Multileaf Faraday cup for beam energy verification in radiation therapy with ultra-high dose-rate electron beams and...

In radiotherapy and particle therapy, the stability of the beam energy must be checked routinely during quality assurance. This is of particular importance when varying the dose rate via the beam intensity of the accelerator in order to study the FLASH effect when comparing irradiations with conventional and ultra-high dose rates.

The standard method of energy verification based on the measurement of percent depth-dose curves in a water phantom is time-consuming and in the case of ultra-high dose-rate beams, may also present a non-negligible radiation protection problem.

A compact, portable multileaf Faraday cup (MLFC) with 128 channels was developed for energy determination based on the measurement of depth-charge curves. Its design is optimized for clinical electron beams in a range between 3 and 25 MeV. The read-out unit of the MLFC displays the beam energy in real time. The device was tested in strongly pulsed electron beams such as those present in irradiations with ultra-high doses per pulse as well as in conventional clinical electron beams. The same detector was used for proof of concept in a single measurement campaign in pulsed beams of therapeutical protons (conventional dose rates) from a synchrotron source using a suitable range shifter. The MLFC was calibrated with monoenergetic electron beams as well as against depth-dose curves. Simulations were carried out for comparison.

The MLFC works well under ultra-high pulse dose rate conditions as well as in conventional electron beams generated by a medical accelerator. Changes in the beam energy of below 20 keV (0.1% of 20 MeV) can be clearly identified by means of the MLFC. For proton beams, well-defined peaks in the depth-charge curves are observed for each single synchrotron spill with a total charge of only about 1 nC. The energy values resulting from MC simulations of the measured MLFC data agree with the actual proton energies within 2 $\%$ .

The MLFC can be used for quick validation of the energy stability when carrying out experiments comparing electron beams of conventional and ultra-high dose rates. The same device can be used in the pulsed ion beams from a synchrotron.