Rhodotron and rotating target: A solution towards micro-spot for high energy and high dose rate bremsstrahlung sources.

High energy over MeV bremsstrahlung sources that employ normal conducting radio frequency linear accelerators have expanding applications in industrial computerized tomography (CT) for non-destructive inspection and evaluation. The X-ray spot size that mainly affects the imaging quality is yet limited by the electron beam width in the high resolution CT systems. In a short exposure time, high beam power is required to generate sufficient photons to improve the signal to noise ratio of imaging. However, with ∼kW level of average beam power these linear accelerators usually have a beam spot size over 1 mm since the temperature rising due to the beam energy deposition in the target should be far below its melting point. We propose a concept of using a Rhodotron-based accelerator to provide high power electron beams in a long duration pulse and a rotating target to mitigate the overheating issue, such that the gap between micro-spot and high dose rate can be bridged in the high energy bremsstrahlung sources. This article presents an in-depth simulation work to discuss and evaluate this scheme of X-ray source. The simulations of beam dynamics in the accelerator and bremsstrahlung process in the target predict the generated X-rays with a spot size as small as 68 µm at full-width half-maximum and a dose rate as high as 4700 cGy/min from a 9 MeV electron beam interacting with a 1 mm thickness tantalum target. Further thermal analysis in the rotating target indicates a significant improvement of beam power handling in comparison with the conventional stationary one.

Solvent Free Laminated Fabrication of Lead Halide Perovskites for Sensitive and Stable X-ray Detection.

The halide perovskite X-ray detector can meet the urgent needs of low-dose medical imaging by X-rays. However, there is still a pressing challenge in lacking robust methods for large-scale fabrication of high-quality perovskite films with tunable thickness. Here we report a laminated fabrication of polycrystalline MAPbI3 by using solvent-free liquid perovskite molten-salt (PMS), that offers reduced toxic issue, scalable fabrication, and highly tunability in film thickness. Nylon membrane was chosen as a scaffold for the infiltration of PMS, which simultaneously acts as a physical barrier to suppress the ionic migration in the MAPbI3-nylon composite (denoted as MAPbI3-LLP). The enhanced material properties result in good stability and high performance of X-ray detectors that show low detection limit and high sensitivity. Additionally, single gamma-ray photon detection was realized by MAPbI3-LLP detectors. The promising performance characteristics of such polycrystalline detectors can accelerate the adoption of polycrystalline perovskites in X-ray imaging and gamma-ray detection.

Efficient X-ray Attenuation Lead-Free AgBi2I7 Halide Rudorffite Alternative for Sensitive and Stable X-ray Detection.

The poor attenuation capability of high-energy X-ray photons hinders the application of X-ray detectors in medical and astrophysical areas. Halide-based perovskites are promising candidates for X-ray detection because of their improved sensitivity. However, their inferior attenuation coefficient is still unsatisfactory for broadband X-ray detection. Here, a new kind of X-ray detection material, AgBi2I7 rudorffite single crystal (SC), is prepared and applied in X-ray detection for the first time, and it shows a higher attenuation coefficient than halide-based perovskites, commercialized a-Se, and the currently outstanding Cd0.9Zn0.1Te (CZT). The AgBi2I7 rudorffite SCs possess outstanding electric properties and excellent stability. AgBi2I7-SC detectors demonstrate a limit of detection (LoD) of 72 nGyair s-1 and a sensitivity of 282.5 µC Gyair-1cm-2 to X-rays and show only a slight performance degradation after ontinuous X-ray irradiation with a total dose of 58 Gyair. This work opens up a new perspective and broad opportunities for halide rudorffite in X-ray detection.

An induction voltage adder with MHz repetition rates and nanosecond duration based on printed circuit board stacked Blumlein lines.

By utilizing the radio-frequency metal-oxide-semiconductor field-effect transistor switch, the multi-stage stacked printed circuit board Blumlein lines, and the induction voltage adder, a voltage generator with MHz repetition rates and nanosecond duration was designed and fabricated. The shunting current of the multi-stage stacked Blumlein lines, which consists of the coupling current at the load ends and the leakage current at the switch ends, is clarified. The circuit simulation and experiment of the three-stage stacked Blumlein lines are carried out; the result shows that the experiment and simulation agree well. The five-stage IVA prototype is designed for generating voltage pulses of ∼10 kV in amplitude and 8-11 ns in duration with MHz burst repetition rates. The voltage generator with MHz repetition rates and nanosecond duration demonstrated an important application prospect to reduce the electron beam initial emittance of the high energy accelerator.

Robust Fabrication of Hybrid Lead-Free Perovskite Pellets for Stable X-ray Detectors with Low Detection Limit.

