Session BH: Nuclear Structure I

High spin structure of the neutron-rich nucleus $^{139}$Cs

High spin excited states in the neutron-rich nucleus $^{139}$Cs were investigated from a study of the prompt $\gamma$ rays emitted in the spontaneous fission of $^{252}$Cf with the Gammasphere detector array. Ten new excited levels with eighteen new deexciting transitions were observed and the level scheme of $^{139}$Cs was extended up to 4670 keV. Spins and parities of levels in $^{139}$Cs were firmly assigned up to 25/2 $^+$ based on measurements of the angular correlations and an internal conversion coefficient. Shell model calculations were performed to interpret the experimental results. A good agreement between theory and experiment was found in the level energies and the mixing ratio of the 595.4 keV transition. This agreement shows the power of the shell model extended to this nucleus with five protons and two neutrons beyond the double magic $^{132}$Sn core. The present experiment has provided further evidence for the similarity of the spectroscopy of the $N=84$ isotones, which is clearly born out by our study.   

Application of the E-Gamma Over Spin (E-GOS) Method to Rare Earth Region Nuclei

The E-GOS (E-Gamma Over Spin) Method [1] provides a purely empirical way to determine the structure of a nucleus as a function of its angular momentum. Investigation of structure as a function of angular momentum complements existing comparisons of structural evolution as a function of nucleon number. No initial assumptions about a nuclide's structure are necessary in the E-GOS Method. Ratios of the experimental energies of gamma ray transitions between two levels divided by the spin of the originating levels can be calculated and plotted versus the spin of the originating levels. In this work, we plotted E-GOS curves using the data for the yrast bands of nuclei from Z = 52 to Z = 78. We then compare the E-GOS curves to the ideal limits of a perfect harmonic vibrator and an axially symmetric rotor. Known structural changes in this well-studied region are clearly observed in the E-GOS curves. \\[4pt] [1] P. H. Regan, \textit{et al}., Phys. Rev. Lett. 90, 152502 (2003).   

Gamma spectroscopy of $^{150}$Sm

Nuclei around N=90 are considered transitional nuclei, since they lie in a region between lighter nuclei which display vibrational-like spectra and heavier isotopes with a rotational-like structure.The structure of these transitional nuclei, and in particular the nature of low-lying excited states, still poses a challenge with many competing theoretical model used with varying degrees of success. Differentiating between the models requires more experimental data especially for non yrast collective structures.As part of a program studying nuclei near N=90, the N=88 nucleus $^{150}$Sm has been produced with a $^{149}$Nd($\alpha $,2n)$^{150}$Sm reaction at the Accelerator Laboratory of Jyv\"{a}skyl\"{a}, Finland, employing a 25 MeV $\alpha $-particle beam.Preliminary results focusing on the observed band structures will be presented.   

Shape coexistence with strong mixing in $^{152}$Sm

The transitional nucleus $^{152}$Sm has been described as a ``soft'' nucleus with low-lying $\beta$- and $\gamma$-vibrational states and with candidate multiple-phonon excitations. However, detailed experimental studies using multiple spectroscopic techniques reveal two extensive families of rotational bands that are remarkably similar. This result suggests that there are coexisting shapes which strongly mix. A prescription of two-state mixing calculations are presented which describe the experimental level energies of the ground-state and first excited ($0^+$ state) rotational bands, electric monopole transition rates, electric quadrupole matrix elements, and the isomer shift of the first excited $2^+$ state.   

Transition Strength Ratios in the Tetrahedral Candidate $^{156}$Dy

A new symmetry has been recently proposed where nuclei may stabilize in a tetrahedral (pyramid) shape. One of the consequences of this symmetry is that the transition strength, $B(E2)$, of the inband transitions should approach zero in the ideal case. Thus, one signal of this exotic shape would be a rotational band where the inband $E2$ transitions are extremely weak or nonexistent. Such bands exist in many of the lowest negative-parity bands in the $N\approx 90$ nuclei, which is also a predicted ``magic" region for tetrahedral symmetry. A Gammasphere experiment was performed to measure the $B(E2)/B(E1)$ ratios of such a negative-parity band in $^{156}$Dy. The results (which are consistent with the theory) will be presented, as well as a discussion of the proposed follow-up experiment to directly measure the $B(E2)$ rates.   

Multitude of bands in $^{156}$Dy

A Gammasphere measurement was performed on rotational bands in $^{156}$Dy using the $^{148}$Nd($^{12}$C,4n) reaction at 65 MeV with the Atlas accelerator at Argonne. The projectile was chosen to populate many bands at low to medium spins. We have added new transitions and new bands to the family of negative-parity structures in this N = 90 nucleus. The lowest lying bands in this region have generally been associated with octupole vibrational modes, converting to two-quasiparticle bands at moderate frequencies. There are deviations from this picture in $^{156}$Dy, due perhaps to the robustness of the octupole vibration through the first band crossing.   

