Optimal Design of Proton Radiography Magnetic Lens

Due to its advantages such as high resolution, high detection efficiency, high signal-to-noise ratio, and strong penetrability, proton radiography technology has become a major research hotspot in the hydrodynamic diagnostic technology. To solve the problem of the radiography blurring caused by multiple Coulomb scatterings between protons and the irradiated object, it is highly efficient and necessary to introduce a magnetic lens. The magnetic lens plays a crucial role in the proton radiography system. Firstly, the traditional Zumbro magnetic lens can only achieve unit radiography magnification, so it is not suitable to observe objects with fine structures. By theoretically analyzing the beam optics characteristics of the magnetic lens and combining with the point-to-point radiography requirements, this paper proposed a new magnetic lens which can achieve different radiography magnifications. More importantly, without changing the strength and position of the quadrupole magnets, just by adjusting the relationship between object distance and image distance, the radiography magnification can be adjusted conveniently. Secondly, by introducing the concept of the chromatic aberration coefficient, the radiography error caused by the beam energy spread was quantified. By comparing the chromatic aberration coefficients with different radiography magnifications, this paper finds that when the radiography magnification exceeds 8, the trend of the chromatic aberration coefficient decaying as the magnification increases becomes extremely weak. Meanwhile, to ensure the compactness of the beam line, the radiography magnification should not be too large. Therefore, it is most appropriate to select a radiography magnification of 8. Thirdly, by simulating the beam envelope trackings with different energy spreads, the physical mechanism of the radiography blurring caused by the beam energy spread was analyzed. There is a Fourier plane where the x and y directions naturally coincide in the optimized magnetic lens. Utilizing the advantage, a collimator was added at the Fourier plane to selectively filter the beam with excessive energy spreads. Eventually, the suppression of the blurring caused by the energy spread is successfully achieved. In addition, by simulating and calculating the boundary resolution and proton utilization rate with different collimator apertures, this paper finds that it is difficult to take both high boundary resolution and high proton utilization rate into account simultaneously. Taking advantage of the proton beam current intensity up to 1 mA at the China Institute of Atomic Energy, it is advisable to appropriately sacrifice the proton utilization rate. This paper finally selects a collimator aperture with 0.20 mm aperture to achieve a boundary resolution of 0.018 8 mm.