Tsekhanovich, I.*; Andreyev, A.; 西尾 勝久; Denis-Petit, D.*; 廣瀬 健太郎; 牧井 宏之; Matheson, Z.*; 森本 幸司*; 森田 浩介*; Nazarewicz, W.*; et al.
Physics Letters B, 790, p.583 - 588, 2019/03
Fragment mass distributions from fission of the excited compound nucleus Pt have been deduced from the measured fragment velocities. The Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction Ar+Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of 1/3. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, / 79/99, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation.
Marsh, B. A.*; Day Goodacre, T.*; 角田 佑介*; Andreyev, A.; 他41名*
Nature Physics, 14(12), p.1163 - 1167, 2018/12
In rare cases, the removal of a single proton (Z) or neutron (N) from an atomic nucleus leads to a dramatic shape change. These instances are crucial for understanding the components of the nuclear interactions that drive deformation. The mercuryisotopes (Z = 80) are a striking example: their close neighbours, the lead isotopes (Z = 82), are spherical and steadily shrink with decreasing N. The even-mass (A = N + Z) mercury isotopes follow this trend. The odd-mass mercury isotopes Hg, however, exhibit noticeably larger charge radii. Due to the experimental difficulties of probing extremely neutron-deficient systems, and the computational complexity of modelling such heavy nuclides, the microscopic origin of this unique shape staggering has remained unclear. Here, by applying resonance ionization spectroscopy, mass spectrometry and nuclear spectroscopy as far as Hg, we determine Hg as the shape-staggering endpoint. By combining our experimental measurements with Monte Carlo shell model calculations, we conclude that this phenomenon results from the interplay between monopole and quadrupole interactions driving a quantum phase transition, for which we identify the participating orbitals. Although shape staggering in the mercury isotopes is a unique and localized feature in the nuclear chart, it nicely illustrates the concurrence of single-particle and collective degrees of freedom at play in atomic nuclei.
Cubiss, J. G.*; Barzakh, A. E.*; Andreyev, A.; 他57名*
Physics Letters B, 786, p.355 - 363, 2018/11
The first accurate measurements of the -decay branching ratio and half-life of the I=1/2 ground state in Tl have been made, along with the first determination of the magnetic moments and I=1/2 spin assignments of the ground states in Au. The results are discussed within the complementary systematics of the reduced -decay widths and nuclear g factors of low-lying, I=1/2states in the neutron-deficient lead region. The findings shed light on the unexpected hindrance of the 1/2 1/2, Tl Au decay, which is explained by a mixing of 3sand 2d configurations in Au, whilst Tl remains a near-pure 3s. This conclusion is inferred from the g factor of Au which has an intermediate value between those of 3s and 2d states. A similar mixed configuration is proposed for the I=1/2 ground state of Au. This mixing may provide evidence for triaxial shapes in the ground states in these nuclei.
Cubiss, J.*; Andreyev, A.; 光岡 真一*; 永目 諭一郎; 西尾 勝久; 太田 周也*; 若林 泰生*; 他60名*
Physical Review C, 97(5), p.054327_1 - 054327_19, 2018/05
Hyperfine-structure parameters and isotope shifts of At have been measured for the first time at CERN-ISOLDE, using the in-source resonance-ionization spectroscopy method. The hyperfine structures of isotopes were recorded using a triad of experimental techniques for monitoring the photo-ion current. The Multi-Reflection Time-of-Flight Mass Spectrometer, in connection with a high-resolution electron multiplier, was used as an ion-counting setup for isotopes that either were affected by strong isobaric contamination or possessed a long half-life; the ISOLDE Faraday cups were used for cases with high-intensity beams; and the Windmill decay station was used for short-lived, predominantly -decaying nuclei. The electromagnetic moments and changes in the mean-square charge radii of the astatine nuclei have been extracted from the measured hyperfine-structure constants and isotope shifts. This was only made possible by dedicated state-of-the-art large-scale atomic computations of the electronic factors and the specific mass shift of atomic transitions in astatine that are needed for these extractions. By comparison with systematics, it was possible to assess the reliability of the results of these calculations and their ascribed uncertainties. A strong deviation in the ground-state mean-square charge radii of the lightest astatine isotopes, from the trend of the (spherical) lead isotopes, is interpreted as the result of an onset of deformation. This behavior bears a resemblance to the deviation observed in the isotonic polonium isotopes. Cases for shape coexistence have been identified in At, for which a significant difference in the charge radii for ground (9/2) and isomeric (1/2) states has been observed.
