Measurement of the Energy Spectrum of a 6 MV Linear Accelerator Using Compton Scattering Spectroscopy and Monte Carlo-Generated Corrections

International Journal of Medical Physics Clinical Engineering and Radiation Oncology · 2020 · 15 citations

• Physics
• Optics
• Computational physics

Purpose: The energy spectrum of a linear accelerator used for dose calculations is determined during beam commissioning by iteratively adjusting the spectrum and comparing calculated and measured percent depth-dose curves. Direct measurement of the energy spectrum using pulse mode detectors is particularly challenging because of the high-energy, high fluence nature of these beams and limitations of the detector systems. This work implements a Compton scattering (CS) spectroscopy setup and presents detector corrections and spectral unfolding techniques to measure the spectrum of a 6 MV linear accelerator using a pulse mode detector. Methods: Spectral measurements were performed using a Varian Clinac 21EX linear accelerator and a high-purity germanium (HPGe) detector. To reduce fluence to the detector, a custom-built lead shield and a CS spectrometry setup were used. The detector was placed at CS angles of 46°, 89°, and 125°. At each of these locations, a detector response function was generated to account for photon interactions within the experimental geometry. Gold’s deconvolution algorithm was used to unfold the energy spectrum. The measured spectra were compared to simulated spectra, which were obtained using an experimentally benchmarked model of the Clinac 21EX in MCNP6. Results: Measurements were acquired and detector response corrections were calculated for all three CS angles. A comparison of spectra for all CS angles showed good agreement with one another. The spectra for all three angles were averaged and showed good agreement with the MCNP6 simulated spectrum, with all points above 400 keV falling within 4%, which was within the uncertainty of the measurement and statistical uncertainty. Conclusions: The measurement of the energy spectrum of a 6 MV linear accelerator using a pulse-mode detector is presented in this work. For accurate spectrum determination, great care must be taken to optimize the detector setup, determine proper corrections, and to unfold the spectrum.

Ionization Chambers to Determine Neutron and Gamma-Ray Kerma in a Research Reactor

IEEE Transactions on Nuclear Science · 2019-08-27 · 1 citations

Ionization chambers were designed and constructed to determine the kerma rates in various materials within several centimeters of a Training, Research, Isotopes, General Atomics (TRIGA) reactor core operating at 1 MW. The primary aim of this article was to compare kerma measurements with the advanced Monte Carlo code calculations of nuclear heating. Wall thickness, collection gap, and fill gas pressure were chosen to satisfy Bragg-Gray criteria, so that the measured ionization current was related to the kerma rate in the wall material. Chamber wall materials composed of low mass number elements, including hydrogen-rich C552 air-equivalent plastic and beryllium, were selected to measure the kerma due to fast neutron elastic scattering. By operating these neutron sensitive chambers coincidentally with relatively neutron insensitive chambers composed of aluminum and Zircaloy-4, we were able to measure the total heating due to fast neutrons and gamma rays in a material and to differentiate these heating components. A chamber composed of borated stainless steel was used in a similar fashion to measure the thermal neutron flux. The total kerma rate was also measured in various materials typically found in a reactor core. Chamber collection volumes were initially determined using ambient air fill gas and National Institute of Standards and Technology (NIST)-traceable air-kerma rates from a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source. All chambers were sealed with argon gas to provide thermal and compositional stability. Chamber properties, including stability, saturation, and gas-phase mass subject to charge collection, were determined using the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source. Chambers were operated for approximately 30 min adjacent to the reactor core, and the integrity of gas seals was subsequently verified by repeating the measurement with the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source.

Characterization of the energy spectrum of a 137 Cs irradiator through measurements using a pulse-mode detector

Radiation Measurements · 2018-04-12 · 4 citations

Ionization Chambers to Measure Neutron and Gamma-Ray Kerma in a Research Reactor

2017-10-01

Ionization chambers were designed and constructed to measure the kerma rates (heating) in various materials within several centimeters of a TRIGA reactor core operating at 1 MW. Wall thickness, collection gap and fill gas pressure were chosen to satisfy Bragg-Gray criteria, so that measured ionization current across the collection gap was proportional to the kerma in the wall material. Chamber wall materials comprised of low mass number elements, including hydrogen-rich C552 air-equivalent plastic and beryllium, were selected to measure the kerma due to fast neutron elastic scattering. By operating these neutron sensitive chambers coincidentally with relatively neutron insensitive chambers composed of aluminum and Zircaloy-4, we were able to measure the relative heating due to neutrons and gamma rays in a material. Chamber collection volumes were initially determined using NIST-traceable kerma rates and a cobalt-60 source. All chambers were sealed with argon gas to provide thermal and compositional stability. Chamber collection gas mass, stability and saturation curves were determined using the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source. Chambers were operated for approximately 30 minutes and the integrity of gas seals was subsequently verified by repeating the measurement with the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co source.

