Charge Yield Research Articles

Measurement of the ionization response of amorphous selenium with 122 keV γ rays

We performed a measurement of the ionization response of 200 μm-thick amorphous selenium (aSe) layers under drift electric fields of up to 50 V/μm.The aSe target was exposed to ionizing radiation from a 57Co radioactive source and the ionization pulses were recorded with high resolution.Using the spectral line from the photoabsorption of 122 keV γ rays, we measured the charge yield in aSe and the line width as a function of drift electric field.From a detailed microphysics simulation of charge generation and recombination in aSe, we conclude that the strong dependence of recombination on the ionization track density provides the dominant contribution to the energy resolution in aSe.These results provide valuable input to estimate the sensitivity of a proposed next-generation search for the neutrinoless ββ decay of 82Se that aims to employ imaging sensors with an active layer of aSe.We estimate the RMS line width of the integrated ionization signal from neutrinoless ββ decay events (of deposited energy 3.0 MeV) to be 2.0% for a drift field of 50 V/μm.The energy resolution can be improved to 1% by correcting for the charge yield as a function of ionization density along the imaged electron tracks.

Determination of responses of liquid xenon to low energy electron and nuclear recoils using a PandaX-II detector * *Supported by a grant from the Ministry of Science and Technology of China (2016YFA0400301), National Science Foundation of China (12090060, 11525522, 11775141, 11755001) and Office of Science and Technology, Shanghai Municipal Government (18JC1410200)

We present a systematic determination of the responses of PandaX-II, a dual phase xenon time projection chamber detector, to low energy recoils. The electron recoil (ER) and nuclear recoil (NR) responses are calibrated, respectively, with injected tritiated methane or 220Rn source, and with 241Am-Be neutron source, in an energy range from 1-25 keV (ER) and 4-80 keV (NR), under the two drift fields, 400 and 317 V/cm. An empirical model is used to fit the light yield and charge yield for both types of recoils. The best fit models can describe the calibration data significantly. The systematic uncertainties of the fitted models are obtained via statistical comparison to the data.

TID Effects Induced by ARACOR, 60Co, and ORIATRON Photon Sources in MOS Devices: Impact of Geometry and Materials

Total ionizing dose (TID) effects induced by different photon sources are studied in MOS devices. The dose factor (DF), defined as the ratio between dose deposition in dosimeter and actual dose deposition in MOS-sensitive oxides (SOs), is estimated using Monte-Carlo simulations of energetic photons striking simple device structures. An effective TID approach is proposed to account for both the real ionizing dose depositions in small volumes and the charge yield. This method is applied to a silicon-on-insulator (SOI) technology using three photon sources: the usual ARACOR and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co sources, which deliver several tens of kiloelectron volt photons and ~1.25-MeV photons, respectively, and a third facility named ORIATRON, a 6-MeV electron linear accelerator that produces photons on a wide energy spectrum. This topic, investigated intensively in the literature a few decades ago, is revisited in the context of several evolutions including: new radiation sources, novel simulation tools, and device scaling, which all impact ionizing dose deposition in microsensitive volumes of current electronic technologies. Dedicated numerical simulations demonstrate the importance of taking into account the entire stack of materials in order to get the most accurate estimations of TID in the SO. Descriptions of the back-end-of-line (BEOL) layers (especially when the technology uses copper), the polysilicon gate, the Buried OXide (BOX) (for SOI devices), but also of the substrate and package are mandatory in these simulations, since dose enhancement and/or backscattered photons can be a significant source of TID variation in the gate oxide. Also the package lid composition and its thickness can be major TID contributors. The results show that the use of a prefilter with lead and aluminum can optimize ionizing dose deposition