Andreyev, A.; 西尾 勝久; Schmidt, K.-H.*
Reports on Progress in Physics, 81(1), p.016301_1 - 016301_61, 2018/01
In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies. This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams. The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and -ray emission will also be reviewed. The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising "next-generation" fission approaches, which might become available within the next decade.
Andel, B.*; Andreyev, A.; Antalic, S.*; Barzakh, A.*; Bree, N.*; Cocolios, T. E.*; Comas, V. F.*; Diriken, J.*; Elseviers, J.*; Fedorov, D. V.*; et al.
Physical Review C, 96(5), p.054327_1 - 054327_11, 2017/12
A detailed -decay spectroscopy study of Tl has been performed at ISOLDE (CERN). Z-selective ionization by the Resonance Ionization Laser Ion Source (RILIS) coupled to mass separation provided a high-purity beam of Tl. Fine-structure decays to excited levels in the daughter Au were identified and an -decay scheme of Tl was constructed based on an analysis of - and - - coincidences. Multipolarities of several -ray transitions deexciting levels in Au were determined. Based on the analysis of reduced -decay widths, it was found that all decays are hindered, which signifies a change of configuration between the parent and all daughter states.
廣瀬 健太郎; 西尾 勝久; 田中 翔也*; Lguillon, R.*; 牧井 宏之; 西中 一朗*; Orlandi, R.; 塚田 和明; Smallcombe, J.*; Vermeulen, M. J.; et al.
Physical Review Letters, 119(22), p.222501_1 - 222501_6, 2017/12
JAEAタンデム加速器施設で行ったO+U反応における多核子移行チャンネルを用いた実験により、U, Np、およびPuの核分裂質量分布を励起エネルギー1060MeVにおいて測定した。これらのうち、U, Npのデータは本実験により初めて観測された。原子核の殻効果の減衰によって対称分裂すると予想されていた高励起エネルギーにおいても、質量分布が非対称を示すことがわかった。搖動散逸定理に基づく動力学モデル計算との比較から、この振る舞いはマルチチャンス核分裂によるものであることを明らかにした。
西尾 勝久; 廣瀬 健太郎; Vermeulen, M. J.; 牧井 宏之; Orlandi, R.; 塚田 和明; 浅井 雅人; 豊嶋 厚史; 佐藤 哲也; 永目 諭一郎; et al.
EPJ Web of Conferences (Internet), 163, p.00041_1 - 00041_6, 2017/11
We are promoting a study of fission using multi-nucleon transfer (MNT) reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. Also, the excited states in the fissioning nucleus are widely populated by the MNT reactions, from which effects of excitation energy on fission properties can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the O beam and several actinide target nuclei such as Th, U, Np, Cm. Ejectile nucleus was identified by a silicon E-E telescope to identify transfer channel and hence the compound nucleus. Fission fragments were detected by multi-wire proportional counters, and fission fragment mass distributions (FFMDs) were measured for each isotope. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model, and importance of multi-chance fission concept is investigated. Fission fragment angular distribution relative to the recoil direction suggested the increase of the spin of the fissioning nucleus with the number of transferred nucleons.
西尾 勝久; 廣瀬 健太郎; Lguillon, R.*; 牧井 宏之; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; 田中 翔也*; et al.
