Estimates of the Nuclear Time Delay in Dissipative U + U and U + Cm Collisions Derived from the Shape of Positron andδ-Ray Spectra

Systematic studies of positron production in heavy-ion collisions near the Coulomb barrier

“…from ~30% at 5.7 MeV/u to ~ 25% at 6.2 MeV/u, as obtained from the U + U system at heavy-ion scattering angles around 45 ~ (lab.). This result, revealing that the probability for atomic positron creation increases even faster with the bombarding energy than the probability for nuclear positron emission, is in agreement with the general trend found by Backe et al [33] and by the TORI group in the beam-energy range (5.9-10) MeV/u [34]. In view of these experimental findings, the original theoretical proposal [44] that favoured bombarding energies slightly below the Coulomb barrier for an optimized comparison between atomic positrons and nuclear background was rather wrong.…”

“…Together with a series of recent studies carried out also at GSI by three independent groups, the ORANGE [8,[11][12][13][14][15][16], the EPOS [17,18,19,51 ], and the TORI collaborations [32][33][34][35], they largely establish the characteristics of positron creation in heavyion collisions predicted by theory. To accomplish it, the smooth positron background, resulting from internal pair decay of excited nuclear states having transition energies larger than 2 m o c 2, had to be determined experimentally as well by measuring the y-ray spectra in coincidence with the scattered heavy ions.…”

“…At present, no convincing explanation can incorporate these findings, despite considerable investigations in heavy-ion collisions [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] and in related Bhabha-scattering experiments [26][27][28][29] as well as theoretical [30,31] efforts in the last years. The TORI group could not confirm unambiguously this phenomenon [36], addressing itself mainly to studies at higher beam energies for which no structures are reported [33][34][35].…”

Systematics of spontaneous positron lines

“…These structures occur due to interference effects between the excitation amplitudes on the incoming and those on the outgoing trajectory, as would also been observed in timedelayed 6-electron emission spectra or in positron spectra of subcritical time-delayed collisions, AEosc=Zrch/T. (13) The fluctuations seem to be of numerical nature. From (10) we have learned that the energy-differential probability, dP/dE~+l .... in the centre of the spontaneous positron line will be proportional to T 2.…”

Conversion processes in superheavy and giant atoms

“…Moreover, it is by no means clear, whether there is one single sharp positron line at more or less the same energy in every system studied so far or whether there are more positron lines, appearing perhaps at different energetical and angular windows for the heavy ions. A third experimental group [21,22] measured positrons in coincidence with deep inelastic nuclear re-actions, for which conversion processes may yield a non-negligible contribution. The various experimental investigations and the difficulties encountered with the conventional scaling of the positron line with charge and radius of the giant nuclear complex represent the basic motivation for our theoretical studies of conversion processes in superheavy systems.…”