No abstract

We report the first observation of the doubly-magic nucleus t~176 This isotope was producedby nuclear fragmentation of t~4 Xe projectiles at 1095 A.MeV using the heavy-ion synchrotron SIS at GSI, Darmstadt. The projectile fragments were separated in flight with the projectile-fragment separator FRS and identified by measuring event by event the magnetic rigidity, the time of flight and the energy deposition.R.Schneider et al.

Abstract:In fusion reactions of 58Ni beams with 54Fe and 58Ni target nuclei neutron deficient evaporation residues have been produced and separated from the beam in an electrostatic deflector with large acceptance. The recoil nuclei were implanted and detected in an array of 100 PIN photodiodes, where also their particle decays were recorded. The assignment of the 0.81 MeV proton line to the decay of 1~ could be experimentally proven by the observation, that it is followed by the 3.31 MeV alpha emission from 1~A proton decay from llYCs was not observed. The decay energies of a number of known c~ emitters have been measured with better precision.

We present new results obtained from a series of follow-up e + e − -coincidence measurements in heavy-ion collisions, utilizing an improved experimental set-up at the double-Orange β-spectrometer of GSI. The collision system 238 U + 181 Ta was reinvestigated in three independent runs at beam energies in the range (6.0−6.4)×A MeV and different target thicknesses, with the objective to reproduce a narrow sum-energy e + e − -line at ∼635 keV observed previously in this collision system. At improved statistical accuracy, the line could not be found in these new data. For the "fission" scenario, an upper limit (1σ) on its production probability per collision of 1.3×10 −8 can be set which has to be compared to the previously reported value of [4.9±0.8(stat.) ± 1.0(syst.)]×10 −7 . Based on the new results, a reanalysis of the old data shows that the continuous part of the spectrum at the line position is significantly higher than previously assumed, thus reducing the production probability of the line by a factor of two and its statistical significance to ≤3.4σ.Previous e + e − -coincidence measurements in heavy-ion collisions at the Coulomb barrier, performed at the UNILAC accelerator of GSI by the EPOS and ORANGE collaborations, have shown narrow e + e − -sum-energy lines with energies in the range 550-810 keV [1][2][3][4]. Their features were found to be complex, and did not fit into any conventional atomic and/or nuclear production process. In particular, the speculation that a hitherto unknown neutral particle (mass∼1.8 MeV/c 2 ), decaying into e + e − pairs, might be involved [1], has conclusively been ruled out by subsequent Bhabha-scattering experiments [5]. Thus, the origin of this phenomenon has remained a puzzle, without a satisfactory explanation until now.On the other hand, all the data reported previously were incomplete as far as a systematic dependence on the collision parameters and the lepton emission scenario is concerned, and were suffering from limited statistical significance. Even more, a consistent description of the line characteristics (i.e., energies, cross sections) could not be achieved by comparing the results of both groups. For instance, the cross sections of the lines reported by us for 238 U + 238 U and 238 U + 208 Pb collisions [2,4] were found to be an order of magnitude smaller than those quoted by the EPOS collaboration [1,3] and, in particular, a line at ∼760 keV, initially reported by EPOS for 238 U + 232 Th collisions [1], has not been observed in our experiments. From our investigations, the most distinct evidence exists for a sum-energy line at ∼635 keV observed in the collision system 238 U + 181 Ta by using a beam energy of 6.3×A MeV and 1000 µg/cm 2 thick 181 Ta target [4]. The line was seen with the so far highest statistical significance (6.5σ) by selecting e + e − -pairs in coincidence with two heavy ions (HI), whose kinematics is consistent with fission of the U after the collision. However, the opening-angle distribution of the e + e − -pairs associated with this line, ...