A measurement of direct photon production in 208 Pb+ 208 Pb collisions at 158 A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5 < pT < 4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at pT > 1.5 GeV/c. The results constitute the first observation of direct photons in ultrarelativistic heavy-ion collisions which could be significant for diagnosis of quark gluon plasma formation. 25.75.+r,13.40.-f,24.90.+p 1

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Fission-fragment angular distributions in a number of heavy-ion-induced fusion reactions are explained on the basis that for composite systems with fission barrier heights comparable to their intrinsic temperatures, a fraction of the fission events take place before the fused system equilibrates fully in the K degree of freedom. A value of about 8 x 10 -21 s is deduced for the characteristic time for ^equilibration. PACS numbers: 25.70.Jj, 25.85.Ge In a number of recent measurements of the fragment angular distributions in heavy-ion-induced fusion-fission reactions, 1 "" 7 the data are not consistent with the predictions of the conventional theory 8,9 of fragment angular distributions in fission following compound-nucleus formation. Theoretical approaches different from the conventional theory have been proposed 10,11 to account for the discrepancy but no single quantitative theory exists at present to explain all the measured distributions. In this work, we present a model based on the suggestion that in heavyion-induced fusion reactions the observed fission events consist of an admixture of events of two types: (i) compound-nucleus fission (CNF), and (ii) noncompound-nucleus fission (NCNF) of a composite system which has equilibrated in all degrees of freedom except the K degree of freedom, where K is the projection of the angular momentum on the nuclear symmetry axis. Reaction mechanisms such as fast fission, 12 taking place for the case of composite systems with zero fission barriers, and quasifission, 13 taking place for composite systems with fission-barrier shapes more compact than the entrance-channel contact configuration, are cases of type (ii). Another class of NCNF events which has not been considered before includes those occurring in a time comparable to the characteristic relaxation time in the K degree of freedom when the fission-barrier heights become comparable to the temperature of the composite system.For the CNF events we use the conventional theory 8,9 with a small modification 14 to include also the dependence of transition-state shape on the quantum number K. For calculating the deformation and rotational energies, we have used the shape parametrization of Brack et al 15 and the liquid-drop-model parameters of Pauli and Ledergerber 16 which were arrived at by them to reproduce the systematics of the liquid-drop-model fission-barrier heights for actinide nuclei. A value of the radius parameter ro=1.22 fm was used to calculate the moments of inertia. For calculating the temperature, a value of the level density parameter a = A/7 was used. The use of another reasonable set of these parameters will result in only slightly different angular distributions (5%-10% change in anisotropy values) for this component. We have also made the reasonable assumption that all CNF events correspond to first-chance fission.There exists at present no theoretical formulation for calculating fragment angular distributions for type-(ii) events. However, in some recent experiments 3 involving very high spin and ...

The kinetic energy distributions have been measured at LOHENGRIN for symmetric fission of U TM and U 236. The pre-neutron emission values for the average total kinetic energy and its rms width have been deduced using a Monte Carlo simulation for neutron evaporation. The total kinetic energy dip between symmetric and asymmetric divisions is 24 MeV for U TM and 29 MeV for U 236. A strong peak in the initial rms width has been observed for masses 111-123 in U 234 and for 111-125 in U 236. A static scission point model is used to understand the origin of these features.