Search

Quality-diversity (QD) algorithms refer to a class of evolutionary algorithms designed to find a collection of diverse and high-performing solutions to a given problem. In robotics, such algorithms can be used for generating a collection of controllers covering most of the possible behaviors of a robot. To do so, these algorithms associate a behavioral descriptor (BD) to each of these behaviors. Each BD is used for estimating the novelty of one behavior compared to the others. In most existing algorithms, the BD needs to be hand-coded, thus requiring prior knowledge about the task to solve. In this article, we introduce: autonomous robots realizing their abilities, an algorithm that uses a dimensionality reduction technique to automatically learn BDs based on raw sensory data. The performance of this algorithm is assessed on three robotic tasks in simulation. The experimental results show that it performs similar to traditional hand-coded approaches without the requirement to provide any hand-coded BD. In the collection of diverse and high-performing solutions, it also manages to find behaviors that are novel with respect to more features than its hand-coded baselines. Finally, we introduce a variant of the algorithm which is robust to the dimensionality of the BD space. [ABSTRACT FROM AUTHOR]

Quality-diversity (QD) algorithms evolve behaviorally diverse and high-performing solutions. To illuminate the elite solutions for a space of behaviors, QD algorithms require the definition of a suitable behavior space. If the behavior space is high-dimensional, a suitable dimensionality reduction technique is required to maintain a limited number of behavioral niches. While current methodologies for automated behavior spaces focus on changing the geometry of the behavior space or on unsupervised learning of its key features, there remains a need for customizing behavioral diversity to a particular meta-objective specified by the end user. In the newly emerging framework of QD Meta-Evolution, or QD-Meta for short, one evolves a population of QD algorithms, each with different algorithmic and representational characteristics, to optimize the algorithms and their resulting archives to a user-defined meta-objective. Despite promising results compared to traditional QD algorithms, QD-Meta has yet to be compared to state-of-the-art behavior space automation methods, such as centroidal voronoi tessellations multidimensional archive of phenotypic elites (CVT-MAP-Elites) and autonomous robots realizing their abilities (AURORAs). This article performs an empirical study of QD-Meta on function optimization and multilegged robot locomotion benchmarks. Results demonstrate that QD-Meta archives provide improved average performance and faster adaptation to a priori unknown changes to the environment when compared to CVT-MAP-Elites and AURORA. A qualitative analysis shows how the resulting archives are tailored to the meta-objectives provided by the end user. [ABSTRACT FROM AUTHOR]

Spin-driven thermoelectric (STE) generation based on the combination of the spin Seebeck effect (SSE) and inverse spin Hall effect is an alternative to conventional semiconductor-based thermocouples as it is large scale, low cost, and environment-friendly. The STE device is thought to be radiation hard, making it attractive for space and nuclear technology applications. By using magnetometry, transmission electron microscopy, and the hard X-ray photoemission spectroscopy (HAXPES) measurements, we show that an STE device made of single-crystalline Y3Fe5O12 (YIG)/Pt heterostructures has tolerance to irradiation of high-energy heavy ion beams. We used 200 MeV gold ion beams modeling cumulative damages due to fission products emitted from the surface of spent nuclear fuels. By varying the dose level, we confirmed that the thermoelectric and magnetic properties of the single-crystalline YIG/Pt STE device are finite at the ion-irradiation dose up to 1012 ions/cm2 fluence. In addition, the HAXPES measurements were performed to understand the effects at the interface of YIG/Pt. The HAXPES data suggest that the chemical reaction regarding Fe and O that diminishes the SSE signals is promoted with the increase of the irradiation dose. The understandings of the damage analysis in YIG/Pt are beneficial for developing better STE devices applicable to harsh environmental usages. [ABSTRACT FROM AUTHOR]

An on-chip measurement circuit that was specifically designed to capture the high-speed transient characteristics of photocurrent is employed to measure total-ionizing-dose (TID)-induced leakage from 10 keV X-rays. This circuit uses a photocurrent-controlled oscillator (PCO) to produce an analog frequency, which provides linearity comparable to traditional, off-chip methods of TID measurement. The circuit was fabricated in a sub-50 nm, fully depleted silicon-on-insulator (FD-SOI) technology. TID-induced leakage currents were measured using the circuit and results were compared to off-chip direct measurements from identical transistor arrays. Additionally, potential in situ applications of this circuit in conjunction with a TID-compensating back-gate biasing technique are explored. [ABSTRACT FROM AUTHOR]

Static random access memories (SRAMs) are prone to a single-event upset (SEU), also known as soft errors, due to transient noise caused by a single strike of radiation. Beam testing has been extensively used to measure the SEU cross section of SRAMs as a function of the linear energy transfer (LET) of charged particle radiation. The evolution of the cross section as a function of LET is called the cross section curve, which plays a vital role in upset rate analysis for hardness assurance. Various analytical models have been developed to describe SRAM SEU cross section curves, and they have proven to be useful in reducing the cost of beam testing as well as revealing the physics behind test results. However, they involve arbitrary parameters, which make it challenging to predict cross section curves without any beam results. Moreover, the current method of analyzing cross section curves or the LET dependence of cross sections relies on a model different from that is used in the analysis of power-supply-voltage dependence, which is becoming increasingly important because of the demand for low-power operation. To overcome these problems, this article proposes a unified equation that describes both LET and the power-supply-voltage dependence of SRAM SEU cross sections. It comprises only parameters that are physically clear and familiar to SEU researchers. As well as giving possible constraints, comparisons with data from the literature suggest it can be applied to SRAMs fabricated in bulk and silicon-on-insulator (SOI) processes across generations from the early 1000-nm-scale to the current 10-nm-scale technology nodes. [ABSTRACT FROM AUTHOR]

