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Electric Quadrupole Moments of the First Excited States of 194 Pt, 196 Pt

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Electric Quadrupole Moments of the First Excited States of 194 Pt, 196 Pt THE AUSTRALIAN NATIONAL* UNIVERSITY RESEARCH SCHOOL OF PHYSICAL SCIENCES ANU-P/948 April 1986 ELECTRIC QUADRUPOLE MOMENTS OF THE FIRST EXCITED STATES OF 19i+Pt, 296 Pt AND X 98Pt G.J. GYAPONG, R.H. SPEAR, M.T. ESAT and M.P. FEWELL Department of Nuclear Physics, Australian National University, Canberra, ACT 2600, Australia and A.M. BAXTER and S.M. BURNETT Department of Physics and Theoretical Physics, Faculty of Science Australian National University. INSTITUTE OF ADVANCED STUDIES ANU-P/948 April 1986 ELECTRIC QUADRUPOLE MOMENTS OF THE FIRST EXCITED STATES OF 194Pt, 196Pt AND 198Pt G. J. GYAPONG, R M. SPEAR, M.T. ESAT and M.P. FEWELL Department of Nuclear Physics, Australian National University, Canberra, ACT 2600, Australia. and A.M. BAXTER and S.M. BURNETT Department of Physics and Theoretical Physics, Faculty of Science, Australian National University, Canberra, ACT 2600, Australia. 12 16 Abstract: Coulomb excitation of iHi96,i98Pt by «He, C and 0 projectiles has + been used to determine the static electric quadrupole moments Q(2t ) of the first excited states of 194« 196« 198Pt, together with values of BfpliOf^f). It is clearly established that Q(2t+) is positive for each nucleus, having values of 0.48(14)eb and 0.66(12)eb for 194Pt and 196Pt, respectively, and 0.42(12)eb or 198 + 0.54(12)eb for Pt, depending on whether the interference term P4(22 ) is positive or negative. Results obtained for B(E2;0J+->2J+) are 1.661(1 l)e2b2, 2 2 2 2 1.382(6)e b and 1.090(7)e b for iHi96,i98Pt> respectively. The results are compared with the predictions of various nuclear models. NUCLEAR REACTIONS 194'196-198Pt(a,a'), E =» 14.0 - 15.6 MeV, 9 = 174.8°; 196Pt(a,a'), E = 16.8 - 18.6 MeV, 9 = 90.0°; i94,i96,i98pt(i2Cfi2C)> E = 41 0 . 45.0 MeV, 8 = 174.8°; I94,i96(i98pt(i60i60.)t E = 55 0 . 63 0 MeV) e = i74.g=; measured + Coulomb excitation probabilities of first 2 states. iM,i96,i98pt deduced + + + B(E2;01 ->21 ) and Q(2j ). Enriched targets. Accepted for publication in Nuclear Physics A 2 1. Introduction The so-called "transition region" near A=190 has, for more than a decade, been a remarkably active area for nuclear spectroscopic investigations. Theoretical and experimental studies of nuclei in this region have produced major advances in our understanding of nuclear strucrore. In 1968, Kumar and Baranger calculated1) that the nuclear shape changes from prolate to oblate in the A = 186-192 region. Their microscopic calculations involved the application of the pairing-plus-quadrupole model to Bohr's collective Hamiltonian. + In particular, they predicted that the static electric quadrupole moment Q(21 ) of the first excited state of even-even nuclei should change from negative (prolate charge distribution) to positive (oblate) in proceeding from the osmium isotopes to the platinum isotopes, and that it should remain positive for the mercury isotopes. They calculated2) that me prolate-oblate shape transition should occur at A = 192. Within a few years these predictions were experimentally verified by Saladin and his collaborators at Pittsburgh. By studying Coulomb excitation with various + projectiles they found that Q(2j ) is clearly negative for 184.186,188,19O,1920S ^.efs 3 4 4 9 19 - ) and positive for i' .i 6. 8pt (refs 5,6) Subsequently, Cline and collaborators at Rochester reported7) substantial confirmation of the Pittsburgh results; however, a detailed presentation of the Rochester data has not been published. In addition, Coulomb excitation measurements at Canberra8'10) and at Koln11) have shown that Q(V) is positive for »W«>.MV04Hg> The discovery of the A = 192 prolate-to-oblate transition has triggered a very large amount of experimental work on the spectroscopy of the Os, Pt and Hg isotopes. In parallel with this, an equally large amount of related theoretical work has been published. For example, the development of the interacting boson model (IBM) [ref. 12)] was greatly stimulated by the suggestion13) that the properties of the low-lying levels of 196Pt are in remarkably good agreement with those predicted by the 0(6) limit of the IBM, and in this context the transition region has been 3 interpreted14) in terms of a progression from the 0(6) ["/-unstable15)] limit for the heavier Pt isotopes toward the SU(3) limit (axially symmetric rotor) for the lighter Os isotopes. Other approaches to understanding the tr?nsition nuclei include interpretations in terms of boson expansion theory16,17), the rigid asymmetric-rotor model18,19), rotation-vibration models, both symmetric20) and asymmetric21), and other more complex geometric models [e.g. refs22"24)]. In all of these considerations, the value of Q(2j+) for the even-even nuclei involved plays a crucial role. For example, Q(2j+) = 0 in the strict 0(6) limit of the IBM. Therefore, the reported positive