电子束放射照相的特性与参数优化

短脉冲强激光产生的电子束具有源尺寸小、脉宽窄、准单能谱等特点, 在放射照相诊断中具有独特作用. 本文通过分析电子在材料中散射并采用蒙特卡罗方法数值模拟, 研究了100 keV到几百MeV能量电子束对有厚度起伏或存在界面的靶的透视, 并与质子、X射线束透视结果比较, 给出了电子束放射照相的特性与参数优化: 基于电子在材料中非弹性散射或能量损失, 选用能量使其射程与靶厚度接近的电子束来诊断靶厚度不均匀性; 基于电子在材料中的弹性散射, 选用射程超过靶厚度的电子束来诊断靶界面.

Characteristics and parameter optimization of electron beam radiography

The electron beam produced by an ultra-short, high-intensity laser pulse is of properties of small source size, short duration, and quasi-monoenergetic energy, and will play a unique role in radiographic diagnostics. By analyzing the scattering processes of electrons in materials and performing Monte-Carlo simulations, electron radiography for probing target surface non-uniformities or material interfaces is studied for electron energy ranging from 100 keV to several hundreds of MeV, and the results are compared with those of proton radiography and X-ray radiography, respectively. Features and parameter optimization of electron radiography are obtained, and some applications are suggested. By taking advantage of inelastic scattering or energy loss of charged particles, target surface nonuniformities could be diagnosed by a charged-particle beam whose range is close to the target thickness. Such a diagnosis would produce a higher detection contrast than that by absorption-type X-ray radiography. For a proton beam, a target thickness variation as small as 0.1% could be detected due to a more evident Bragg peak of the stopping power near its range. Nevertheless, the energy of laser-accelerated proton beams being up to 100 MeV would limit the applications. For an electron beam, since a thickness variation of 0.3% could be detected, its energy over 1 GeV has been realized by laser acceleration, the electron radiography could be extended to diagnose thicker targets. When using an electron beam to radiograph a thin or a foil target, for example, of thickness on the order of 100 μm, a spatial resolution of 11 μm or better could be achieved due to the reduced elastic scattering and angular deflection. By taking advantage of elastic scattering of electrons, an electron beam whose range is much greater than the target thickness could be used to diagnose a target interface composed of different materials or even a multilayered capsule, and a higher contrast of the electron fluence modulation at interfaces would be realized than that by absorption-type X-ray radiography, which is caused by stronger scattering of electrons as the electron scattering cross section is several orders of magnitude greater than that of X-ray scattering such as the Thomson scattering. As a laser-produced electron beam is prone to have an ultrafast pulse duration of 100’s of femtoseconds or less, it is anticipated that the electron radiography will produce an ultrasfast temporal resolution. These results and conclusions would be helpful to the applications and parameter optimization of electron radiography.