Poleward moving auroral forms (PMAFs) are one of the most common dayside auroral phenomena, which are important to study the dynamics of the polar cusp. In this article, we propose an unsupervised auroral optical flow (UnAurFlow) model for recognizing the PMAFs from auroral image sequences captured by all-sky imagers. Since auroral shape and illumination change continuously and randomly during the evolution, and auroral motion violates the brightness constancy assumption, UnAurFlow designs an auroral deformation detection module based on bidirectional optical flow and exploits the census transform to compensate for the auroral luminosity changes. UnAurFlow is trained in an unsupervised manner, which circumvents the difficulty of manually labeling auroral optical flow. The validity of UnAurFlow is qualitatively and quantitatively evaluated using auroral observations in 2003–2005 at the Arctic Yellow River Station. Based on the auroral optical flow computed by UnAurFlow, and combining with ResNet and the attention mechanism, PMAFs are automatically recognized with a precision of 84.77% from continuous auroral observations, which outperform the state-of-the-art unsupervised optical flow algorithms. The temporal occurrence distribution of PMAFs is prenoon–postnoon asymmetric under negative interplanetary magnetic field (IMF) By conditions, which is in good agreement with the previous statistical study. [ABSTRACT FROM AUTHOR]

Retrieving informative images from the large-scale aurora data is of great significance in the field of space physics. In this article, we propose a hierarchical deep embedding (HDE) model to assist scientists for their aurora image retrieval. Other than conventional bag-of-words (BoW) models employing local cues individually, HDE performs visual matching in a hierarchical way, that is, only keypoints which are similar on local, regional, and global simultaneously can be treated as a true match. The added contextual evidences can effectively alleviate the occurrence of false matches and improve the precision of visual matching. Specifically, to complement the local SIFT feature, the convolutional neural network (CNN) is refined with a polar region pooling (PRP) layer to extract features from regional patches and global image, forming a group of hierarchical deep features with strong discriminative power. Also, an improved polar meshing (IPM) scheme is presented to determine the positions of keypoints, which is more suitable for images captured by circular fisheye lens and capable of reflecting the physical information in aurora images. Extensive experiments are conducted on the big aurora data, which indicate that the proposed HDE model greatly promotes the retrieval accuracy with acceptable memory cost and efficiency. In addition, the effectiveness of the IPM scheme and the superiority of the hierarchical deep feature integration are separately demonstrated. [ABSTRACT FROM AUTHOR]

In recent years, automatic auroral image classification has been actively investigated. The baseline method has relied on supervised learning. As this approach requires a large amount of labeled teacher data, it is necessary to collect the data manually and label them, which is a time-consuming task. In this study, we proposed a method to extend an image data set by inputting training images into a deep convolutional generative adversarial network (DCGAN) and generating images in this manner. The proposed approach implied using both generated and original images to train the classifier. It could reduce the number of labeling operations performed manually. As an evaluation experiment, we performed classifier learning on the data sets before and after extension and confirmed that the classification accuracy was improved because of training on the data set after the extension. [ABSTRACT FROM AUTHOR]

In conventional aurora image classification methods, it is general to employ only one single feature representation to capture the morphological characteristics of aurora images, which is difficult to describe the complicated morphologies of different aurora categories. Although several studies have proposed to use multiple feature representations, the inherent correlation among these representations are usually neglected. To address this problem, we propose a group-wise learning (GWL) method for the automatic aurora image classification using multiple representations. Specifically, we first extract the multiple feature representations for aurora images, and then construct a graph in each of multiple feature spaces. To model the correlation among different representations, we partition multiple graphs into several groups via a clustering algorithm. We further propose a GWL model to automatically estimate class labels for aurora images and optimal weights for the multiple representations in a data-driven manner. Finally, we develop a label fusion approach to make a final classification decision for new testing samples. The proposed GWL method focuses on the diverse properties of multiple feature representations, by clustering the correlated representations into the same group. We evaluate our method on an aurora image data set that contains 12 682 aurora images from 19 days. The experimental results demonstrate that the proposed GWL method achieves approximately 6% improvement in terms of classification accuracy, compared to the methods using a single feature representation. [ABSTRACT FROM AUTHOR]

Post-Traumatic Stress Disorder (PTSD) is a psychiatric condition resulting from threatening or horrifying events. We hypothesized that circadian rhythm changes, measured by a wrist-worn research watch are predictive of post-trauma outcomes. Approach: 1618 post-trauma patients were enrolled after admission to emergency departments (ED). Three standardized questionnaires were administered at week eight to measure post-trauma outcomes related to PTSD, sleep disturbance, and pain interference with daily life. Pulse activity and movement data were captured from a research watch for eight weeks. Standard and novel movement and cardiovascular metrics that reflect circadian rhythms were derived using this data. These features were used to train different classifiers to predict the three outcomes derived from week-eight surveys. Clinical surveys administered at ED were also used as features in the baseline models. Results: The highest cross-validated performance of research watch-based features was achieved for classifying participants with pain interference by a logistic regression model, with an area under the receiver operating characteristic curve (AUC) of 0.70. The ED survey-based model achieved an AUC of 0.77, and the fusion of research watch and ED survey metrics improved the AUC to 0.79. Significance: This work represents the first attempt to predict and classify post-trauma symptoms from passive wearable data using machine learning approaches that leverage the circadian desynchrony in a potential PTSD population. [ABSTRACT FROM AUTHOR]

