Volume 257, number 3,4 PHYSICS LETTERS B 28 March 1991 The collectivity and the de-excitation of the yrast superdeformed band in 15°Gd P. Fallon, A. Alderson, I. Ali, D.M. Cullen, P.D. Forsyth, M.A. Riley, J.W. Roberts, J.F. Sharpey-Schafer, P.J. Twin Oliver Lodge Laboratory, Universityof Liverpool, LiverpoolL69 3BX, UK M.A. Bentley and A.M. Bruce SERC DaresburyLaboratory, Warrington WA4 4AD, UK Received 18 October 1990 A Doppler shift attenuation measurement has been carried out to determine the collectivity of the superdeformed band in ~5°Gd. The data was found to be consistent with a constant inband quadrupole moment of 17 + 3 eb. This corresponds to a quadrupole deformation offl; ~ 0.58. In addition the measurement has resolved important questions regarding the de-excitation of the band, confirming the rapid de-excitation of the superdeformed band in ~S°Gd with more than 80% of the band intensity being lost over one transition. The observation of y-ray cascades with small and constant energy spacings can be understood in terms of collective rotational bands built upon superde- formed structures [ l ]. In the A = 150 mass region the large dynamic moments of inertia ,Y (21 suggest that these exotic shapes have quadrupole deformations of fl2~0.6 [2,3]. However such evidence is not conclu- sive as single-particle alignment effects [ 4,5 ] may also lead to large moments of inertia and a more satisfac- tory determination of the deformation must be made from a measurement of the nuclear quadrupole mo- ment. A technique to do this involves measuring the lifetimes of the nuclear states and relating the transi- tion probabilities to the quadrupole moment via a ro- tational model [ 6 ]. The quadrupole moment can then be associated with a deformation (e.g./?2). It has been predicted [7 ] that chains of superde- formed nuclei exist within a given mass region and that the deformation of these superdeformed nuclei decreases with decreasing N and Z; thus ~5°Gd is ex- pected to have a lower quadrupole deformation than ~s2 Dy [2]. Furthermore the yrast superdeformed band in ~5°Gd [8] has the largest variation in .~12) of all the superdeformed bands and a measurement of the quadrupole transition moment of the band would confirm that the variation in ,¢~2) is not accom- panied by a change in the deformation, in agreement with the superdeformed bands in both the A = 130 [ 9 ] and A = 190 [ 10 ] mass regions. In this letter we re- port on an experiment to determine the collectivity of the superdeformed band in 15°Gd using the Dop- pler shift attenuation method (DSAM). Superdeformed states in ~5°Gd were populated by the reaction 13°Te(26Mg, 6n)~5°Gd at a beam energy of 145 MeV. The beam was supplied by the Nuclear Structure Facility Tandem Van de Graaff at Dares- bury Laboratory and the target consisted of 1 mg/ cm 2 of tellurium on 5 mg/cm 2 of gold. The gold back- ing was of sufficient thickness to bring the recoils to rest. The y-decay was observed with the TESSA3 [ 11 ] multidetector spectrometer comprising of sixteen es- cape suppressed germanium counters and a fifty-ele- ment bismuth germanate (BGO) ball [ 12]. In total around 1.8 × 108 germanium-germanium-BGO ball coincidence events were recorded corresponding to events where twelve or more of the BGO ball ele- ments registered. The data were stored as Ev-Ev correlation matri- ces, where, since the Doppler shifted energy is depen- dent on the angle (0) between the recoil direction and 0370-2693/91/$ 03.50 © 1991 - Elsevier SciencePublishers B.V. ( North-Holland ) 269