Re: roentgen to rad factor

["Otto G. Raabe" <ograabe@ucdavis.edu>]

November 17, 1997
Davis, CA

According to ICRU-30 (1979) the W ev/ion-pair in air for gamma radiation is
a function of photon energy and is equal to about 33.7 ev/ion-pair for
photon energies from about 0.2 to 3 MeV. Hence, for an air ionization
intensity of 1 C/kg the absorbed dose to air is given by 33.7 Gy in this
range of photon energies. Since 1 R = 0.000258 C/kg, an air ionization
intensity of 1 R results in an air dose of 0.00870 Gy = 0.87 rad.

The corresponding tissue dose depends on the composition of the tissue, but
is about given by a typical body tissue/air dose ratio of 1.105 (ICRU-30,
1979). Therefore, the  shallow tissue dose adjacent to an exposure of 1 R
is given by 0.00870 air Gy times 1.105 = 0.00961 tissue Gy = 0.961 tissue rad.

If you choose a higher value of W or make some adjustments for photon
energy, you will probably obtain a higher value of the corresponding air
dose, such as 0.00879 if W = 33.97 and there is a 0.0032 fractional loss of
energy by Bremmstrahlung for higher MeV photons. The values 0.0087 to
0.00879 are in the range of uncertainty associated with the value of W as
given in ICRU-31 (1979).
 
The more important point is that in order to prevent confusion, the
absorbed dose must always have a well defined antecedent material and
radiation identity associated with it. Otherwise, someone might be mislead
to believe that 1 R gives a tissue dose of 0.87 rad rather than the correct
value of 0.96 rad.

Otto