Re: I-125 gamma constant

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

Jeanne,

The data on dose constants in that book are based
on an ANSI standard that was withdrawn.  The current
ANSI standard more closely agrees with other values
I have seen.  I haven't used it to calculate I-125
directly though as the new standard still diverges
somewhat at around 30 KeV.  There was some discussion
about this on the net awhile back and Paul Frame and I
have had some additional discussions about the 
origin of the discrepancy with no resolution.
The RHH value converts to 274 mRem/hr/Ci.  I have seen
values listed as low as 70 mRem/hr/Ci, but other
sources have values closer to 140 mRem/hr/Ci.
The 140 mRem/hr/Ci value agrees with what one can calculate 
using the mass energy absorption coefficient for tissue.

The dose constants calculated with the expired ANSI standard
are a factor of 2 to 3 higher at low energies than what
one can calculate using the mass energy absorption
coefficient.  The difference is largest at lower energies.
at higher energies the two calculation converge somewhat.
Above 1 MeV the Old ANSI values are only about 10% higher.

At the lowest energies the energy absorption given by
the old ANSI exceeds what one would calculate with the
mass attenuation coefficient which I believe is physically
impossible.

The current ANSI standard agrees fairly closely with
the values calculated using the mass energy absoption
coefficients except at the lowest energy.

Hope this helps.

Dale Boyce
dale@radpro.uchicago.edu