value of Q(2j+) for 196Pt [refs.5"7)] raises complications25) for the suggestion that this nucleus is an 0(6) paradigm. However, a recent redetermination of Q(2j+) for 194Pt by the Pittsburgh group26) has produced a value consistent with zero (0.13 ± 0.17 eb), in contrast to their earlier value of 0.64 ± 0.16 eb. They attribute the discrepancy to difficulties associated with impurity subtractions owing to the lower isotopic enrichments of the targets used in the earlier work. Their published target compositions6) suggest that those problems could haw, been at least as great for 196Pt and 198Pt, which raises doubts about their results for those nuclei, and, incidentally, about the nature of the prolate-oblate transition. Furthermore, there have been some difficulties26) in reconciling values of Q(2j+) obtained for some of the Cs and Pt isotopes using muonic X-ray techniques27*29) with those obtained from Coulomb-excitation data. It is therefore highly desirable to make new and independent measurements of Q(2j+) for the Pt isotopes. The present paper presents the results of such measurements for 194Pt, 196Pt and 198Pt. A brief report of the results for 196Pt has already been published30). 2. Experimental Procedure The use of Coulomb excitation to determine nuclear properties such as Q(2j+) + + and the reduced transition probability B(E2;01 -»21 ) has been comprehensively described in several review articles31'33). The basic experimental procedures used in 4 the present work have been described in previous publications from this laboratory [e.g. refs. 34"37)). Although two independent measurements of the excitation probability of the 2j+ + + + state usually suffice to determine Q(2j ) and B(E2;01 ->21 ), we have, in order to obtain substantial redundancy and increased confidence in the results, made measurements under four different experimental arrangements for 19oPt (backscattering of 4He, 12C and 160 and scattering of 4He at 90°) and three different arrangements for 154Pt and 198Pt (backscattering of 4He, 12C and 160). Charged-particle beams were obtained from the ANU MUD pelletron accelerator, the beam energy having been previously calibrated38) to better than 0.1%. Targets, which consisted of isotopically enriched Pt metal evaporated onto thin carbon foils, had thicknesses in the range 2-15 (ig cm"2. Isotopic compositions of the enriched Pt used are listed in table 1. Backscattered particles were detected with an annular silicon surface-barrier detector, the mean laboratory scattering angle being 174.8 ± 0.2°. Particles scattered at 90° were analysed using an Enge split-pole magnetic spectrometer with a position-sensitive multi-electrode proprotional counter at its focal plane39). Since the Coulomb-excitation probability varies rapidly with angle at 90°, the scattering angle must be measured accurately. This was done using the kinematic technique described by Kuehner et al.40) and resulted in a value of 90.0 ± 0.1°. 3. Analysis and Results 3.1 SPECTRUM ANALYSIS The experimentally determined Coulomb-excitation probability PMO of the first V~ state is defined as Thus for each spectrum obtained, the number of counts in each of the "elastic" (0j+) and "inelastic" (2,+) peaks must be determined. Representative spectra are shown in 5 figs. 1 and 2. As is normal in this type of experiment, the 2j+ peak sits on a tail extending down in energy from the much larger elastic peak. Peak areas were extracted using well-established procedures34,35*37), with the modification that x2 was 2 40 replaced by the log-likelihood function x i p of ref. ) as a measure of goodness of fit. The elastic-scattering peak was fitted with a lineshape consisting of a skewed gaussian plus one or more exponential functions tc represent the low-energy tail of the peak. This lineshape was used to estimate the magnitude of the elastic-peak tail underneath the inelastic peak. When analysing spectra from the Enge spectrometer, allowance was made for the variation of peak shape along the focal plane. Small contributions to the spectra from Pt isotopes other than the one of primary interest ("isotopic impurities") were accounted for using the supplier's assay (table 1) and B(E2) values from the literature25,42,43); these contributions are shown by the broken curves in figs. 1 and 2. Values obtained for P are listed in table 2. The bombarding energies (E) given are the values obtained after applying small corrections (ranging from 1 to 15 keV) for the effects of finite target thickness. Target thicknesses were determined from Rutherford-scattering measurements. The experimental uncertainties assigned to values of P arise from statistical uncertainties, and from uncertainties involved in estimating the background beneath the 2j+ peak and in correcting for isotopic impurities. 3.2 INVESTIGATION OF TARGET CONTAMINANTS Elastic scattering from target contaminants other than Pt isotopes could distort the results by contributing to the spectrum in the region of the Pt inelastic peaks.
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