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An Investigation Into the Measurementof the Energy Spectrum of a 6MV Linear Accelerator Using a Pulse-Mode Detector

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S Taneja

S Taneja1*, L Bartol1 , W Culberson1 , L DeWerd1 , (1) Univ of Wisconsin-Madison, Madison, WI


SU-F-108-1 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 108

Purpose: Measurement of the energy spectrum of 6MV linear accelerator using a pulse-mode detector has not been successful due to the high-fluence, high-energy nature of these photon beams. As a result, the energy spectrum is typically determined by unfolding data from percent depth dose curves, which is indirect and insensitive to small spectral variations. This work provides results from spectral measurements in the form of background-corrected pulse height distributions (PHD) using a high purity germanium (HPGe) pulse-mode spectrometer and simulations run in Monte-Carlo transport code, MCNP6.

Methods: Spectrum measurements were taken using a HPGe spectrometer housed in a custom-built shielding with a 16 cm cylindrical lead enclosure featuring a 30 cm long tungsten collimator. A Compton Scattering (CS) spectrometry setup was used with the spectrometer placed at an angle of 130 degrees from an aluminum scattering rod located at isocenter. The measured PHD was energy-calibrated using a Eu-152 standardized source. A model of a Varian Clinac 21EX linear accelerator was experimentally benchmarked in MCNP6 to determine the photon energy distribution below the jaws of a (5x5) cm² field, which was subsequently used in all simulations. The CS spectrometry setup was modeled in MCNP6, including a previously verified detector and shield geometry, and an expected PHD was simulated and binned with an energy resolution comparable to measurements. PHD shape from measurements and simulations were compared.

Results: Simulated and measured PHDs were normalized to their respective total response to allow for comparison of spectral shape, which showed agreement. The detector deadtime was less than 5%, indicating minimal pulse pileup during measurements.

Conclusion: This work demonstrated that the measurement of the energy spectrum is feasible using a pulse-mode detector, and the intended signal is distinguishable above background. Future work will optimize and apply corrections to the PHD to determine the true energy spectrum.

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