VI Reunião de Trabalho Física Nuclear.pdf - Sociedade Brasileira ...

of clear channels correlations, some type of resonancebased
phenomenon is, however, certainly taking place and generating,
at least the fine structure seen in most excitation functions.
Simple "direct" models have also been proposed for
' the purpose of explaining the gross features of the cross
section at back angles. These range from simple changes in the
"normal" optical potentials, to simple changes in the "normal"
elastic S-matrix. The necessity for invoking these changes in
the normal "E-18" type description arose from two important
observations; (a) the quite conspicuous rise in --2-- (180 ° 1
°Ruth
to a value, at Ecm = 35 MeV, almost four orders of magnitude
bigger than the corresponding "E-18" value, and (b) the period
of the angle oscillations, AO , supplies a value of the contributing
angular momentum £ 6 (E) through AO n 7-1-Er , which is twice
0'
as large as the angular momentum, t 8 (E) that determines the
period, AE , of the energy oscillation in the 180 0-excitation
DI E (E)
function, AE a 1/ DE
The first anomaly has been accounted for through the
use of the so-called surface transparent potentials. These
optical potentials are characterized by an imaginary part with
very small diffusivenese which results in an increased
reflection. However, these" potentials, though quite adequate
in describing the angular distributions, fail dramatically in
describing the second anomaly associated with the excitation
function. This'clearly points . to the need for a second .'
imp6rtant modification of the normal optical E.18 potential,
namely the 'addition of a'small, albeit important parity-dependent
component (proportional - to (-) t , which would-not modify the
angular distribution since.'it contributes mostly at lack-angles.
The 180° excitation function would then behave approkimately
- sin
IE
(E)
2
2
n) , thus giving rise to a local period 'AE =
110