ABSTRACT
Compressed ultrafast photography (CUP) is a computational imaging technology capable of capturing transient scenes in picosecond scale with a sequence depth of hundreds of frames. Since the inverse problem of CUP is an ill-posed problem, it is challenging to further improve the reconstruction quality under the condition of high noise level and compression ratio. In addition, there are many articles adding an external charge-coupled device (CCD) camera to the CUP system to form the time-unsheared view because the added constraint can improve the reconstruction quality of images. However, since the images are collected by different cameras, slight affine transformation may have great impacts on the reconstruction quality. Here, we propose an algorithm that combines the time-unsheared image constraint CUP system with unsupervised neural networks. Image registration network is also introduced into the network framework to learn the affine transformation parameters of input images. The proposed algorithm effectively utilizes the implicit image prior in the neural network as well as the extra hardware prior information brought by the time-unsheared view. Combined with image registration network, this joint learning model enables our proposed algorithm to further improve the quality of reconstructed images without training datasets. The simulation and experiment results demonstrate the application prospect of our algorithm in ultrafast event capture.
ABSTRACT
This paper present a novel, integrated compressed ultrafast photography system for comprehensive measurement of the aluminium planar wire array Z-Pinch evolution process. The system incorporates a large array streak camera and embedded encoding to improve the signal-to-noise ratio. Based on the "QiangGuang-I" pulsed power facility, we recorded the complete continuous 2D implosion process of planar wire array Z-Pinch for the first time. Our results contribute valuable understanding of imploding plasma instabilities and offer direction for the optimization of Z-Pinch facilities.
ABSTRACT
Neutron and x-ray imaging are essential ways to diagnose a pulsed radiation source. The three-dimensional (3D) intensity distribution reconstructed from two-dimensional (2D) radiation images can significantly promote research regarding the generation and variation mechanisms of pulsed radiation sources. Only a few (≤5) projected images at one moment are available due to the difficulty in building imaging systems for high-radiation-intensity and short-pulsed sources. The reconstruction of a 3D source with a minimal number of 2D images is an ill-posed problem that leads to severe structural distortions and artifacts of the image reconstructed by conventional algorithms. In this paper, we present an iterative method to reconstruct a 3D source using spherical harmonic decomposition. Our algorithm improves the representation ability of spherical harmonic decomposition for 3D sources by enlarging the order of the expansion, which is limited in current analytical reconstruction algorithms. Prior knowledge of the source can be included to obtain a reasonable solution. Numerical simulations demonstrate that the reconstructed image quality of the iterative algorithm is better than that of the analytical algorithm. The iterative method can suppress the effect of noise in the integral projection image and has better robustness and adaptability than the analytical method.
ABSTRACT
Time response and light yield are two of the most important features of a scintillation detector, and are mostly determined by the luminescence properties of the scintillator. Here we have investigated the radioluminescence (RL) characteristics of a single-crystalline hybrid lead halide perovskite at both room temperature and low temperature. A dual-channel single photon correlation (DCSPC) system with a vacuum chamber is employed for the measurement. A rise time faster than 100 ps and several times enhancement of the crystal scintillation performances at low temperature have been observed. These behaviors demonstrated that bulk solution-grown single crystals of hybrid lead halide perovskites (MAPbCl3 and Br-doped MAPbBr0.08Cl2.92, where MA = CH3NH3) can serve as stable scintillating materials for pulsed gamma detectors. In addition, this work provides a pathway for perovskite application and also attracts attention to investigating low-temperature scintillators.
ABSTRACT
Lutetium-yttrium oxyorthosilicate doped with cerium (LYSO:Ce) is a widely used scintillator, and the study of its nonlinear behavior under high excitation density is very significant owing to its direct influence on radiation measurements. Using a 266 nm ultraviolet laser to excite an LYSO:Ce crystal, the relationship between the photoluminescence (PL) light yield and excitation density was studied by Z scan experiments. The excitation threshold of the LYSO:Ce was obtained, which is about 2.3 J cm-3. Picosecond transient absorption of LYSO:Ce at 800 nm was obtained and used to analyze the dynamic process of carriers. The physical mechanism behind the nonlinearity was discussed and analyzed using the Förster dipole-dipole interaction model, and the interaction characteristic radius was obtained by fitting. This work can help us understand the nonlinearity phenomenon in scintillators and provide references for related radiation detection applications.
