Monte-Carlo simulation of Callisto's exosphere

We model Callisto's exosphere based on its ice as well as non-ice surface via the use of a Monte-Carlo exosphere model. For the ice component we implement two putative compositions that have been computed from two possible extreme formation scenarios of the satellite. One composition represents the oxidizing state and is based on the assumption that the building blocks of Callisto were formed in the protosolar nebula and the other represents the reducing state of the gas, based on the assumption that the satellite accreted from solids condensed in the jovian sub-nebula. For the non-ice component we implemented the compositions of typical CI as well as L type chondrites. Both chondrite types have been suggested to represent Callisto's non-ice composition best. As release processes we consider surface sublimation, ion sputtering and photon-stimulated desorption. Particles are followed on their individual trajectories until they either escape Callisto's gravitational attraction, return to the surface, are ionized, or are fragmented. Our density profiles show that whereas the sublimated species dominate close to the surface on the sun-lit side, their density profiles (with the exception of H and H-2) decrease much more rapidly than the sputtered particles. The Neutral gas and Ion Mass (NIM) spectrometer, which is part of the Particle Environment Package (PEP), will investigate Callisto's exosphere during the JUICE mission. Our simulations show that NIM will be able to detect sublimated and sputtered particles from both the ice and non-ice surface. NIM's measured chemical composition will allow us to distinguish between different formation scenarios. (C) 2015 Elsevier Inc. All rights reserved.

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. ^

Philippe de Monte, Brustbild

Signatur des Originals: S 36/G03406

Ms. lat. oct. 50 - Responsionale secundum usum ecclesiae Beatae Mariae Virginis in monte Francofurti

Vorbesitzer: Gerhard Kirn; Bartholomaeusstift Frankfurt am Main

Verification of intensity modulated stereotactic radiotherapy using Monte Carlo calculations and EPID dosimetry

The purpose of this work was to develop a comprehensive IMSRT QA procedure that examined, using EPID dosimetry and Monte Carlo (MC) calculations, each step in the treatment planning and delivery process. These steps included verification of the field shaping, treatment planning system (RTPS) dose calculations, and patient dose delivery. Verification of each step in the treatment process is assumed to result in correct dose delivery to the patient. ^ The accelerator MC model was verified against commissioning data for field sizes from 0.8 × 0.8 cm 2 to 10 × 10 cm 2. Depth doses were within 2% local percent difference (LPD) in low gradient regions and 1 mm distance to agreement (DTA) in high gradient regions. Lateral profiles were within 2% LPD in low gradient regions and 1 mm DTA in high gradient regions. Calculated output factors were within 1% of measurement for field sizes ≥1 × 1 cm2. ^ The measured and calculated pretreatment EPID dose patterns were compared using criteria of 5% LPD, 1 mm DTA, or 2% of central axis pixel value with ≥95% of compared points required to pass for successful verification. Pretreatment field verification resulted in 97% percent of the points passing. ^ The RTPS and Monte Carlo phantom dose calculations were compared using 5% LPD, 2 mm DTA, or 2% of the maximum dose with ≥95% of compared points required passing for successful verification. RTPS calculation verification resulted in 97% percent of the points passing. ^ The measured and calculated EPID exit dose patterns were compared using criteria of 5% LPD, 1 mm DTA, or 2% of central axis pixel value with ≥95% of compared points required to pass for successful verification. Exit dose verification resulted in 97% percent of the points passing. ^ Each of the processes above verified an individual step in the treatment planning and delivery process. The combination of these verification steps ensures accurate treatment delivery to the patient. This work shows that Monte Carlo calculations and EPID dosimetry can be used to quantitatively verify IMSRT treatments resulting in improved patient care and, potentially, improved clinical outcome. ^

Monte Carlo and analytical dose calculations for ocular proton therapy

Uveal melanoma is a rare but life-threatening form of ocular cancer. Contemporary treatment techniques include proton therapy, which enables conservation of the eye and its useful vision. Dose to the proximal structures is widely believed to play a role in treatment side effects, therefore, reliable dose estimates are required for properly evaluating the therapeutic value and complication risk of treatment plans. Unfortunately, current simplistic dose calculation algorithms can result in errors of up to 30% in the proximal region. In addition, they lack predictive methods for absolute dose per monitor unit (D/MU) values. ^ To facilitate more accurate dose predictions, a Monte Carlo model of an ocular proton nozzle was created and benchmarked against measured dose profiles to within ±3% or ±0.5 mm and D/MU values to within ±3%. The benchmarked Monte Carlo model was used to develop and validate a new broad beam dose algorithm that included the influence of edgescattered protons on the cross-field intensity profile, the effect of energy straggling in the distal portion of poly-energetic beams, and the proton fluence loss as a function of residual range. Generally, the analytical algorithm predicted relative dose distributions that were within ±3% or ±0.5 mm and absolute D/MU values that were within ±3% of Monte Carlo calculations. Slightly larger dose differences were observed at depths less than 7 mm, an effect attributed to the dose contributions of edge-scattered protons. Additional comparisons of Monte Carlo and broad beam dose predictions were made in a detailed eye model developed in this work, with generally similar findings. ^ Monte Carlo was shown to be an excellent predictor of the measured dose profiles and D/MU values and a valuable tool for developing and validating a broad beam dose algorithm for ocular proton therapy. The more detailed physics modeling by the Monte Carlo and broad beam dose algorithms represent an improvement in the accuracy of relative dose predictions over current techniques, and they provide absolute dose predictions. It is anticipated these improvements can be used to develop treatment strategies that reduce the incidence or severity of treatment complications by sparing normal tissue. ^