Proceedings of 6th International Conference on Fission and Properties of Neutron-rich Nuclei (ICFN-6), p.590 - 597, 2017/11
We are promoting a fission study using multi-nucleon transfer reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. This allows us to study fission in the new region of chart of nuclei. Also, the excited states in the fissioning nucleus are widely populated in the reactions, thus the effects of excitation energy on fission can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the O beam (9MeV/u) and several actinide target nuclei such as Th, U, Np, Cm. Ejectile nuclei and thus the transfer channels were identified by a newly developed silicon E-E detectors. The produced nuclei in one experiment reached more than fifteen. Fission fragment mass distributions (FFMDs) were measured for each isotopes. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model (Langevin-type calculation), where effects of multi-chance fission were included. We also started to measure the prompt neutrons accompanied by fission.
Wilson, G. L.*; 武山 美麗*; Andreyev, A.; Andel, B.*; Antalic, S.*; Catford, W. N.*; Ghys, L.*; 羽場 宏光*; Heberger, F. P.*; Huang, M.*; et al.
Physical Review C, 96(4), p.044315_1 - 044315_7, 2017/10
The exotic decay process of -delayed fission has been studied in the neutron-deficient isotope Am at RIKEN, Japan. The Am nuclei were produced in the complete fusion reaction Pb(Al,4n)Am and separated by using the GARIS gas-filled recoil ion separator. A lower limit for the -delayed fission probability larger than 0.30 was deduced, which so far is the highest value among all known nuclei exhibiting this decay mode.
西尾 勝久; 廣瀬 健太郎; Lguillon, R.*; 牧井 宏之; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; 田中 翔也*; et al.
EPJ Web of Conferences (Internet), 146, p.04009_1 - 04009_6, 2017/09
The objective of this work is to extend fission data for actinide nuclei not investigated so far in order to understand the fission mechanism, especially for neutron-rich nuclei. Multi-nucleon transfer reactions were used to populate the compound nuclei which cannot be accessed by particle capture or fusion reactions. The experiment was carried out at the tandem facility of Japan Atomic Energy Agency. In the multi-nucleon transfer reactions using O beam, a wide variety of nuclei around a target nucleus was produced. By identifying transfer channels, fission data of nuclei more than fifteen compound nuclei were generated in one experiment. Another feature of transfer reaction is that the excitation energies of a compound nucleus can be populated continuously from the ground state up to several tens MeV, allowing us to study the excitation energy dependence of fission properties. From the excitation function of fission probabilities, fission-barrier heights of neutron-rich actinide nuclei were obtained. Experiments were carried out in the reactions of O + U, Th, Cm, Np. The obtained fission fragment mass distributions were reproduced by a model calculation based on a fluctuation dissipation model.
廣瀬 健太郎; 西尾 勝久; 牧井 宏之; 西中 一朗*; 太田 周也*; 永山 達郎*; 田村 信行*; 後藤 真一*; Andreyev, A.; Vermeulen, M. J.; et al.
Nuclear Instruments and Methods in Physics Research A, 856, p.133 - 138, 2017/06
Fission and capture reactions were simultaneously measured in the neutron-induced reactions of Am at J-PARC. Data were taken with the TOF method in = 0.120 eV. The fission events were observed by detecting prompt fission neutrons using liquid organic scintillators. The capture reaction was measured by detecting rays from the compound nuclei using the same detectors, where the prompt fission neutrons and capture rays were separated by a pulse shape analysis. The cross sections were obtained by normalizing the relative yields at the first resonance to evaluations or other experimental data. The ratio of the fission to capture cross sections at each resonance is compared with those from an evaluated nuclear data library and other experimental data. Some differences were found between the present values and the library/literature values at several resonances.
Lic, R.*; Rotaru, F.*; Borge, M. J. G.*; Grvy, S.*; Negoita, F.*; Poves, A.*; Sorlin, O.*; Andreyev, A.; Borcea, R.*; Costache, C.*; et al.