The effects of buried-well potential perturbation under the buried-oxide (BOX) layer are studied in both a heavy-ion single event upset (SEU) test and a high-energy proton-SEU test of a silicon-on-insulator (SOI) dual interlocked storage cell (DICE)-based flip-flop. Their dependence on incident angle and back bias is discussed. We fabricated both DICE-based flip-flop and conventional flip-flop, which are designed as 80 000-stage shift-register chains. In a heavy-ion test, a considerable number of SEUs were observed at back bias exceeding 2.4 V, and a ten-times larger SEU-cross section was finally recorded at back bias of 3.0 V compared with the total active area of a DICE-based flip-flop cell. This marks the first case where DICE topology was found to be broken by buried-well potential perturbation on an SOI DICE-based flip-flop. In a proton test, one error was observed at back bias of 2.0 V. The SEU rate in the Van Allen belt at an altitude of 2300 km and an inclination of 90° was estimated as being once every 5 years. [ABSTRACT FROM AUTHOR]

This article investigates heavy-ion irradiation effects on perpendicular magnetic anisotropy spin-transfer torque magnetic tunnel junction devices (PMA STT-MTJs). The radiative campaign took place at the Université Catholique de Leuven (UCL) facility. The considered devices consist of STT p-MTJs purely magnetic memories and they were fabricated at SPINTEC using the most advanced CoFeB-MgO MTJ technology. Single-event upset (SEU) tolerance and modification of magnetic properties has been investigated. [ABSTRACT FROM AUTHOR]

The history of integrated circuit (IC) development is another record of human challenges involving space. Efforts have been made to protect ICs from sudden malfunctions due to single-event effects (SEEs). These effects are triggered by only a single strike of particle radiation, such as an α-ray or cosmic ray, originating from our solar activity and galactic events including supernovas. This article explores how SEEs have evolved along with the progress in complementary metal-oxide-semiconductor (CMOS) digital IC technology, or device scaling, from the early micrometer-scale generations to the current nanometer-scale generations. For this purpose, focusing on basic digital elements, that is, inverters and static random access memories (SRAMs), this study collected more than 100 sets of data on four characteristic parameters of single-event upsets (SEUs) and single-event transients (SETs), both of which are undesired flips in digital logic states. The results show that all the examined parameters, such as the SEU critical charge, decrease with the device feature size. Analysis involving structure classification, such as bulk versus silicon-on-insulator (SOI) substrates and planar versus fin channels, reveals relationships between the examined SEE parameters and other device features such as the power supply voltage. All the data collected in this survey are explicitly given in tables for future exploration of IC reliability. [ABSTRACT FROM AUTHOR]

Today users’ private data in edge computing devices (desktops, laptops, and tablets, etc.) is at high risk because they run applications on potentially compromised or malicious systems. To address this problem, hardware vendors propose Trusted Execution Environment (TEE). Particularly, Intel has released a new processor feature called Software Guard eXtension (SGX), and provisions shielded executions (i.e., enclaves) for security-sensitive computations. Regrettably, Intel SGX’s design objectives omit trusted I/O paths. Without such guarantees, it is unlikely for an enclave to fulfill its security and privacy purposes because the source or sink of data may have been corrupted. To this end, we propose a novel architecture called Aurora to provide trusted I/O paths for enclave programs even in the presence of untrusted system software. Specifically, Aurora exploits two commercial-off-the-shelf features (System Management Mode, SMM and SGX) and establishes a secure channel between an enclave program and target device. Furthermore, we design and implement trusted paths for HID keyboard, serial port printer, hardware clocks, and USB mass storage, respectively. Leveraging these trusted paths, we protect real-world applications including OpenSSH client, OpenSSL server/client and SQLite database. Security and performance evaluations show that Aurora mitigates several kinds of I/O related attacks and introduces acceptable overheads. Our framework has been open-sourced and is available to the security community. [ABSTRACT FROM AUTHOR]

The radiation response of TaOx-based resistive memory (RRAM) devices fabricated in academic (Set A) and industrial (Set B) settings was compared. Ionization damage from a 60Co gamma source did not cause any changes in device resistance for either device type, up to 45 Mrad(Si). Displacement damage from a heavy ion beam caused a decrease in resistance at 1 × 1021 oxygen displacements per cm3 in Set B devices in the high-resistance state (HRS); meanwhile, Set A devices did not exhibit any decrease in resistance due to displacement damage. Both types of devices exhibited an increase in resistance around 3 ×1022 oxygen displacements per cm3, possibly due to the damage at the oxide/metal interfaces. These extremely high levels of damage represent near-total atomic disruption, and if this level of damage was ever reached, other circuit elements would likely fail before the RRAM devices in this article. Overall, both sets of devices were much more resistant to radiation effects than the similar devices reported in the literature. Displacement damage effects were only observed in the Set A devices once the displacement-induced oxygen vacancies surpassed the intrinsic vacancy concentration in the devices, suggesting that high oxygen vacancy concentration played a role in the devices’ high tolerance to displacement damage. [ABSTRACT FROM AUTHOR]

