SU-GG-T-240: New Method of An HPGe Detector Precise Efficiency Calibration with Experimental Measurements and Monte Carlo Simulations

Abstract

Purpose: Developing a method of an HPGe detector precise γ efficiency calibration which is very important for accurate radiation detection during cancer radiotherapy practices. Method and Materials: 24Al radioactive nucleus produced and separated with Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University has positron decays followed by γ transitions up to 8 MeV from 24Mg excited states, which is used for a β‐γ coincidence measurement with a 1‐mm‐thick BC404 plastic scintillator, an HPGe detector and a fast tape‐transport system to calibrate the HPGe detector. Results: By carefully considering the effects of summing, positron annihilation, internal conversion, and β detector efficiency when analyzing 24Al spectrum, we got the efficiency for γ‐ray 7070 keV at 49 mm distance away from the source sample 24Al, which was 0.192(6)%. The Monte Carlo (MC) simulation with CYLTRAN code gave a value of 0.189%, which was in agreement with our measurements. The precise efficiency calibration curve of the HPGe detector up to 7070 KeV at 49 mm distance away from the source sample was obtained. By using the same procedure, we got the efficiency for the 7070 keV γ‐ray at 151 mm distance away from the source sample 24Al, which was 0.0385(8)%. MC simulation value was 0.0399%, which differed from measurement by 4(2)%. This discrepancy led us to assign an uncertainty of 4% to our efficiencies at 151 mm up to 7070 KeV. The Monte Carlo calculations also reproduced the intensity of observed single‐and double‐escape peaks, providing that the effects of positron annihilation‐in‐flight were incorporated. Conclusion: A new method was established. The precise calibration curves obtained from this work are useful for accurate radiation detection and improving quality control to quality assurance (QA) for intensity‐modulated radiation therapy (IMRT). Research sponsored by Department of Energy and Robert Welch Foundation.

Monte Carlo calculated calibration settings for commercially available nuclear medicine ionization chambers

“…A more comprehensive approach of characterizing the PDE of the detector with Monte Carlo modeling provides a means of accurately interpolating the PDE over a validated energy range for various extended source geometries and accounts for each isotope‐specific PDE at $$\hskip.001pt 511$$ keV due to differences in beta spectra. Monte Carlo models for calculating planar HPGe detector PDE have been developed prior to this work and validated against physical measurements to within $$5\%$$ at $$95\%$$ confidence 12,13 and as little as $$2\%$$ in other instances when a more detailed detector geometry is considered, 14 all of which provide sufficient accuracy to identify discrepancies in NMIC activity readings $$>5\%$$ like those reported in the literature 8–10 …”

“…A more comprehensive approach of characterizing the PDE of the detector with Monte Carlo modeling provides a means of accurately interpolating the PDE over a validated energy range for various extended source geometries and accounts for each isotope-specific PDE at 511 keV due to differences in beta spectra. Monte Carlo models for calculating planar HPGe detector PDE have been developed prior to this work and validated against physical measurements to within 5% at 95% confidence 12,13 and as little as 2% in other instances when a more detailed detector geometry is considered, 14 all of which provide sufficient accuracy to identify discrepancies in NMIC activity readings > 5% like those reported in the literature. [8][9][10] The linearity of the NMIC response implies that the relative difference in energy deposition per decay for different isotopes is directly proportional to the correct calibration settings required for the respective isotope, therefore calibration settings for any isotope can be calculated from decay data and knowledge of the NMIC signal as a function of decay product energy.…”