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for the sum of isobaric abundances £c^ the even A soecias has a zigzag-shaped
irregular appearance and the abundance values depend strongly on the neutron
excess nunber*
At mass numbers where two stable isobars exist, the one with the
smaller neutron excess has the higher nuclear abundance in the light mass reglono
Going to higher mass nunfcers the curves for the sum of the isobaric abundances
get more and more regular and the isobars with t^e
higher neutron excess become the
more abundant oneso
(toe may try to uMerstand this difference by considering two types of
nuclear reactionst
(1) A reaction leading to the f ornation of nuclear species an the neutronrich
si da of the energy valleyo
This reaction predominated at higher mass
nuiri)er3 and yielded a "smoothed out" abundance distribution.
(2) A reaction leading to nuclei on the neutron-deficient side of the energy
valleyo
This zeaetion predominated and led to the "fine structxure" in the abundance
distribution of the lower mass regiono
It is feasible to assxme that reaction (1) was a "neutron capture, build up''^,
that is 0?:^)
followed by p-deesQr, as postulated by the neutron capture theoryo
No theory has yet been proposed, that could account for a reaction of the type
(2) but the empirical evidence indicates that a reaction of this kind is required,
as will be shown later-
A steady change of the abundances with mass nunftjer has to be expected from
any theory in which very high temperatures (kP >* 1 Mev) are postulated
for
the transmutation of nuclei of one given nass nunfijer into those of another mass
number»
In the case of a very high temnerature, not only the ground state but SLLS-:
many excited states will be involved in the reactions so that the effect