Relative Response to Low-Energy Photons and Determination of Instrument Correction Factors for Portable Radiation Instrumentation
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Practically all portable radiation instruments come from the manufacturer with a graph of photon energy response. However, many of these graphs are in log-log format which can disguise relatively large variations in response, particularly for low-energy photons. Additionally, many only include one specific orientation. Thus, in many cases, it is left up to the user to determine for which orientation and photon energies the instrument will be calibrated and ultimately used in the field. It is known that many instruments can have inconsistent responses below ~300 keV, which may lead to under or over-estimation of exposure rate. However, based on relative response plots, one can derive an instrument correction factor that can be applied to the measured exposure rate to yield a constant response curve and more accurately estimate the exposure rate. Using a combination of irradiator systems, six different types of radiation instrumentation were irradiated with photons with energies from 38 to 1253 keV in various orientations. A calibrated ion chamber, in conjunction with an electrometer, was used to determine the conventionally true exposure rates for various x-ray beam codes and radionuclides contained in the irradiator systems. The conventionally true exposure rates were compared to the measured values for each instrument type and relative response plots were constructed. These plots were used to determine an ideal orientation and correction factors were chosen for responses > ±20 percent. From the relative response plots, instrument correction factors are not necessary for the following; Eberline RO-20, Thermo RadEye B20, and Bicron Micro Rem LE. Correction factors of 0.7 and 1.5 should be applied for photons between 80 – 120 keV for the Eberline Teletector 6112B low and high-range detectors, respectively. A correction factor of 0.8 should be applied for photons below 120 keV for the Eberline RO-7-BM. For the Thermo Mk2 EPD, a correction factor of 1.25 should be applied for photons below 40 keV. The primary causes of under and/or over-responses were found to be window attenuation, varying interaction cross-sections, and the range of secondary electrons. Angular dependence and calibrations for specific applications are also discussed.
Wagoner, David Andrew (2010). Relative Response to Low-Energy Photons and Determination of Instrument Correction Factors for Portable Radiation Instrumentation. Master's thesis, Texas A&M University. Available electronically from