Bulletin of the American Physical Society
3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009; Waikoloa, Hawaii
Session EH: Nuclear Structure III |
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Chair: I-Yang Lee, Lawrence Berkeley National Laboratory Room: Kings 3 |
Friday, October 16, 2009 9:00AM - 9:15AM |
EH.00001: $\gamma$-ray spectroscopy of $^{213,214}$Th using SASSYER Tan Ahn, Andreas Heinz, Jing Qian, Ryan Winkler, Robert Casperson, Gabriela Ilie, David McCarthy, Axel Schmidt, J. Russell Terry, Elizabeth Williams Data for excited states have been scarce in the proton-rich region above $^{208}$Pb near the $N=126$ shell closure due to the low fusion cross section and high fission background present when producing these nuclei, but measurements of new excited states have helped elucidate the local single particle structure and can be a good test for various nuclear models. A recent measurement by Khuyagbaatar et al. (Eur. Jour. Phys. A 34 335 (2007)) of the first excited states of $^{213,214}$Th, including the discovery of an ($8^+$) isomer, helped shed light on the single particle structure in the region. These excited states have been identified through the observation of $\gamma$ rays coming from the decay of the $(8^+)$ isomeric state. An experiment to extend the known levels and transitions in these nuclides by measuring prompt $\gamma$ rays from $^{213,214}$Th has been performed at the WNSL at Yale University. The gas-filled separator SASSYER was used to identify $\gamma$ rays from the nuclei of interest by gating on evaporation residue recoils and their subsequent known alpha decays, a technique known as recoil-decay tagging. Current results from this experiment will be presented. [Preview Abstract] |
Friday, October 16, 2009 9:15AM - 9:30AM |
EH.00002: Investigation of the N=127 nucleus $^{217}$Th R. Chevrier, A. Heinz, J. Qian, T. Ahn, R. Casperson, G. Ilie, J.R. Terry, R. Winkler, E. Williams, D. McCarthy, L. Kucuk, G. Susoy The evolution of the N=126 neutron shell is interesting, since it can be compared to large-scale shell model calculations and the structure of heavy-proton-rich nuclei exhibit a number of interesting structural phenomena. The recoil decay tagging technique, which provides an extremely clean method for channel selection, is the key to the investigation of heavy nuclei near N=126. Here we report on results of an experiment performed with the gas-filled recoil separator SASSYER which attempted, for the first time, in-beam gamma-ray spectroscopy of the nucleus $^{217}$Th with one neutron above the 126-neutron shell. [Preview Abstract] |
Friday, October 16, 2009 9:30AM - 9:45AM |
EH.00003: Determination of the $^{229}$Th isomer half-life Jason Burke, Bret Beck, John Becker, Michael Haydell, Rick Norman, Nicholas Scielzo, Steven Sheets, Erik Swanberg Recently there has been renewed interest in studying the nuclear properties of the $^{229}$Th isomer. $^{229}$Th has the lowest known isomer at 7.6 eV [1]. Direct laser manipulation of the ground and first excited states could lead to the realization of the world's first nuclear clock. To understand the linewidth of the isomeric state we are conducting experiments to directly observe the half-life of the isomer decay. We use a novel ``hot-atom'' technique in which we catch the recoiling $^{229}$Th nuclei following the alpha decay of $^{233}$U $\Rightarrow \quad ^{229}$Th + $\alpha $. On average 2{\%} of the $^{229}$Th populate the isomeric 3/2+ state compared to the 5/2+ ground state. Recoils are collected on various catcher plate materials, rotated in vacuum in front of an einzel lense and multi-channel plate detector. The internal conversion electrons are counted as a function of time to determine the half-life. Varying the catcher plate material we can investigate the effect that the materials have on the half-life. Determination of the half-life we will provide valuable guidance to Th trapping research [2]. 1) Beck et al., PRL 98, 142501 (2007) 2) Campbell et al., PRL 102, 233004 (2009) This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Friday, October 16, 2009 9:45AM - 10:00AM |
