Session X14: Nuclear Structure: Heavy Nuclei

Transition rates of high spin bands in $^{136}$Nd

Electromagnetic transition probabilities have been measured for the transitions in the two multi-quasiparticle rotational bands in the nucleus $^{136}$Nd. Lifetimes were obtained in a DSAM measurement at Gammasphere, using the $^{100}$Mo($^{40}$Ar, 4n)$^{136}$Nd reaction. The measurements are compared with new results of TAC and RPA calculations. The bands are identified as being built on two different quasiparticle configurations, with very different transition rates. These results contradict the speculation of a chiral-band pair [1] in this even-even nucleus. [1] E. Mergel \textit{et al.}, Eur. Phys. J. A \textbf{15}, 417 (2002).   

SU(3)-Basis Description of the Alhassid-Whelan Arc of Regularity

More than a decade ago Alhassid and Whelan identified an interior path connecting the U(5) and SU(3) vertices of the Casten symmetry triangle which unlike most of the rest of the interior does not exhibit chaos but rather preserves regularity. Recently, 12 nuclei whose parameters lie along this regularity were found. They all exhibit an almost one-to-one correspondence between the gamma band head and the K=02+ band head. If wave functions of the nuclei on the arc of regularity are complicated when expressed in a U(5) basis, the basis in which most of the IBA calculations are done, they may be easier to work with in a SU(3) basis. Based on the SU(3) description of the wave functions, we found a couple of different degeneracies taking place around the Arc. Although the degeneracy condition disappears rapidly as one goes away from the Arc, the wave functions change much more slowly. The existance of these degeneracies are leading us to the conclusion that there may be some underlying quantum number(s) that may (approximately) be valid in the regular region.   

Decay pathways and rotational properties of strongly deformed bands in $^{168}$Hf

Three strongly deformed bands were observed previously in $^{168} $Hf [1] and proposed as candidates of triaxial strongly deformed (TSD) bands. However, none of the bands was linked to known levels. Without the knowledge of level spins, parities, and excitation energies, it was difficult to gain a clear understanding of these bands. We have performed an extensive spectroscopic analysis for the $\gamma$-ray coincidence data obtained from a Gammasphere experiment at ANL. The decay pathways of TSD2 band to low-spin structures have been established, and the approximate spin values of levels in TSD1 band obtained. A detailed comparison of experimental properties of these bands and theoretical calculations, as well as the intrinsic configurations of the bands will be discussed. Work supported by U.S. DOE grant DE-FG02-95ER40939. \newline \noindent [1] H. Amro et al., Physics Letters B 506 (2001) 39-44.   

Study of $^{171}$Hf at High Rotional Frequency

High-spin properties of the nucleus $^{171}$Hf were studied through the fusion evaporation reaction $^{48}$Ca($^{128}$Te,5n)$^{171}$Hf at a beam energy of 209 MeV at ANL. Decay gamma rays were measured with Gammasphere detector array. Previously known [1] rotational bands were extended to considerably higher spins. Six new bands were established. One of them was identified as a prolate band with a deformation enhanced than others, with an intrinsic configuration of $\pi (i_ {13/2} h_{9/2}) \nu(h_{9/2})$. The proton alignments were observed at rotational frequency $\hbar\omega \sim$ 0.5 MeV in several bands. The intrinsic configurations and band crossings for other bands will also be discussed based on comparisions of their properties with cranked shell model calculations. Work supported by U.S. DOE grant DE-FG02-95ER40939. \newline \noindent [1] D. M. Cullen $\it {et al.}$, Nucl. Phys. A673, 3 (2000).   

