Variability of radioiodine measurements in the thyroid

Monte Carlo simulations were carried out to study the response of a thyroid monitor for measuring intake activities of 125I and 131I. The aim of the study was 3-fold: to cross-validate the Monte Carlo simulation programs, to study the response of the detector using different phantoms and to study the effects of anatomical variations. Simulations were performed using the Swiss reference phantom and several voxelised phantoms. Determining the position of the thyroid is crucial for an accurate determination of radiological risks. The detector response using the Swiss reference phantom was in fairly good agreement with the response obtained using adult voxelised phantoms for 131I, but should be revised for a better calibration for 125I and for any measurements taken on paediatric patient

Monte Carlo simulation of a whole-body counter using IGOR phantoms

Whole-body counting is a technique of choice for assessing the intake of gamma-emitting radionuclides. An appropriate calibration is necessary, which is done either by experimental measurement or by Monte Carlo (MC) calculation. The aim of this work was to validate a MC model for calibrating whole-body counters (WBCs) by comparing the results of computations with measurements performed on an anthropomorphic phantom and to investigate the effect of a change in phantom's position on the WBC counting sensitivity. GEANT MC code was used for the calculations, and an IGOR phantom loaded with several types of radionuclides was used for the experimental measurements. The results show a reasonable agreement between measurements and MC computation. A 1-cm error in phantom positioning changes the activity estimation by >2 %. Considering that a 5-cm deviation of the positioning of the phantom may occur in a realistic counting scenario, this implies that the uncertainty of the activity measured by a WBC is ∼10-20

Silane depletion dependent ion bombardment and material quality of microcrystalline silicon deposited by VHF-PECVD

Microcrystalline silicon is a composite material embedding silicon nanocrystals in an amorphous matrix [1]. It has attracted much research efforts in the photovoltaic domain [2], because of its potential for integration in a tandem cell concept as bottom cell with an amorphous silicon top cell. Efficiencies of micromorph tandem cells and modules well above 10% have thus been demonstrated [3]. However, due to its complex structure that depends on deposition conditions [1, 4] and substrate properties [5], and due to the difficulty of characterizing plasma deposition regimes, the impact of these parameters on the microcrystalline material quality is still an open field of research. In this paper, microcrystalline silicon thin films are deposited in different conditions of silane depletion following a recent publication [6] and the material quality is investigated. It is shown that by simply reducing the hydrogen flow, the microcrystalline material quality can be greatly improved. This improvement is correlated with the reduced ion bombardment energy in high depletion regimes, leading to lower defect densities in the microcrystalline intrinsic layer

Influence of the substrate's surface morphology and chemical nature on the nucleation and growth of microcrystalline silicon

Hydrogenated microcrystalline silicon (μc-Si:H) layers about 500 nm thick were deposited in the same run on flat and rough substrates (rms = 60 nm) of various chemical nature. This study reveals that the spatial distribution of the microcrystalline/amorphous phases within the layer depends on the substrate's topography. The influence of the chemical nature of the substrate is shown to be preponderant on the layers nucleation. In particular, this study shows that nucleation density is the highest on plasma enhanced chemical vapor deposited silicon dioxide, whereas it is independent of the substrate's surface topography. Finally, the interpretation of Micro-Raman experiments for the evaluation of the respective volume fractions of amorphous/microcrystalline phases in the layers is discussed in relation with their spatial distribution. © 2005 Elsevier B.V. All rights reserved

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R&D KAI (TM) reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improving the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally, the material quality of a novel regime for high rate deposition is reviewed on the basis of these finding

Calibration of the Politrack® system based on CR39 solid-state nuclear track detectors for passive indoor radon concentration measurements

Swiss national requirements for measuring radon gas exposures demand a lower detection limit of 50 kBq h m−3, representing the Swiss concentration average of 70 Bq m−3 over a 1-month period. A solid-state nuclear track detector (SSNTD) system (Politrack, Mi.am s.r.l., Italy) has been acquired to fulfil these requirements. This work was aimed at the calibration of the Politrack system with traceability to international standards and the development of a procedure to check the stability of the system. A total of 275 SSNTDs was exposed to 11 different radon exposures in the radon chamber of the Secondary Calibration Laboratory at the Paul Scherrer Institute, Switzerland. The exposures ranged from 50 to 15000 kBq h m−3. For each exposure of 20 detectors, 5 SSNTDs were used to monitor possible background exposures during transport and storage. The response curve and the calibration factor of the whole system were determined using a Monte Carlo fitting procedure. A device to produce CR39 samples with a reference number of tracks using a 241Am source was developed for checking the long-term stability of the Politrack system. The characteristic limits for the detection of a possible system drift were determined following ISO Standard 1192

Species recognition limits mating between hybridizing ant species

Identifying mechanisms limiting hybridization is a central goal of speciation research. Here, we studied pre-mating and post-mating barriers to hybridization between two ant species, Formica selysi and Formica cinerea. These species hybridize in the Rhône valley in Switzerland, where they form a mosaic hybrid zone, with limited introgression from F. selysi into F. cinerea. There was no sign of temporal isolation between the two species in the production of queens and males. With choice experiments, we showed that queens and males strongly prefer to mate with conspecifics. Yet, we did not detect post-mating barriers caused by genetic incompatibilities. Specifically, hybrids of all sexes and castes were found in the field and F1 hybrid workers did not show reduced viability compared to non-hybrid workers. To gain insights into the cues involved in species recognition, we analyzed the cuticular hydrocarbons of queens, males and workers and staged dyadic encounters between workers. Cuticular hydrocarbon profiles differed markedly between species, but were similar in F. cinerea and hybrids. Accordingly, workers also discriminated species, but they did not discriminate F. cinerea and hybrids. We discuss how the CHC-based recognition system of ants may facilitate the establishment of pre-mating barriers to hybridization, independent of hybridization costs. This article is protected by copyright. All rights reserved