Test of A I = 2 Staggering in the superdeformed bands of Ig 4 Hg

Superdeformed (SD) nuclei are some of the best quantum rotors known. Their characteristic long sequences of equally spaced transition energies provide a unique opportunity to search for unexpected effects on an energy scale rarely achieved elsewhere in nuclear physics. In this context the recent observation of a regular staggering pattern of the transition energies in the yrast SD band in 14'Gd [l], where states differing by four units of angular momentum show a similar energy shift of about 60 eV relative to a (smooth) rotational sequence, is particularly intriguing. Evidence for similar effects has been reported in '%Hg [2], 14'Gd [3], Ig2Tl [4] and in some Ce nuclei [5]. These observations have triggered an intense theoretical effort to understand this phenomenon [6, 7,8, 9, 10, 111. Some discussions connect this effect with the presence of a C4 symmetry of the nuclear Hamiltonian [S, 7, 81. Other studies [9, 10, 111 argue that the measured energy differences could be related to band-crossings. Since the observed energy shifts are only of the order of lOOeV or less it is essential to confirm the reported effects by new measurements with higher statistics utilizing the currently available detector arrays, like Gammasphere [12] and Eurogam I1 [13]. In this contribution we report results from a new experiment on the known SD bands in Ig4Hg[14, 151 investigating the previously reported[2] staggering in those bands. The very high statistics obtained in this experiment has made possible the determination of the relative transition energies for these bands with a precision of 60eV or better for most transitions. No extended regular AI = 2 staggering was found in the three SD bands of lg4Hg and the new results do not confirm those previously reported in Ref. 121. However, deviations of the y-ray energies from a smooth reference have been established for the two excited SD bands in lg4Hg. Superdeformed states in lg4Hg were populated in the reaction '50Nd(48Ca,4n) using a 201 MeV 48Ca beam provided by the 88-inch Cyclotron of the Lawrence Berkeley National Laboratory, The emitted y rays were detected by the Gammasphere array which at the time of the experiment consisted of 70 Compton-suppressed Ge detectors. A stack of two 500,ug/cm2 thick Nd targets was used with both sides of each target foil covered with a thin layer of gold (450pg/cm2 facing the beam and 220pg/cm2 on the other side). A total of 1.4 x lo9 coincidence events with fold 2 4 were recorded on magnetic tape. A gain of 0.125 keV per ADC channel was chosen in order to achieve a high resolution. The effects we were looking for are only of the order of lOOeV and consequently the analysis plays an important role. Therefore we give hereafter a brief description of the analysis performed