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