Search for Element 120
The present experimental status of superheavy element research does not allow to fix the proton numbers where the "island of stability" is located. Different theoretical models predict the region of Z=114, 120 or 126. Since the available data do not clearly identify Z=114 as shell closure, we investigated as further candidate Z=120. Three different projectile-target combinations lead to the same compound nucleus 302120: 64Ni + 238U, 58Fe + 244Pu and 54Cr + 248Cm. The neutron number of the compound nucleus 302120 is N=182. This is only 2 neutrons below N=184 where the neutron shell closure is expected. Therefore, 302120 is closer to the N=184 shell than any other so far produced compound nucleus with lower Z.
The largest production rate for Z=120 is predicted for the most mass asymmetric projectile/target combination 54Cr + 248Cm. However, at SHIP no radioactive targets could be used up to now. Therefore, we took the reaction 64Ni + 238U. If the proton shell closure is at Z=120 it would be reflected in an enhanced production rate and half-live of the element 120. Depending on the magnitude of the stabilization due to the closed shell, one could expect up to a few events per week for the isotopes 299120 and 298120 produced in 64Ni + 238U reactions. The half-lives are expected on the order of some 10 μs.
We run the experiment 120 days, day and night but no decay chain of element 120 was found. With the total number of 2.6×1019 projectiles which impinged the target, it converts to an upper cross-section limit of 90 fb. From this result we can deduce, that the stability in the region around Z=120, N=184 is not exceptionally high with respect to the neighbouring regions.