Truman P. Kohman Carnegie Institute of Technology Pittsburgh, Pennsylvania
Chronology of Nucleosynthesis and Extinct Natural Radioactivity
T h e simple question ''HOW old are the elements?", often asked (and frequently answered!) in the past,, must on the basis of present knowledge be replaced hy a more meaningful one, "When were the elements of the solar system synthesized?" It is now evident that the shuffling of nucleons t o form the nuclei of the elements as t,hey now exist must have been a complicated process involving a variety of mechanisms operat,ing a t several or many different times, and that a particular sample of matter may have had an ent,irely different history t,han a sample in another part of the universe. We limit our inquiry for the present, accordingly, to t,he "local" sample of elements, about. which wc have by far the most information, and in which we have the greatest personal interest. At t,he same time we broaden the questlion to concern the entire span of time within which nucleosynthesis of "our" elements occurred, as well as t o include something about t,he time dependence of the synthesis within that span. General Ideas on the Time of Nucleosynthesis Until recentsly it was supposed that all of the elements were c r e a t ~ dmuch in their present form in one violent event, referr~d to as "nucleogenesis," which may have bwn int,imat,ely connected with the initial stage of t,he expanding universe (1, 2). According to t,his idea all of t,he atoms would have the same definit,e "age," which might be determined either from the mutual recession t,ime of the extragalactic nebulae or from the residual content of long-lived natural radionuclides; t,he lower limits of this "age" would be given by t,he ages of any bodies or substances in t,he universe. Serious difficulties with the development of this idea and recent developments in astrophysics have subsequently led t , general ~ acceptance of the alternate idea t,hat "nucleosynthesis," or the building up of t,he heavier atomic nurlei from light particles, proceeds mainly in stellar interiors wit,h redist,ribution of the matter mainly through supernova explosions (3-7). This idea is compatible both with a single nucleogenic event coincident w ~ t h"cosmogenesis" and with "continuous creation" of matter in a steady-&ate expanding universe. h~ucleosynthesis thus follows but is not directly connected with "nucleogenesis," t,he initial creation of the element,ary particles and hence of matter itself. P r e ~ e n t das part of the Symposium, "Geochemistry: Analysis
and Synthesis," before the Divisions of Inorganic Chemistry, Physical Chemistry, and Chemical Education a t the 137th Meeting of the ACS, Cleveland, Ohio, April, 1060. Preparation of this paper was supported by the IT. S. Atomia Energy Commission.
Among the hundreds of nuclides composing "our" matter, those mhirh are most useful for giving information about the time of its formation are those which are radioactive wit,h half-livcs sufficiently short for appreciable decay t , have ~ occurred during or subsequmt to the formation period. Of the score of "primary" natural radionuclides, only four, UZg5[half-life (7.0 + 0.1) X 108 y], IO cimenlo, Ser. 9, 10, 1623 (1953). , E., Phl,.~.Reu. 73, 1209 (1948). (9) S u ~ s s H. (10) ATEN,A. \\-. H., JR., Physica 23, 1073 (1957). (11) ANDERS,I?., A S D GOLEG,G. G., J. CHEM.EDTIC., 38, 58
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