Physical Review C, 95(2), p.021301_1 - 021301_6, 2017/02
The decay of Mg was used to study the Al nucleus through spectroscopy at the Isotope Separator On-Line facility of CERN. Previous studies identified two -decaying states in Al having spin-parity assignments =4 dominated by the normal configuration (d5/2) (f7/2) and = 1 by the intruder configuration (d5/2) (d3/2)(f7/2). Their unknown ordering and relative energy have been the subject of debate for the placement of Al inside or outside the = 20 "island of inversion". We report here that the 1 intruder lies only 46.6 keV above the 4 ground state. In addition, a new half-life of =44.9(4)ms, that is twice as long as thepreviously measured 20(10) ms, has been determined for Mg. Large-scale shell-model calculations with the recently developed SDPF-U-MIX interaction are compared with the new data and used to interpret the mechanisms at play at the very border of the = 20 island of inversion.
Barzakh, A.*; Andreyev, A.; Cocolios, T. E.*; de Groote, R. P.*; Fedorov, D. V.*; Fedosseev, V. N.*; Ferrer, R.*; Fink, D. A.*; Ghys, L.*; Huyse, M.*; et al.
Physical Review C, 95(1), p.014324_1 - 014324_12, 2017/01
Hyperfine structure and isotope shifts have been measured for the ground and isomeric states in the neutron deficient isotopes Tl using the 276.9 nm transition. The experiment has been performed at the CERN Isotope Separator On-Line facility using the in-source resonance-ionization laser spectroscopy technique. Spins for the ground states in Tl have been determined as = 1/2. Magnetic moments and changes in the nuclear mean-square charge radii have been deduced. By applying the additivity relation for magnetic moments of the odd-odd Tl nuclei the leading configuration assignments were confirmed. A deviation of magnetic moments for isomeric states in Tl from the trend of the heavier Tl nuclei is observed. The charge radii of the ground states of the isotopes Tl follow the trend for isotonic (spherical) lead nuclei. The noticeable difference in charge radii for ground and isomeric states of Tl has been observed, suggesting a larger deformation for the intruder-based 9/2 and 10 states compared to the ground states. An unexpected growth of the isomer shift for Tl has been found.
Lund, M. V.*; Andreyev, A.; Borge, M. J. G.*; Cederkll, J.*; De Witte, H.*; Fraile, L. M.*; Fynbo, H. O. U.*; Greenlees, P. T.*; Harkness-Brennan, L. J.*; Howard, A. M.*; et al.
European Physical Journal A, 52(10), p.304_1 - 304_14, 2016/10
Beta-delayed proton emission from Mg has been measured at ISOLDE, CERN, with the ISOLDE Decay Station setup including both charged-particle and -ray detection capabilities. A total of 27 delayed proton branches were measured including seven so far unobserved. An updated decay scheme, including three new resonances above the proton separation energy in Na and more precise resonance energies, is presented. Beta-decay feeding to two resonances above the Isobaric Analogue State in Na is observed. This may allow studies of the 4032.9(2.4) keV resonance in Ne through the beta decay of Mg, which is important for the astrophysically relevant reaction O(,)Ne. Beta-delayed protons were used to obtain a more precise value for the half-life of Mg, 91.4(1.0) ms.
Lguillon, R.; 西尾 勝久; 廣瀬 健太郎; 牧井 宏之; 西中 一朗*; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; et al.
Physics Letters B, 761, p.125 - 130, 2016/10
It is shown that the multinucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multinucleon transfer channels of the O + Th reaction are used to study fission of fourteen nuclei Th, Pa, and U. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel, in selected bins of excitation energy. In particular, the mass distributions of Th and Pa are measured for the first time. Predominantly asymmetric fission is observed at low excitation energies for all studied cases, with a gradual increase of the symmetric mode towards higher excitation energy. The experimental distributions are found to be in general agreement with predictions of the fluctuation-dissipation model.
Truesdale, V. L.*; Andreyev, A.; Ghys, L.*; Huyse, M.*; Van Duppen, P.*; Sels, S.*; Andel, B.*; Antalic, S.*; Barzakh, A.*; Capponi, L.*; et al.
Physical Review C, 94(3), p.034308_1 - 034308_11, 2016/09
A nuclear-decay spectroscopy study of the neutron-deficient isotope At is reported where an isotopically pure beam was produced using the selective Resonance Ionization Laser Ion Source and On-Line Isotope Mass Separator (CERN). The fine-structure decay of At allowed the low-energy excited states in the daughter nucleus Bi to be investigated. A -delayed fission study of At was also performed. A mixture of symmetric and asymmetric fission-fragment mass distributions of the daughter isotope Po (populated by decay of At) was deduced based on the measured fission-fragment energies. A DF probability (At) = 9(1)10 was determined.
