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of the neutron excess number.
In the higher mass number region there is no
obvious correlation of abundances with neutron excess, and therefore no ain?3le
correlation of the abundance values with the drop of the binding energy of the
last particle at a magic number can be expected-
Hughes and Sierman (1950) have shown that the neutron capture cross section
of nuclei containing a magic number of neutrons is exceptionally small-
This
expeidnental result has been taken as strong evidence atDeaking In favor of the
neutron capture theory of the origin of the elements, because in this theory
the high cosmic abuiKiances of the magic neutron nuclei follow from the low
neutron caoture cross sections in a very satisfactory way-
A neutron build-up
process taking place in the region of the stable nuclei leads, according to this
theory, to a sharp rise at a neutron shell edge and a gradual smooth leveling off
of the nuclear abundances when going to hi^er mass numbers.
If the neutron
build-up takes place in tte region of the neutron-rich & unstable nuclei,
the maximum to be expected will be flattened out and be displaced toward lower
mass nuniborso
It is possible that the broad maxima in the abundance curves
around mass number 130 and 19U are magic nuntoor effects of the shell closures at
N equal to 82 and 126 fTcm a neutron build-up in the M - Z equal to 3U and 5U
regions-
With decreasing neutron density the center of the build-up reaction
will shift to lower neutron excess numbers, so that the sharp maxima at A equal to
138 and 208 nay have piled up subsequent 1» the formation of the bulk of nuclei
in this itfiss range-
It has not yet been possible to describe the kinetics of such reactions in
a mocpe rigorous quantitative way and to find out what assumntions are necessary
to explain the sharp minima arotuid A equal to 135 and 206 imnedlately proceeding
the N equal to 82 and 126 shell closvres-