Ultrafast laser-driven ion interactions in matter: Evolving dose distribution at the nanoscale and nonlinear response

在物理学中，标度定律提供了双重功能。首先，它们可以通过建立系统如何对变化或扰动做出反应来揭示管理系统的潜在物理机制。对于非线性标度律尤其如此，其中输入扰动的微小变化可以导致系统响应的剧烈变化。其次，标度定律为研究人员提供了一种工具，他们可以使用该工具来预测系统将如何在给定的一组输入参数下演化。这是向提供具有高度针对性的尖端应用程序迈出的关键一步。正是在这个框架下，我们建议研究物质中离子相互作用所产生的超快动力学，以确定介质的特征响应如何随入射离子通量的变化而变化。要直接研究任何超快过程，引起系统变化的微扰明显短于系统的自然恢复时间是至关重要的。如果微扰的时间比恢复的时间长得多，那么在一次相互作用中就会有反复的激发和松弛循环。这抑制了在没有复杂的近似和假设的情况下提取关于系统的基本信息的能力。不幸的是，到目前为止，这一直是研究物质中离子相互作用的首要问题。从大型加速器装置上获得的离子脉冲的持续时间为100‘S皮秒，明显长于飞秒，物质对辐射的响应特征恢复时间很少。因此，与离子物质相互作用的最早可达阶段相关的现有实验结果具有令人望而却步的相关不确定性。我们的方法克服了这个问题，通过使用激光驱动的离子加速器产生超快离子脉冲。这一性能将允许在飞秒和皮秒时间尺度上研究物质中的高能离子(&gt；1 MeV/核子)的停止。我们将利用这一能力来了解如何通过改变离子的入射通量来控制最终的平衡路径，并研究这为放射化学和强效疗法的高级应用提供的新的可能性。贝尔法斯特女王大学等离子体物理中心目前正在建造世界上能量最高的少光周期激光系统Taranis-X，预计将于2016年底上线。这种独特的环境将使我们能够产生迄今为止在实验室中产生的最短的离子脉冲。有了这种最先进的设备，就有可能实时测试物质中离子相互作用的基本极限。理解这种行为是这项研究的一个关键目标。特别是，将这些实验扩展到水中的离子相互作用将使我们能够研究在强子(或离子束)治疗期间提供新剂量的可能性。这是因为水占人体细胞的70%以上，因此它是研究人体内电离辐射影响的理想系统。最后，这一提议的关键动机之一是在为支持这项工作的科学理由而进行的低时间分辨率实验中关于离子通量的非线性响应的指示。我们将与我们在德国(慕尼黑)和美国(德克萨斯州)的国际合作伙伴一起研究多种不同的相互作用机制，以确定这种非线性响应的比例，并与GEANT4 DNA协作合作，开发数值方法，以形成对管辖它的比例定律(或多个定律)的清晰理解。

In physics, scaling laws provide a dual function. First, they can reveal the underlying physical mechanisms that govern a system by establishing how the system responds to changes or perturbations. This is particularly true of nonlinear scaling laws where small changes in an input perturbation can lead to dramatic changes in the response of the system. Secondly scaling laws provide researchers with a tool that they can use to predict how a system will evolve for a given set of input parameters. This is a crucial step towards providing highly-targeted, cutting-edge applications. It is within this framework that we propose to study the ultrafast dynamics that result from ion interactions in matter to determine how the characteristic response of the medium scales with the incident ion flux. To study any ultrafast process directly it is critical that the perturbation causing the system to change is significantly shorter than the natural recovery time of the system. If the perturbation is significantly longer that this recovery there will be repeated cycles of excitation and relaxation within a single interaction. This inhibits the ability to extract fundamental information about the system without complicated approximations and assumptions. Unfortunately, to date, this has been the overriding problem for the study of ion interactions in matter. The ion pulses that have been available from large accelerator facilities have been 100's of picoseconds in duration which is significantly longer than the femtosecond and few picosecond characteristic recovery times of matter in response to irradiation. Accordingly, existing experimental results relating to the earliest accessible stages of ion matter interactions have prohibitively large associated uncertainties. Our approach overcomes this issue by generating ultrafast pulses of ions using laser driven ion accelerators. This performance will allow the stopping of energetic ions (> 1 MeV/nucleon) in matter to be studied on femtosecond and picosecond timescales. We will use this capability to understand how the resulting pathways to equilibrium can be controlled by varying the incident flux of ions and investigate the new possibilities this offers for advanced applications in both radiation chemistry and hadrontherapy. The Centre for Plasma Physics in Queen's University Belfast is currently constructing the world's highest energy few-optical-cycle laser system, TARANIS-X, due to come online in late 2016. This unique environment will allow us to generate the shortest pulses of ions produced in the laboratory to date. With this state of the art facility it will be possible to test, in real time, the fundamental limits of ion interactions in matter. Understanding this behaviour is a key goal of this research. In particular extending these experiments to ion interactions in water will allow us to investigate the potential for new modalities of dose delivery during hadron (or ion beam) therapy. This is because water makes up over >70% of human cells and so it makes for an ideal system in which to study the effects of ionising radiation in the human body. Finally, one of the key motivators for this proposal is the indication of nonlinear response with respect to ion flux in low temporal resolution experiments performed to support the scientific case for this work. Together with our international partners in Germany (Munich) and the U.S. (Texas) we will investigate multiple different interaction regimes to determine the scaling of this nonlinear response and, in partnership with the GEANT4 DNA collaboration, we will develop numerical approaches to form a clear understanding of the scaling law (or laws) that governs it.