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Solution grown metal halide perovskites (MHPs) are a class of low-cost, direct conversion semiconducting materials with the potential to meet the need for large areas, high stopping power, and high sensitivity x-ray detectors. While polycrystalline MHP thin films have shown significant potential for meeting this need, their efficiencies for detecting x rays are limited by their thickness. This article presents an MHP variant, methylammonium lead tribromide (MAPB) in the form of 3D polycrystalline mosaic wafers with relatively high hole transport properties that demonstrated stable sensitivity during irradiation. An expedited and efficient detector fabrication method that did not include polishing or sintering was evaluated, and a radiograph of a brass key was produced using electron collection from the resulting wafer. However, a high dose rate was required due to very low sensitivity values. The effects of surface inhomogeneity and radiation damage were investigated as explanatory factors, and these phenomena were further characterized through IV measurements, current response as a function of electric field and x-ray dose rate, and alpha particle irradiation. The results indicated that although compromising surface quality for fabrication efficiency was the primary hindrance to the x-ray detection performance of 3D polycrystalline MAPB, stable performance could still be achieved under reasonable dose rates. [ABSTRACT FROM AUTHOR]

We study the self-collimation phenomenon in photonic crystals (PhC) of wide bandgap materials for ultra-fast and high spatial resolution x-ray detection. We work on various heavy inorganic scintillators: BaF2, GaN, ZnO, CsI:Tl, NaI:Tl, LYSO, WO4 compounds, and plastic scintillators. Conventional scintillator detectors do not rely on a direct detection mechanism; hence, they require intricate design and fabrication processes. We offer a PhC design to observe self-collimation phenomena and overcome the ongoing spatial resolution challenges with these types of materials. We investigate the photonic band diagrams and iso-frequency contours. Fourier transforms based on finite-difference time-domain and frequency domain simulations are done for verifying and analyzing the self-collimation with the selected material. Light extraction efficiency at the PhC–air interface, depending on the truncation distance from the excitation point, is measured. Beam divergence values are calculated at 1 mm propagation distance. The vertical field profiles are obtained to observe the confinement. For the spatial resolution analysis, cross-sectional beam profiles have been examined. Gaussian envelopes are fitted to beam profiles for a consistent data analysis, and full-width-at-half-maximum values are considered. As a result, we theoretically prove and demonstrate the spatially resolved x-ray detection at the sub-micrometer level for a wide range of scintillator materials. [ABSTRACT FROM AUTHOR]

We explore the ultrafast generation of spin currents in magnetic multilayer samples by applying fs laser pulses to one layer and measuring the magnetic response in the other layer by element-resolved x-ray spectroscopy. In Ni(5 nm)/Ru(2 nm)/Fe(4 nm), the Ni and Fe magnetization directions couple antiferromagnetically due to the Ruderman–Kittel–Kasuya–Yosida interaction but may be oriented parallel through an applied magnetic field. After exciting the top Ni layer with a fs laser pulse, we also find that the Fe layer underneath demagnetizes, with a 4.1 ± 1.9 % amplitude difference between parallel and antiparallel orientation of the Ni and Fe magnetizations. We attribute this difference to the influence of a spin current generated by the fs laser pulse that transfers angular momentum from the Ni into the Fe layer. Our results confirm that superdiffusive spin transport plays a role in determining the sub-ps demagnetization dynamics of synthetic antiferromagnetic layers, but also evidence large depolarization effects due to hot electron dynamics, which are independent of the relative alignment of the magnetization in Ni and Fe. [ABSTRACT FROM AUTHOR]

A model for calculating the sensitivity of direct conversion X-ray image detectors is developed incorporating the space charge effect. The space charge perturbs the applied electric field across the thickness of the photoconductor, and thus the electric field becomes nonuniform. First, Poisson's equation is solved to obtain the electric field distribution. Then, the continuity equations for both holes and electrons are solved considering the perturbed electric field. Finally, the collected charge is determined from the charge carrier distributions and electric field profile using the Shockley–Ramo theorem. The developed X-ray sensitivity model is applied to polycrystalline mercuric iodide based detectors. The electric field collapses for a critical amount of trapped charge if the ratio of trapped electrons to holes is high (∼0.5). The pinch in electric field slows down the drift of free electrons and hence reduces the sensitivity of the detector. The developed model is fitted with the published experimental data, and the fitted values of effective mobility-lifetime products of charge carriers agree well with the published results. It is found that the space charge reduces the sensitivity in the screen-printed sample more significantly than in the vacuum deposited sample. [ABSTRACT FROM AUTHOR]

Developments in the field of organic semiconductors have generated organic photodetectors with high quantum efficiency, wide spectral sensitivity, low power consumption, and unique form factors that are flexible and conformable to their substrate shape. In this work, organic photodetectors coupled with inorganic CsI(Tl) scintillators are used to showcase the low dose rate sensitivity that is enabled when high performance organic photodetectors and scintillator crystals are integrated. The detection capability of these organic-inorganic coupled systems to high energy radiation highlights their potential as an alternative to traditional photomultiplier tubes for nuclear spectroscopy applications. When exposed to Bremsstrahlung radiation produced from an X-ray generator, SubPc:C60, AlPcCl:C70, and P3HT:PC61BM thin film photodetectors with active layer thicknesses less than 100 nm show detection of incident radiation at low and no applied bias. Remarkably low dose rates, down to at least 0.18 μGy/s, were detectable with a characteristic linear relationship between exposure rate and photodetector current output. These devices also demonstrate sensitivities as high as 5.37 mC Gy-1cm-2 when coupled to CsI(Tl). Additionally, as the tube voltage across the X-ray generator was varied, these organic-inorganic systems showed their ability to detect a range of continuous radiation spectra spanning several hundred keV. [ABSTRACT FROM AUTHOR]

Although amorphous selenium (a-Se) has a long and successful history of application in optical and X-ray imaging, some of its fundamental properties are still puzzling. In particularly, the mechanism of carrier recombination following x-ray excitation and electric field and temperature dependences of the electron-hole pair creation energy (Wehp) remain unclear. Using the combination of X-ray photocurrent and pulse height spectroscopy measurements, we measure Wehp in a wide range of temperatures (218-320K) and electric fields (10-100 V/μm) and show that the conventional columnar recombination model which assumes Langevin recombination within a column (a primary electron track) fails to explain experimental results in a wide range of electric fields and temperatures. The reason for the failure of the conventional model is revealed in this work, and the theory of the columnar recombination is modified to include the saturation of the recombination rate at high electric field in order to account for the experimental results in the entire range of fields and temperatures. [ABSTRACT FROM AUTHOR]

We have performed X-ray synchrotron micro-diffraction measurements to study the processes controlling the formation of hillocks and whiskers in Sn layers on Cu. The studies were done in real-time on Sn layers that were electro-deposited immediately before the X-ray measurements were started. This enabled a region of the sample to be monitored from the as-deposited state until after a hillock feature formed. In addition to measuring the grain orientation and deviatoric strain (via Laue diffraction), the X-ray fluorescence was monitored to quantify the evolution of the Sn surface morphology and the formation of intermetallic compound (IMC) at the Sn-Cu interface. The results capture the simultaneous growth of the feature and the corresponding film stress, grain orientation, and IMC formation. The observations are compared with proposed mechanisms for whisker/hillock growth and nucleation. [ABSTRACT FROM AUTHOR]