The width of auroral arc is one of the most important factors in understanding and examining its physical mechanisms. In this letter, we propose a fully automatic method for computing the width of auroral arcs based on the instance segmentation of auroral images. To accurately detect and segment auroral arcs with oriented bounding boxes, we adapt a state-of-the-art instance segmentation model, Mask region-based convolutional neural network, by designing a two-stage inference process combined with an indispensable random rotation training strategy and designing an effective feature extraction architecture. Given the segmented masks of individual auroral arcs, we present a method for computing the arc width automatically. In our experiments, the instance segmentation model achieves 86.8% of mean average precision on the human-labeled data set. By automatically evaluating the width of 29 938 detected auroral arcs in 18 417 auroral arc images, we obtain a similar arc width distribution to that evaluated by the semiautomatic approach, which demonstrates the effectiveness of our proposed method. [ABSTRACT FROM AUTHOR]

High-level surface-charging events are frequently observed on spacecrafts in aurora region. Besides the sunlit condition, two major factors causing charging events are the ambient plasma density and the precipitating electron flux. For the first factor, when the ambient plasma density is relatively low due to weak solar UV output during solar minimum, more surface-charging events are reported to occur for polar satellite. Moreover, the increased energetic electron precipitation during high geomagnetic activities in solar cycle decent years, may also contribute to the charging events. To evaluate these two factors comprehensively, we present a statistical survey of over 2000 surface charging events recorded by Defense Meteorological Satellite Program (DMSP) satellites over one solar cycle. The relationship between charging events and Kp index, F10.7 index, and monthly average sunspot number is investigated. It is shown that the occurrence frequency of charging events is larger at descending years of solar cycle. In addition, the conditional probabilities of the charging events increase as Kp increase. There is a threshold for Kp index, under which most of the charging events cannot occur, and this threshold grows as F10.7 increases. Finally, it is demonstrated that the surface charging potential is strongly correlated with the Kp index. Although there is no obvious correlation between the surface charging potential and F10.7 index, we still observe a decreasing trend of the surface potential range as F10.7 increases. [ABSTRACT FROM AUTHOR]

Efficiently exploiting thread-level parallelism has been challenging for software developers. As many parallel applications do not scale with the number of cores, the task of rightly choosing the ideal amount of threads to produce the best results in performance or energy is not straightforward. Moreover, many variables may change according to the system at hand (e.g., application, input set, microarchitecture, number of cores) and even during execution. Existing solutions lack transparency (demand changes in the original code) or adaptability (do not automatically adjust to applications at run-time). In this scenario, we propose Aurora, an OpenMP framework that is completely transparent to both the designer and end-user. Without any code transformation or recompilation, it is capable of automatically finding, at run-time and with minimum overhead, the optimal number of threads for each parallel loop region and re-adapt in cases the behavior of a region changes during execution. When executing fifteen well-known benchmarks on four multi-core processors, Aurora improves the Energy-Delay Product by up to 98, 86 and 91 percent over the standard OpenMP execution, the OpenMP feature that dynamically adjusts the number of threads, and the Feedback-Driven Threading, respectively. [ABSTRACT FROM AUTHOR]

Static random access memory (SRAM) is one the most sensitive devices to radiation. It may often exhibit undesired reversals of memory bits, called single-event upsets or soft errors. Sensitivity to such undesired events is widely quantified with the parameter of cross sections, which varies from cell to cell due to process variations. Many efforts have been made to quantify such variations, most commonly using simulations that often become a hurdle especially for commercial-off-the-shelf SRAMs. In this regard, an analysis method is proposed to extract such variations easily from radiation test data. It relies on the data retention voltage, which is an electrical parameter associated with the static noise margin. This voltage can be measured on a desk without special circuits. The validity of the proposed method is experimentally demonstrated using SRAM fabricated in a 65-nm silicon-on-insulator technology, which was exposed to various high-energy heavy ions simulating the effects of galactic and terrestrial cosmic rays. Theoretical discussion provides further evidence, while revealing an interesting relationship between the data retention voltage and power supply voltage. This relationship suggests power-supply voltage tuning to deal with chip-to-chip variations in cross sections and also with similar variations caused by total-dose effects and aging effects such as negative bias temperature instability. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel weakly supervised semantic segmentation (WSSS) method that uses image tags as supervision to achieve joint pixel-level localization of the key local structure (KLS) and image-level classification of the aurora images captured by the ground-based optical all-sky imager. First, a patch-scale model (PSM) based on the small-scale structure of aurora is designed to identify the type-specific regions for each training image. Second, a region-scale model is trained with the identified type-specific regions to coarsely localize the KLS from multiple sizes of field of view, based on which the aurora image is classified. Finally, given the predicted image type, the PSM further refines the KLS in a pixel level. By localizing KLS from coarse to fine, the proposed method captures both overall shape with a bottom–up processing and local structure details of aurora in a top–down manner. Extensive experiments on the expert labeled data sets have demonstrated the efficacy of the proposed method in benchmarking with the state-of-the-art WSSS methods. [ABSTRACT FROM AUTHOR]