EH.00004: Multiple Octupole-Band Structures in $^{238}$U Shaofei Zhu, R.V.F. Janssens, M.P. Carpenter, T.L. Khoo, F.G. Knodev, T. Lauritsen, C.J. Lister, D. Seweryniak An experiment with a $^{207}$Pb beam (1400 MeV) has been carried out on a thick $^{238}$U target at Gammasphere. The level scheme of the $^{238}$U has been extended significantly. The signature-partner bands of the known K=1 and K=2 octople bands were uncovered for the first time, in addition to another newly observed postive-parity band. This band decays to all the K=0, 1 and 2 octuple bands with an intensity much stronger than that observed for the deexcitation to the groundstate band. Its most important features can be related to a double octuple phonon excitation. The comparison between this band and a similar one in $^{240}$Pu [1] sheds more light on the recently proposed concept of octuple phonon condensation [2].\\[4pt] [1] X. Wang et al., Phys. Rev. Lett. 102, 122501(2009).\\[0pt] [2] S. Frauendorf, Phys. Rev. C 77, 021304(R)(2008) [Preview Abstract] |
Friday, October 16, 2009 10:00AM - 10:15AM |
EH.00005: Observation of new neutron-rich micro-second isomers among fission products of $^{238}$U at $345$~MeV/u Daisuke Kameda, Taro Nakao, Toshiyuki Kubo, Tetsuya Ohnishi, Hiroyuki Takeda, Naoki Fukuda, Kensuke Kusaka, Atsushi Yoshida, Koichi Yoshida, Masao Ohtake, Naohito Inabe, Yoshiyuki Yanagisawa, Kanenobu Tanaka, Yasuyuki Gono In the production of the radioactive isotope (RI) beam using projectile fragment separators, $\gamma$ rays emitted from metastable states, isomers, of the reaction products can be used as a fingerprint of the isotope that is analyzed in the separator. In the operation of the superconducting in-flight RI beam separator BigRIPS [1] at RIKEN RI Beam Factory, the detection of such $\gamma$ rays plays important roles not only in the identification of the RI beam [2] but also in searching for new isomers, providing valuable spectroscopic information on the isotopes. In the recent new-isotope production experiment with BigRIPS using in-flight fission of $345 $~MeV/nucleon $^{238}$U at the beam intensity around $0.3$~pnA, we have observed a number of short-lived isomeric decays which include unknown decays from more than $10$ isotopes, using three clover-type Ge detectors with a beam stopper of aluminum. The primal result of the observations will be reported. [1] T. Kubo: Nucl. Instr. and Meth. B {\bf 204} (2003) 97. [2] T. Ohnishi et al.: J. Phys. Soc. Japan, {\bf 77} (2008) 083201. [Preview Abstract] |
Friday, October 16, 2009 10:15AM - 10:30AM |
EH.00006: $^{238}$U shape isomer population with a neutron beam J.A. Caggiano, J.J. Ressler, P. Peplowski, H. Weller Actinide nuclei are expected to have isomeric nuclear states that are significantly. These isomers are called shape isomers, or ``fission'' isomers, because of the preference of the higher-Z isomers to decay via fission. Shape isomers can decay by gamma-ray emission or fission and have been identified in some of the thorium, uranium and the trans-uranic nuclei. Semi-empirical studies of these shape isomers indicate that the lower-Z actinides such as uranium and neptunium may preferentially decay via gamma-ray emission. $^{236}$U and $^{238}$U are known examples. The $^{238}$U shape isomer population via neutron bombardment has been measured in the energy range of approximately 2-6 MeV and at 14.5 MeV (using a d-t neutron generator), but no measurements have been reported between 6 and 14.5 MeV. In previous measurements, fission fragments were used to characterize the population instead of the (dominant) gamma ray branch. Thus, we performed an experiment to investigate shape isomer population using a neutron beam in the energy range of 5-15 MeV to look for the characteristic shape isomer gamma-rays using the neutron irradiation facility at TUNL. Results of our measurements will be presented. [Preview Abstract] |
