| Abstract: | The nature of the background optical density on films exposed to orthovoltage x-rays and electron beams has been studied for correction purposes. A higher than expected background value can be demonstrated by comparing the film scanned beam profile with water phantom ionisation scans of the same beam. A range of 3-5% increased background in the penumbral tail, with energy dependence, has been shown experimentally. Testing the assumption that this increased background is due to Cerenkov radiation produced in the film, Filmstrips were interleaved in a solid water equivalent phantom and exposed to 300kV orthovoltage x-ray beams and 5MeV to 12MeV electron beams. The film stacks were made up of single or multiple bare filmstrips, multiple filmstrips interleaved with black paper, and multiple filmstrips interleaved with overhead transparency sheet. The experimental result demonstrated that visible light was not significantly responsible for an enhanced film optical density, but rather that this was due to scattered radiation, with a complex low energy spectrum, arising from the film silver halide emulsion or base. An improved background correction technique is developed which incorporates this unexpected background value as an added component in the correction applied to the measured optical density. The resulting profiles exhibit improved agreement between film and ionization chamber measurements in the penumbra and tail regions. |