Effect of Prosopis flexuosa on understory species and its importance to pastoral management in woodlands of the Central Monte Desert

In the Monte Biogeographic Province, located in the arid region of Argentina, the presence of Prosopis flexuosa DC. produces spatial heterogeneity through edaphic modifications and microclimate changes. This results in vegetation patches differing in species composition and abundance. However, this interaction can be modified by the occurrence of gradients of biotic stress or disturbance intensity. In particular, grazing has been observed to enhance or reduce vegetation heterogeneity. Such complex of interactions could determine forage availability for cattle in one of the driest areas of the Monte Desert. We assessed the effect of Prosopis on understory species and analyzed whether the outcomes of this interaction differed with distance to watering points, as a proxy of grazing intensity, in the Northeast of Mendoza Province, Argentina. We used a two-way factorial design including the following factors: 1) microsite (under the cover of P. flexuosa trees and in intercanopy microsites) and 2) distance to watering points ("near the watering point", 500-700 m away, and "far from the watering point", 3-4 km away). Cover of each species, total cover, bare soil, and litter were recorded, and plant diversity, richness, and evenness were estimated with the modified Point Quadrat method. Results showed that P. flexuosa cover, distance from watering points, and the interaction between them determined species composition, abundance and spatial distribution of understory species, and were, consequently, a determining factor for forage availability. The presence of P. flexuosa enhances carrying capacity by supporting higher abundance of grasses under its canopy. Near watering points, high grazing intensity appears to disrupt the patches formed under P. flexuosa canopies, reducing the differences between microsites.

Distribución potencial de los bosques de prosopis flexuosa en la provincia biogeográfica del monte

El objetivo de esta investigación fue generar un modelo de distribución potencial de los bosques de Prosopis flexuosa, en la provincia Biogeográfica del Monte, desde Catamarca hasta Mendoza, para luego comparar con distribuciones actuales y poder inferir acerca de los principales procesos y cambios en la distribución que pudieron haber sufrido estos bosques ya sea por causas naturales o antrópicas. Para obtener el mapa modelo se utilizó el software Maxent, el cual utiliza como datos los registros de presencia (georreferenciados) de la especie y las variables ambientales que se consideran relevantes para su distribución. Para correr el modelo, se utilizaron las variables bioclimáticas (obtenidas del sitio Worldclim) y las variables altura, pendiente, orientación, subórdenes de suelos y freática. Como registros, se utilizaron puntos de presencia de bosques (tomados con GPS) en diferentes zonas del Monte. El mapa de distribución potencial resultante concuerda con la ubicación de los principales valles, bolsones y llanuras del Monte, en donde se conoce la existencia actual de los bosques de Prosopis flexuosa. En comparación con estas distribuciones reales, existe una importante coincidencia pero, en general, el área abarcada por los bosques potenciales es superior a lo determinado por las imágenes actuales, como así también existen casos particulares en los cuales las probabilidades de ocurrencia de bosques son muy bajas y las imágenes nos muestran existencia de los mismos. A su vez, al comparar con fuentes y registros históricos, con documentos oficiales (cartas IGM) y estadísticas del ferrocarril se pueden inferir posibles procesos de desmontes, que estarían determinando la ausencia de bosques actuales en sitios donde nuestro mapa potencial presenta altas probabilidades.

Minimum water input event for seedling emergence of three native perennial grasses of the Central Monte desert (Argentina) influenced by the effect of shade and the season of the year

In deserts, seedling emergence occurs only after precipitation threshold has been exceeded, however, the presence of trees modifies microenvironmental conditions that might affect the effectiveness of a water pulse. In the Monte desert, Prosopis flexuosa trees generate different micro-environmental conditions that might influence grass seedlings establishment. The objective of this work was: a) to know the effective minimum water input event that triggers the emergence of native perennial grass seedlings; b) to relate this fact with the effect of the shade of P. flexuosa canopy and the seasonal temperatures. Three important forage species of the Monte were studied: Pappophorum caespitosum and Trichloris crinita, with C4, and Jarava ichu, with C3 metabolism. Each season, seeds of these species were sown in pots placed at two light conditions: shade (similar to P. flexuosa cover) and open area, and with seven irrigation treatments (0, 10, 20, 30, 40, 2*10 and 3*10 mm). J. ichu did not emerge in any of the treatments. Significant seedling emergence was registered for P. caespitosum and T. crinita in shade conditions with 40 mm irrigation treatment in summer. Since 40 mm precipitation events are infrequent in the Monte, seedling emergence for these species would be restricted to exceptional rainy years. The facilitating effect of P. flexuosa shade would be important during the hot season.

