Numerical optimization of transmission bremsstrahlung target for intense pulsed electron beam

Design of high-energy X-ray conversion target based on linear electron accelerator

High-energy X-rays can be employed to inspect fuel elements, even weld seams, cracks, and internal structural deformations of the components due to their density-sensitive characteristics and strong penetration capabilities. The quality of these X-rays is determined by various factors such as the X-ray conversion target, the cooling system for the conversion target, and the filter type, thereby influencing the detection accuracy. This study was conducted using an electron linear accelerator with an energy of 10 MeV and an average current of 150 μA. Firstly, we designed the conversion target material and geometry structure by Monte Carlo method. The results indicate that the optimal target material is tungsten and the optimum shape of the conversion target is a cylinder with dimensions of Φ 40 × 2.2 mm. Second, finite element analysis was applied to design the conversion target cooling system. After 60 min of electron beam irradiation, the local maximum temperature of the conversion target is approximately 1330 °C. Lastly, theoretical analysis and Monte Carlo simulations were used to design the X-ray filters. The result demonstrates that a double-layer filter consisting of 8 mm bismuth and 4 mm iron can increase the proportion of high-energy X-rays by 13.89 ± 0.05 %. Finally, the results of using the attenuation method to measure the X-ray energy spectrum produced by an electron linear accelerator indicate that without a filter 78.81 % of the X-rays are above 1 MeV and with a filter this proportion increases to 80.55 %.

Megavoltage photon FLASH for preclinical experiments

BACKGROUND

FLASH radiotherapy using megavoltage (MV) photon beams should enable greater therapeutic efficacy, target deep seated tumors, and provide insights into mechanisms within FLASH.

PURPOSE

In this study, we aim to show how to facilitate ultra-high dose rates (FLASH) with MV photons over a field size of 12-15 mm, using a 6 MeV (nominal) preclinical electron linear accelerator (linac). Our intention is to utilize this setup to deliver FLASH with MV photons in future preclinical experiments.   METHODS: An electron linear accelerator operating at a pulse repetition frequency of 300 Hz, a tungsten target, and a beam hardening filter were used, in conjunction with beam tuning and source-to-surface distance (SSD) reduction. Depth dose curves, beam profiles, and average dose rates were determined using EBT-XD Gafchromic film, and an Advanced Markus ionization chamber was used to measure the photon charge output.

RESULTS

A 0.55 mm thick tungsten target, in combination with a 6 mm thick copper hardening filter were found to produce photon FLASH dose rates, with minimal electron contamination, delivering dose rates > 40 Gy/s over fields of 12-15 mm. Beam flatness and symmetry were comparable in horizontal and vertical planes.

CONCLUSION

Ultra-high average dose rate beams have been achieved with MV photons for preclinical irradiation fields, enabling future preclinical FLASH radiation experiments.

Optimization of Transmission X-ray Target for Intense Pulsed Electron Beam Accelerators

X-ray sources based on pulsed electron accelerators stimulate the development of bremsstrahlung converter designs. The numerical optimization of transmission-type X-ray targets for maximum X-ray output by pulsed electron beams was carried out in the present work. The targets featured a combination of a heavy element (tungsten or molybdenum) X-ray conversion layer and a titanium membrane that served as the vacuum window, thermal shielding for converter heat, and an electron dump. The energy spectrum of the electron beam generated via explosive emission was analyzed via the space-charge effect, and was utilized for the source sampling algorithm for electron transportation simulation with a Monte Carlo method for X-ray emission analysis. It was revealed that the transmission photon intensity of a mono-material target is primarily affected by the thickness of the target, and there exists an optimal target thickness within which the photon fluence is restricted by insufficient electron stopping; when exceeded, the extra thickness of the X-ray converter target imposes absorption and attenuates the generated X-ray. Analysis on dual-layer targets proved that this optimized converter target thickness, combined with a proper titanium window, produces the highest X-ray photon emissions.