Improvements to the macroscopic-microscopic approach of nuclear fission

The well established macroscopic-microscopic (mac-mic) description of nuclear fission enables the prediction of fission fragment yields for a broad range of fissioning systems. In this work, we present several key enhancements to this approach. We improve upon the microscopic sector of nuclear potential energy surfaces by magnifying the resolution of the Lipkin-Nogami equations and strengthening the Strutinsky procedure, thus reducing spurious effects from the continuum. We further present a novel deterministic method for calculating fission dynamics under the assumption of strongly damped nucleonic motion. Our technique directly determines the evolution of the scissioned shape distribution according to the number of random walk steps rather than the statistical accumulation of fission events. We show that our new technique is equivalent to the Metropolis random walk pioneered over the past decade by Randrup and colleagues. It further improves upon it, as we remove the need for altering the nuclear landscape via a biased potential. With our final improvement, we calculate fission fragments mass and charge distributions using particle number projection, which affords the simultaneous calculation of both mass and charge yield distributions. Fission fragments are thus calculated from the quantum mechanical $A$-body states of the potential energy surface rather than the collective mass asymmetry variable ($\alpha_{\rm g}$) of the Finite-Range Liquid-Drop Model (FRLDM) used in past work. We highlight the success of our enhancements by predicting the odd-even staggering and the charge polarization for the neutron-induced fission of ${}^{233}$U and ${}^{235}$U.

Unraveling the Temperature Dependence of Exciton Dissociation and Free Charge Generation in Nonfullerene Organic Solar Cells

Organic solar cells based on nonfullerene acceptor molecules show high charge generation yields with negligible driving force at the donor–acceptor (D/A) interface to drive exciton dissociation. Understanding the underlying charge generation dynamics in these material systems is crucial for further development of this technology. Herein, the acceptor exciton dissociation dynamics in these materials is studied using transient optical spectroscopy. The results show that exciton dissociation at the D/A interface takes up to ≈100 ps to complete, and this process is not significantly affected by temperature. A similar timescale for charge transfer (CT) state separation into free electrons and holes is observed at room temperature. But in contrast to the weak temperature dependence of exciton dissociation, the free charge generation rate and yield are significantly reduced at low temperature. This suggests that overcoming the Coulomb barrier for charge separation at the D/A interface is the main reason for the endothermic charge separation observed for these material systems, instead of diffusion‐limited exciton dissociation.

Properties of liquid argon scintillation light emission

Liquid argon is used as active medium in a variety of neutrino and dark matter experiments thanks to its excellent properties of charge yield and transport and as a scintillator. Liquid argon scintillation photons are emitted in a narrow band of 10 nm centered around 127 nm and with a characteristic time profile made by two components originated by the decay of the lowest lying singlet, ${^{1}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, and triplet states, ${^{3}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, of the excimer ${\mathrm{Ar}}_{2}^{*}$ to the dissociative ground state. A model is proposed which takes into account the quenching of the long lived triplet states through the self-interaction with other triplet states or through the interaction with molecular ${\mathrm{Ar}}_{2}^{+}$ ions. The model predicts the time profile of the scintillation signals and its dependence on the intensity of an external electric field and on the density of deposited energy, if the relative abundance of the unquenched fast and slow components is know. The model successfully explains the experimentally observed dependence of the characteristic time of the slow component on the intensity of the applied electric field and the increase of photon yield of liquid argon when doped with small quantities of xenon (at the part per million level). The model also predicts the dependence of the pulse shape parameter, ${F}_{\text{prompt}}$, for electron and nuclear recoils on the recoil energy and the behavior of the relative light yield of nuclear recoils in liquid argon, ${\mathcal{L}}_{\mathrm{eff}}$.

Ionization yield in silicon for eV-scale electron-recoil processes

The development of single charge resolving, macroscopic silicon detectors has opened a window into rare processes at the O(eV) scale. In order to reconstruct the energy of a given event, or model the charge signal obtained for a given amount of energy absorbed by the electrons in a detector, an accurate charge yield model is needed. In this paper we review existing measurements of charge yield in Silicon, focusing in particular on the region below 1 keV. We highlight a calibration gap between 12-50 eV (referred to as the "UV-gap") and employ a phenomenological model of impact ionization to explore the likely charge yield in this energy regime. Finally, we explore the impact of variations in this model on a test case, that of dark matter scattering off electrons, to illustrate the scientific impact of uncertainties in charge yield.

