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Journal Articles

Stability and synthesis of superheavy elements; Fighting the battle against fission - Example of $$^{254}$$No

Lopez-Martens, A.*; Henning, G.*; Khoo, T. L.*; Seweryniak, D.*; Alcorta, M.*; Asai, Masato; Back, B. B.*; Bertone, P. F.*; Boilley, D.*; Carpenter, M. P.*; et al.

EPJ Web of Conferences, 131, p.03001_1 - 03001_6, 2016/12

 Times Cited Count:1 Percentile:47.47

Fission barrier height and its angular-momentum dependence have been measured for the first time in the nucleus with the atomic number greater than 100. The entry distribution method, which can determine the excitation energy at which fission starts to dominate the decay process, was applied to $$^{254}$$No. The fission barrier of $$^{254}$$No was found to be 6.6 MeV at zero spin, indicating that the $$^{254}$$No is strongly stabilized by the nuclear shell effects.

Journal Articles

Population and decay of a $$K^{pi}$$ = 8$$^{-}$$ two-quasineutron isomer in $$^{244}$$Pu

Hota, S.*; Tandel, S.*; Chowdhury, P.*; Ahmad, I.*; Carpenter, M. P.*; Chiara, C. J.*; Greene, J. P.*; Hoffman, C. R.*; Jackson, E. G.*; Janssens, R. V. F.*; et al.

Physical Review C, 94(2), p.021303_1 - 021303_5, 2016/08

 Times Cited Count:5 Percentile:47.58(Physics, Nuclear)

The decay of a $$K^{pi}$$ = 8$$^{-}$$ isomer in $$^{244}$$Pu and the collective band structure populating the isomer are studied using deep inelastic excitations with $$^{47}$$Ti and $$^{208}$$Pb beams, respectively. Precise measurements of $$M1/E2$$ branching ratios in the band confirm a clean 9/2$$^-$$[734]$$_{nu}$$$$otimes$$7/2$$^+$$[624]$$_{nu}$$ for the isomer, validating the systematics of K$$^{pi}$$ = 8$$^{-}$$ two-quasineutron isomers observed in even-$$Z$$, $$N$$ = 150 isotones. These isomers around the deformed shell gap at $$N$$ = 152 provide critical benchmarks for theoretical predictions of single-particle energies in this gateway region to superheavy nuclei.

Journal Articles

Identification of deformed intruder states in semi-magic $$^{70}$$Ni

Chiara, C. J.*; Weisshaar, D.*; Janssens, R. V. F.*; Tsunoda, Yusuke*; Otsuka, Takaharu*; Harker, J. L.*; Walters, W. B.*; Recchia, F.*; Albers, M.*; Alcorta, M.*; et al.

Physical Review C, 91(4), p.044309_1 - 044309_10, 2015/04

 Times Cited Count:25 Percentile:9.58(Physics, Nuclear)

The neutron-rich isotope $$^{70}$$Ni was produced by multi-nucleon transfer reactions of $$^{70}$$Zn in the Argonne National Laboratory, and an in-beam $$gamma$$-ray experiment were performed using the GRETINA array. The $$2^+_2$$ and $$4^+_2$$ levels of $$^{70}$$Ni were observed for the first time. Those levels are regarded as large deformed states associated with proton excitation from the $$f_{7/2}$$ orbit because they cannot be reproduced by a shell-model calculation assuming a small valence space without $$f_{7/2}$$. A theoretical analysis based on the Monte Carlo shell model published in 2014 indicates that those levels corresponds to a prolate deformed band. The present result demonstrates the occurrence of shape coexistence in neutron-rich Ni isotopes other than a known case of $$^{68}$$Ni, and confirms the predictive power of the Monte Carlo shell-model calculation.

Journal Articles

Fission barrier of superheavy nuclei and persistence of shell effects at high spin; Cases of $$^{254}$$No and $$^{220}$$Th

Henning, G.*; Khoo, T. L.*; Lopez-Martens, A.*; Seweryniak, D.*; Alcorta, M.*; Asai, Masato; Back, B. B.*; Bertone, P. F.*; Boilley, D.*; Carpenter, M. P.*; et al.

