espacial da transmitância espectral, possibilitando o uso de técnicas especiais para se obter uma variação em faixas de bandas de grande amplitude, em um único filtro. Um trabalho de modelagem analítica e análise de tolerância de espessuras dos filmes depositados foram necessários para a seleção da estratégia do \"mascaramento\" seletivo do material evaporado formado na câmara e-Beam (elétron-Beam) com o objetivo da obtenção do filtro espectral linear variável de características desejadas. Para tanto, de acordo com os requisitos de projeto, foram necessárias adaptações em uma evaporadora por e-Beam para receber um obliterador mecânico especialmente projetado para compatibilizar os parâmetros das técnicas convencionais de deposição com o objetivo de se obter um perfil inclinado, perfil este previsto em processos de simulação para ajustar e calibrar a geometria do obliterador e se obter um filme depositado na espessura, conformação e disposição pretendidos. Ao final destas etapas de modelagem analítica, simulação e refinamento recorrente, foram determinados os parâmetros de projeto para obtenção de um determinado FIV (Filtro de Interferência Variável) especificado. Baseadas nos FIVs muitas aplicações são emergentes: dispositivos multi, hiper e ultra espectral para sensoriamento remoto e análise ambiental, sistemas Lab-on-Chip, biossensores, detectores chip-sized, espectrofotometria de fluorescência on-chip, detectores de deslocamento de comprimento de onda, sistemas de interrogação, sistemas de imageamento espectral, microespectrofotômetros e etc. No escopo deste trabalho se pretende abranger um estudo de uma referência básica do emprego do (FIV) filtro de interferência variável como detector de varredura de comprimento de ondas em sensores biológicos e químicos compatível com pós processamento CMOS. Um sistema básico que é constituído por um FIV montado sobre uma matriz de sensores ópticos conectada a um módulo eletrônico dedicado a medir a intensidade da radiação incidente e as bandas de absorção das moléculas presentes em uma câmara de detecção de um sistema próprio de canais de microfluidos, configurando-se em um sistema de aquisição e armazenamento de dados (DAS), é proposto para demonstrar as possibilidades do FIV e para servir de base para estudos exploratórios das suas diversas potencialidades que, entre tantas, algumas são mencionadas ao longo deste trabalho. O protótipo obtido é capaz de analisar fluidos químicos ou biológicos e pode ser confrontado com os resultados obtidos por equipamentos homologados de uso corrente.

Total glossectomy: Is it justified?

Background: The treatment of advanced malignancies of the tongue with total glossectomy is controversial. Methods: The present study retrospectively reviews 20 patients that had total glossectomy with or without laryngectomy performed at the Princess Alexandra Hospital (Woolloongabba, QLD, Australia) over the past 10 years. The clinicopathologic parameters, perioperative morbidity and mortality were assessed. Results: The 5 years disease free and disease specific survival rates were 34% and 38%, respectively. Ninety per cent of patients acquired independent oral feeding prior to discharge, while 57% of patients that had laryngeal preservation performed achieved satisfactory vocal rehabilitation. Conclusion: Total glossectomy is an oncologically sound surgical procedure that should only be performed in carefully selected patients with advanced malignancies affecting the tongue.

A detailed study of the mechanical and electrical aspects of microparticle impact phenomena

An experimental and theoretical study of the impact behaviour of charged microparticles in a high voltage vacuum gap has been carried out to investigate under controlled conditions the role of low velocity microparticles (ζ 500 ms-1) in initiating electrical breakdown in such gaps. This has involved developing a unique (UHV) low-velocity source of micron-sized charged particles to study the underlying mechanical and electrical aspects of micro-particle impact on a range of target materials e.g. Pb, Ti, C, stainless-steel and mica etc., having atomically clean or oxidised surfaces. Argon-ion etching and electron-beam heating has been used for in-situ surface treatment and ellipsometry for characterising the target surfaces. An associated sphere/plane theoretical model has been developed for detailed analysis of the many complex electrical (in-flight in-field emission, M.I.M. tunnelling and ohmic conduction) and mechanical (impact dynamics, deformation and heating) phenomena that are involved when a microparticle closely approaches and impacts on a plane target. In each instance the influence of parameters such as particle radius, particle/target impact velocity, surface field, surface condition and material has been determined.

Ion plating

Under ideal conditions ion plating produces finely grained dense coatings with excellent adhesion. The ion bombardment induced damage initiates a large number of small nuclei. Simultaneous coating and sputtering stimulates high rates of diffusion and forms an interfacial region of graded composition responsible for good adhesion. To obtain such coatings on components far industrial applications, the design and construction Of an ion plater with a 24" (O.6rn) diameter chamber were investigated and modifications of the electron beam gun were proposed. A 12" (O.3m) diameter ion plater was designed and constructed. The equipment was used to develop surfaces for solar energy applications. The conditions to give extended surfaces by sputter etching were studied. Austenitic stainless steel was sputter etched at 20 and 30 mTorr working pressure and at 3, 4 and 5 kV. Uniform etching was achieved by redesigning the specimen holder to give a uniform electrostatic field over the surfaces of the specimens. Surface protrusions were observed after sputter etching. They were caused by the sputter process and were independent of grain boundaries, surface contaminants and inclusions. The sputtering rate of stainless steel was highly dependent on the background pressure which should be kept below 10-5 Torr. Sputter etching improved the performance of stainless steel used as a solar selective surface. A twofold improvement was achieved on sputter etching bright annealed stainless steel. However, there was only slight improvement after sputter etching stainless steel which had been mechanically polished to a mirror finish. Cooling curves Were used to measure the thermal emittance of specimens.The deposition rate of copper was measured at different levels of power input and was found to be a maximum at 9.5 kW. The diameter of the copper feed rod was found to be critical for the maintenance of a uniform evaporation rate.

Why is it possible to detect doped regions of semiconductors in low voltage SEM:a review and update

There is an urgent need for fast, non-destructive and quantitative two-dimensional dopant profiling of modern and future ultra large-scale semiconductor devices. The low voltage scanning electron microscope (LVSEM) has emerged to satisfy this need, in part, whereby it is possible to detect different secondary electron yield values (brightness in the SEM signal) from the p-type to the n-type doped regions as well as different brightness levels from the same dopant type. The mechanism that gives rise to such a secondary electron (SE) contrast effect is not fully understood, however. A review of the different models that have been proposed to explain this SE contrast is given. We report on new experiments that support the proposal that this contrast is due to the establishment of metal-to-semiconductor surface contacts. Further experiments showing the effect of instrument parameters including the electron dose, the scan speeds and the electron beam energy on the SE contrast are also reported. Preliminary results on the dependence of the SE contrast on the existence of a surface structure featuring metal-oxide semiconductor (MOS) are also reported. Copyright © 2005 John Wiley & Sons, Ltd.