Entrance Surfaces Research Articles

Investigation of a MOSFET dosimetry system for midpoint dose verification in prostate 3D CRT/IMRT

The suitability of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) for use in in-vivo dosimetry for IMRT prostate treatment and patient setup errors has been investigated in this work. MOSFETs were placed on entrance and exit surfaces of a number of different phantoms (with varying complexities from homogeneous to anthropomorphic). Dose measurements were then used to calculate a midpoint dose, which was compared with an IC placed at the isocentre. The agreements found between the calculated (MOSFETs) and the measured midpoint dose (IC) was: 0.7% for a prostate treatment verification and 3.5% for an IMRT treatment. MOSFETs placed on entry and exit surfaces can detect patient setup offsets of 2 cm, but do not have the sensitivity to confidently detect offsets of 1 cm or smaller.

Transformation-optical design of adaptive beam bends and beam expanders

We describe the design of adaptive beam bends and beam splitters with arbitrary bend and split angles by use of finite embedded coordinate transformations. The devices do not exhibit reflection at the entrance or exit surfaces. It is shown that moderate and practically achievable values of the relative permittivity and permeability can be obtained for beam bends and splitters with both small and large bend radius. The devices are also discussed in the context of reconfigurable metamaterials, in which the bend and split angles can be dynamically tuned. The performance of adaptive beam bends and splitters is demonstrated in full wave simulations based on a finite-element method. Furthermore, the design of an adaptively adjustable transformation-optical beam expander/compressor is presented. It is observed that a pure transformation-optical design cannot result in a reflectionless beam expander/compressor.

Focus on lasers and optics

Focus on lasers and optics

Excavation rate of silicon surface nanoholes

Silicon surface nanoholes, that are small pits introduced spontaneously on electron exit surfaces of silicon foils under electron irradiation, are excavated at a constant rate with increasing electron dose. On the other hand, surface nanoholes on electron entrance surfaces become shallow under electron irradiation. The mechanism for the excavation and shallowing of surface nanoholes is discussed in terms of the movement of surface vacancies assisted by irradiating electrons.

RADIATION DOSES TO PATIENTS FROM EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY

The aim of this study was to determine the radiation doses to patients during extracorporeal shock wave lithotripsy (ESWL) and compare them with the available bibliographical data. In this method localization of the renal stones is attained by the use of fluoroscopy, and thus ESWL is included among those medical practices associated with patient radiation exposure. The entrance surface dose was measured using 3-4 thermoluminescent dosimeters positioned on the patients' backs at the entrance surfaces of the two x-ray beams for 50 ESWL procedures. Fluoroscopy time and number of spot films were also recorded. The average entrance surface dose at the patient's side with the renal stone was estimated to be 76.5 mGy for the oblique x-ray beam and 44.5 mGy for the PA x-ray beam. The mean fluoroscopy time was 204 s while 4 spot films were acquired on average. The mean effective dose (E) was estimated as 1.63 mSv per patient. The mean entrance surface dose values recorded in this study are comparable to but smaller than the values reported in the literature for ESWL, while the mean fluoroscopy time is within the range of values reported by other authors. On the other hand, the estimated E value is relatively higher compared to the corresponding values given in the literature based on patient measurements. However, it is comparable to recent published data acquired using extended measurements in an anthropomorphic phantom.

Femtosecond laser ablation of carbon reinforced polymers

Interaction of intense ultrashort laser pulses (120 fs at 795 nm) with polymer based composites has been investigated. We have found that carbon filled polymers exhibit different ultrafast ablation behaviour depending on whether the filling material is carbon black or carbon fiber and on the polymer matrix itself. The shape and dimensions of the filling material are responsible for some geometrical bad quality effects in the entrance and inner surfaces of drilled microholes. We give an explanation for these non-quality effects in terms of fundamentals of ultrafast ablation process, specifically threshold laser fluences and material removal paths. Since carbon fiber reinforced polymers seemed particularly concerned, this could prevent the use of ultrafast ablation for microprocessing purposes of some of these materials.

Classical trajectory calculations for collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra of N2, CO, and CH3CN with metastable He*(2 3S) atoms

Collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectra (2D-PIES) of N2, CO, and CH3CN with metastable He*(2 3S) atoms are measured, and classical trajectory calculations with anisotropic entrance and exit potential energy surfaces are performed for these systems. Numerical qualities of the entrance potential surfaces are decisively important to understand the collisional ionization dynamics as well as to reproduce observed 2D-PIES, whereas the exit potential surfaces are less sensitive to the collisional ionization dynamics and the electron spectra except for special cases in which a deep potential well is relevant in the entrance potential surface. Ab initio calculations of both entrance and exit potentials as well as ionization widths are found to be reliable in obtaining their anisotropy and radial dependence with good quantitative accuracy.