Physical Review Letters, 113(26), p.262505_1 - 262505_6, 2014/12

 Times Cited Count:26 Percentile:16.35(Physics, Multidisciplinary)

Fission barrier heights of a shell-stabilized superheavy nucleus $$^{254}$$No have been determined as a function of spin up to 19$$hbar$$ through the measured distribution of entry points of $$gamma$$ deexcitations in the excitation energy vs. spin plane. The fission barrier height of $$^{254}$$No was determined to be 6.0 MeV at spin 15$$hbar$$, and 6.6 MeV at spin 0$$hbar$$ by extrapolation. This demonstrates that the shell effect actually enlarges the fission barrier in such heavy nuclei and keeps the barrier high even at high spin.

Journal Articles

Exploring the stability of super heavy elements; First measurement of the fission barrier of $$^{254}$$No

Henning, G.*; Lopez-Martens, A.*; Khoo, T. L.*; Seweryniak, D.*; Alcorta, M.*; Asai, Masato; Back, B. B.*; Bertone, P. F.*; Boilley, D.*; Carpenter, M. P.*; et al.

EPJ Web of Conferences, 66, p.02046_1 - 02046_8, 2014/03

 Times Cited Count:3 Percentile:22.93

Fission barrier heights of $$^{254}$$No have been determined through the entry distribution method. The entry distribution is the initial distribution of excitation energy and spin from which the $$gamma$$ deexcitation starts in the fusion-evaporation reaction. The initial distribution is extracted from measured $$gamma$$-ray multiplicity and total $$gamma$$-ray energy. This paper describes the details of the entry distribution method, and reports the first determination of the fission barrier heights of $$^{254}$$No, which is the heaviest nucleus whose fission barrier has been measured.

Journal Articles

Evidence for rigid triaxial deformation at low energy in $$^{76}$$Ge

Toh, Yosuke; Chiara, C. J.*; McCutchan, E. A.*; Walters, W. B.*; Janssens, R. V. F.*; Carpenter, M. P.*; Zhu, S.*; Broda, R.*; Fornal, B.*; Kay, B. P.*; et al.

Physical Review C, 87(4), p.041304_1 - 041304_5, 2013/04

 Times Cited Count:58 Percentile:3.33(Physics, Nuclear)

Excited states of $$^{76}$$Ge have been investigated via the $$^{76}$$Ge + $$^{238}$$U reaction with $$E(^{76}$$Ge$$)=530$$ MeV by use of in-beam $$gamma$$-ray spectroscopy using the $$gamma$$ sphere array. The $$gamma$$ band was extended considerably and one new band was identified. Comparisons of the $$gamma$$ band with collective- and shell-model calculations suggest that $$^{76}$$Ge may be a rare example of a nucleus exhibiting rigid triaxial deformation in the low-lying states.

Journal Articles

First prompt in-beam $$gamma$$-ray spectroscopy of a superheavy element; The $$^{256}$$Rf

Rubert, J.*; Dorvaux, O.*; Gall, B. J. P.*; Greenlees, P. T.*; Asfari, Z.*; Piot, J.*; Andersson, L. L.*; Asai, Masato; Cox, D. M.*; Dechery, F.*; et al.

Journal of Physics; Conference Series, 420, p.012010_1 - 012010_10, 2013/03

 Times Cited Count:0 Percentile:100

The first prompt in-beam $$gamma$$-ray spectroscopy of a superheavy element, $$^{256}$$Rf, has been performed successfully. A development of an intense isotopically enriched $$^{50}$$Ti beam using the MIVOC method enabled us to perform this experiment. A rotational band up to a spin of 20 $$hbar$$ has been discovered in $$^{256}$$Rf, and its moment of inertia has been extracted. These data suggest that there is no evidence of a significant deformed shell gap at $$Z$$ = 104.