Novel design techniques for efficient testability are developed and have been implemented in a 14-/16-nm bulk FinFET node technology characterization vehicle. The result of this paper was the measurement of over 300 000 SETs across 12 combinational logic variants and 415 000 SEUs in three D-flip-flop designs over a large test matrix of heavy-ion linear energy transfer, angle of incidence, and supply voltage in only 319 test runs with a total test time of 108 h. A similar-sized data set with equivalent technology coverage, using the traditional serial testing approach as used in previous work, would typically require thousands of test runs and a significant number of additional hours of testing. The methodologies developed in this paper are technology agnostic and have provided the capability to efficiently characterize the radiation-induced response of the 14-/16-nm FinFET technology generation and yielded insights for assessing the feasibility for incorporation in systems requiring radiation resiliency. [ABSTRACT FROM PUBLISHER]

This paper describes a circuit called dynamic and applicative measurement of single events in logic (DAMSEL), which can individually measure the impact of single event upsets and single-event transients (SETs) in realistic, synchronous digital circuits. Heavy-ion and pulsed laser test results are presented. Both a simple digital and an analog simulation methodology are demonstrated to provide similar cross sections to those observed during the tests. Overall, the DAMSEL structure has many advantages over current circuits used for measuring SETs, including the ease with which test results can be extrapolated to real designs. [ABSTRACT FROM PUBLISHER]

Heavy-ion-induced degradation in the reverse leakage current of SiC Schottky power diodes exhibits a strong dependence on the ion angle of incidence. This effect is studied experimentally for several different bias voltages applied during heavy-ion exposure. In addition, TCAD simulations are used to give insight on the physical mechanisms involved. [ABSTRACT FROM PUBLISHER]

The Hecht equation can be used to model the nonlinear degradation of charge collection efficiency (CCE) in response to radiation-induced displacement damage in both fully and partially depleted GaAs photodiodes. CCE degradation is measured for laser-generated photocurrent as a function of fluence and bias in Al 0.3 Ga 0.7 As/GaAs/Al 0.25 Ga 0.75 As p-i-n photodiodes which have been irradiated with 12 MeV C and 7.5 MeV Si ions. CCE is observed to degrade more rapidly with fluence in partially depleted photodiodes than in fully depleted photodiodes. When the intrinsic GaAs layer is fully depleted, the 2-carrier Hecht equation describes CCE degradation as photogenerated electrons and holes recombine at defect sites created by radiation damage in the depletion region. If the GaAs layer is partially depleted, CCE degradation is more appropriately modeled as the sum of the 2-carrier Hecht equation applied to electrons and holes generated within the depletion region and the 1-carrier Hecht equation applied to minority carriers that diffuse from the field-free (non-depleted) region into the depletion region. Enhanced CCE degradation is attributed to holes that recombine within the field-free region of the partially depleted intrinsic GaAs layer before they can diffuse into the depletion region. [ABSTRACT FROM AUTHOR]

The development of accelerators technology offers a new window for the creation of surface nanostructures in an efficient and accurate way. The use of 30 MeV C 60 cluster ions enables the creation of nano-hillocks of size larger than the ones produced by GeV monoatomic ions. The physical mechanism underlying the realization of such nanostructures is elucidated using a plasma expansion approach. Numerical analysis showed that increasing the ionic temperature (number density) ratios would lead to decrease (increase) the nano-hillocks height. [ABSTRACT FROM AUTHOR]

Experimental results on ion-induced leakage current increase in 4H-SiC Schottky power diodes are presented. Monte Carlo and TCAD simulations show that degradation is due to the synergy between applied bias and ion energy deposition. This degradation is possibly related to thermal spot annealing at the metal semiconductor interface. This thermal annealing leads to an inhomogeneity of the Schottky barrier that could be responsible for the increase leakage current as a function of fluence. [ABSTRACT FROM PUBLISHER]

Purpose: Gas-filled ionization chambers are the most important radiation detectors in radiotherapy. The collected charge at the electrodes does not represent the total released charge due to the unavoidable recombination processes. This needs to be considered for precise dose measurements. A quantitative description and correction of the recombination effects is established for two cases: continuous radiation exposure and pulsed radiation fields of single pulses with vanishing pulse duration. This work derives formulas for calculating the saturation correction for pulsed beams of nonvanishing pulse duration. Methods: Recursive formulas are derived describing the spatio-temporal development of the charge density distributions in plane-parallel ionization chambers starting at neglected recombination. Pulse duration dependent effective chamber parameters are calculated, by comparing the coefficients of a series expansion for small recombination effects. These parameters are used afterward in the known formula for the saturation correction factor for pulsed irradiation with increased recombination effects which was established with the assumption of vanishing pulse duration. Results: The formulas should be valid for pulse durations shorter than half the collection time of the chamber. They allow calculating the saturation correction factor for pulsed beams of nonvanishing pulse duration and arbitrary pulse dose, if chamber, beam, and filling gas parameters are known. The filling gas parameters could be determined from experimental data. The calculation results are in good agreement with already published experimental and simulated results for a Roos chamber. Conclusions: The new formulas can be applied to determine the expected saturation correction for pulsed beams of different pulse duration and pulse dose including new beams at accelerators of new technologies. More experimental validation by using other chambers and an extension of the new formalism to pulse durations longer than half the collection time of the chamber is desired. [ABSTRACT FROM AUTHOR]

