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

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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.