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The structure of the odd-$A$ isotopes $^{193,195,197}\mathrm{Pt}$, which lie along the line of stability, has been studied up to high spin through multinucleon transfer reactions. Positive- and negative-parity sequences in $^{193,195}\mathrm{Pt}$ have been considerably extended and multiple band crossings established. An isomer with ${T}_{1/2}=5.0(5)$ ns and ${I}^{\ensuremath{\pi}}=25/{2}^{\ensuremath{-}}$ is present in $^{195}\mathrm{Pt}$. The isotopes $^{193,195}\mathrm{Pt}$ are characterized by moderate oblate deformation, and angular momentum generation at high spin in the yrast, positive-parity sequences is attributed to the rotation alignment of ${i}_{13/2}$ neutrons and ${h}_{11/2}$ protons. A detailed understanding of the observed features is presented using calculations based on tilted axis cranking covariant density functional theory as well as others with the ultimate cranker code.

Garrett et al. systematically investigated band-crossing frequencies resulting from the rotational alignment of the first pair of i13/2 neutrons (AB) in rare-earth nuclei. In that study, evidence was found for an odd-even neutron number dependence attributed to changes in the strength of neutron pairing correlations. The present paper carries out a similar investigation at higher rotational frequencies for the second pair of aligning i13/2 neutrons (BC). Again, a systematic difference in band-crossing frequencies is observed between odd-N and even-N Er, Yb, Hf, and W nuclei, but in the BC case, it is opposite to the AB neutron-number dependence. These results are discussed in terms of a reduction of neutron pairing correlations at high rotational frequencies and of the effects of Pauli blocking on the pairing field by higher-seniority configurations. Also playing a significant role are the changes in deformation with proton and neutron numbers, the changes in location of single-particle orbitals as a function of quadrupole deformation, and the position of the Fermi surface with regard to the various ω components of the neutron i13/2 shell.

The 22Ne($ \alpha$, n) reaction is expected to provide the dominant neutron source for the weak s process in massive stars and intermediate-mass (IM) Asymptotic Giant Branch (AGB) stars. However, the production of neutrons in such environments is hindered by the competing 22Ne($\alpha$,$\gamma$)26Mg reaction. Here, the 11B(16O,p) fusion-evaporation reaction was used to identify $\gamma$-decay transitions from 22Ne + $\alpha$ resonant states in 26Mg. Spin-parity restrictions have been placed on a number of $ \alpha$-unbound excited states in 26 Mg and their role in the 22Ne($\alpha$,$\gamma$)26Mg reaction has been investigated. In particular, a suspected natural-parity resonance at Ec.m. = 557(3) keV, that lies above the neutron threshold in 26Mg, and is known to exhibit a strong $ \alpha$-cluster character, was observed to $ \gamma$ decay. Furthermore, a known resonance at $ E_{c.m.} = 466(4)$ keV has been definitively assigned 2+ spin and parity. Consequently, uncertainties in the 22Ne($ \alpha$,$ \gamma$) stellar reaction rate have been reduced by a factor of ∼ 20 for temperatures ∼ 0.2 GK.

The yields of 42 projectile-like fragments (PLFs) and fission fragments and 36 target-like fragments (TLFs) were measured using off-line $\ensuremath{\gamma}$-ray spectroscopy in a thin target experiment involving the $^{136}\mathrm{Xe}+^{198}\mathrm{Pt}$ reaction. The center of target beam energy was 760.5 MeV $({\mathrm{E}}_{\mathrm{c}.\mathrm{m}.}=450$ MeV). The reported yields are compared with those from previous measurements for this reaction and with predictions of the GRAZING, di-nuclear systems (DNS), and improved quantum molecular dynamics (ImQMD) models. The yields of the TLFs and PLFs are, in general, substantially smaller than those previously observed at a beam energy of 1085 MeV. Neither the GRAZING nor the DNS model correctly describes the measured TLF and PLF yields in this lower- energy reaction but the ImQMD model describes these yields adequately.