An Eco-friendly Porous Nanocomposite Fabric-Based Triboelectric Nanogenerator for Efficient Energy Harvesting and Motion Sensing.

A wearable and effective tribopositive material, especially an economical and eco-friendly triboelectric fabric developed from biomaterials, is highly crucial for the development of green wearable triboelectric nanogenerators. In this work, we design a porous nanocomposite fabric (PNF) with strong charge accumulation capacity through a facile dry-casting method and use it as a tribopositive material to construct attractive wearable triboelectric nanogenerators (abbreviated as TENGs). Specifically, the porous nanocomposite is developed by the incorporation of nano-Al2O3 fillers into cellulose acetate networks. By adjusting the concentration of casting solution and the content of nano-Al2O3 fillers, we systematically engineer the physical properties of the PNF for obtaining a large triboelectric charge yield. When a 10 wt % solution concentration and 10 wt % nanofiller content are adopted for the PNF, the corresponding PNF-TENG can deliver an electrical performance of ∼2.5 mW/cm2 on a 0.8 MΩ external resistor. This remarkable output can be ascribed to the synergistic effect between the appropriate porous network and improved dielectric properties of the nanocomposite. Moreover, the PNF-TENG also exhibits good reliable electrical outputs under multiple stain-washing measurements or after experiencing cyclical contact-separation 13,500 times. Also, the device is capable of charging various capacitors, lighting LED arrays, and driving commercial wrist watches and is proven to be an efficient and reliable green wearable power source. Furthermore, a PNF-TENG-based elbow supporter and a grip ball, as self-powered sensors, are proposed to realize real-time detection for human actions during sports exercise. This work proposes an eco-friendly nanocomposite fabric as an effective tribopositive material, verifies the feasibility of developing environmentally friendly wearable power sources and sensors, and provides new insights into the design of green wearable triboelectric nanogenerators.

A 3D multi-interface structure of coral-like Fe-Mo-S/Ni3S2@NF using for high-efficiency and stable overall water splitting

Rational interface designing of noble-metal-free electrocatalysts is vital for the enhancement of efficiency and durability towards overall water splitting. In this study, the optimal Fe-Mo-S/Ni3S2@NF with a coral-like microstructure and multiple interfaces is fabricated successfully by a facile one-step hydrothermal method. Systematic experiments, including X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and electrochemical impedance spectroscopy (EIS), verify that these multiple interfaces in Fe-Mo-S/Ni3S2@NF will promote charge transfer and yield abundant active sites, leading to a synergistic effect on electrocatalytic reactions. As a result, this electrode material exhibits high hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity. When employing Fe-Mo-S/Ni3S2@NF as a bifunctional electrode for overall water splitting, the integrated electrolysis device can achieve an extremely low cell voltage of 1.51 V at 20 mA cm−2 in alkaline solution, outperforming the noble Pt/C-IrO2 couple (1.8 V @20 mA cm−2), as well as exhibits remarkable durability. Additionally, the favourable contributions of the multi-interface toward HER and OER are deeply investigated by density functional theory (DFT) calculations. This study provides insights into the significance of coupling interfaces, and it also suggests a possible design for future electrolysis catalysts.

Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells.