Secondary electron emission near the electronic stopping power maximum

We report on measurements of light ion $(Z=2)$ and heavy ion $(Z=14)$ induced electron emission yields from the entrance and exit surfaces of thin carbon foils near the electronic energy loss per unit path length $(dE/dx)$ maximum. At constant $dE/dx,$ secondary electron yields are lower for high projectile velocities than for lower projectile velocities. This velocity effect, which is well known for secondary ion emission, is observed for backward electron emission for both He and Si. In the case of Si projectiles such a velocity effect is also observed for forward electron emission. The results can be qualitatively understood in the framework of recent electron transport and nuclear track models.

Elastic scattering of partially coherent beams of fast electrons by a crystal with a defect.

Scattering of a quasi-monochromatic electron beam by a crystal with a defect is described with the use of the mutual coherency function and the formalism of quasi-Bloch waves. An expression correlating the mutual intensity on the exit and entrance surfaces of the crystal in terms of the scattering matrix has been found. The matrix elements are determined by a system of integro-differential equations, which have been obtained without using the column approximation. It has been shown that calculations of the matrix elements can be significantly simplified when the approximation of the small-angle scattering of quasi-Bloch waves by the defect displacement field is satisfied. Such an approximation can be applied in many cases, e.g. to a crystal with a dislocation. The mutual intensity on the crystal entrance surface has been found for the general case of defocused illumination. As an example of applying the new approach, expressions for the intensity in convergent-beam electron diffraction (CBED) and large-angle CBED (LACBED) patterns have been obtained. The LACBED patterns of a crystal with a dislocation have been simulated. It has been shown that the developed approach allows a more exact simulation of the LACBED than do the conventional approaches using the column approximation and the approximation of independent plane waves filling the illumination cone.

Shaping of the end of a multimode optical fiber for efficient coupling light from a laser diode

An entrance surface of a multimode optical fiber with the highest efficiency of coupling radiation from a laser diode is considered. The geometry of the surface is designed with the purpose of keeping a refracted ray inside the fiber with simultaneous minimization of the reflection coefficient in each point of the surface. As a good approximation to the ideal surface, the composite entrance surface of a cylinder-wedge shape is proposed. It has clearly improved the coupling efficiency as compared with that of the purely cylindrical surface. Analysis of the entrance surfaces is based on their transmission coefficients, depending on the incident ray direction.

Application of self-focusing of ps laser pulses for three-dimensional microstructuring of transparent materials

Self-focusing of ps laser pulses (λ=790 nm) due to the nonlinear Kerr effect is shown to provide an easy and flexible method for producing bulk structures with dimensions on the order of 10 μm without damaging the entrance or exit surfaces of the material. The depth at which damage occurs can be controlled by adjusting either the pulse energy or pulse length. A study of the dependence of structure depth on the square root of the laser power for a given pulse length provides a straightforward method for determining the nonlinear index of refraction (Kerr coefficient), n2.

Primary extinction in cylinders and spheres

By transforming the Takagi equations into a representation using angular coordinates, it is in principle possible to obtain analytical expressions for the coefficients in a series expansion for the primary extinction factor in perfect crystals with a circular diffraction plane. In practice, it has been possible to obtain the first five terms in the expansion. This involves establishing recurrence relations for the families of Bragg and Laue boundary-value Green functions combined with integrations over the entrance and exit surfaces. The calculations, which cover the whole range of values for the scattering angle, theta oh, are performed using the mathematical software systems Mathematica and Maple.

Tracks in YIG induced by MeV C 60 ions

Samples of yttrium iron garnet (Y 3Fe 5O 12 or YIG) were irradiated at 300 K with MeV C 60-ions at normal and tilted incidence. As observed by transmission electron microscopy, the cluster ions create amorphous and continuous tracks. Craters generated both at the entrance and exit surfaces of the samples are clearly visible at tilted incidence. Moreover, hillocks are seen at the entrance and exit of the tracks. On the sample surface matter ejected from the tracks has been deposited. Some of the tracks induced by high energy C 60-ions contain a “bubble-like structure” inside the amorphous part of the tracks, which appears only in very thin zones of the samples and when the projectiles exit from the sample. This is in contrast to the tracks created by smaller cluster ions and by GeV heavy ions for which no structure was seen.

Experimental and computational comparative study of electron emission of thin carbon foils traversed by MeV protons and hydrogen atoms in frozen charge states

The secondary electron emission has been measured for 2 MeV H + and for H 0 projectiles passing through very thin carbon foils without charge changing. We have studied the statistics of the emission from both entrance and emergence surfaces. Simultaneously, Monte Carlo calculations have been performed for similar experimental conditions (500 keV < E < 2 MeV). Both experimental and theoretical distributions can be fitted adequately by Pólya distributions. The measured and calculated differences of the yields and of the distribution widths between the H + and “transmitted” H 0 cases are interpreted in terms of screening effects on the large impact parameter collisions with target electrons.