Purpose: To present and characterize a Monte Carlo (MC) tool for the simulation of the relative biological effectiveness for the induction of direct DNA double strand breaks (RBEdirectDSB ) for protons and light ions. Methods: The MC tool uses a pregenerated event-by-event tracks library of protons and light ions that are overlaid on a cell nucleus model. The cell nucleus model is a cylindrical arrangement of nucleosome structures consisting of 198 DNA base pairs. An algorithm relying on k-dimensional trees and cylindrical symmetries is used to search coincidences of energy deposition sites with volumes corresponding to the sugar-phosphate backbone of the DNA molecule. Strand breaks (SBs) are scored when energy higher than a threshold is reached in these volumes. Based on the number of affected strands, they are categorized into either single strand break (SSB) or double strand break (DSB) lesions. The number of SBs composing each lesion (i.e., its size) is also recorded. RBEdirectDSB is obtained by taking the ratio of DSB yields of a given radiation field to a 60Co field. The MC tool was used to obtain SSB yields, DSB yields, and RBEdirectDSB as a function of linear energy transfer (LET) for protons (1H+), 4He2+, 7Li3+, and 12C6+ ions. Results: For protons, the SSB yields decreased and the DSB yields increased with LET. At ≈24.5 keV μm-1, protons generated 15% more DSBs than 12C6+ ions. The RBEdirectDSB varied between 1.24 and 1.77 for proton fields between 8.5 and 30.2 keV μm- 1, and it was higher for iso-LET ions with lowest atomic number. The SSB and DSB lesion sizes showed significant differences for all radiation fields. Generally, the yields of SSB lesions of sizes ⩾2 and the yields of DSB lesions of sizes ⩾3 increased with LET and increased for iso-LET ions of lower atomic number. On the other hand, the ratios of SSB to DSB lesions of sizes 2-4 did not show variability with LET nor projectile atomic number, suggesting that these metrics are independent of the radiation quality. Finally, a variance of up to 8% in the DSB yields was observed as a function of the particle incidence angle on the cell nucleus. This simulation effect is due to the preferential alignment of ion tracks with the DNA nucleosomes at specific angles. Conclusions: TheMCtool can predict SSB and DSB yields for light ions of various LET and estimate RBEdirectDSB . In addition, it can calculate the frequencies of different DNA lesion sizes, which is of interest in the context of biologically relevant absolute dosimetry of particle beams. [ABSTRACT FROM AUTHOR]

Purpose: Accurate dose measurement in radiotherapy is critically dependent on correction for gain drop, which is the difference of the measured current from the ideal saturation current due to general ion recombination. Although a correction method based on the Boag theory has been employed, the theory assumes that ionized charge density in an ionization chamber (IC) is spatially uniform throughout the irradiation volume. For particle pencil beam scanning, however, the charge density is not uniform, because the fluence distribution of a pencil beam is not uniform. The aim of this study was to verify the effect of the nonuniformity of ionized charge density on the gain drop due to general ion recombination. Methods: The authors measured the saturation curve, namely, the applied voltage versus measured current, using a large plane-parallel IC and 24-channel parallel-plate IC with concentric electrodes. To verify the effect of the nonuniform ionized charge density on the measured saturation curve, the authors calculated the saturation curve using a method which takes into account the nonuniform ionized charge density and compared it with the measured saturation curves. Results: Measurement values of the different saturation curves in the different channels of the concentric electrodes differed and were consistent with the calculated values. The saturation curves measured by the large plane-parallel IC were also consistent with the calculation results, including the estimation error of beam size and of setup misalignment. Although the impact of the nonuniform ionized charge density on the gain drop was clinically negligible with the conventional beam intensity, it was expected that the impact would increase with higher ionized charge density. Conclusions: For pencil beam scanning, the assumption of the conventional Boag theory is not valid. Furthermore, the nonuniform ionized charge density affects the prediction accuracy of gain drop when the ionized charge density is increased by a higher dose rate and/or lower beam size. [ABSTRACT FROM AUTHOR]

Heavy ion irradiation increases the leakage current in reverse-biased SiC Schottky diodes. This letter demonstrates, via molecular dynamics simulations, that a combination of bias and ion-deposited energy is required to produce the degradation. [ABSTRACT FROM AUTHOR]

We report on a measurement of low energy ion irradiation effects on as-grown films of SiO_2 on a Si substrate. Beams of normally incident Na^ + ions with kinetic energies of 2 keV to 5 keV were focused onto \sim 1900 \ \AA SiO_2 films. Aluminum top metal contacts were subsequently deposited onto these targets such that irradiated regions and unexposed (pristine) regions of the target could be compared using capacitance–voltage (C–V) measurements of individual metal-oxide-semiconductor (MOS) devices. The C–V data reveal an energy-dependent shift in the flatband voltage (VFB) that can be returned to its near-pristine value by a low temperature anneal. An increase in the density of interface states (Dit) inferred from the C–V curves is found to have a superlinear dependence on the incident kinetic energy. These data are consistent with previously observed UV radiation effects on MOS oxides, where transferred energy leads to electron-hole pair production and the diffusion and trapping of holes throughout the oxide. Our measured trapped hole densities are compared with calculated densities, which are based on the incident ion dose and the predicted ion implantation range, to arrive at a fractional yield for hole survival and measurement within an encapsulated MOS device. [ABSTRACT FROM AUTHOR]

