Researchers at Queen's University Belfast in Northern Ireland (UK) are using laser-based particle acceleration with picosecond time resolution to investigate ultrafast radiation chemistry that occurs immediately after the interaction of protons in water.
As much as 66% of the radiation deposited into a tumor volume is initially absorbed by water molecules in the cancer cells. Thus, it is critical to unravel the ensuing processes to eventually advance radiation therapy. Proton therapy is a promising form of radiation treatment in the fight against cancer. The purpose of the research work therefore is to gain better fundamental understanding of these dynamics.
Tracking proton-induced radiolysis in water
“This project demonstrates for the first time that it is possible to track proton-induced radiolysis in pure H2O with picosecond temporal resolution,” confirms lead researcher Dr Brendan Dromey, a lecturer in the University’s Centre for Plasma Physics. “We achieve this by capitalizing on the ultrafast nature of laser accelerated proton bursts generated using the TARANIS Laser Facility at Queen’s University Belfast and the correspondingly high synchronicity with optical probe radiation from the same system.”
A game-changing approach
Previous attempts to track early stage radiation chemistry in water induced by proton pulses relied on the use of radio-frequency cavity accelerators, which according to the expert suffer from inherently longer pulse duration and timing jitter with respect to optical probe pulses. “Overcoming this limitation required the introduction of chemical scavenging agents in ever increasing concentrations to access the earliest stages of the radiation chemistry history,” Dromey says. The problem he adds is that to access time windows below 100 picoseconds, the concentration of chemical scavenger becomes sufficiently high that it can start to take part in the radiation chemistry itself, which introduces a correspondingly higher uncertainty in unravelling the exact timeline of proton-induced processes. “Our technique overcomes this as it does not require chemical scavenger agents to access these early times,” Dromey notes. “Instead we rely on the picosecond temporal resolution provided by laser driven proton accelerators. This allows us to study the real time evolution of the solvated electron due to proton irradiation in a pristine H2O environment.”
Impact on future of light-based cancer therapy
“While it is still very early, one possible outcome of this research is that it provides an opportunity for us to build up a complete picture from first principles of how proton stopping in matter can lead to cell death,” Dromey says. By cross-referencing their results with those of numerical simulation, the team hopes to be able to understand the full history of radiation chemistry in water arising from protons.
Dromey stresses that his team is not suggesting that laser driven proton accelerators can provide an alternative source for cancer therapy at this stage. He says there is still significant work to be done to improve energy stability and beam profile from this relatively new technology. “At this stage, we see the benefit of this work purely in the enhanced temporal resolution of our approach over existing sources and how this unique capability can contribute to the larger aim of efficient hadron therapy for cancer treatment.”
“By far the biggest challenge in this work was convincing ourselves that the overserved signal was indeed due to protons and not other ionizing species (X-rays and fast electrons) that can also be generated during the intense light-matter interaction,” Dromey says. “We overcame this by carefully filtering the signal and adjust the source — sample distance to determine time of flight.”
Dromey says the most surprising outcome of the research is the very long growth time of more than 100 picoseconds of the photo-absorption band for the solvated electron in water under irradiation from few picosecond proton pulses. “This is significant because the solvated electron is a key player in the subsequent radiation chemistry and this delay may play a significant role in the processes that follow.”
What is next
Going forward, Dromey and his colleagues aim to study exactly how the growth and evolution of the solvated electron population differs under irradiation by ions (large-mass particles), fast electrons (low-mass particles) and X-rays. “The advantage of our laser based accelerator is that it is relatively simple to switch between these species by altering our intense light-matter interaction,” he says. “This capability, coupled with the ability to change the wavelength of our highly synchronized optical probe, will allow us to build up a complete picture of the early stages of radiation chemistry in water.
The research work is detailed in the article “Experimental investigation of picosecond dynamics following interactions between laser accelerated protons and water,” published in Applied Physics Letters.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today