Stem corrections for ionization chambers

Authors

  • Ibbott Geoffrey S.,

    1. University of Colorado Medical Center, Denver, Colorado 80220
    2. University of New Mexico School of Medicine, Albuquerque, New Mexico 87131
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  • Barnes J. Edward,

    1. University of Colorado Medical Center, Denver, Colorado 80220
    2. University of New Mexico School of Medicine, Albuquerque, New Mexico 87131
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  • Hall G. Richard,

    1. University of Colorado Medical Center, Denver, Colorado 80220
    2. University of New Mexico School of Medicine, Albuquerque, New Mexico 87131
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  • Hendee William R.

    1. University of Colorado Medical Center, Denver, Colorado 80220
    2. University of New Mexico School of Medicine, Albuquerque, New Mexico 87131
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Abstract

Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem effect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 60Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25- and 100-R chambers), a Capintec 192 dosimeter with a Farmer 0.6-cm3 probe, a PTW transit dose probe, and an EG&G IC-18 probe with a Keithley 610-B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.

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