Source expiration-added comment

[Les Slaback <lester.slaback@nist.gov>]

Not mentioned so far is the uncertainty in the value for the half-life,
leading to an added uncertainty in the decayed source activity.  At some
point that uncertainty will exceed that which is acceptable for the
intended use.  For shorter lived stds this would probably be determining.
Disclaimer:  the above are the personal musings of the author, and do not
represent any past, present, or future position of NIST, the U.S. government,
or anyone else who might think that they are in a position of authority. 
Lester Slaback, Jr.  [Lester.Slaback@NIST.GOV] 
NBSR Health Physics 
Center for Neutron Research 
NIST
100 Bureau Dr.  STOP 3543 
Gaithersburg, MD  20899-3543 
301 975-5810 
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