X-ray Profiling with an Enhanced Backscattering Optical Fibre in Single Mode-Multimode-Single Mode Configuration

Ionizing radiation are widely employed in the medical field both for diagnosis and for radiotherapy treatments, where it is often required to measure the precise dose delivered along with radiation distribution in the target volume. This is even more important in emerging techniques such as the FLASH radiotherapy [1], in which very high dose rates (e.g. >40 Gy/s) are delivered over short times (<1 s). Quality Assurance in the FLASH radiotherapy planning process is essential to ensure the accurate dose delivery to the patient and to minimize the possibility of accidental exposure. In this framework, commissioning procedures involve the characterization of radiation beam using a water phantom during the simulated treatment. Furthermore, periodical tests are carried out in a similar way to ensure that there are no drifts in the machine performance, ensuring that measured and calculated doses lie within the agreement criteria. The characterization of the beam in the water phantom is performed by probes such as ionization chambers or scintillators that map the dose distribution in the target volume. A possible more convenient alternative for in-situ, real-time dose profiling is represented by optical fibres used as radiation sensors. However, standard silicate optical fibres for telecom applications exhibit little sensitivity to ionizing radiations. This issue can be addressed by developing ad-hoc silicate fibres, like those doped with aluminium or magnesium nanoparticles, which for their higher Rayleigh scattering behaviour have been named Enhanced Backscattering Fibres (EBFs). In a previous work [2], it was demonstrated the possibility to recover the radiation profile of an X-ray beam through the Radiation Induced Refractive Index Change (RRIC) in EBFs, by means of Optical Frequency Domain Reflectometry (OFDR). OFDR is a powerful tool, but the measurement is performed within seconds and may not be compatible with measurement of FLASH pulses. Fibre Bragg gratings (FBGs) may be used as pinpoint sensors, since they can be interrogated with fast FBG interrogators and spectrometers, but their radiation sensitivity is usually low. To overcome this problem, a novel pinpoint radiation sensor is being developed, based on an interferometric structure made by offset splicing an EBF, working as a multimode fibre, between two single mode pigtails [3]. This single mode-multimode-single mode (SMS) structure produces a periodic spectral response that is sensitive to temperature, strain and radiation. The SMS is also equipped with a FBG, inscribed in the EBF section by a femtosecond laser, for differential compensation of the strain (the SMS and the FBG exhibit opposite wavelength shift in response to strain). The sensor was characterized in a radiation chamber by exposure to X-rays at a dose rate of 11.6 Gy/s for up to 10 minutes. Although these testing conditions are not resembling FLASH treatments, they were chosen to provide a magnified effect while providing a reliable indication of the sensing capability. Fig. 1(a) shows the characterization setup that also includes a commercial FBG interrogator to track the shift of the spectrum under x-ray exposure. Fig. 1(b) depicts the shift of the SMS spectral pattern when exposed at a dose rate of 11.6 Gy/s for a total cumulated dose of 4900 Gy. Fig. 1(c) highlights the higher sensitivity of the SMS with respect to the FBG. This feature, combined with the easy fabrication of the SMS (which only requires a standard fusion splicer), makes it an attractive platform for radiation sensing.