Reduction of the Charge Build-up Effect on the Beam Current Transformers for Real-Time Monitoring of Ultra-High Dose per-Pulse Electron Beams 📝

Author: Wesley S. Culberson, Miguel Angel Flores Mancera, Jeff Radtke 👨‍🔬

Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison 🌍

Abstract:

Purpose: Beam current transformers (BCTs) are a promising alternative to ion chambers for real-time monitoring of ultra-high dose per-pulse (UHDPP) beams. The signal of the BCT is insensitive to ion recombination, which is the main drawback with ion chambers. Nonetheless, the BCT signal may be susceptible to electric fields produced by the charge accumulation in plastic phantoms, which are commonly used in radiotherapy QA measurements. This work seeks a solution to the issues with plastic phantoms in UHDPP electron beams to recover a pristine signal to monitor the beam.

Methods: Beam pulses from a 9 MeV UHDPP (1 – 4 µs pulse width) electron beam were obtained using an IntraOp® Mobetron®. An upper and lower BCT were installed inside the LINAC head. The BCTs were connected to an oscilloscope (PicoScope) to visualize the pulses, and the pulse signal was analyzed using a customized MATLAB-based program. The charge build-up was studied from the BCT signal using a 5 cm water-equivalent slab placed at 2 cm from the collimator output and using a 3D-printed water tank. An aluminized mylar window (1.8 mgꞏcm-2) was designed to shield the BCT from charge build-up. Additionally, the stability of the BCT signal as a function of the number of pulses used per-delivery and the variability of the first pulse across multiple deliveries was investigated. Shielded BCT signals were cross-calibrated using film to determine dose in real-time.

Results: The charge build-up effect was visualized, and the BCT signal was successfully shielded using the mylar window. The signal from the lower BCT showed less sensitivity to the number of pulses per-delivery than the upper BCT. The initial pulse can vary up to 2% across multiple deliveries.

Conclusion: A BCT may be successfully shielded from inadvertent charge build-up on plastic phantoms in UHDPP electron beams with a mylar window.