Ultrafast Nanodosimetry - the role of the nanoscale in radiation interactions in matter.

随着基于辐射的技术继续针对过程和应用进行更严格的控制，我们对材料在最小尺度上对辐射的反应的理解的限制正在成为进步的障碍。随着中央激光设施的极端光子学应用中心将于2024年投入使用，现在越来越需要开发在纳米尺度上询问和理解这些相互作用所需的方法，以加速这种新的国家能力将解锁的下一波创新。在“超快纳米剂量学”中，我们将通过研究超快过程与电离辐射相互作用的物质纳米结构之间的相互作用来解决这一挑战。目前，在应用于扩展长度尺度的模型中，标准做法是假设物质在纳米尺度上均匀或均匀分布。这是因为包括扩展卷中典型的无序将在计算上非常昂贵。此外，均匀近似准确地预测了入射辐射在介质中损失能量的范围，使其成为一种通用而有效的方法。然而，虽然范围对应用来说当然很重要，但辐射化学和电离物质通过造成的永久性损害同样重要。例如，由于制造要求更高的精度，例如量子点发光二极管的离子诱导缺陷，很明显，将达到一个极限，对纳米结构相关过程的理解将是实现这些目标的关键。此外，即使是宏观应用，如放射治疗，也将越来越依赖于对纳米放射化学途径的理解，例如，使用金纳米颗粒剂量增强治疗的患者特异性模式。因此，我们必须开始建立一个全面的图像，了解能量是如何在纳米尺度上在辐照物质中沉积和传输的。我们提出，在纳米尺度上存在对纳米尺度异质性高度敏感的过程，因此，对于在预测框架中充分理解这些相互作用至关重要。这一假设是基于最近对物质中超快质子相互作用的实验，这些实验对静态、均匀密度分布的假设提出了质疑。通过利用激光驱动加速器的独特能力，从单一源提供x射线和质子的超快脉冲，将实现对这一点的测试。这两种物种在物质中具有根本不同的相互作用，我们将利用这些相互作用来询问辐照系统中的“局部”和“非局部”过程。这可以理解如下。如果主电离电子具有高能量（即那些被x射线激发的电子），它们在第一次碰撞之前平均会远离电离点。在这种情况下，他们没有“看到”材料的局部纳米结构，并且初始剂量迅速变得均匀（非局部）。相反，如果初级电子的能量较低（即那些被质子激发的电子），它们在第一次碰撞之前不会移动很远。在这种情况下，它们将在初始电离点（局部）附近与材料相互作用，成为纳米结构的探针。与我们在德国、中国和美国的合作伙伴一起，我们将开发新的方法来跟踪这些过程。特别是，我们的目标是展示异质性如何通过调整纳米尺度上的物质结构来影响远离平衡态的物质动力学。这将为当前的“同质”模型的崩溃提供一个硬性限制。我们的首要目标是揭示纳米过程如何影响复杂系统中的宏观现象学和能量输运。

As radiation-based technologies continue to target tighter controls over processes and applications, limits in our understanding of how materials respond to irradiation on the very smallest scales is becoming a barrier to progress. With the commissioning of the Extreme Photonics Applications Centre at the Central Laser Facility due in 2024, there is now a growing need to develop the methodologies required for interrogating and understanding these interactions on the nanoscale to accelerate the next wave of innovation that will be unlocked by this new national capability. In 'Ultrafast Nanodosimetry' we will address this challenge by investigating the interplay between ultrafast processes and the nanoscopic structure of matter for ionising radiation interactions. Currently, in models for applications that operate over extended length scales it is standard practice to assume that matter is evenly, or uniformly, distributed on the nanoscale. This is because including the disorder typical in extended volumes would be computationally very expensive. Also, the uniform approximation accurately predicts the range over which the incident radiation loses energy in the medium, making this a versatile and efficient approach. However, while range is certainly important for applications, the radiation chemistry and permanent damage caused by the passage of ionising species is equally important. For instance, as manufacturing demands greater precision e.g. ion-induced defects for quantum dot light emitting diodes, it is clear that a limit will be reached where an understanding of nanostructure-dependent processes will be crucial to match these ambitions. Furthermore, even macroscopic applications such as radiotherapy will increasingly rely on understanding nanoscopic radiation chemistry pathways to open, for example, routes towards patient-specific modalities using gold nanoparticle dose-enhanced treatments. Therefore, it is essential that we begin to build a comprehensive picture of how energy is deposited and transported on the nanoscale in irradiated matter. We propose that there are processes that persist on the nanoscale that are highly sensitive to nanoscopic heterogeneity and, as such, are crucial for fully understanding these interactions in a predictive framework. This hypothesis is based on recent experiments examining ultrafast proton interactions in matter that have called into question the assumption of a static, uniform density distribution. Testing this will be achieved by harnessing the unique capability of laser-driven accelerators to provide ultrafast pulses of both X-rays and protons from a single source. Both of these species have fundamentally different interactions in matter that we will exploit to interrogate both 'local' and 'non-local' processes in irradiated systems. This can be understood as follows. If the primary ionised electrons have high energy (i.e. those excited by X-rays), they will, on average, travel far from the point of ionisation before their first collision. In this case they do not 'see' the local nanostructure of the material and the initial dose becomes rapidly homogenous (non-local). Conversely, if the primary electrons have low energy (i.e. those excited by protons), they will not travel far before their first collision. In this case they will interact with the material near the point of initial ionisation (local), becoming a probe of nanostructure. Together with our partners in Germany, China and the US we will develop new methods to track these processes. In particular, we aim to show how heterogeneity can influence dynamics in matter far from equilibrium by tuning the structure of matter on the nanoscale. This will provide a hard limit for which current 'homogenous' models break down. Our overarching goal is to reveal how nanoscopic processes can influence macroscopic phenomenology and energy transport in complex systems.