Exploring the potential of Raman micro-spectroscopy of radiochromic films for experimental microdosimetry

Micrometer-scale dosimetry is crucial when estimating the energy deposited within micrometer-scale biological targets exposed to low doses or high dose gradients. Raman micro-spectroscopy read-out of radiochromic films (RCFs) permits micrometer-scale resolution; this presents a novel opportunity to explore its feasibility for experimental microdosimetry.

The purpose of this work was to develop a novel approach towards generating data for experimental microdosimetry. The objective was to develop a method based on high (1-2 µm) spatial resolution Raman micro-spectroscopy of RCFs, ensuring reproducibility of data while producing two-dimensional intensity maps of the Raman response.

EBT3 RCFs were irradiated to doses between 0.2 Gy and 2 Gy using a clinical linear accelerator. Raman spectra were collected using a custom Raman microscope fitted with 40× and 60× water immersion (WI) objectives, and a commercial Raman microscope utilizing a 100× dry objective. The excitation source of the custom setup was circularly polarized to minimize the influence of polarization on the film read-out. The Raman response of the RCFs was measured over a 100 × 100 µm² region of interest (ROI) with a 10 × 10 grid. The Raman response of the active layer of the film was normalized to the radiation-insensitive monomer peak at 2260 cm^-1. The Raman intensities of the 1445 cm^-1 and 2060 cm^-1 peaks were used to generate dose response curves for each microscope setup. Maps of the Raman intensity over the 100 × 100 µm² ROI for the 60× WI setup were used to quantify the heterogeneity in the film response. Higher resolution point-scans were performed over a 20 × 20 µm² ROI for 0 Gy and 2 Gy samples.

The dose response of each Raman microscope setup over the 0-2 Gy dose range was linear (r² of 0.98) after normalization to the 2260 cm^-1 Raman peak in the active layer. The slope of the dose response curve of the commercial microscope exhibited dependence on the film orientation; this was minimized with the custom Raman setup by using the circularly polarized excitation source. The relative standard deviation (RSD) of the 1445 cm^-1 peak Raman intensity over the 100 × 100 µm² ROIs was significant (∼11%) for each microscope setup, and independent of dose. The Raman intensity distribution maps revealed that the heterogeneity in Raman response across the ROI was on the same size-scale (1.62 µm × 9.4 µm) of the lithium salt of pentacosa-10,12-diynoic acid (LiPCDA) crystals comprising the active layer of the film.

This work explored the feasibility of a Raman micro-spectroscopy-based read-out technique of RCFs for experimental microdosimetry. Utilizing the 2260 cm^-1 peak in the Raman spectrum as an internal standard for normalization produced a linear (r² of 0.98) dose-response curve in the 0-2 Gy dose range. Utilizing circularly polarized laser excitation minimized the polarization dependence of the film and increased the reproducibility of the Raman measurements. Spatial heterogeneity in the concentration of PCDA crystals in the active layer was visualized based on two-dimensional maps of the Raman intensity response to explore the implications on microdosimetry.