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 🌍
Purpose: Electrometers are fundamental instruments for absorbed dose determination, and their accuracy is strongly dependent on their calibration. The standard procedure of electrometer calibration is based on a continuous input, which does not replicate the instantaneous current produced in actual beam pulse deliveries. In this regard, the calibration procedure may be inaccurate for electrometers exposed to ultra-high dose per-pulse (UHDPP) beams with higher instantaneous currents than conventional beams. In this work, UHDPP beam pulses are acquired using a commercial ionization chamber. The goal is to replicate beam pulses using a wavefunction generator (WFG) and propose an innovative pulse-based method of electrometer calibration with NIST-traceable electrical standards.
Methods: Multiple conventional (0.2 – 1 mGy per-pulse) and UHDPP (3 – 12 Gy per-pulse) beam pulses were acquired using a 9 MeV electron beam using a Varian TrueBeam and an IntraOp® Mobetron®, respectively. For the visualization and analysis of the pulses, an Advanced Markus® chamber was used and connected to a Femto® amplifier and then to an oscilloscope (PicoScope). Beam pulses were integrated to determine the charge per pulse (CPP) using a customized MATLAB-based program. A single pulse was selected as a template based on the CPP median value from the delivery. The template pulses were simulated using a WFG, and the output was connected to a resistance and, finally, to a Max-4000 electrometer. The average current from the pulse train and the electrometer reading were estimated and used for the pulse-based calibration method.
Results: The simulated pulses were reproduced using the WFG, and the Type-A uncertainty of the average current produced was within 0.1% (k=1). The pulsed-based calibration method showed the same calibration coefficient as the standard calibration (1.001 pA/reading) for the conventional and UHDPP pulses.
Conclusion: The pulse-based calibration showed capacity as a calibration method. Realistic instantaneous currents will be investigated.