TH-CD-201-02: A Monte Carlo Investigation of a Novel Detector Arrangement for the Energy Spectrum Measurement of a 6MV Linear Accelerator

Medical Physics · 2016-06-01

Purpose: Direct measurement of the energy spectrum of a 6MV linear accelerator has not been successful due to the high fluence rate, high energy nature of these photon beams. Previous work used a Compton Scattering (CS) spectrometry setup with a shielded spectrometer for spectrum measurements. Despite substantial lead shielding, excessive pulse pile-up was seen. MCNP6 transport code was used to investigate the feasibility and effectiveness of performing measurements using a novel detector setup. Methods: Simulations were performed with a shielded high-purity germanium (HPGe) semiconductor detector placed in the accelerator vault's maze, with a 2 cm diameter collimator through a 92 cm thick concrete wall. The detector was positioned 660 cm from a scattering rod (placed at isocenter) at an angle of 45° relative to the central axis. This setup was compared with the shielded detector positioned in the room, 200 cm from the scattering rod at the same CS angle. Simulations were used to determine fluence contributions from three sources: (1) CS photons traveling through the collimator aperture, the intended signal, (2) CS scatter photons penetrating the detector shield, and (3) room-scattered photons penetrating the detector shield. Variance reduction techniques including weight windows, DXTRAN spheres, forced collisions, and energy cutoffs were used. Results: Simulations showed that the number of pulses per starting particle from an F8 detector tally for the intended signal decreased by a factor of 102 when moving the detector out of the vault. This reduction in signal was amplified for the unwanted scatter signal which decreased by up to a factor of 109. Conclusion: This work used MCNP6 to show that using a vault wall to shield unwanted scatter and increasing isocenter-to-detector distance reduces unwanted fluence to the detector. This study aimed to provide motivation for future experimental work using the proposed setup.

Air-kerma modulation effects on the energy spectrum of a 137CS irradiator using Monte-Carlo techniques

Radiation Measurements · 2016-11-01 · 4 citations

Impact of the differential fluence distribution of brachytherapy sources on the spectroscopic dose‐rate constant

Medical Physics · 2015-04-17

PURPOSE: To investigate why dose-rate constants for (125)I and (103)Pd seeds computed using the spectroscopic technique, Λ spec, differ from those computed with standard Monte Carlo (MC) techniques. A potential cause of these discrepancies is the spectroscopic technique's use of approximations of the true fluence distribution leaving the source, φ full. In particular, the fluence distribution used in the spectroscopic technique, φ spec, approximates the spatial, angular, and energy distributions of φ full. This work quantified the extent to which each of these approximations affects the accuracy of Λ spec. Additionally, this study investigated how the simplified water-only model used in the spectroscopic technique impacts the accuracy of Λ spec. METHODS: Dose-rate constants as described in the AAPM TG-43U1 report, Λ full, were computed with MC simulations using the full source geometry for each of 14 different (125)I and 6 different (103)Pd source models. In addition, the spectrum emitted along the perpendicular bisector of each source was simulated in vacuum using the full source model and used to compute Λ spec. Λ spec was compared to Λ full to verify the discrepancy reported by Rodriguez and Rogers. Using MC simulations, a phase space of the fluence leaving the encapsulation of each full source model was created. The spatial and angular distributions of φ full were extracted from the phase spaces and were qualitatively compared to those used by φ spec. Additionally, each phase space was modified to reflect one of the approximated distributions (spatial, angular, or energy) used by φ spec. The dose-rate constant resulting from using approximated distribution i, Λ approx,i, was computed using the modified phase space and compared to Λ full. For each source, this process was repeated for each approximation in order to determine which approximations used in the spectroscopic technique affect the accuracy of Λ spec. RESULTS: For all sources studied, the angular and spatial distributions of φ full were more complex than the distributions used in φ spec. Differences between Λ spec and Λ full ranged from -0.6% to +6.4%, confirming the discrepancies found by Rodriguez and Rogers. The largest contribution to the discrepancy was the assumption of isotropic emission in φ spec, which caused differences in Λ of up to +5.3% relative to Λ full. Use of the approximated spatial and energy distributions caused smaller average discrepancies in Λ of -0.4% and +0.1%, respectively. The water-only model introduced an average discrepancy in Λ of -0.4%. CONCLUSIONS: The approximations used in φ spec caused discrepancies between Λ approx,i and Λ full of up to 7.8%. With the exception of the energy distribution, the approximations used in φ spec contributed to this discrepancy for all source models studied. To improve the accuracy of Λ spec, the spatial and angular distributions of φ full could be measured, with the measurements replacing the approximated distributions. The methodology used in this work could be used to determine the resolution that such measurements would require by computing the dose-rate constants from phase spaces modified to reflect φ full binned at different spatial and angular resolutions.