Role of projectile electrons in secondary electron emission from solid surfaces under fast-ion bombardment

We report on measurements of the number of electrons per incoming projectile emitted from the projectile entrance and exit surfaces of thin carbon foils as function of the projectile atomic number Z (1{le}Z{le}32), incident charge states, and velocities below, near, and above the stopping power maximum. The screening of the projectile charge by the projectile electrons reduces the backward electron emission. The corresponding reduction parameters have been determined as function of the number of projectile electrons. For all ions but protons we note as a general trend that the forward to backward yield ratio R{sub {gamma}} increases with the projectile velocity. In addition a pronounced increase of the R{sub {gamma}} with increasing Z is also observed. It was found that the increase of the R{sub {gamma}} with Z in the medium velocity region ({approximately}1MeV/u) is not a {open_quotes}nuclear charge{close_quotes} effect, but is simply due to the difference in the incident number of projectile electrons. The experimental results are compared to Monte Carlo simulations based on the work by Gervais and Bouffard. A reasonable agreement (within 20{percent}) between experimental and calculated results for heavy ions is observed. However, the difference is larger for light ions. Finally, the results are discussed withinmore » the framework of previously published semiempirical models and the relation between electron yields and the electronic stopping power is elaborated. {copyright} {ital 1997} {ital The American Physical Society}« less

Sputtering of thin carbon foils by 20 keV and 40 keV Ar + bombardment

Sputtering of nominal 2 μg/cm 2 carbon foils by 20 keV and 40 keV Ar + bombardment has been measured using thickness measurements derived from the angular scatter distribution of 2 keV protons transiting the foils. The total sputtering yield is observed to reach a constant rate of 3.5 atoms removed per incident Ar + and is independent of both the foil thickness and the incident energy. This is attributed to a nearly constant deposited energy density at the entrance and exit surfaces, which results from a nuclear stopping power that varies little over the energy range of 6–40 keV. By extrapolating measured backsputtering yield data to the energies used in this study, we obtain a transmission sputtering yield of approximately 1.7. Holes were also formed in the foil, and the fraction of holes created by Ar + bombardment linearly increases with increasing fluence above a threshold fluence of 4.5 × 10 15 Ar +/cm 2.

On the scintillation response of NaI(TI)-crystals

Several articles have been reviewed on the scintillation response of NaI(Tl). Those publications which report results obtained with radioactive sources either dispersed throughout the NaI(Tl)-lattice or located inside the wells of NaI(Tl)-crystals match within a /spl plusmn/0.9% standard deviation of a linear scintillation response between 3 MeV and 3 keV. Several authors who plated their sources outside the crystals measured nonlinear responses of NaT(TI) with up to /spl plusmn/7% of standard deviations between 700 and 4.5 keV. The presented analysis suggests that the observed nonlinearity is not an intrinsic effect of NaI(TI) but could be caused, at least partly, by effects occurring at the radiation entrance surfaces of the crystals.

Laue-Transmission Integrated Reflectivities for ‘Thin’ Crystal Diamond, Si and Ge X-Ray Monochromators

Optimised theoretical (111) and (220) Laue-diffracted integrated reflectivities vs. crystal thickness (0 to 100 μm) are determined for perfect diamond, silicon and germanium monochromator crystals in which both symmetric and asymmetric Laue modes are compared. Oscillations in transmitted integrated reflectivity vs. crystal thickness are shown to fluctuate between 10 to 40% for crystals > 50μm thick which demands an accuracy of a few μm in parallelism between entrance and exit surfaces to achieve optimum efficiency. The special case for diamond, when used as a first white synchroton radiation (WSR) Laue monochromator is also discussed. Less perfect diamonds with higher point defect concentration are simulated by decreasing the static Debye-Waller factor.

Elastic Scattering of Partially Coherent Beams of Fast Electrons in a Perfect Crystal

Scattering of a quasi-monochromatic electron beam emitted by an extended source by a crystal is described with the use of the mutual coherency function and formalism Bloch waves. An expression correlating the mutual intensities on the exit and entrance surfaces of the crystal under partially coherent illumination is obtained. The case of illumination by an effective incoherent source filling the condenser aperture is considered. The effect of coherence of an incident beam on the intensity distribution in electron-microscopy images has been studied. For a wedge-shaped crystal, intensity profiles of a transmitted electron beam in images obtained at different values of incident-beam divergency and defocusing of the objective lens have been calculated.

Transport of swift heavy ion induced electrons in solids

We investigated the transport of heavy ion induced electrons in solids by both numerical simulations and measurement: Electron yields and bouble differential spectra of electrons have been studied as a function of target thickness d. We measured electron yields from the beam entrance and exit surfaces of thin carbon foils bombarded with swift highly charged heavy ions at 13.6 MeV/u (Ar, foil thickness d ≈ 4 μg/cm 2–360 μg/cm 2) and 74 MeV/u (Ni, d ≈ 3 μg/cm 2–50 mg/cm 2) and compared the results to numerical simulations. Also, double differential (in velocity and angle) spectra of electrons emitted into the forward direction from C foils induced by Ar 17+ at 13.6MeV/u reveal the dependence of the shape of the “binary encounter” electron peak on thickness d.