Friday, October 16, 2009 10:30AM - 10:45AM |
EH.00007: Recent Experimental Searches for Fission Isomers in U and Np Isotopes P.N. Peplowski, J.A. Caggiano, C.F. Francy, D.V. Jordan, J.J. Ressler, G.A. Warren Nuclei in the actinide mass region are subject to high deformations, which can produce a second minimum in the potential energy of the nucleus. Excited states within this second potential minimum are inhibited from decaying to the ground state and also have a small probability to decay via alpha emission, resulting in unusually long decay half-lives. These states are known as fission isomers, and despite the interest in these states they are largely unidentified for U and Np isotopes. An experimental search for these fission isomers was carried out using light ion reactions and neutrons on actinide targets. Gamma rays were detected in a high-purity germanium detector array surrounding the actinide target within varying time windows which were chosen based on theoretical predictions for the lifetimes of fission isomers of interest. Results from these experiments will be presented. [Preview Abstract] |
Friday, October 16, 2009 10:45AM - 11:00AM |
EH.00008: Spectroscopy of Short-Lived Fission Fragment Isomers J.J. Ressler, C.F. Francy, J.A. Caggiano, D.V. Jordan, P. Peplowski, G.A. Warren Fission is a well-known mechanism to populate excited states in neutron-rich isotopes. Fission is also an effective tool for studying isomeric decays, as they are often well-populated and relatively easy to detect. Isomeric states with half-lives in the $\sim $5 -- 100 $\mu $s range were examined using $^{6}$Li-induced fission on $^{232}$Th. A 45-MeV $^{6}$Li beam from the 88-Inch Cyclotron of LBNL was alternately blocked to provide beam on/off periods to populate and observe the isomeric decays. Fission fragments were tagged using a thin Si detector near the $^{232}$Th target, and coincident gamma rays were detected using six clover and one LEPS HPGe detectors of the LiBerACE array. Several isomers were identified in the A$\sim $95 and A$\sim $140 mass regions, as expected. Numerous isomers were also observed near A$\sim $120, due to the significant contribution from symmetric fission. Characteristics of the induced fission, with observed isomer populations and decays, will be discussed. [Preview Abstract] |
Friday, October 16, 2009 11:00AM - 11:15AM |
EH.00009: Rotational alignments in $^{235}$Np Aaron Hurst The $j_{15/2}$ neutron-orbital alignment has been studied by exploring $\gamma$- ray transitions of yrast states in $^{235}$Np, populated utilizing the nucleon-transfer reaction $^{237}$Np($^{116}$Sn,$^{118}$Sn). The experiment was carried out at Argonne National Laboratory using the {\small GAMMASPHERE/CHICO} setup. The ground-state band for this nucleus has been delineated to high spin for the first time, with the $\alpha = +1/2$ and $\alpha = -1/2$ signature partners reaching spin- state population levels of $49/2^{+}$~$\hbar$ and $47/2^{+}$~$\hbar$, respectively. Definite assignments for these intraband transitions were established through $\gamma$-ray cross correlations between $^{235}$Np and $^{118}$Sn and events where at least three $\gamma$ rays corresponding to Np-like particles were detected. These transitions reveal clear upbends in the aligned-angular momentum and kinematic moment of inertia plots; this observation is indicative of a strong interaction between an aligned $s$-band and the $g$-band in $^{235}$Np. The role of the $\nu j_{15/2}$, $\pi h_{9/2}$, and $\pi i_{13/2}$ alignment mechanisms in the deformed U$-$Pu region will be discussed in light of the current spectroscopic data and in the framework of the CSM. [Preview Abstract] |