X-ray detectors are widely utilized in medical diagnostics and nondestructive product inspection. Halide perovskites are recently demonstrated as excellent candidates for direct X-ray detection. However, it is still challenging to obtain high quality perovskites with millimeter-thick over a large area for high performance, stable X-ray detectors. Here, methylammonium bismuth iodide (MA3 Bi2 I9 ) polycrystalline pellets (PPs) are developed by a robust, cost effective, and scalable cold isostatic-pressing for fabricating X-ray detectors with low limit of detection (LoD) and superior operational stability. The MA3 Bi2 I9 -PPs possess a high resistivity of 2.28 × 1011 Ω cm and low dark carrier concentration of ≈107 cm-3 , and balanced mobility of ≈2 cm2 V-1 s-1 for electrons and holes. These merits enable a sensitivity of 563 µC Gyair -1 cm-2 , a detection efficiency of 28.8%, and an LoD of 9.3 nGyair s-1 for MA3 Bi2 I9 -PPs detectors, and the LoD is much lower than the dose rate required for X-ray diagnostics used currently (5.5 µGyair s-1 ). In addition, the MA3 Bi2 I9 -PPs detectors work stably under high working bias field up to 2000 V cm-1 after sensing an integrated dose >320 Gyair with continuous X-ray radiation, demonstrating its competitive advantage in practical application. These findings provide an approach to explore a new generation of low LoD, stable and green X-ray detectors based on MA3 Bi2 I9 -PPs.

Measurement of yield and spectrum of secondary electron emission and their characteristics under modification of conductive materials.

Surface modification technique of secondary electron emission (SEE) characteristics of materials, which is utilized to suppress or promote the SEE from material surface under electron bombardment, has extensive applications in a variety of fields. Measurement of the secondary electron yield (SEY) and the secondary electron spectrum (SES) before and after surface modification is essential for the evaluation of effectiveness and the investigation of mechanism of material modification. A SEY and SES measurement system is reported in this article. The comparative measurements of the total SEY, the true SEY, the backscattered electron yield, and the SES of nickel and free-standing vertical graphene, which was grown in situ on the surface of nickel substrate by plasma enhanced chemical vapor deposition, were performed using this system. The measurement results demonstrate that this system could facilitate the study of surface modification on the SEE characteristics of conductive materials.

Note: Stability and lifetime of scandium deuteride film cathode in a vacuum arc ion source.

This paper reports the properties of the plasma and gas produced in a vacuum arc discharge with scandium deuteride (ScD1.8) film cathodes. The thickness of the ScD1.8 film influences the quantity of the gases released from the cathode material. The deuterium gas releasing in the discharge process was in a depth range from the cathode surface to the cathode interior, that is, between 3 and 6 µm. Surprisingly, after discharge, the deuterium ion ratio remains the same in the film with different thicknesses. That indicates that the release of deuterium gas in a 3 µm-thick ScD1.8 film is enough for ionization. In addition, as the number of discharge increases, the stability of atomic fraction ratio gets worse and the ratio of deuterium ions decreases.

Diffusion and distribution of deuterium in scandium deuteride thin films under irradiation of deuterium ion beam.

Scandium deuteride (ScDx) thin films, as an alternative target for deuterium-deuterium (D-D) reaction, are a very important candidate for detection and diagnostic applications. Albeit with their superior thermal stability, the ignorance of the stability of ScDx under irradiation of deuterium ion beam hinders the realization of their full potential. In this report, we characterize ScDx thin films with scanning electron microscopy (SEM) and X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA). We found with increased implantation of deuterium ions, accumulation and diffusion of deuterium are enhanced. Surprisingly, the concentration of deuterium restored to the value before implantation even at room temperature, revealing a self-healing process which is of great importance for the long-term operation of neutron generator.

Homogeneous nanocrystalline cubic silicon carbide films prepared by inductively coupled plasma chemical vapor deposition.

Silicon carbide films with different carbon concentrations x(C) have been synthesized by inductively coupled plasma chemical vapor deposition from a SiH(4)/CH(4)/H(2) gas mixture at a low substrate temperature of 500 °C. The characteristics of the films were studied by x-ray photoelectron spectroscopy, x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared absorption spectroscopy, and Raman spectroscopy. Our experimental results show that, at x(C) = 49 at.%, the film is made up of homogeneous nanocrystalline cubic silicon carbide without any phase of silicon, graphite, or diamond crystallites/clusters. The average size of SiC crystallites is approximately 6 nm. At a lower value of x(C), polycrystalline silicon and amorphous silicon carbide coexist in the films. At a higher value of x(C), amorphous carbon and silicon carbide coexist in the films.