What a nice question, Steve, since this is exactly one of the debates in our
lab at the moment. Not really for 14C but for tritium, but anyway: I am
really looking forward to the debate in this forum.

Here are my thoughts: (PC = proportional counter, LSC = liquid scintillation
counter)

It clearly depends on what you want.

I do not know anybody selling proportional counters for radiocarbon dating
on a comercial basis. So if you do not have a good electronic engineer and
precision mechanic at hand, you have no choice. If you have them at hand
anyway, you might take into consideration, that the material for a PC system
is much cheaper than a commercial LSC. But the problem is, that you need a
lot of experience (several years) to run a state of the art proportional
counting system. Taking this into acount for somebody without experience,
LSC wins.

I hope the community will agree, that LSC is the technique of choice, if you
have samples of elevated activity, as for example in biology where you use
14C-labelled substances. LSC have automated sample-changers which you will
hardly find for PC. The PC inlet needs a vacuum system and it is more
difficult to handle it. So for high activities and a sample throughput of
tens of samples per day, again LSC clearly wins.

There is an old paper coming to the result, that PC is the better system
concerning background and stability of parameters for radiocarbon dating:

Geyh, M.A. (1972): A Comparison: Proportional Counter and Liquid
Scintillation Spectrometer for Radiocarbon Dating.  Proceedings of the
VIIIth International Conference on Radiocarbon Dating, B, pp. 81-93;
Wellington, New Zealand.

But as you already stated, things changed in the last quarter of the
century.

And there is the question of sample preparation: For LSC you need at least
three steps: conversion to CO2, conversion to acetylene, conversion to
benzene. For elevated activites there are commercial systems on the market
doing that, but I do not know anybody using these for radiocarbon dating. A
PC system can run with pure CO2, which brings you to two steps (conversion
to and purification of CO2). Slight advantage for the PC systems.

Take into consideration that you might not want to measure only 14C, but
perhaps also tritium. There you need again the conversion into a gas for the
PC (there are systems running with a Hydrogen-Argon-CH4 mixture, or with
CH4, or with C2H6), but you just mix the destilled water with the coctail
for LSC. Clear advantage for LSC on this side. On the other hand, for
environmental samples you will have to electrolytically enrich the water,
since the rain nowadays has something like 10-20TU on the northern
hemisphere, and the detection limit of a good LSC is around 5-10TU (our PC
for tritium without enrichment have a detection limit of 1TU). If you do not
want to enrich, PC has an advantage, if you have to enrich anyway, LSC is
the clear choice.

To bring the things to the point: if you have money and no experience, take
the LSC. If you are looking for the scientific adventure and have no money
at all, you might try PC.

Axel

Dr. Axel Suckow
Institut für Geowissenschaftliche
Gemeinschaftsaufgaben (GGA);
Joint Geoscientific Research Institute (GGA);
Geochronology and Isotope Hydrology (S3);
Stilleweg 2;
30655 Hannover;
Phone: +49 511 6432527;
Fax:   +49 511 6433665;
e-mail: [log in to unmask] <mailto:[log in to unmask]>


-----Ursprüngliche Nachricht-----
Von: Steve Nelson [mailto:[log in to unmask]]
Gesendet am: Freitag, 8. September 2000 18:11
An: [log in to unmask]
Betreff: 14C counting

I'd welcome the thoughts of the community on the relative merits of
gas proportional versus liquid scintillation counting for 14C
measurements--especially in the light of modern equipment.
--

********************************
Steve Nelson
Dept. of Geology
S389 ESC
Brigham Young University
Provo. Utah  84602

voice:  1-801-378-8688
lab:    1-801-378-5124
FAX:    1-801-378-8143

CONTROL, ALT, DELETE:  the 3 most
frequently used keys on a PC.
*********************************