Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!cmcl2!beta!a!mwj From: mwj@a.UUCP (William Johnson) Newsgroups: sci.med,sci.bio Subject: Re: Dating age of humans Message-ID: <295@a.UUCP> Date: Fri, 18-Sep-87 19:31:15 EDT Article-I.D.: a.295 Posted: Fri Sep 18 19:31:15 1987 Date-Received: Sun, 20-Sep-87 05:33:09 EDT References: <26333@sun.uucp> <1960@kitty.UUCP> <3836@cit-vax.Caltech.Edu> <763@sugar.UUCP> Organization: Los Alamos Natl Lab, Los Alamos, N.M. Lines: 37 Keywords: Ageing Human Carbon Xref: mnetor sci.med:3230 sci.bio:651 In article <763@sugar.UUCP>, peter@sugar.UUCP (Peter da Silva) writes: > Carbon-14 isn't the only radioisotope used in this sort of thing, is it? What > about shorter-lived isotopes that (while not longevious enough for archeology) > would decay fast enough to be useful here? The issue isn't just how long-lived an isotope is; it also matters whether uptake of the isotope (as a fraction of all isotopes of the element) is constant. The reason carbon-14 dating has worked well is that the C-14 production rate has been roughly constant for thousands of years. (In fact, there have been minor glitches in the rate due to disturbances in the earth's magnetic field, etc., but those are incremental effects.) In principle, one might use beryllium-7, which has a much shorter half life (53 days), if there was enough beryllium in the sample to be analyzable. (Remember, beryllium is EXTREMELY toxic.) However, beryllium-7 gets produced erratically (primarily through cosmic-ray interactions in the atmosphere, which are more or less time-independent, and atmospheric nuclear tests, which definitely are NOT time-independent) and doesn't get homogenized through "native" terrestrial beryllium. Consequently the rate of uptake of Be-7 per gram of beryllium isn't constant, and the technique wouldn't work. About the same reasoning more or less precludes use of any other radionuclide: if it's short-lived enough to be useful, about the only things that can make it near earth's surface are cosmic rays and nuclear explosions -- and the latter screw up production rates of the former in spectacular fashion. (Don't blame me -- I just work here.) Between this and the problem of getting the radionuclide mixed in with the stable isotopes, it just don't work. Incidentally, the REALLY bad news is that atmospheric testing (and maybe also Chernobyl to some fantastically tiny extent) also releases C-14 into the environment -- not a lot, but it doesn't take much. Consequently, C-14 dating of biological samples that were alive after August 1945 won't work either. The archaeologists of the year 6000 are going to have to think of some other dating technique. Life is hard. -------------------------------------------------------------- Bill Johnson Opinions? Who Los Alamos National Laboratory has opinions?