Circuit parameters and configuration are very important when studying the synergistic effects total dose/SET. We explore a method combining dynamic parameter measurement and spectrum analysis which lead to a better understanding of this complex phenomenon. In this paper symbolic circuit analysis is used to obtain the relationship between input, output and noise injection due to photocurrent generation in the form of a rational function using symbolic variables in the complex frequency domain. This simulation technique was able to predict the impact of (i) total dose level, (ii) circuit parameters, and (iii) the injected energy on the ASET shapes. Basic mechanisms as field collapse and collection efficiency were also predicted and assessed. [ABSTRACT FROM PUBLISHER]

Charge collection mechanisms are investigated in surface channel GaAs MOSFETs under broadbeam heavy ion irradiation and pulsed two-photon-absorption laser irradiation. The large barrier between the gate dielectric and GaAs eliminates gate conduction current, but there is significant gate displacement current. Charge enhancement occurs because radiation-generated holes accumulate in the substrate, which increases the local electrostatic potential. The increased potential enhances the source-to-drain current, resulting in excess collected charge. The collected charge increases significantly with gate bias, due to the long tails of the charge waveforms that occur for higher gate bias. The collected charge increases with increasing drain bias. [ABSTRACT FROM PUBLISHER]

Purpose: The purpose of this study was to experimentally assess the possibility to monitor carbon ion range variations--due to tumor shift and/or elongation or shrinking--using prompt-gamma (PG) emission with inhomogeneous phantoms. Such a study is related to the development of PG monitoring techniques to be used in a carbon ion therapy context. Methods: A 95 MeV/u carbon ion beam was used to irradiate phantoms with a variable density along the ion path to mimic the presence of bone and lung in homogeneous humanlike tissue. PG profiles were obtained after a longitudinal scan of the phantoms. A setup comprising a narrow single-slit collimator and two detectors placed at 90° with respect to the beam axis was used. The time of flight technique was applied to allow the selection between PG and background events. Results: Using the positions at 50% entrance and 50% falloff of the PG profiles, a quantity called prompt-gamma profile length (PGPL) is defined. It is possible to observe shifts in the PGPL when there are absolute ion range shifts as small as 1-2 mm. Quantitatively, for an ion range shift of -1.33 ± 0.46 mm (insertion of a Teflon slab), a PGPL difference of -1.93 ± 0.58 mm and -1.84 ± 1.27 mm is obtained using a BaF2 and a NaI(Tl) detector, respectively. In turn, when an ion range shift of 4.59 ± 0.42 mm (insertion of a lung-equivalent material slab) is considered, the difference is of 4.10 ± 0.54 and 4.39 ± 0.80 mm for the same detectors. Conclusions: Herein, experimental evidence of the usefulness of employing PG to monitor carbon ion range using inhomogeneous phantoms is presented. Considering the homogeneous phantom as reference, the results show that the information provided by the PG emission allows for detecting ion range shifts as small as 1-2 mm. When considering the expected PG emission from an energy slice in a carbon ion therapy scenario, the experimental setup would allow to retrieve the same PGPL as the high statistics of the full experimental dataset in 58% of the times. However, this success rate increases to 93% when using a better optimized setup by means of Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

We studied the radiation hardness of 4H-SiC Schottky barrier diodes (SBD) for the light ion detection and spectroscopy in harsh radiation environments. n -Type SBD prepared on nitrogen-doped (∼4 × 10 14 cm −3 ) epitaxial grown 4H-SiC thin wafers have been irradiated by a raster scanning alpha particle microbeam (2 and 4 MeV He 2+ ions separately) in order to create patterned damage structures at different depths within a sensitive volume of tested diodes. Deep Level Transient Spectroscopy (DLTS) analysis revealed the formation of two deep electron traps in the irradiated and not thermally treated 4H-SiC within the ion implantation range (E1 and E2). The E2 state resembles the well-known Z 1/2 center, while the E1 state could not be assigned to any particular defect reported in the literature. Ion Beam Induced Charge (IBIC) microscopy with multiple He ion probe microbeams (1–6 MeV) having different penetration depths in tested partly damaged 4H-SiC SBD has been used to determine the degradation of the charge collection efficiency (CCE) over a wide fluence range of damaging alpha particle. A non-linear behavior of the CCE decrease and a significant degradation of the spectroscopic performance with increasing He ion fluence were observed above the value of 10 11 cm −2 . [ABSTRACT FROM AUTHOR]

