Investigation of the decay out of superdeformed bands in 194 Hg by lifetime measurements A. Dewald, R. Ku ¨ hn, R. Peusquens, and P. von Brentano Institut fu ¨r Kernphysik, Universita ¨t Ko ¨ln, Ko ¨ln, Germany R. Kru ¨ cken,* M.A. Deleplanque, I. Y. Lee, R. M. Clark, P. Fallon, A. O. Macchiavelli, R. W. MacLeod, and F. S. Stephens Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 094720 T. L. Khoo and P. Reiter Physics Division, Argonne National Laboratory, Argonne, Illinois 60439 K. Hauschild Lawrence Livermore National Laboratory, Livermore, California 94550 Received 7 June 2001; published 12 October 2001 The lifetimes of low-lying states in the superdeformed SD bands of 194 Hg were measured by means of the recoil distance method using Gammasphere and the Cologne plunger device. The deduced transitional quad- rupole moments in all three bands were found to be constant within the experimental uncertainties and equal those extracted from Doppler-shift attenuation method measurements for the higher-lying states, confirming that the decay out does not strongly affect the structure of the SD bands. The experimental findings are used to discuss the different mechanisms proposed for the decay out of SD bands. DOI: 10.1103/PhysRevC.64.054309 PACS numbers: 21.10.Tg, 21.10.Re, 27.80.+w In recent years much experimental and theoretical work has been devoted to the interesting topic of the sudden decay out of superdeformed SD bands. Various theoretical con- cepts have been employed 1–9 to describe the mechanism involved. Besides 194 Pb and 152 Dy, the nucleus 194 Hg is the ideal candidate for the further investigation of the decay mechanism leading to an abrupt decay out of SD bands at low spins because only in these nuclei have direct linking transitions been observed so far 10–12,14–16. Thus for these three nuclei the absolute excitation energies, spins, and for some cases also the parities are known. In addition to these experimental data knowledge of the absolute transition probabilities for intraband transitions as well as for transi- tions depopulating the SD bands are important experimental observables for testing the different mechanisms proposed. In this article, we report on a measurement of the life- times of low-lying states in the SD bands in 194 Hg using the recoil distance method RDM. We have successfully deter- mined the lifetimes of the three lowest-lying states in bands SD1, SD2, and SD3, respectively. Using intraband intensities for the yrast SD band from an experiment with high statistics 17, transition quadrupole moments Q t were deduced di- rectly from these lifetimes. Comparison of these Q t values with those extracted from measurements 18,19 using the Doppler-shift attenuation method DSAM for transitions in the upper part of the bands shows that the deformation re- mains constant down to the bottom of the band and is not appreciably disturbed by the decay-out process. Superdeformed states of 194 Hg were populated using the reaction 150 Nd( 48 Ca,4n ) 194 Hg at a beam energy of 210 MeV. The beam was supplied by the 88-Inch Cyclotron at the Lawrence Berkeley National Laboratory. The plunger target consisted of an approximately 1 mg/cm 2 150 Nd layer evapo- rated onto a 1.47-mg/cm 2 -thick tantalum foil. The recoiling nuclei had a mean velocity of 1.861% c and were stopped in a 11 mg/cm 2 gold foil. Target and stopper were mounted in the Cologne plunger apparatus surrounded by 97 large- volume Compton-suppressed Ge detectors of the Gamma- sphere array 20. Coincidence events with at least four Compton-suppressed gamma rays were recorded onto mag- netic tape. In total, 72.810 9 unfolded triple events were recorded at nine target-to-stopper distances ranging from 8 to 103 m separation of the foils. The lifetimes of the low-lying states within the SD bands were determined using the differential decay curve method DDCM21,22. The germanium detectors at Gammasphere can be grouped into 17 rings whereby the detectors belong- ing to one ring have the same polar angle. Double-gated spectra with gates set on the shifted components of higher- lying SD transitions were produced for each distance and each detector ring. For the lifetime determination we could only analyze the ring spectra taken at the polar angles 17°, 35° combination of detectors positioned at 32° and 37°, 50°, 58°, 122°, 130°, 146° combination of detectors posi- tioned at 143° and 148°, and 163° as the diminishing Dop- pler shift, proportional to cos, made it impossible to dis- tinguish between shifted and unshifted peaks for the remaining rings, which were only used to set gates. Spectra taken at forward and backward angles were modified in such a way that the Doppler-shifted peaks came to the same posi- tion as those in the spectra taken at 17° and 163°, respec- tively, while the position of the unshifted peaks remained fixed. This can be achieved by splitting the spectra and in- serting the appropriate space between unshifted peak and Doppler-shifted peak to adjust for the polar angle. Thus we were able to add up all analyzed ring spectra for each dis- *Also atA.W. Wright Nuclear Structure Laboratory, Yale Univer- sity, New Haven, CT 06520. PHYSICAL REVIEW C, VOLUME 64, 054309 0556-2813/2001/645/0543096/$20.00 ©2001 The American Physical Society 64 054309-1