Andel, B.*; Andreyev, A.; Antalic, S.*; Heberger, F. P.*; Ackermann, D.*; Hofmann, S.*; Huyse, M.*; Kalaninov, Z.*; Kindler, B.*; Kojouharov, I.*; et al.
Physical Review C, 93(6), p.064316_1 - 064316_12, 2016/06
Isomeric states in Po and Po were studied at the velocity filter SHIP. The isotopes were produced in the fusion-evaporation reactions Pr(Fe, )Po and Sm(V, )Po. Several new -ray transitions were attributed to the isomers and - coincidences for both isomers were studied for the first time. The 459-keV transition earlier, tentatively proposed as de-exciting the isomeric level in Po, was replaced by a new 248-keV transition, and the spin of this isomer was reassigned from (11) to (10). The de-excitation of the (11) isomeric level in Po by the 154-keV transition was confirmed and a parallel de-excitation by a 733-keV (E3) transition to (8) level of the ground-state band was suggested. Moreover, side feeding to the (4) level of the ground-state band was proposed. The paper also discusses strengths of transitions de-exciting 11 isomers in neighboring Po and Pb isotopes.
Lic, R.*; Mach, H.*; Fraile, L. M.*; Gargano, A.*; Borge, M. J. G.*; Mrginean, N.*; Sotty, C. O.*; Vedia, V.*; Andreyev, A.; Benzoni, G.*; et al.
Physical Review C, 93(4), p.044303_1 - 044303_7, 2016/04
The levels in Sn populated from the decay of In isomers were investigated at the ISOLDE facility of CERN using the newly commissioned ISOLDE Decay Station. The lowest 1/2 state and the 3/2 ground state in Sn are expected to have configurations dominated by the neutron (=0) and (=2) single-particle states, respectively. Consequently, these states should be connected by a somewhat slow -forbidden transition. Using fast-timing spectroscopy we have measured the half-life of the 1/2 315.3-keV state, = 19(10) ps, which corresponds to a moderately fast transition. Shell-model calculations using the CD-Bonn effective interaction, with standard effective charges and factors, predict a 4-ns half-life for this level. We can reconcile the shell-model calculations to the measured value by the renormalization of the effective operator for neutron holes.
Van Beveren, C.*; Andreyev, A.; Barzakh, A. E.*; Cocolios, T. E.*; de Groote, R. P.*; Fedorov, D.*; Fedosseev, V. N.*; Ferrer, R.*; Ghys, L.*; Huyse, M.*; et al.
Journal of Physics G; Nuclear and Particle Physics, 43(2), p.025102_1 - 025102_22, 2016/02
-decay spectroscopy of Tl has been performed at the CERN isotope separator on-line (ISOLDE) facility. New fine-structure decays have been observed for both isotopes. -decay branching ratios of 0.089(19), 0.047(6) and 1.22(30) have been deduced for the (10), (7) and (2) states respectively in Tl and a lower limit of 0.49 for the -decay branching ratio of Tl. A new half-life of 9.5(2) s for the (2) state in Tl and 1.9(1) s for the low-spin state in Tl has been deduced. Using - coincidence analysis, multiple rays were observed de-exciting levels in Au fed by Tl decays. The transitions connecting these low-lying states in Au are essential to sort the data and possibly identify bands from inbeam studies in these isotopes. Owing to the complex fine-structure decays and limited knowledge about the structure of the daughter nuclei, only partial level schemes could be constructed for both gold isotopes in the present work. Reduced -decay widths have been calculated and are compared with values obtained in neighboring odd-A and even-A thallium isotopes. Except for the allowed decay of the Tl (10) state, the other fine-structure decays observed in this study are hindered. This points to strong structural changes between parent thallium and daughter gold isotopes.