ABSTRACT
Scintillators form the chief device for radiation detection, and the study of their characteristics and their related theories is very significant. Specifically, the nonlinear behavior of scintillators under high excitation density has been closely studied owing to its direct influence on the measurements of radiation. We propose a new method to calibrate the nonlinearity of scintillators based on the electron pulse generated by a linear electron accelerator. The nonlinear light yield of several commonly used scintillators versus fluence of 70 MeV electrons in a 10 ps pulse has been measured by adjusting the charge of electron pulses, and the deposition energy threshold is also simulated and calculated. The results show that the deposition energy threshold for the occurrence of 5% nonlinearity is highest for two types of oxide scintillators, viz. LSO and PbWO4, followed by fluoride scintillators, viz. BaF2 and CeF3, and the threshold for a plastic scintillator EJ232 is lowest.
ABSTRACT
An interferometer, directly coupling a single-mode fiber with a bulk semiconductor, was used to one-shot record the instantaneous change in the refractive index of bulk semiconductors. A practical contrast level can be achieved in the interferometer with different types of polished bulk materials several hundred micrometers in thickness. The temporal resolution was approximately 50 ps and can be improved by using thinner materials or higher bandwidth detection. This interferometer can also be used to study the fast recording technology based on pulsed radiation-induced changes in optical characteristics.
ABSTRACT
We fabricated a liquid scintillator capillary array (LSCA) for gamma imaging with the aim of developing a one-dimensional detector system utilizing a streak camera for high temporal and spatial resolution pulsed gamma radiation detection. The detector's performance was studied in a simulation and via an experiment. The maximum efficiency of the LSCA's emission was at a wavelength of 420 nm. To establish a high fidelity representation of the detector's edge spread function and modulation transfer function (MTF), a slanted edge algorithm was introduced to calculate the edge spread function of the discrete sampling array's image screen. The simulation results showed that the spatial resolution of the LSCA was better for 14 MeV neutrons than for 1.25 MeV gamma radiation. The experimental results show that in comparison with a 6-mm-thick LaBr3 image plate, the LSCA had a higher temporal and spatial resolution when used as a gamma detector. The spatial resolution was 1.1 lp/mm (MTF = 0.1) for the LSCA. In addition, when an ultra-violet streak camera was coupled with the LSCA, it had a comparable sensitivity to that of a 6-mm-thick LaBr3 image plate.
ABSTRACT
For the quantitative investigation of MeV-photon-induced changes in the refractive indices of bulk semiconductors, a model was established to describe the evolution of the excess carrier density, including the generation and recombination processes. The two key parameters of the evolution model, namely, the summed injection intensity and the gamma intensity curve, were obtained via dose measurements and gamma pulse monitoring, respectively. An interferometric method of measuring instantaneous changes in the refractive index and obtaining real-time measurements of the excess carrier density in bulk materials was successfully implemented. The probe beam was transmitted through a single-mode fiber to form double-beam interference in a slab geometry. Two bulk samples, one consisting of intrinsic GaAs and one of intrinsic ZnO, were tested. The recombination time constant of the intrinsic GaAs sample was found to be approximately 0.6 ns and did not vary distinctly with the photon energy, whereas the ZnO sample's recombination behavior consisted of two components. The short component was evident when short and intense pulses were incident, whereas the long component dominated under long and relatively weak pulses. The method reported in this work can be used to study the excess carrier dynamics induced by pulsed gamma radiation and to investigate the mechanisms of refractive index modulation under pulsed gamma conditions; thus, it is expected to be beneficial for guiding the development of RadOptic systems based on bulk materials.
ABSTRACT
A normal-incident flattop laser with a tapered end is proposed as an optical undulator to achieve a high-gain and high-brightness X-ray free electron laser (FEL). The synchronic interaction of an electron bunch with the normal incident laser is realized by tilting the laser pulse front. The intensity of the flattop laser is kept constant during the interaction time of the electron bunch and the laser along the focal plane of a cylindrical lens. Optical shaping to generate the desired flattop pulse with a tapered end from an original Gaussian pulse distribution is designed and simulated. The flattop laser with a tapered end can enhance the X-ray FEL beyond the exponential growth saturation power by one order to reach 1 Gigawatt as compared to that without a tapered end. The peak brightness can reach 1030 photons/mm2/mrad2/s/0.1% bandwidth, more than 10 orders brighter than the conventional incoherent Thompson Scattering X-ray source.
