Trivia question, sort of... -Reply

[FRAMEP@ORAU.GOV]

TGIF

I asked Mike Stabin to respond to Greg's query and he was gracious
enough to do so.

Mike's answer refers to the ICRP's SEE and the Us values. SEE is the
specific Effective Energy i.e. the energy (MeV) absorbed by a gram of
the target tissue/organ  as a result of a single decay/transformation in
the source tissue. The SEE also factors in the quality factor. Us is the
number of decays/transformations in the source tissue over the fifty
year period following intake.   I also asked  Mike why the alpha emitting
Gd-150 had not been assigned an ALI.  He explained that in many cases
the ICRP ignored those nuclides that are not commonly encountered and
that Gd-150 might be one of those. Paul Frame 

Mike's response follows:


There are two effects going on here - one involving the Us and the other
involving the SEE. Gd-148 and 152 have considerably longer half-lives
than the other isotopes so this causes their Us's to be about 100-1000
times larger than the others. Then, they are alpha emitters, as you
mentioned, and their SEE's for trabecular bone irradiating bone surface
cells (which turns out to be the limiting dose tissue) are 3-4 orders of
magnitude greater than those for the others which primarily decay by
electron capture. (Gd-151 is also an alpha emitter, but with yeilds of only
about 1E-9). But, as dose is the product of Us and SEE, the product will
be 5 to 7 orders of magnitude higher for these isotopes, and so the ALIs
and DACs are that much lower than for the others. So the effect
appears to be real.

Mike Stabin                                         P.O. Box 117
Radiation Internal Dose                      Oak Ridge  TN37831-0117
  Information Center                           423-576-3449 ph 
Oak Ridge Institute for                       423-576-8673 fax
 Science and Education                     stabinm@orau.gov 
http://www.orau.gov/ehsd/ridic.htm