Answer to Question #337 Submitted to "Ask the Experts"

Q

What is the conversion factor for ¹³⁷Cs air exposure to rem (R to rem)corrected for phantom backscattering effect.

A

For ¹³⁷Cs, the dose equivalent in rem is calculated by multiplying the free air exposure in R by 1.06. At this energy the conversions are the same for both deep dose equivalent (10 mm depth in an individual person's body) and shallow dose equivalent (0.07 mm depth). According to information presented in HPS N13.11-1993, The American National Standard for Dosimetry -- Personnel Dosimetry Performance -- Criteria for Testing, the ¹³⁷Cs conversion from air kerma (Gy or rad) to dose equivalent (Sv or rem) is 1.21 rem/rad(air), and the suggested ¹³⁷Cs air kerma to exposure equivalency is 0.878 rad(air kerma)/R. Thus: 1.21 rem/rad(air) x 0.878 rad(air)/R = 1.06 rem/R. This is slightly different from the value of 1.03 published 10 years earlier in ANSI N13.11-1983. Your reference to "phantom backscattering effect" suggests that your interest may be personnel dosimeter calibration. For personnel dosimeter calibration, the dosimeter should be placed at the surface of an appropriate phantom; the exposure (or air kerma) value is the value measured (or calculated) at the location of the center of the front surface of the phantom, but in the absence of a phantom. The corresponding dose equivalent value is the value expected at the appropriate depth in the body of an individual person if he or she was located at the position of the phantom.

Charles E. Roessler, CHP, PhD