Session Item

The ARRONAX facility can deliver proton (68MeV) and alpha (67.4MeV) beams at ultra-high dose rates (UHDR). Our preclinical beamlines are specifically set up to easily shift between conventional and UHDR irradiation conditions. A major challenge of these irradiations is to control their dosimetry in both modes. As the use of two common radiochromic films, the GAFchromic™ EBT3 and EBT-XD films (Ashland, Inc.), was found to lead to non-negligible overestimations of the net optical density (netOD) and hence the absorbed dose, we evaluated the usability and the dose rate dependency of novel films from OrthoChromic Inc., the OC-1 films. Additionally, the dose conversion process was investigated for these films.

We compared the response in netOD of OC-1 films after proton and alpha irradiations at various mean dose rates (0.25, 40, 1500, 7500 and 60000 Gy/s), for 10 different doses (2-130 Gy). Additionally, these irradiations were compared with photon beam irradiations at conventional dose rate. We also monitored the stability over time of OC-1 films after a 20 Gy irradiation, using a 6 MV photon beam from Varian Clinac. To compensate for the scanner variability, we simultaneously monitored non-irradiated OC-1 and EBT-XD films, as well as a 3 Gy EBT-XD film (similar in netOD to a 20 Gy OC-1). These four films were repeatedly digitized, up to 16 days after irradiation, using a V700 Photo Epson Perfection flatbed color scanner. The films were analyzed using ImageJ software v1.53e, and mean pixel values and associated standard deviations were measured on both red and green channels. 

OC-1 films did not show any significant difference in the netOD between conventional dose rate and UHDR dose rates up to 60000 Gy/s, except at very low doses associated to higher uncertainties, for 3 Gy and below. The netOD of OC1-films was found to increase substantially in the first hours following the irradiation, then to stabilize about 100h after irradiation.  

OC-1 films, that present the advantage of a large dynamic dose range (0.1 – 100 Gy), did not show any impact of the mean dose rate up to 60000 Gy/s above 3Gy. Despite their unlamination, making them fragile and difficult to handle, OC-1 films seem appropriate for a good control dosimetry of our future UHDR experiments at the ARRONAX cyclotron. Users should however observe a four days delay before film reading and analysis.