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

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.

The impacts generated by this research will cut right across the socio-economic spectrum. For the first time it will be possible to quantify the fundamental interaction of ions with matter allowing researchers to determine how these interactions scale with the flux of incident ions. This opens the possibility of harnessing these interactions directly to increase efficiency and safety for applications in manufacturing and healthcare. The details of the expected impacts are described below under 4 distinct headings.

Knowledge
The development of cutting edge science using the world leading laser capabilities available in Queen's University Belfast will contribute directly to the UKs ability to drive the next generation of technology and applications for a knowledge based economy. We will also capitalise on the outputs of our research to inspire the next generation of young children to become involved in science. Through our extensive public engagement programme that includes the Northern Ireland Science Festival, we will demonstrate to a brand new audience how ion beams can change the world around us and how the TARANIS-X laser is leading the way in shaping this future technology.

People
This project will train at least 5 people (3 PDRA and 2 PhDs, with the potential for an additional 2 PhDs in coming years, bringing the total to 7) in world leading technologies based on ultrafast science. These people will become the next generation of scientists who lead and innovate. This project will also offer them the opportunity to travel to international laboratories and to present results at international conferences creating the ideal platform for exposure to different practices and to engage in knowledge transfer with groups from outside the UK.

Economy
Ion beam based manufacturing processes for future technologies in the semiconductor industry have the potential to revolutionise how integrated circuits are constructed and metrologised on the production line. This proposal will study how radiation induced processes in semiconductor industry relevant materials (i.e. SiO2) scale with the incident ion flux so that novel capabilities can be tailored to an exceptionally high degree of accuracy. Also, as electronic devices shrink to ever smaller dimensions it will become increasingly important to understand how radiation induced processes scale with dimensionality for the deployment of circuits in radiation harsh environments such as in nuclear engineering and deep space travel. Through our use of nanostructured media it will be possible to determine exactly how reducing the dimensionality can affect the recovery time of the material and how solutions can be found to overcome the resulting problems.

Society
One very clear impact from the proposed research is the advance in healthcare for society it will deliver. At key objective of our proposed research is to examine the fundamental interaction of ions in water and reveal how ion flux determines the production of the radical species that take part in killing cancer cells for hadrontherapy. We will also the study the role of predicted shock induced delay in the formation of these radical species, its potential role in dose fractionating (or time scale for recovery) and how this process scales with ion flux. The predicted nonlinearity in the production of radicals with respect to ion flux also offers a brand new approach to enhanced protection for tissue surrounding a deep seated tumour volume. In particular, the pathways towards a 'crossfire' treatment technique will be investigated. In this scheme multiple low dose beams are overlapped in the tumour volume leading to nonlinear growth of radicals in the tumour volume while leaving the surrounding tissue exposed to significantly lower risk of damage and cell death through radiation induced processes. This, by extension, will lead to increased patient safety and reduced requirement for treatment aftercare.