Investigation of Low and Medium Energy X-Ray Calibrations for Survey Meters
S Jollota*, N Nelson, A Khan, L DeWerd, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison
Presentations
TH-B-BRA-2 (Thursday, 7/14/2022) 8:30 AM - 9:30 AM [Eastern Time (GMT-4)]
Ballroom A
Purpose: Recent policy changes by the Nuclear Regulatory Commission (NRC) have encouraged the replacement of Cs-137 irradiators with X-ray irradiators. Considering Cs-137 is used for exposure rate calibrations of radiation survey meters (RSMs), this work investigates the feasibility using a 320 kV x-ray tube as a source for RSM exposure rate calibrations.
Methods: Nine moderately filtered NIST traceable x-ray beam qualities were used to measure the energy response of three RSMs. For all the beam qualities considered, the exposure rates at the reference tube currents exceed the highest operating ranges of most RSMs (<5 R/hr). The x-ray tube output was characterized as a function of tube current down to 0.1 mA using a built-in monitor chamber to produce exposure rates within the operating ranges of the RSMs. RSM exposure rate measurements were then performed at 0.1 mA and 0.5 mA for all beam qualities to measure the energy response. EGSnrc Monte Carlo simulations were used to optimize additional amounts of added filtration to produce exposure rates within the RSM operating ranges. For each proposed filter, associated energy spectra are reported.
Results: The energy response of ionization chamber-based RSMs approached unity with increasing effective energies while the Geiger-Mueller-based RSM exhibited a strong over-response above 40 keV. DOSRZnrc simulations showed that all RSM operating ranges could be achieved with an added 5 mm Pb in front of the current M200 NIST beam quality. FLURZnrc simulations show that an added 5 mm Pb filter would increase the average energy from 99.6 to 172.5 keV.
Conclusion: X-ray RSM calibrations are feasible with a 320 kV x-ray tube, however, higher energy beams will be needed to overcome the energy dependence of most GM RSMs. A combination of tube current modulation and the proposed filter designs demonstrate a potential method for X-ray RSM calibrations.
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