Dear Anne, it has to do with the fact that  although the Libby half life is
slightly wrong, it only introduces a systematic bias in BP (radiocarbon
ages, a discrepancy that is taken out anyhow by the process of calibration.
It is perfectly possible to work with a BP time scale (without calibration)
when you only intercompare radiocarbon ages (and the radiocarbon dater in me
has to say that most other dating methods lack the precision to become a
problem, ha ha) . If you want to be as close as possible to an absolute time
scale you should calibrate. Historically,  the Libby half life had been in
use for some time when it was found slightly out. It was decided not to
change the "conventional " half life not to create possible confusion
between two differently calculated 14C ages. And as I said, in practice it
is no problem.
Best regards,
GJ

 At 16:58 24/10/97 +0200, you wrote:

>Does anyone of you know about a good reference or have an
> explanation why the Libby half live time is used when
>computing the radiocarbon age of samples.
>
---------------------------------------
Dr. Gert Jaap ("GJ") van Klinken,
Radiocarbon Accelerator Unit,
Research Laboratory for Archaeology and the History of Art,
University of Oxford
6 Keble Road, Oxford OX1 3QJ

Tel (+44) (01865) 273939
                                 283941
          Fax (01865) 273932

http://www.rlaha.ox.ac.uk/orau.html