Gel dosimetry

We have developed a radio-fluorogenic (RFG) gel which becomes fluorescent in UV light after exposure to ionizing radiation. This radiation-induced fluorescent property is permanent in time, distinguishing it from transitory optical emission or scintillation effects, and in space. Also, the fluorescence is proportional to the local radiation dose and can be easily recorded with a commercial camera.

Our ultimate aim is to provide a method of quantitative, 3D resolved dosimetry applicable to particle beams and other forms of high-energy radiation that result in complex dose distributions.

The quasi-rigid RFG gel consists of tertiary-butyl acrylate (TBA) with ca 100 ppm maleimido-pyrene (MPy). First, a gel matrix is formed by radiation-induced, free-radical polymerization of pure monomeric TBA. Residual monomer is evaporated and replaced by the low concentration, second monomer component, MPy, in TBA. This non-fluorescent MPy is converted into the fluorescent succinimido derivative, -SPy-, on incorporation into the poly-TBA chain upon irradiation.

We have initiated a programme of research into the fundamental radiation chemistry and photophysics of the medium with the ultimate goal of providing a quantitative fluorescence-based method of in-situ 3D dosimetry, dose and dose-rate dependence of polymer-gel formation on irradiation (pure TBA and MPy/TBA), and the effect of MPy concentration.

Combined with optical read-out methods, the gel fluorescent emission gives a measurement of the deposited dose in the (nearly water equivalent) medium. The fluorescent signal is currently being recorded by a CCD commercial camera (Ricoh GX200), via the blue pixel value of linear DNG files, as 2D images and quasi-3D images using partial gel excitation (slit width = 2 mm). We are working on providing 3D images, using optical-emission computed tomography (OECT) methods of the RFG gel with specifically designed apparatus and read-out algorithms.

This RFG gel was already used to make fixed fluorescent images of complex radiation fields, including those produced by overlapping X-ray beams, 3MeV electrons, 80 MeV protons and an Ir-192 brachytherapy seed. In the last case RFG images could be video-recorded in situ during seed insertion and retraction.