Purpose: Different ion types offer different physical and biological advantages for therapeutic applications. The purpose of this work is to assess the advantages of the most commonly used ions in particle therapy, i.e., carbon (12C), helium (4He), and protons (¹H) for different treatment scenarios. Methods: A treatment planning analysis based on idealized target geometries was performed using the treatment planning software TRiP98. For the prediction of the relative biological effectiveness (RBE) that is required for biological optimization in treatment planning the local effect model (LEM IV) was used. To compare the three ion types, the peak-to-entrance ratio (PER) was determined for the physical dose (PERPHYS), the RBE (PERRBE), and the RBE-weighted dose (PERBIO) resulting for different dose-levels, field configurations, and tissue types. Further, the dose contribution to artificial organs at risk (OAR) was assessed and a comparison of the dose distribution for the different ion types was performed for a patient with chordoma of the skull base. Results: The study showed that the advantages of the ions depend on the physical and biological properties and the interplay of both. In the case of protons, the consideration of a variable RBE instead of the clinically applied generic RBE of 1.1 indicates an advantage in terms of an increased PERRBE for the analyzed configurations. Due to the fact that protons show a somewhat better PERPHYS compared to helium and carbon ions whereas helium shows a higher PERRBE compared to protons, both protons and helium ions show a similar RBE-weighted dose distribution. Carbon ions show the largest variation of the PERRBE with tissue type and a benefit for radioresistant tumor types due to their higher LET. Furthermore, in the case of a two-field irradiation, an additional gain in terms of PERBIO is observed when using an orthogonal field configuration for carbon ions as compared to opposing fields. In contrast, for protons, the PERBIO is almost independent on the field configuration. Concerning the artificial lateral OAR, the volume receiving 20% of the prescribed RBE-weighted dose (V20) was reduced by over 35% using helium ions and by over 40% using carbon ions compared to protons. The analysis of the patient plan showed that protons, helium, and carbon ions are similar in terms of target coverage whereas the dose to the surrounding tissue is increasing from carbon ions toward protons. The mean dose to the brain stem can be reduced by more than 55% when using helium ions and by further 25% when using carbon ions instead of protons. Conclusions: The comparison of the PERRBE and PERPHYS of the three ion types suggests a strong dependence of the advantages of the three ions on the dose-level, tissue type, and field configuration. In terms of conformity, i.e., dose to the normal tissue, a clear gain is expected using carbon or helium ions compared to protons. [ABSTRACT FROM AUTHOR]

Single crystals of z- cut SrWO 4 are irradiated at room temperature with 6.0 MeV O ions at a fluence of 1.6×10 14 ions/cm 2 . The effects of irradiation on the structural properties are studied by a variety of techniques, including X-ray diffraction (XRD), Raman spectroscopy and Rutherford backscattering spectrometry in channeling geometry (RBS/C) along the 〈001〉 axial direction. The optical properties are investigated by prism coupling technique with a wavelength of 633 nm; the results indicate that both the ordinary ( n o ) and extraordinary indices ( n e ) of SrWO 4 crystals are raised together after implantation. [ABSTRACT FROM AUTHOR]

Purpose: The authors investigated the performance of several patient-specific intensity-modulated radiation therapy (IMRT) quality assurance (QA) dosimeters in terms of their ability to correctly identify dosimetrically acceptable and unacceptable IMRT patient plans, as determined by an in-house-designed multiple ion chamber phantom used as the gold standard. A further goal was to examine optimal threshold criteria that were consistent and based on the same criteria among the various dosimeters Methods: The authors used receiver operating characteristic (ROC) curves to determine the sensi-tivity and specificity of (1) a 2D diode array undergoing anterior irradiation with field-by-field evaluation, (2) a 2D diode array undergoing anterior irradiation with composite evaluation, (3) a 2D diode array using planned irradiation angles with composite evaluation, (4) a helical diode array, (5) radiographic film, and (6) an ion chamber. This was done with a variety of evaluation criteria for a set of 15 dosimetrically unacceptable and 9 acceptable clinical IMRT patient plans, where acceptability was defined on the basis of multiple ion chamber measurements using independent ion chambers and a phantom. The area under the curve (AUC) on the ROC curves was used to compare dosimeter performance across all thresholds. Optimal threshold values were obtained from the ROC curves while incorporating considerations for cost and prevalence of unacceptable plans Results: Using common clinical acceptance thresholds, most devices performed very poorly in terms of identifying unacceptable plans. Grouping the detector performance based on AUC showed two significantly different groups. The ion chamber, radiographic film, helical diode array, and anterior-delivered composite 2D diode array were in the better-performing group, whereas the anterior-delivered field-by-field and planned gantry angle delivery using the 2D diode array performed less well. Additionally, based on the AUCs, there was no significant difference in the performance of any device between gamma criteria of 2%/2 mm, 3%/3 mm, and 5%/3 mm. Finally, optimal cutoffs (e.g., percent of pixels passing gamma) were determined for each device and while clinical practice commonly uses a threshold of 90% of pixels passing for most cases, these results showed variability in the optimal cutoff among devices Conclusions: IMRT QA devices have differences in their ability to accurately detect dosimetrically acceptable and unacceptable plans. Field-by-field analysis with a MapCheck device and use of the MapCheck with a MapPhan phantom while delivering at planned rotational gantry angles resulted in a significantly poorer ability to accurately sort acceptable and unacceptable plans compared with the other techniques examined. Patient-specific IMRT QA techniques in general should be thoroughly evaluated for their ability to correctly differentiate acceptable and unacceptable plans. Additionally, optimal agreement thresholds should be identified and used as common clinical thresholds typically worked very poorly to identify unacceptable plans. [ABSTRACT FROM AUTHOR]