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

Primary fission fragment mass yields across the chart of nuclides

We have calculated a complete set of primary fission fragment mass yields, $Y(A)$, for heavy nuclei across the chart of nuclides, including those of particular relevance to the rapid neutron capture process ($r$ process) of nucleosynthesis. We assume that the nuclear shape dynamics are strongly damped which allows for a description of the fission process via Brownian shape motion across nuclear potential-energy surfaces. The macroscopic energy of the potential was obtained with the Finite-Range Liquid-Drop Model (FRLDM), while the microscopic terms were extracted from the single-particle level spectra in the fissioning system by the Strutinsky procedure for the shell energies and the BCS treatment for the pairing energies. For each nucleus considered, the fission fragment mass yield, $Y(A)$, is obtained from 50,000 -- 500,000 random walks on the appropriate potential-energy surface. The full mass and charge yield, $Y(Z,A)$, is then calculated by invoking the Wahl systematics. With this method, we have calculated a comprehensive set of fission-fragment yields from over 3,800 nuclides bounded by $80\leq Z \leq 130$ and $A\leq330$; these yields are provided as an ASCII formatted database in the supplemental material. We compare our yields to known data and discuss general trends that emerge in low-energy fission yields across the chart of nuclides.

Mapping photocathode quantum efficiency with ghost imaging

Measuring the quantum efficiency (QE) map of a photocathode injector typically requires laser scanning, an invasive operation that involves modifying the injector laser focus and rastering the focused laser spot across the photocathode surface. Raster scanning interrupts normal operation and takes considerable time to setup. In this paper, we demonstrate a novel method of measuring the QE map using a ghost imaging framework that correlates the injector laser spatial variation over time with the total charge yield. Ghost imaging enables passive, real-time monitoring of the QE map without manually modifying the injector laser or interrupting injector operation. We first demonstrate the method at the UCLA Pegasus photoinjector with the help of a digital micromirror device (DMD) and a piezoelectric mirror to increase our control of the overall transverse variance of the illumination profile. The reconstruction algorithm parameters are fine-tuned using simulations and the results are validated against the ground truth map acquired using the traditional rastering method. Finally, we apply the technique to data acquired parasitically from the LCLS photoinjector, showing the feasibility of this method to retrieve a QE map without interrupting normal operation.

Slow charge transfer from pentacene triplet states at the Marcus optimum.

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

Gate Damages Induced in SiC Power MOSFETs During Heavy-Ion Irradiation—Part II

This article presents the results of a 2-D finite element simulation study of the gate damages induced by heavy-ion irradiation in SiC power metal–oxide–semiconductor field-effect transistors (MOSFETs). The time evolution of the electric field in the gate oxide is studied. Two effects are investigated: the first is associated with the charge deposition in the SiC portion of the MOSFET, with the time evolution studied using the 2-D finite element simulator; the second one results from holes generated during the ion transit, trapped in the gate oxide after the fast electrons have been quickly swept away by the electric field. Two different techniques have been combined for estimating the hole concentration in the gate oxide: the well-known recombination rate was modified to consider the trapped charge yield, as was recently done to better interpret single event gate rupture (SEGR) failure of silicon power MOSFETs. Under ion irradiation test conditions at which the gate damage experimentally starts to be observed, we demonstrate that, because of the ion impact, regardless of the ion linear energy transfer (LET), the peak value of the electric field in the gate oxide becomes practically equal to the oxide breakdown field (~12–15 MV/cm). Moreover, we show that simulations can be used to predict the test conditions at which gate damage starts to appear as a function of LET and the range of heavy ions used in the irradiation experiments.

Calibration and electric characterization of p-MNOS RADFETs at different dose rates and temperatures

Abstract This paper describes the radiation response and I-V characteristics of the stacked p-MNOS based RADFETs measured at different dose rates and irradiation temperatures. It is shown that the enhanced charge trapping takes place at the interface of the thick gate dielectrics in the MNOS transistors at low dose rates (ELDRS). The sensitivity of the radiation effect to irradiation temperature has also experimentally revealed. We associate both effects with the temperature and dose rate dependence of the effective charge yield in the thick oxides described within the framework of the previously proposed model. We have also simulated the I-V characteristics of the transistors for different total doses and irradiation conditions. It has been found the used electric and radiation models qualitatively and semi-quantitatively describe the observed dependencies of the RADFETs’ sensitivity on dose rates and irradiation temperatures for the devices with different thickness of insulators.

