Re: Trivia question, sort of...

[Elizabeth Brackett <ebracket@freenet.columbus.oh.us>]

On Fri, 26 Apr 1996 Slaback@micf.nist.gov wrote:

> First, because they are alpha emitters.
> Second, because of long biological halflife.
> Third, because with the ICRP dose-commitment
> scheme long effective halflives lowered the
> DACs proportionally.
> 
> More interestingly, why is the 'D' form more
> restrictive than the 'Y' form?
> 
I didn't keep the original post, but I believe it asked about gadolinium,
which doesn't have any forms assigned to class Y (sorry if I'm mistaken
about this).  It does have forms assigned to class W, and these ALIs are
larger than the class D ALIs for most of the isotopes.  The following
applies regardless of the nuclide discussed.

The material class refers only to the half time of the material in the
lungs.  When it leaves the lungs, a portion of it goes to body fluids and
is subsequently deposited in various organs where it continues to
irradiate the body, and a portion of it goes to the GI tract where it is
eliminated or re-absorbed into the blood stream (this is a function of the
f1 value).  If the isotope has a long half life, it will continue to
irradiate the organs of deposition for a significant amount of time
relative to the time it spent in the lungs.  So, it is possible for a
class D material of an isotope to give you a larger dose per intake
quantity  depending on its half life, organ(s) of deposition, decay mode,
and f1 value.  Plutonium is another example of this; Pu-238 and Pu-239
have smaller class W ALIs than the class Y ALIs.

Liz Brackett, CHP
Consultant
ebracket@freenet.columbus.oh.us