Dinámica temporal de la masa de broza en distintos microhábitats del desierto del Monte Central (Mendoza, Argentina)

Uno de los mecanismos propuestos para el enriquecimiento del suelo generado bajo la cobertura de leñosas en zonas áridas es la relocalización de nutrientes absorbidos por sus extensos sistemas radicales hacia el área bajo su dosel. Sin embargo, el efecto final sobre la fertilidad del suelo depende de los distintos procesos que transforman la broza y liberan nutrientes. Prosopis flexuosa D.C. (algarrobo) es la especie leñosa de mayor producción de broza y genera islas de fertilidad bajo su dosel en el Monte Central. En este trabajo analizamos la dinámica temporal de la masa de la broza caída bajo P. flexuosa, en distintos microhábitats (bajo la copa de P. flexuosa, bajo Larrea divaricata, en áreas próximas a árboles talados, y en áreas expuestas). Encontramos una mayor disminución en la masa de broza en invierno, sin diferencias entre microhábitats, y menores tasas de pérdida y mayor heterogeneidad espacial en primavera y verano. Nuestros resultados sugieren que la dinámica de la broza depende principalmente de su composición, ya que es mayor la tasa de pérdida, inmediatamente después del ingreso de broza producida por la caída de hojas de P. flexuosa no obstante las condiciones ambientales desfavorables para la actividad de microorganismos. A pesar de observarse diferencias en la dinámica de la broza entre los microhábitats, la magnitud total de los cambios de masa de broza no presenta una variabilidad espacial importante. Por el contrario, se detectó relocalización secundaria de broza, producto de la actividad de artrópodos y posiblemente otros factores (agua-viento), los que podrían actuar como moderadores de las diferencias generadas por la concentración de broza bajo las leñosas.

¿Varía la dieta de las aves granívoras en áreas pastoreadas en el desierto del Monte central?

El ganado vacuno consume principalmente plantas de gramíneas, lo que provoca una reducción en la abundancia de sus semillas tanto en pie como en el banco del suelo. Estas semillas son el principal alimento de las aves granívoras en el desierto del Monte central, quienes las prefieren por encima de las herbáceas dicotiledóneas. Se evaluó si el pastoreo genera cambios en la dieta de cinco especies de aves granívoras: Saltatricula multicolor, Diuca diuca, Poospiza torquata, Catamenia analis y Zonotrichia capensis (la cual posee tres subespecies con diferente estatus de residencia). Durante el invierno de 2014 se capturaron aves en algarrobales no pastoreados dentro de la Reserva de Biósfera de Ñacuñán (Mendoza) y en dos estancias aledañas con distinto grado de pastoreo. Se obtuvieron muestras de contenido estomacal por medio de la técnica de lavaje y evacuación del tracto digestivo. Las muestras fueron analizadas cuantificando y clasificando las distintas semillas presentes. Zonotrichia capensis y P. torquata se comportaron como especies plásticas incorporando una mayor cantidad de semillas sub-óptimas a la dieta. Saltatricula multicolor, D. diuca y C. analis mostraron, aunque en distinto grado, una dieta poco flexible basada en la ingesta casi exclusiva de semillas de gramíneas. Las tres subespecies de Z. capensis consideradas en este trabajo mostraron una dieta plástica consumiendo una gran cantidad de ítems sub-óptimos. El pastoreo, al disminuir la abundancia de semillas de gramíneas, provoca variaciones especie-específicas en la dieta de las especies de aves granívoras

Isotopic composition of water in fog, rain, throughfall and stemflow in the Monte Verde, Costa Rica

Fog deposition, precipitation, throughfall and stemflow were measured in a windward tropical montane cloud forest near Monteverde, Costa Rica, for a 65-day period during the dry season of 2003. Net fog deposition was measured directly using the eddy covariance (EC) method and it amounted to 1.2 ± 0.1 mm/day (mean ± standard error). Fog water deposition was 5-9% of incident rainfall for the entire period, which is at the low end of previously reported values. Stable isotope concentrations (d18O and d2H) were determined in a large number of samples of each water component. Mass balance-based estimates of fog deposition were 1.0 ± 0.3 and 5.0 ± 2.7 mm/day (mean ± SE) when d18O and d2H were used as tracer, respectively. Comparisons between direct fog deposition measurements and the results of the mass balance model using d18O as a tracer indicated that the latter might be a good tool to estimate fog deposition in the absence of direct measurement under many (but not all) conditions. At 506 mm, measured water inputs over the 65 days (fog plus rain) fell short by 46 mm compared to the canopy output of 552 mm (throughfall, stemflow and interception evaporation). This discrepancy is attributed to the underestimation of rainfall during conditions of high wind.