E0 transitions in the deformed nucleus $^{158}$Dy

The physics governing large E0 strengths between low-lying collective $0^+$ states has recently been a topic of considerable debate. In the deformed limit, where data is scarce, the $sd$-Interacting Boson Model-1 unequivocally predicts large E0 strengths between the first excited collective $0^+$ state and the ground state. To test this prediction, and further explore the as of yet ill-understood structure of the $0_2^+$ state in rare earth nuclei, the deformed nucleus $^{158}$Dy was populated via electron capture decay from $^{158}$Er and $^{158}$Ho. Gamma-rays and internal conversion electrons were measured; internal conversion coefficients and B(E0)$/$B(E2) ratios were obtained. Preliminary results of this work will be presented. Work supported by US DOE grant number DE-FG02-91ER-40609 and The ANU.   

Lifetime measurements of the yrast band states of \textsuperscript{184}Pt and \textsuperscript{186}Pt

Recoil distance Doppler-shifted data was taken in a plunger experiment investigating \textsuperscript{184}Pt and \textsuperscript{186}Pt. Lifetime measurements were made of yrast states ranging in spin from 4\textsuperscript{+} to 12\textsuperscript{+} in those nuclei using the differential decay curve method. Reduced transition strengths were calculated. This data was examined in the context of the Interacting Boson Approximation (IBA) model which has been found to give good predictions for both energy levels and branching ratios for nuclei in this region. A strong agreement with IBA predictions was found in the B(E2)s for \textsuperscript{184}Pt though the results seen in \textsuperscript{186}Pt, where lifetimes had not previously been measured, could not be said to be in similar agreement and require further interpretation.   

Investigation of excited states in Pt isotopes: $^{188}$Pt, $^{192}$Pt, $^{194}$Pt

The Pt isotopic chain is located in a transitional region between well-deformed rare-earth and the spherical nuclei near the doubly-magic $^{208}$Pb. They are known to exhibit a range of interesting structural phenomena. Platinum isotopes, $^{188}$Pt, $^{192}$Pt and $^{194}$Pt, were investigated in different reactions. Excited states in $^{188}$Pt were populated in $\beta$-decay and studied through off-beam $\gamma$-ray spectroscopy. The energies and decay properties of the low-lying levels were measured. A (p, t) experiment was performed to study $^{192}$Pt and $^{194}$Pt. The (p, t) reactions are particularly sensitive to 0$^+$ excited states. The purpose of these studies is to obtain information which can help to discriminate between alternative structural interpretations. Results of this work will be presented. Research was supported by the U.S. DOE under Grant No. DE-FG02-91ER-40609, MLL, and DFG (C4-Gr894/2-3, Jo391/2-3).   

Nuclear photon scattering experiments by quasi-monochromatic, linearly polarized light sources

Magnetic dipole (M1) transitions in atomic nuclei have attracted increasing attention in nuclear physics and nuclear astrophysics. The knowledge of the M1 response allows one to elucidate the details of nuclear dynamics. It is also important for the estimate neutral current neutrino-nucleus cross sections for supernova explosion, because of the close relationship between the M1 excitation and neutrino-nucleus processes. Low-lying electromagnetic transitions can be studied by the method of nuclear resonance fluorescence (NRF) or photon scattering. Recently, it has been shown that quasi-monochromatic, linearly polarized photon beams from inverse laser Compton scattering has considerably increased experimental sensitivity and to enable one to detect the fine structure of relatively weak M1 transitions. In this report, results of the NRF measurements on $^{208}$Pb using a linearly polarized photon beam will be presented. The M1 resonance below the neutron separation energy is resolved into several individual transitions. The experimental results are compared with an estimation of self-consistent random phase approximation using a semi-realistic interaction.   

Compton Scattering on $^{209}Bi$ at HI$\gamma$S From $E_{\gamma}=11-30$ MeV

New data collected at the High Intensity $\gamma$-ray Source (HI$\gamma$S) were taken to investigate the Iso-Vector Giant Dipole (IVGDR) and Giant Quadrupole (IVGQR) Resonance Regions in $^{209}$Bi, $E_{\gamma}=11-30$ MeV. Linearly polarized $\vec{\gamma}$-rays were incident upon an isotopically pure ($>$99.9\%) $^{209}$Bi target and the scattered $\gamma$-rays were detected using the HI$\gamma$S NaI Detector Array (HINDA). HINDA is an array of large (10''x10'') core detectors surrounded by 3'' thick NaI shields that are segmented optically into 8 individual segments. This assembly was run in an anti-coincidence mode to reduce background and improve the resolution as well as a coincidence mode to isolate the first escape peak. During the 150 hour run, the nearly mono-energetic $\vec{\gamma}$-ray intensities were $1\times10^{5}-1\times10^{7}\gamma$'s/sec on target and statistical accuracies of 1-3\% were achieved. Preliminary analysis of angular distributions of cross sections and analyzing powers as well as absolute cross sections will be reported.   

Spectroscopy of the N=126 Nucleus $^{215}$Ac

The investigation of heavy nuclei becomes increasingly difficult because of the fact that cross sections for the production of fusion-evaporation residues decrease while the probabilities for background channels like fission increase dramatically. This is the reason for the lack of data on prompt gamma radiation of many heavy nuclei. Here we report on an experiment which used the gas-filled recoil separator SASSYER and recoil-decay tagging to study the semi-magic nucleus $^{215}$Ac in order to improve our understanding of the evolution of single-particles energies along the N=126 neutron shell. Results linking prompt gamma transitions with isomeric ones are presented and discussed in the framework of the shell model.