I-125 gamma constant -Reply

[FRAMEP@ORAU.GOV]

Lots of fun with this one:

Don't put much importance on the eary constants you see floating
around. Even the old 1970 RHH put an "approximately" sign in front of
its 0.7 R/h per mCi at 1 cm value. Among other things, there was
probably considerable uncertainty about the low energy x-ray and
gamma intensities (yields) in those days.

One of the more important early listings of exposure rate constants
was Nachtigall's (1969). Its my understanding, he typically ignored
x-rays in his calculations - with something like I-125 he'd be way
out of line with other sources of these numbers.

As Dale mentioned, using modern data, and including the x-rays,  
I-125's exposure rate constant would be around 1.4 R/hr per mCi at 1
cm. 

The new RHH numbers came from Unger and Trubey's ORNL/RSIC -45/R1. I
calculated the I-125 dose constant from scratch using their equations
and it came out exactly as listed: 2.75 rem/hr per mCi at 1 cm. Its
not a mistake. As Dale mentioned, the discrepancies between the
exposure rate constants and the new RHH dose rate constants occur at
low energies. For example, see listings for I-129 and Sr-82 exposure
rate constants and compare them with the new RHH numbers. For what
its worth, the new RHH (Unger and Trubey) numbers include the
contribution from anything over 10 keV. In other words, they include
the contributions from the x-rays.

A key point is that the new RHH numbers are  in terms of dose
equivalent (eg Sv/hr or rem/hr) and not exposure rate (R/h). Just as
important, the new numbers are dose equivalents, not effective dose
equivalents. I'm looking forward to Dale's calculation using the new
ANSI fluence to effective dose equivalent data. I only have a draft
of the new ANSI  but it makes the following point: " ..the data for
the 1977 [ANSI] standard [from which the new RHH numbers are derived]
... have little to recommend their use other than the fact they are
inherently conservative". 

As a guess, Dale's new number might even be as low as 0.3 rem/hr per
mCi at 1 cm - looking forward to it. 

Paul Frame
Professional Training Programs
Oak Ridge Institute for Science and Education
framep@orau.gov