# Optimization of Transmission X-ray Target for Intense Pulsed Electron Beam Accelerators
## 1 Introduction  
After their discovery by [PERSON] from research on cathode rays [1], X-rays generated by kinetic electrons on solid targets have been studied and applied widely in fields such as medical diagnostics and treatment [2; 3], material characterization [4; 5; 6; 7], security checks [8; 9; 10], and industrial inspection [11; 12]. Although with the development in physics and technology, some other types of X-ray sources, such as synchrotron radiation [13] and laser-plasma [14; 15], have exhibited advantages including continuous broad-spectrum emissions and higher radiation intensity, X-ray sources with kinetic electrons on solidtargets stand out after over one century of application for their mature and robust design and adequateness in low-energy X-ray applications. Compared with isotopes, X-ray sources from electron-solid interactions such as X-ray tubes and electron accelerators have advantages in higher controllability, as the radiation can be shut off immediately after being powered off. In low-energy X-ray applications, such as medical imaging with photon energy of tens of keV, X-rays are typically generated with a reflective X-ray target. The transmission-type X-ray targets are preferable for X-ray sources with accelerators with higher electron energies from hundreds of keV to several MeV. The design and optimization of X-ray targets require consideration in many aspects; besides the manipulation of X-ray parameters such as fluence and spectrum, other issues in practical application, such as the heating and discharge by electrons, also need to be well solved. Behind this, the basic process in physics is to study the stopping of electrons in the target and relevant secondary photon emission, as issues such as X-ray distribution, target heating, and charge accumulation on parts are intrinsically determined. In recent years, trials with intense pulsed electron beams (IPEBs) for electron and X-ray radiation have been extensively developed. These IPEB accelerators with compact designs, producing pulsed electron beams with energy of 6-200 J per pulse, electron energy in the range from 100 to 500 keV, beam current in kA scale, sub-microsecond (75-250 ns) pulse duration, and repetition up to 50 pulses per second (pps) [16; 17], have been widely studied in material synthesis and processing [18], wastewater [19] and gas [20] purification, agricultural irradiation [21], and medical material preparation [22]. When used for X-rays, these IPEB sources also exhibit virtues such as compact design, high mobility, and flexibility in deployment [17]. Most IPEB sources extract electrons from plasma formed by explosive emissions induced by applying pulsed negative high voltages on cold solid cathodes made of graphite, metals, or composited metal-dielectric materials. The cathode-anode gap of the IPIB diode is typically within 1 cm to allow the strong electric field on the cathode surface to enhance explosive emission and achieve much higher current intensity than conventional methods such as thermal and field emission. Then, an IPEB can be accelerated from the plasma and used in a vacuum directly, bombarded on a target for X-ray generation [23], or be extracted into the atmosphere through a membrane window for irradiation purposes [24]. Although the physics of electron-matter interaction has been extensively studied and some tests on IPEB X-ray sources have been carried out, an optimized X-ray target design to match the IPEB has not been fully elaborated. In previous studies, for robustness, a single sheet of metal [17] or some dual-layer structure was used and tested with the X-ray converter [23]. The selection of the target thickness ensured it was roughly thick enough to avoid breaking under repetitive operation or adopted the electron range for complete electron absorption.