Simultaneous fluorescence and Compton scattering computed tomography based on linear polarization X-ray

Purpose To propose a method for simultaneous fluorescence and Compton scattering computed tomography by using linearly polarized X-rays.Methods Monte Carlo simulations were adopted to demonstrate the feasibility of the proposed method.In the simulations,the phantom is a polytetrafluoroethylene cylinder inside which are cylindrical columns containing aluminum,water,and gold(Au)-loaded water solutions with Au concentrations ranging between 0.5 and 4.0 wt%,and a parallel-hole collimator imaging geometry was adopted.The light source was modeled based on a Thomson scattering X-ray source.The phantom images for both imaging modalities were reconstructed using a maximumlikelihood expectation maximization algorithm.Results Both the X-ray fluorescence computed tomography(XFCT)and Compton scattering computed tomography(CSCT)images of the phantom were accurately reconstructed.A similar attenuation contrast problem for the different cylindrical columns in the phantom can be resolved in the XFCT and CSCT images.The interplay between XFCT and CSCT was analyzed,and the contrast-to-noise ratio(CNR)of the reconstruction was improved by correcting for the mutual influence between the two imaging modalities.Compared with K-edge subtraction imaging,XFCT exhibits a CNR advantage for the phantom.Conclusion Simultaneous XFCT and CSCT can be realized by using linearly polarized X-rays.The synergy between the two imaging modalities would have an important application in cancer radiation therapy.

Beam and image experiment of beam deflection electron gun for distributed X-ray sources

Distributed X-ray sources comprise a single vacuum chamber containing multiple X-ray sources that are triggered and emit X-rays at a specific time and location. This process facilitates an application for innovative system concepts in X-ray and computer tomography. This paper proposes a novel electron beam focusing, shaping,and deflection electron gun for distributed X-ray sources.The electron gun uses a dispenser cathode as an electron emitter, a mesh grid to control emission current, and two electrostatic lenses for beam shaping, focusing, and deflection. Novel focusing and deflecting electrodes were designed to increase the number of focal spots in the distributed source. Two identical half-rectangle opening electrodes are controlled by adjusting the potential of the two electrodes to control the electron beam trajectory, and then, multifocal spots are obtained on the anode target. The electron gun can increase the spatial density of the distributed X-ray sources, thereby improving the image quality. The beam experimental results show that the focal spot sizes of the deflected(deflected amplitude 10.5 mm)and non-deflected electron beams at full width at half maximum are 0.80 mm 90.50 mm and 0.55 mm 90.40 mm, respectively(anode voltage 160 kV; beam current 30 mA). The imaging experimental results demonstrate the excellent spatial resolution and time resolution of an imaging system built with the sources, which has an excellent imaging effect on a field-programmable gate array chip and a rotating metal disk.

Beam dynamics optimization of very-high-frequency gun photoinjector

Very-high-frequency(VHF)gun photoinjectors,capable of producing high-brightness and high-repetitionrate electron bunches,are some of the best electron sources for driving MHz-class repetition-rate free-electron lasers.In this study,the beam dynamics optimization of a VHF gun photoinjector for Shanghai HIgh Repetition Rate X-ray Free Electron Laser and Extreme Light Facility(SHINE)is systematically demonstrated using a genetic algorithm.Through the inclusion of the solenoid geometry as an optimization variable into the genetic algorithm,the optimum projected normalized emittance for 100 pC bunches with bunch length of 1 mm rms is reduced to 0.1 mm mrad for 100%of the particles and 0.075 mm mrad for 95%of the particles,proving that sub-100 nm emittance can be achieved in the SHINE injector using a single-cell Tsinghua University(THU)VHF gun.This emittance fulfills the requirements not only of SHINE and Linac Coherent Light Source(LCLS)-II but also of LCLS-II-High Energy(LCLS-II-HE).We demonstrate that the optimal emittance in the VHF gun injector is reduced via the optimization of the solenoid geometry,thereby reducing solenoid spherical aberration.Through the inclusion of high-order(H.O.)energy spread among the optimization objectives,the H.O.energy spread can be reduced by a factor of nearly six using a high-harmonic cavity despite a 38%emittance growth.Finally,the beam dynamics in the SHINE main accelerator show that reducing the H.O.energy spread in the injector is of great significance to improving compression efficiency and reducing bunch current spike.

Analytic RF design of a linear accelerator with a SLED-Ⅰ type RF pulse compressor

This study presents the RF design of a linear accelerator(linac)operated in single-bunch mode.The accelerator is powered by a compressed RF pulse produced from a SLED-I type RF pulse compressor.The compressed RF pulse has an unflattened shape with a gradient distribution which varies over the structure cells.An analytical study to optimize the accelerating structure together with the RF pulse compressor is performed.The optimization aims to maximize the efficiency by minimizing the required RF power from the generator for a given average accelerating gradient.The study shows that,owing to the compressed RF pulse shape,the constant-impedance structure has a similar efficiency to the optimal structure using varying iris apertures.The constant-impedance structure is easily fabricated and is favorable for the design of a linac with a pulse compressor.We utilize these findings to optimize the RF design of a X-band linac using the constant-impedance accelerating structure for the Tsinghua Thomson X-ray source facility.

Laser-RF synchronization based on digital phase detector

Laser oscillator synchronization with RF reference signal is ultra-important for a modern light source based on the accelerator.For Tsinghua Thomson scattering X-ray source,we have synchronized the mode-locked laser oscillator to RF reference signal with 48.2 fs RMS relative jitter.Both fundamental and harmonic signals derived from photo diode detection are used for laser-RF synchronization in our scheme.The fundamental signal is for coarse laser-RF synchronization and multiple laser oscillator synchronization.The harmonic signal is for high precise phase locking.The digital phase detector is implemented in the synchronization scheme for less noise,replacing the mixing to DC phase detection scheme.The digital processing algorithm for synchronization is commonly used in low-level RF control field.In order to test the phase locking loop logic without damaging the real laser oscillator,a laser oscillator emulator was developed for phase locking.This paper will report the laser-RF synchronization scheme and its performance.The laser oscillator emulator system will also be introduced here.

Observation and mitigation of image distortion in high-energy electron radiography

Image distortion caused by the angular misalignment of quadrupole magnets in high-energy electron radiography has been studied systematically.We propose that the distortion originates from the coupling of the electron motions in the transverse directions,based on a theoretical analysis and the transfer-matrix method.The relative angular rotation between the second and third magnetic quadrupoles was identified as the main contributor to image distortion.This was verified by both a beam-dynamics simulation and experiments.Different strategies to mitigate this image distortion are also explored,including magnets online tuning,higher beam energy and larger magnification factor.This study provides criteria for designing experiments and paves the way for achieving higher image precision.