寻找“万能”X射线源

在工业中,X射线成像对尺寸、密度和分辨率的要求范围较大,因此没有“万能”的发射源。例如,对于小型生物样本成像所需的X射线源在光谱和空间需求方面便与飞机焊接有显著的不同。但是,在对发射源特征进行研究并调控后,这些应用便可以通过激光驱动系统实现。

近日,英国Strathclyde大学的科研人员将X射线靶由平面改为丝状,并分别研究了强激光与两种形状靶作用的X射线发射,相关成果发表在High Power Laser Science and Engineering第7卷第2期上(Armstrong, C. D. , et al. Bremsstrahlung emission from high power laser interactions with constrained targets for industrial radiography)。

本文通过与高强度激光器固体靶的相互作用来表征和优化X射线发射[1],相比平面靶,采用丝状靶预期会改善X射线源的空间分辨率和总通量。

这种无需侵入性或复杂实验设置的靶向变化适用于具有固体靶的X射线生成。该技术可以在无需更高能量激光驱动器的前提下提高图像质量。模拟结果表明,从电子到X射线的转换效率提高了3倍,实验数据显示,射线通量增加1.5 - 2倍,工业样品的空间分辨率提高2.6倍。

研究中还对比了25 - 100 μm丝状靶和25 - 600 μm厚平面靶的通量。在厚靶中,电子更可能与靶材料碰撞,并在与后表面上的鞘层相互作用之前发射轫致辐射[2]。当电子与鞘层相互作用时,通常会损失一些能量,随后通过靶回流。随着电子继续横向穿过靶,这种回流会增加作用范围。这些回流的电子仍然具有足够的能量,足以在继续循环中产生X射线。

由于丝状靶去除了基底产生的横向通量,实验表明,当在靶后表面上迅速建立电场,并且电场覆盖大部分可用表面积时,从平面靶到丝状靶的变化会限制电子膨胀。鞘层的变化导致更多的低能回流电子群,这反过来会增加X射线通量。

线形和薄膜靶的比较:a)X射线发射区域的空间轮廓,b)电子密度(红色)和场生成(蓝色),c)来自GEANT4模拟的X射线源位置,d)多个 X射线源的示意图,e)不同靶类型样本的边缘扩散函数(ESF)。

[1] 本文的数据是在使用Target Area West公司的Vulcan激光器进行的实验中测得的。

[2] 轫致辐射又称刹车辐射或制动辐射 ,原指高速运动的电子骤然减速时发出的辐射,后来泛指带电粒子与原子或原子核发生碰撞时突然减速发出的辐射。

Bremsstrahlung emission from high power laser interactions with constrained targets for industrial radiography

Due to the range of size, density, and resolution demands associated with industrial x-ray radiography, there is not a source that is “one-size fits all”. Compromises and optimisations must be made depending on the object of study. For example, the X-ray source required to image a small biological sample is significantly different in both spectral and spatial demands to that for an aircraft weld. Both examples, however, are readily achieved with laser driven systems. Altering the source characteristics to deliver what is needed requires continued study. This publication explores the X-ray emission from spatially constrained targets compared to standard foil targets. The research results are published in High Power Laser Science and Engineering, Volume 7, No. 2, 2019 (Armstrong, C. D. , et al. Bremsstrahlung emission from high power laser interactions with constrained targets for industrial radiography.)

The data within this publication was measured during an experimental campaign using the Vulcan laser in Target Area West. We worked in conjunction with industrial partners to characterise and optimise the X-ray emission from solid target interactions with high intensity lasers. Changing the target from a foil configuration to a wire configuration was expected to improve the spatial profile of the X-ray source since there is a confined volume from which X-rays can be generated. The flux of X-ray sources is also investigated, a comparison between 25-100 μm wires and 25-600 μm thick foil is shown.

In thick targets, electrons are more likely to collide with the target material and emit bremsstrahlung prior to interacting with the sheath on the rear surface. When interacting with the sheath, electrons typically lose some energy and subsequently recirculate through the target. This recirculation causes an increase in the spatial extent of the source, as the electrons continue to travel laterally through the target. These recirculating electrons still have significant energy enough to readily generate X-rays as they continue to circulate the target.

Switching to a wire target geometry removes the flux produced from the substrate, in the transverse direction, as there is no material from which to generate X-rays. Experimentally, we show that changing from a foil target to a wire target constricts the electron expansion as the electric field on the rear-surface of the target builds rapidly and covers a high proportion of the available surface area. The change in the sheath field results in a higher population of cooler recirculating electrons, which in turn results in an increase in the measured X-ray flux. Simulations using EPOCH in 2D show the sheath field developing faster on the wire target geometry, and by using the recirculating population outputted from EPOCH in a GEANT4 simulation, the increase in x-ray emission is demonstrated by applying electric fields to the target surfaces.

This simple targetry change is readily applicable to X-ray generation with solid targets, demonstrating a significant improvement in both the spatial resolution and the overall flux of the source, without necessitating invasive or complex experimental set ups. Going forward this technique can be applied to improve the image quality without necessitating a higher energy laser driver, the simulations demonstrate a 3x improvement in the conversion efficiency from electrons to X-rays and the experimental data shows a 1.5-2x increase in the detected X-ray flux and a 2.6x increase in the spatial resolution for an industrial sample.

Comparison of wire and foil targets, a) spatial profile of X-ray emission area, b) electron density (red) and field generation (blue), c) X-ray source location from GEANT4 simulations, d) Schematic of multiple X-ray source characterisation, e) ESF from sample object for each target type.