SHAPE OF COLLECTIVE FLOW IN HIGHLY CENTRAL AU(150 A MEV)+AU COLLISIONS

Using the FOPI facility at GSI, charged particles (1 less than or equa l to Z less than or equal to 6) produced in the Au(150 A MeV)+Au react ion have been measured at laboratory angles 1.2(0) < Theta(lab) < 30(0 ). Highly central collisions have been selected with two criteria, bot h dealing with the longitudinal and transverse degrees of freedom of t he reaction. The relevance of this selection method is supported by QM D calculations which indicate that such criteria are able to select me an impact parameters less than 2 fm. Bias effects introduced by the cr iteria have been evaluated. The centre-of-mass polar angle distributio ns of low energy clusters emitted in these central collisions, have be en extracted : the intensity ratio deduced for a transverse to longitu dinal emission is found to be R=1.4(-0.4)(+0.2). Model comparisons usi ng QMD are presented. The value of R appears to depend sensitively on the nucleon-nucleon cross section, sigma(nn). Within this model, a val ue of sigma(nn)=25+/-5 mb is derived.