Journal Articles

Shell-structure and pairing interaction in superheavy nuclei; Rotational properties of the $$Z$$=104 nucleus $$^{256}$$Rf

Greenlees, P. T.*; Rubert, J.*; Piot, J.*; Gall, B. J. P.*; Andersson, L. L.*; Asai, Masato; Asfari, Z.*; Cox, D. M.*; Dechery, F.*; Dorvaux, O.*; et al.

Physical Review Letters, 109(1), p.012501_1 - 012501_5, 2012/07

 Times Cited Count:46 Percentile:10.77(Physics, Multidisciplinary)

Rotational band structure of the $$Z$$=104 nucleus $$^{256}$$Rf has been observed for the first time using an in-beam $$gamma$$-ray spectroscopic technique. This nucleus is the heaviest among the nuclei whose rotational band structure has ever been observed. Thus, the present result provides valuable information on the single-particle shell structure and pairing interaction in the heaviest extreme of nuclei. The deduced moment of inertia indicates that there is no deformed shell gap at $$Z$$=104, which is predicted in a number of current self-consistent mean-field models.

Journal Articles

Multi-intruder structures in $$^{34}$$P

Bender, P. C.*; Tabor, S. L.*; Tripathi, V.*; Hoffman, C. R.*; Hamilton, L.*; Volya, A.*; Clark, R. M.*; Fallon, P.*; Macchiavelli, A. O.*; Paschalis, S.*; et al.

Physical Review C, 85(4), p.044305_1 - 044305_10, 2012/04

 Times Cited Count:12 Percentile:37.44(Physics, Nuclear)

The available experimental information on $$^{34}$$P has been greatly increased through the analysis of $$gamma$$ decays in coincidence with protons from the interaction of an $$^{18}$$O beam at 24 MeV with an $$^{18}$$O target. Light charged particles from the reaction were detected with Microball, and multiple $$gamma$$ ray coincidences with Gammasphere. Many newly observed $$gamma$$ transitions have been identified and placed in the level scheme. Additionally, for most states, spins have been assigned based on measured $$gamma$$ ray angular distributions while parities were inferred from lifetimes determined through Doppler-broadened line-shape analysis. Most of the states observed have been interpreted in terms of shell-model calculations using the WBP-a and SDPF-NR interactions having one particle in the 0$$f$$$$_{7/2}$$ or 1$$p$$$$_{3/2}$$ orbital. The two calculations agree almost equally well with the data resulting in root-mean-square differences of about 200 keV. However, two of a few high-lying states may be associated with stretched $$pi$$$$f$$$$_{7/2}$$ $$otimes$$ $$nu$$$$f$$$$_{7/2}$$ states, but the calculations over-predict their energies by 2-3 MeV. Furthermore, a newly observed long-lived 7919-keV state is established for which no explanation is available at present.

Journal Articles

Search for a 2-quasiparticle high-$$K$$ isomer in $$^{256}$$Rf

Robinson, A. P.*; Khoo, T. L.*; Seweryniak, D.*; Ahmad, I.*; Asai, Masato; Back, B. B.*; Carpenter, M. P.*; Chowdhury, P.*; Davids, C. N.*; Greene, J.*; et al.

Physical Review C, 83(6), p.064311_1 - 064311_7, 2011/06

 Times Cited Count:25 Percentile:16.16(Physics, Nuclear)

We have identified an isomer with a half-life of 17 $$mu$$s in $$^{256}$$Rf through a calorimetric conversion electron measurement tagged with implanted $$^{256}$$Rf nuclei using the fragment mass analyzer at Argonne National Laboratory. The low population yield for this isomer suggests that this isomer should not be a 2-quasiparticle high-$$K$$ isomer which is typically observed in the N = 152 isotones, but should be a 4-quasiparticle one. Possible reasons of the non-observation of a 2-quasiparticle isomer are this isomer decays by fission with a half-life similar to that of the ground state of $$^{256}$$Rf. Another possibility, that there is no 2-quasiparticle isomer at all, would imply an abrupt termination of axially symmetric deformed shape at Z=104.