Friday, October 16, 2009 11:15AM - 11:30AM |
EH.00010: Search for a new element Z=117 among the $^{249}$Bk + $^{48}$Ca reaction products K. Rykaczewski, Yu Oganessian, S. Dmitriev, V. Utyonkov et al, J. Hamilton, A. Ramayya, R. Henderson, K. Moody, D. Shaughnessy, M. Stoyer et al, J. Roberto et al, M. Ryabinin et al Following irradiation of Am and Cm seed isotopes at the ORNL High Flux Isotope Reactor, the $^{249}$Bk activity (T$_{1/2} $=320 d) has been separated at the ORNL Radiochemical Engineering Development Center [1] to be used as target material for a search for element Z=117 using an intense $^{48} $Ca beam from the U-400 cyclotron at JINR Dubna. The targets will be made from Bk nitrate at the IAR Dmitrovgrad. The search for the A=294 and A=293 isotopes of the new element Z=117 and their decay products, see [2], is scheduled to begin at the Dubna Gas Filled Recoil Separator at JINR Flerov Laboratory of Nuclear Reactions in August 2009. Details of the $^{249}$Bk separation and experiment will be reported. [1] C.Alexander, P.Bailey, J.Ezold, M.Ferren, C.Porter, F.Riley et al., HFIR/REDC campaign 74, 2009. [2] Yu. Oganessian, J.Phys.G 34, R164,2007. [Preview Abstract] |
Friday, October 16, 2009 11:30AM - 11:45AM |
EH.00011: Alpha-gamma coincidence spectroscopy of $^{259}$Rf using a mixed Cf target Masato Asai, Kazuaki Tsukada, Yoshitaka Kasamatsu, Tetsuya K. Sato, Atsushi Toyoshima, Yasuo Ishii, Ryuta Takahashi, Yuichiro Nagame, Tetsuro Ishii, Ichiro Nishinaka, Daiya Kaji, Kouji Morimoto, Yasuaki Kojima Gamma rays following the $\alpha$ decay of $^{259}$Rf have been observed for the first time by means of $\alpha$-$\gamma$ coincidence spectroscopy. $^{259}$Rf was produced via the $^ {251}$Cf($^{12}$C,4n)$^{259}$Rf reaction at the JAEA tandem accelerator. The target consists of 63\% $^{249}$Cf, 12\% $^ {250}$Cf, and 25\% $^{251}$Cf with a thickness of 420~$\mu$g/cm$^{2}$. Reaction products were transported with a He/KCl aerosol jet into a rotating wheel $\alpha$-$\gamma$ detection system equipped with two sets of two Si detectors and two Ge detectors. Two $\gamma$ lines were observed at 97.3 and 146.7~keV in coincidence with the 8770~keV $\alpha$ transition of $^{259}$Rf. In addition, a few $\gamma$ events appeared at $\sim$125~keV. The energy differences and intensities of these $\gamma$ transitions, which are very similar to those in the $\alpha$ decay of $^{257}$No, allow us to assign the 3/2$^{+} $[622] configuration to the 146.7~keV level in $^{255}$No as well as to the ground state of $^{259}$Rf. This result indicates that the order of neutron orbitals should be inverted between $^{255}$Fm and $^{257}$No in $N=155$ isotones. [Preview Abstract] |
Friday, October 16, 2009 11:45AM - 12:00PM |
EH.00012: Decay Properties of $^{266}$Bh and $^{262}$Db Produced in the $^{248}$Cm + $^{23}$Na Reaction Kosuke Morita, Kouji Morimoto, Daiya Kaji, Hiromitsu Haba, Kazutaka Ozeki, Yuki Kudou, Nozomi Sato, Akira Yoneda, Takatoshi Ichikawa, Kenji Katori, Atsushi Yoshida, Sin-ichi Goto, Hisaaki Kudo, Takayuki Sumita, Eiji Ideguchi, Yoshitaka Kasamatsu, Hiroyuki Koura, Kazukaki Tsukada, Yasuyuki Fujimori, Fuyuki Tokanai, Yukiko Komori, Kazuhiro Ooe, Akira Ozawa, Takayuki Yamaguchi Decay properties of an isotope $^{266}$Bh and its daughter nucleus $^{262}$Db produced by the $^{248}$Cm($^{23}$Na, 5 \textit{n}) reaction were studied by using a gas-filled recoil separator coupled with a position-sensitive semiconductor detector. $^{266}$Bh was clearly identified from the correlation of the known nuclide, $^{262}$Db. The obtained decay properties of $^{266}$Bh and $^{262}$Db are consistent with those observed in the $^{278}$113 chain, which provided further confirmation of the discovery of $^{278}$113. [Preview Abstract] |
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