Shape coexistence and electric monopole transitions inPt184

Abstract

Levels in 184 Pt were studied from the decay of mass-separated 184 Au by gamma-ray and conversionelectron spectroscopy and by gamma-ray angular distributions from low-temperature nuclear orientation. In addition to coexisting K*=0 + bands, coexisting A^ = 2 + bands are observed at low energy. Strong interband electric monopole transitions are observed between states of the two K Xsss 0* bands and also between states of the two K x -2 + bands. The coexistence of K Kmm 2 + bands at low energy represents a new type of collective structure. PACS numbers: 23.20.Lv, 23.20.En, 23.20.Nx, 27.70.+q Shape coexistence is widely manifested [1] in nuclei. A dramatic example occurs in the neutron-deficient Hg (Z -80) isotopes, where weakly deformed oblate and strongly deformed prolate shapes coexist at low energy [2,3]. We report details here of coexisting shapes at low energy in 184 Pt (Z=78) with a completely new feature: In addition to two bands of states with /*-0 + , 2 + , 4 + , 6 + (K Xss 0 + bands), we observe two bands of states with /* = 2 + , 3 + , 4 + (/T=2 + bands). These two K* = 2 + bands, which occur at low excitation energy, are connected by strong electric monopole (£0) transitions. The association of E0 transitions with shape coexistence is well recognized [4] but has previously been established only in the case of K x -0 + bands; here we report that E0 transitions are also associated with coexistence of K x=s 2^ bands.Excited states of 184 Pt were studied through the radioactive decay of mass-separated ,84 Au 01/2=53 s, 7=6) by using the UNISOR isotope separator operated on line [5] to the 25-MV folded tandem accelerator at the Holifield Heavy-Ion Research Facility (HHIRF). The 184 Au was produced through the ( I2 C,9«) reaction on a target of 181 Ta using a beam of 140-MeV 12 C ions. Gamma-ray and conversion-electron (ce) spectrum multiscaling and y-y~t, and y-x-t, y-ce-f, and ce-x-/ coincidence measurements were conducted on line. Conversion-electron spectra were taken with a 200 mm 2 x3 mm cooled Si (Li) detector. All assignments of 7-ray and internally converted transitions were made on the basis of coincidence information.In a separate experiment, angular distributions of the y rays were measured following the decay of 184 Au oriented at low temperatures. The nuclei were produced using the same reaction as the spectroscopy experiment. The activity was mass separated on line in the UNISOR separator and then transported to the UNISOR Nuclear Orientation Facility [6], a 3 He-4 He dilution refrigerator that is on line to the mass separator. The radioactive nuclei were implanted at 50 kV into a polarized Fe target in the refrigerator, which operates at a temperature below 7 mK with beam on target. The angular distributions of the y rays were observed with an array of six Ge detectors surrounding the target at a distance of about 10 cm.The 184 Au decay scheme is exceedingly complex; in total we have assigned more than 150 y transitions and have established 52 levels in l84 Pt. Here we discuss only the low-lyi...

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