TH‐AB‐BRA‐08: Impact of Approximating the Differential Photon Fluence On the Accuracy of the Spectroscopic Dose‐Rate Constant

Medical Physics · 2015-06-01

Purpose: To investigate why dose‐rate constants computed using the spectroscopic technique do not agree with those computed using standard Monte Carlo (MC) techniques. Published work shows that the disagreement between the two techniques is not attributable to any currently characterized source of uncertainty. This work examined if approximations to the differential photon fluence made by the spectroscopic technique are the cause of the disagreement. Specifically, approximations of the spatial, angular and energy distributions of photons leaving the source were studied. Methods: Phase spaces of the fluence leaving the encapsulation of 14 125‐I and 6 103‐Pd models were modified to reflect one of the approximated distributions while the other distributions were left unaltered. The dose‐rate constant was computed from the modified phase space using MC and compared to that computed using standard MC techniques. This process was repeated for each assumption with each source model. Results: Approximations to the differential photon fluence used in the spectroscopic technique cause offsets in the dose‐rate constant of up to 5.3% relative to standard MC techniques. The largest offsets were introduced by the approximated angular fluence distribution. Approximations of the spatial distribution leaving the encapsulation caused smaller discrepancies with an average discrepancy of −0.4% relative to standard MC techniques. Approximations to the energy distribution introduced an average discrepancy of 0.1% relative to standard MC techniques, which is within the uncertainty of the computations. Conclusion: Current approximations of the differential photon fluence used in the spectroscopic technique reduce the accuracy of the technique and are a source of uncertainty not currently considered with the technique. Either more accurate fluence distributions need to be used when computing the dose‐rate constant with the spectroscopic technique, or the additional uncertainties arising from the approximated distributions need to be included when using the current calculation technique.

SU‐E‐T‐107: An Investigation Into Attenuation Effects On the Energy Spectrum for ¹³⁷Cs Using Monte Carlo Techniques

Medical Physics · 2015-06-01 · 1 citations

Purpose: The calibration of radiation protection instrumentation including ionization chambers, scintillators, and Geiger Mueller (GM) counters used as survey meters are often done using 137 Cs irradiators. During calibration, irradiators use a combination of attenuators with various thicknesses to modulate the beam to a known air‐kerma rate. The variations in energy spectra as a result of these attenuators are not accounted for and may play a role in the energy‐dependent response of survey meters. This study uses an experimentally validated irradiator geometry modeled in the MCNP5 transport code to characterize the effects of attenuation on the energy spectrum. Methods: A Hopewell Designs G‐10 137 Cs irradiator with lead attenuators of thicknesses of 0.635, 1.22, 2.22, and 4.32 cm, was used in this study. The irradiator geometry was modeled in MCNP5 and validated by comparing measured and simulated percent depth dose (PDD) and cross‐field profiles. Variations in MCNP5 simulated spectra with increasing amounts of attenuation were characterized using the relative intensity of the 662 keV peak and the average energy. Results: Simulated and measured PDDs and profiles agreed within the associated uncertainty. The relative intensity of the 662 keV peak for simulated spectra normalized to the intensity of the unattenuated spectra ranged from 0.16% to 100% based on attenuation thickness. The average energy for simulated spectra for attenuators ranged from 582 keV with no attenuation to 653 keV with 5.54 cm of attenuation. Statistical uncertainty for MCNP5 simulations ranged from 0.11% to 3.69%. Conclusion: This study successfully used MCNP5 to validate a 137 Cs irradiator geometry and characterize variations in energy spectra between different amounts of attenuation. Variations in the average energy of up to 12% were determined through simulations, and future work will aim to determine the effects of these differences on survey meter response and calibration.

Spectroscopic characterization of high-energy and high fluence rate photon beams

2013-01-01 · 2 citations