Level and life-time studies in odd-odd $^{202,204}$Tl

The $^{203,205}$Tl($n,2n\gamma$) reactions were used to study excited states in odd-odd $^{202,204}$Tl isotopes. The data were taken using the GEANIE spectrometer, a Compton-suppressed array of 26 Ge detectors. The pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided neutrons in the energy range from 0.6 to 250~MeV. The time-of-flight technique was used to determine the incident neutron energies. Partial $\gamma$-ray cross sections were measured from the beam-on data while half-lives of isomers were determined from the beam-off data (typically, the half-lifes that can be currently measured with GEANIE vary between a few $\mu$s to a few ms). The level schemes of $^{202,204}$Tl have been considerably enriched and extensive similarities observed between the two level schemes are discussed. The previously reported first excited state in $^{202}$Tl has been decomposed into two close-lying states. In $^{204}$Tl, the structure above the previously known 7$^{+}$ isomer (from the $\pi s_{1/2} \nu i_{13/2}$ configuration) at 1104-keV excitation energy has been established for the first time up to $E_{x} \sim$2.3~MeV, while, at higher excitation energies, a lower limit for the excitation energy of the $\pi h_{11/2} \nu i_{13/2}$ configuration is proposed. The half-lives of the 7$^{+}$ isomers of $^{202,204}$Tl were measured with more precise results than the values adopted in the literature. This work was supported by the U.S. Department of Energy under Contracts No. DE-AC52-06NA25396 (LANL) and DE-AC52-07NA27344 (LLNL).   

Shears bands in $^{204}$At and $^{206}$Fr?

Excited states above the (10$^{-})$ isomers in the N = 119 isotones $^{204}$At and $^{206}$Fr have been observed for the first time. The experimental setup consisted of Gammasphere and the evaporation-residue detector Hercules. A beam-target combination of $^{30}$Si + $^{181}$Ta (E$_{lab}$ =152 MeV) was utilized for 20 hours in order to obtain these data (via the $\alpha $3n and 5n channels, respectively). Both nuclides display a sequence of transitions with $\gamma $-ray energies between 130 and 300 keV. These structures are similar to the shears bands observed primarily in Pb nuclei [1]; however, the dipole character of the $\gamma $-rays must be verified. Alignment plots for the band structures indicate that a crossing occurs at a relatively low frequency of $\omega \quad \approx $ 0.23 MeV. This is in contrast with the shears bands in the N = 119 Bi and Pb nuclei where no crossings are observed, and in N $<$ 119 Pb nuclei where crossings are normally found at $\omega \quad \approx $ 0.30 MeV. Possible interpretations of the nature of this crossing will be discussed. [1] R.M. Clark and A.O. Macchiavelli, Annu. Rev. Nucl. Part. Sci. 501, 1 (2001).   

Non-yrast states in $^{221}$Th

The nucleus $^{221}$Th has been studied, using the $^{207}$Pb($^{18}$O,4n) E$_{lab}$=96 MeV fusion-evaporation reaction and the Gammasphere + HERCULES detector combination. The $^{18}$O beam, provided by the ATLAS accelerator, had a 247.5-ns pulse structure. Based on evaporation-residue selected $\gamma $--ray coincidence data, the previously reported [1] octupole band (yrast) has been extended to higher spin, but also the non-yrast structure of $^{221}$Th has been delineated. The latter includes another octupole-type band. A comparison with the neighboring nuclei $^{223}$Th and $^{219}$Ra will be presented. [1] M. Dahlinger et al., Nucl. Phys. A 484, 337 (1988).   

Investigation of K-isomers in $^{255}$Lr and $^{256}$Rf

Recently, K-isomers have been observed in very heavy nuclei around A$\sim $250 ($^{252,254}$No). The investigation of the decay of K-isomer states in the near super heavy nuclei gives very valuable information on the ordering of single particle orbitals in these nuclei. I would like to discuss our recent results on K-isomers in $^{256}$Rf and $^{255}$Lr. The experiments were performed at the Lawrence Berkeley National Laboratory's 88-Inch Cyclotron and the decay of the isomers were studied at the focal plane of the Berkeley Gas-filled Separator (BGS). The nuclei of interest were produced via the $^{208}$Pb($^{50}$Ti, $2n)^{256}$Rf and $^{209}$Bi($^{48}$Ca, $2n)^{255}$Lr reactions.   

Search for Dipole States in $^{235,238}$U

There is considerable interest in isotope-specific material identification. The presence of a particular isotope can be inferred by observing deexcitations of nuclear levels of $\gamma$-ray transitions characteristic for the isotope of interest using nuclear resonance fluorescence techniques. These high energy $\gamma$-transitions would penetrate protective shielding, thus acting as an identifier of hidden nuclear materials. Nearly monoenergetic, high-intensity and 100\% polarized $\gamma$-ray beam from the HI$\gamma$S facility was used to search for low-spin states in $^{235,238}$U at excitation energies between 3 and 5 MeV. The resulting data on the distribution of dipole strength below particle emission threshold will be presented.