לוח שנה משולב

Scientifically based attempts to synthesize new elements beyond uranium started in the middle of the 1930's, when the atomic model was established and the constituents of the atomic nucleus, protons and neutrons, were known. In this talk I will present a short history from early searches for ‘trans-uraniums’ up to the production and safe identification of shell-stabilized ‘Super-Heavy Nu-clei (SHN)’. The nuclear shell model reveals that these nuclei should be located in a region with closed shells for the protons at Z = 114, 120 or 126 and for the neutrons at N = 184. The outstanding aim of experimental investigations is the exploration of this region of spherical SHN. Experimental methods are described, which allowed for the identification of these nuclei. Systematic studies of heavy ion reactions for the synthesis of SHN revealed production cross-sections which reached values down to one picobarn and even below for the heaviest species. The systematics of measured cross-sections can be understood only on the basis of relatively high fission barriers as predicted for nuclei in and around the island of SHN. A key role in answering some of the open questions plays the synthesis of isotopes of element 120. Attempts aiming for synthesizing this element at the velocity filter SHIP will be reported. Finally, plans are presented for the further development of the experimental set-ups. Efforts performed at various laboratories will eventually reveal the change of shell strength as function of proton and neutron number, the location of the most stable nuclei and how long their lifetime will be, the optimum method of their production, and, possibly, the existence of nucleonic arrangements and shapes, which are presently still objects of speculation.

I will report on some recent computational modeling work on the Macaque visual cortex. My co-authors Bob Shapley, Logan Chariker and I have constructed a semi-realistic model of LGN-to-4Ca, the input layer to V1 in the magnocellular pathway. As with most modeling work, our aim was to understand how cortex responds to stimuli. To do that, many authors have postulated transducer functions for specific sets of stimuli. We have chosen to take a fundamentally different route: we have chosen to simulate how cortex works, by simulating cortical dynamics on the level of neuron-to-neuron interactions. Using a single network model, we have been able to reproduce as emergent phenomena a fairly comprehensive set of experimental observations, including orientation selectivity, simple and complex cells, gamma rhythms etc. Specific aims of this project were (1) to reconcile the picture of Hubel & Wiesel with the sparseness of LGN, (2) to address the extent to which cortex is driven by feedforward vs recurrent inputs, (3) to replicate and explain the diversity of neuronal responses seen in real cortex, and (4) to connect all of the above to dynamical interactions in local neuronal populations.