Damage Separation in a Bipolar Junction Transistor Following Irradiation With 250-MeV Protons

Irradiation of 2N5339 n-p-n bipolar junction transistors (BJTs) with either 250-MeV protons or 10-keV X-rays shifts the input characteristics to lower values of $V_{\mathrm {BE}}$ while degrading the current gain and drive current. The degradation is consistent with increased recombination in the neutral base and emitter–base (E–B) depletion region. A decrease in collector current following proton irradiation suggests that recombination in the neutral base is significant. At a given ionizing dose, degradation is worse for proton irradiation than for X-ray irradiation due to the presence of displacement damage and a higher charge yield. A comparison of degradation produced by the two radiation sources suggests that ~40% of the excess base current resulting from 250-MeV proton irradiation is due to ionization damage. When compared with previous results for other devices, these results suggest that ionization-to-displacement damage ratios in bipolar devices may increase with proton energy in a way that is consistent with trends in charge yield and non-ionizing energy loss (NIEL).

Combine harvesting efficiency as affected by rice field size and other factors and its implication for adoption of combine contracting service

ABSTRACTSmall combine harvesters have become popular in SE Asia recently, but small rice fields appear to limit its field operation and hence its wide adoption by smallholders. Combine harvesting efficiency, the area of paddy field harvested per unit time, was determined for 6 seasons in Central Lao PDR for over 400 fields varying in size and toposequence position, rice varieties grown, crop establishment methods and crop conditions particularly lodging at harvest. Combine harvesting efficiency was commonly about 3 ha/day in small fields of less than 1000 m2 and increased with the increase in field size up to 5 ha/day in the size of about 2000–3000 m2, but not beyond this range. Lower efficiency was also found in wet season than dry season, and also crops established from broadcasting compared with transplanting and drill seeding, at least partly because broadcasted crops tended to lodge more often than crops established from other methods. Photoperiod sensitive, long duration varieties grown in lower field positions also tended to have lower combine efficiency. Using the data obtained in this work, net return of combine adoption to farmers was estimated for different combine charge fees and yield levels. Increasing combine harvesting efficiency by 50% with increased field size would increase the net income of combine harvesting contractors by around 50% at current charges, allowing them to reduce the combine harvesting fees they charge. This would aid the adoption of combine harvesting services.

Simultaneous measurement of the light and charge response of liquid xenon to low-energy nuclear recoils at multiple electric fields

Dual-phase liquid xenon (LXe) detectors lead the direct search for particle dark matter. Understanding the signal production process of nuclear recoils in LXe is essential for the interpretation of LXe based dark matter searches. Up to now, only two experiments have simultaneously measured both the light and charge yield at different electric fields, neither of which attempted to evaluate the processes leading to light and charge production. In this letter, results from a neutron calibration of liquid xenon with simultaneous light and charge detection are presented for energies from 3-74 keV, at electric fields of 0.19, 0.49, and 1.02 kV/cm. No significant field dependence of the yields is observed.

Impact of Heavy Ion Energy on Charge Yield in Silicon Dioxide

The impact of heavy ion energy on charge yield in silicon dioxide was investigated by irradiating power VDMOSFETs with heavy ions at energies from 3 to 25 MeV/amu. The ion charge yield was determined by analyzing the gate voltage shifts of power VDMOSFETs induced by irradiation with 60Co $\gamma $ -rays and several different heavy ion species. A strong dependence of the charge yield on the ion energy is revealed. The charge yield at ion energy of 25 MeV/amu is greater by nearly an order of magnitude than that at the ion energy of 3 MeV/amu. The results are consistent with simulations of heavy ion track structures for low- and high-energy ions.

Toward an understanding of the anomaly in charge yield of Mo and Sn fragments in the fission reaction U238(n,f )

Using the improved scission-point model, the charge distribution of fission fragments is studied in neutron-induced fission of the $^{238}\mathrm{U}$ nucleus. A possible explanation of the anomaly in charge yield of Mo and Sn fragments is presented.