Development and Applications of a Real-time Magnetic Electron Energy Spectrometer for Use with Medical Linear Accelerators

Purpose – This work presents a design for a real-time electron energy spectrometer, and provides data analysis methods and characterization of the real-time system. This system is intended for use with medical linear accelerators (linacs). The goal is 1 Hz acquisition of the energy range 4-25 MeV, reconstructed in 0.1 MeV increments. Methods – Our spectrometer uses a nominal 0.54 T permanent magnet block as the dispersive element and scintillating fibers coupled to a CCD camera as the position sensitive detector. A broad electron beam produced by a linac is collimated by a 6.35 mm dimeter aperture at the entrance to the spectrometer. The collimated beam is dispersed by the magnetic field onto a row of 60 vertical 1 mm x 1 mm square scintillating fibers mounted to a lateral face of the magnet. Detector response functions (DRFs) were created using a simplified physics model of the spectrometer to determine electron trajectories within the magnet block from the entrance aperture to the detector plane. The DRFs were used in an iterative method to transform the fiber signal intensity versus position into an energy spectrum. We made measurements on an Elekta Infinity linac; each available energy (7, 9, 10, 11, 13, 16, 20 MeV) was investigated. Measurements were used to assess setup reproducibility, pinhole mismatch, dose rate effects, temporal stability, and linac detuning. Results – Our reconstruction method was able to reconstruct energy spectra from idealized simulations to within 0.14 MeV ± 0.28 MeV of the ideal FWHM value, and 0.06 MeV ± 0.12 MeV of the ideal most probable energy, Ep0. The measured spectral stability was consistent with the expected linac operating stability. The system achieved a refresh rate of 0.8 Hz during real-time operation. Conclusions – We developed a real-time electron energy spectrometer that measures electron energies from 4 to 25 MeV with a continuous readout rate of 0.8 Hz. The device can be used for assessing linac performance as a routine clinical tool, assist in diagnostic maintenance and repair, or potentially provide a more efficient method for beam tuning and matching