Journal Articles

Cross-shell excitations near the "island of inversion"; Structure of $$^{30}$$Mg

Daecon, A. N.*; Smith, J. F.*; Freeman, S. J.*; Janssens, R. V. F.*; Carpenter, M. P.*; Hadinia, B.*; Hoffman, C. R.*; Kay, B. P.*; Lauritsen, T.*; Lister, C. J.*; et al.

Physical Review C, 82(3), p.034305_1 - 034305_7, 2010/09

 Times Cited Count:15 Percentile:28.88(Physics, Nuclear)

no abstracts in English

Journal Articles

Bridging the nuclear structure gap between stable and super heavy nuclei

Seweryniak, D.*; Khoo, T. L.*; Ahmad, I.*; Kondev, F. G.*; Robinson, A.*; Tandel, S. K.*; Asai, Masato; Back, B. B.*; Carpenter, M. P.*; Chowdhury, P.*; et al.

Nuclear Physics A, 834(1-4), p.357c - 361c, 2010/03

 Times Cited Count:7 Percentile:48.28(Physics, Nuclear)

Experimental data on single-particle energies in nuclei around Z=100 and N=152 play an important role to test validity of theoretical predictions for shell structure of superheavy nuclei. We found high-K two-quasiparticle isomers in $$^{252}$$No and $$^{254}$$No, and evaluated energies of proton single-particle orbitals around Z=100. We also found a new high-K three quasiparticle isomer in $$^{257}$$Rf. Energies of neutron single-particle orbitals were also evaluated from experimental data of the $$alpha$$ decay of $$^{257}$$Rf. Comparisons between the present experimental data and various theoretical calculations for the proton single-particle orbitals indicate that the calculation by using the Woods-Saxon potential gives the best agreement with the data.

Journal Articles

Spectroscopy of $$^{257}$$Rf

Qian, J.*; Heinz, A.*; Khoo, T. L.*; Janssens, R. V. F.*; Peterson, D.*; Seweryniak, D.*; Ahmad, I.*; Asai, Masato; Back, B. B.*; Carpenter, M. P.*; et al.

Physical Review C, 79(6), p.064319_1 - 064319_13, 2009/06

 Times Cited Count:27 Percentile:15.53(Physics, Nuclear)

$$alpha$$-, $$gamma$$-, and conversion electron spectroscopy experiments for $$^{257}$$Rf have been performed using Fragment Mass Analyzer at Argonne National Laboratory. A new isomer with a half-life of 160 $$mu$$s has been discovered in $$^{257}$$Rf, and it is interpreted as a three-quasiparticle high-$$K$$ isomer. Neutron configurations of one-quasiparticle states in $$^{253}$$No, the $$alpha$$-decay daughter of $$^{257}$$Rf, have been assigned on the basis of $$alpha$$-decay hindrance factors. Excitation energies of the 1/2$$^{+}$$[620] states in $$N$$=151 isotones indicate that the deformed shell gap at $$N$$=152 increases with the atomic number.

Journal Articles

$$K^{pi}=8^{-}$$ isomers and $$K^{pi}=2^{-}$$ octupole vibrations in $$N=150$$ shell-stabilized isotones

Robinson, A. P.*; Khoo, T. L.*; Ahmad, I.*; Tandel, S. K.*; Kondev, F. G.*; Nakatsukasa, Takashi*; Seweryniak, D.*; Asai, Masato; Back, B. B.*; Carpenter, M. P.*; et al.

Physical Review C, 78(3), p.034308_1 - 034308_6, 2008/09

 Times Cited Count:42 Percentile:9.37(Physics, Nuclear)

Isomers have been identified in $$^{246}$$Cm and $$^{252}$$No with quantum number $$K^{pi}=8^{-}$$, which decay through $$K^{pi}=2^{-}$$ rotational bands built on octupole vibrational states. For $$N=150$$ isotones with atomic number $$Z=94$$$$sim$$102, the $$K^{pi}=8^{-}$$ and 2$$^{-}$$ states have remarkably stable energies, indicating neutron excitations. An exception is a singular minimum in the 2$$^{-}$$ energy at $$^{246}$$Cm, due to the additional role of proton configurations.

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