Real-time electron spectrometer utilizing a permanent magnet and diode detector array

Magnetic spectrometers have been previously described for measuring energy spectra of therapeutic electron beams. However, challenges for clinical utilization have been their size, weight, and limited real-time capabilities. Development of a compact, lightweight, and inexpensive device with real-time readout will make an electron spectrometer a practical clinical tool.

This work integrates a commercial diode detector array with a permanent dipole magnet to create a practical real-time energy spectrometer for therapeutic electron beams.

A 4 kg, 0.55 T permanent dipole magnet was coupled to two PC boards of a Sun Nuclear SRS MapCHECK device, which provided two interlaced diode arrays that sampled a radiation field with an effective spacing of 0.175 cm. These components were rigidly attached to a copper insert in an Elekta 14 × 14 cm² electron applicator. The insert's 0.6 cm diameter aperture on central axis selected the electron beam entering the magnet. The Lorentz force spatially dispersed the electron beam onto the diode arrays, which measured the spectrometer response, $R_{meas}\left(z_{diod{e}_i}\right)$ , at diode location $z_{diod{e}_i}$ along beam central axis. Background X-ray dose to the diode detectors and its integrated circuits (IC), shielded by a 5.75 cm thick Cerrobend block attached to the copper insert, was measured. Corrections were made to $R_{meas}\left(z_{diod{e}_i}\right)$ for individual diode sensitivity, the 0.5 cm separation of the two diode array planes, and use of only four (three) diode columns from the proximal (distal) arrays. Modeling the magnet to have constant primary and fringe fields, while correlating peak positions of the energy spectra with E_{p ,0} from central-axis dose versus depth curves in water, produced an energy calibration curve (E vs. z). $R_{meas}\left(z_{diod{e}_i}\right)$ measurements were evaluated for 15 and 1 s (real-time) intervals. Monte Carlo-calculated, monoenergetic detector response functions, DRF(E, z), were used to extract the incident energy spectrum, ${\Phi }^p\left(E\right)$ , from the corrected $R_{meas}\left(z_{diod{e}_i}\right)$ .

Background dose to IC electronics was 11.6 ± 1.7 µGy/MU and 42.8 ± 4.4 µGy/MU for 9 and 20 MeV beams, respectively, allowing >100 h of use at 400 MU/min before receiving 100 Gy. Background to diode detectors was less than 10% of peak signal for 7-20 MeV beams. Individual diode sensitivities varied ±6%, each varying insignificantly with energy. Mapping distal and proximal diode array readings with 0.175 cm spacing provided $R_{meas}\left(z_{diod{e}_i}\right)$ curves with 45 data points. Energy spectra from 1 and 15 s measurement times were identical, demonstrating real-time measurement rates ≥1 Hz. Energy spectra measured for six matched Elekta accelerators showed notable differences in shape (peak location and FWHM), demonstrating the spectrometer's potential for beam tuning and quality assurance.

This study showed that an SRS MapCHECK diode array coupled with a 0.55 T permanent magnet can be used to construct a real-time electron energy spectrometer that should be compact, lightweight, inexpensive, and practical. It offers accelerator engineers a new tool to improve efficiency and effectiveness of electron beam tuning during commissioning and maintenance. Also, it offers medical physicists a potentially efficient and effective paradigm for machine quality assurance.