Excited states of $^{202}\mathrm{Hg}$ have been studied via the $^{12}\mathrm{C}(^{202}\mathrm{Hg},^{202}\mathrm{Hg}^{*})$ Coulomb excitation reaction at a beam energy of 890 MeV. The $\ensuremath{\gamma}$-ray transitions from the excited states of $^{202}\mathrm{Hg}$ were detected by the Gammasphere array. The intensities of the observed $\ensuremath{\gamma}$ rays determined the relative populations of the excited states which were used to extract the absolute $M1$ and $E2$ transition strength distributions for excited ${2}^{+}$ states of $^{202}\mathrm{Hg}$ up to 2 MeV. The measured absolute $B(M1;{2}_{7}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ strength of $0.18(8){\ensuremath{\mu}}_{\mathrm{N}}^{2}$ indicates that the ${2}_{7}^{+}$ level of $^{202}\mathrm{Hg}$ is the main fragment of the proton-neutron mixed-symmetry ${2}_{1,\mathrm{ms}}^{+}$ state. Upper limits for the $F$-spin mixing matrix elements of ${}^{202,204}\mathrm{Hg}$ are determined as well.

Excited states in the neutron-rich 70 Fe nucleus were populated in a one-proton removal reaction from 71 Co projectiles at 87 MeV/nucleon. A new transition was observed with the γ -ray tracking array GRETINA and shown to feed the previously assigned 4+ 1 state. In comparison to reaction theory calculations with shell-model spectroscopic factors, it is argued that the new γ ray possibly originates from the 6+ 1 state. It is further shown that the Doppler-reconstructed γ -ray spectra are sensitive to the very different lifetimes of the 2+ and 4+ states, enabling their approximate measurement. The emerging structure of 70 Fe is discussed in comparison to LNPS-new large-scale shell-model calculations.

An extensive, model-independent analysis of the nature of triaxial deformation in $^{76}$Ge, a candidate for neutrinoless double-beta ($0\nu\beta\beta$) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $\gamma$ bands. Both sequences were determined to be characterized by the same $\beta$ and $\gamma$ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in $^{76}$Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured $E2$ matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to $0\nu\beta\beta$., Comment: 6 pages, 5 figures

Spectroscopic data, such as precise γ-ray branching and E2/M1 multipole-mixing ratios, provide vital constraints when performing multi-dimensional Coulomb-excitation analyses. Consequently, as part of our new Coulomb-excitation campaign aimed at investigating the role of exotic non-axial (triaxial) deformations in the unstable refractory Ru-Mo isotopes, additional beta-decay data was obtained. These measurements make use of ANL’s CARIBU facility, which provides intense beams of radioactive refractory isotopes along with the excellent efficiency and angular resolution of the GRETINA γ-ray tracking array. In this article, we report on the analysis of the A = 110 decay chain, focussing on the identification of previously unreported states in 110Ru following the decay of 110Tc.

A number of 3-quasiparticle isomers have been found and characterised in the odd-mass, neutron-rich, 187Re, 189Re and 191Re nuclei, the latter being four neutrons beyond stability. The decay of the isomers populates states in the rotational bands built upon the 9 / 2 − [ 514 ] Nilsson orbital. These bands exhibit a degree of signature splitting that increases with neutron number. This splitting taken together with measurements of the M 1 / E 2 mixing ratios and with the changes observed in the energy of the gamma-vibrational band coupled to the 9 / 2 − [ 514 ] state, suggests an increase in triaxiality, with γ values of 5°, 18° and 25° deduced in the framework of a particle-rotor model.

We report the first observation of the ^{108}Xe→^{104}Te→^{100}Sn α-decay chain. The α emitters, ^{108}Xe [E_{α}=4.4(2) MeV, T_{1/2}=58_{-23}^{+106} μs] and ^{104}Te [E_{α}=4.9(2) MeV, T_{1/2}

As zirconium gains neutrons, its nucleus morphs in shape---changing from a soccer ball to an American football---and researchers have found the exact moment of the transition.

Yrast and near-yrast levels up to an I=17ℏ spin value and a 6-MeV excitation energy have been delineated in the “two-neutron” 210Pb nucleus following deep-inelastic reactions involving 208Pb targets and a number of heavy-ion beams at energies ∼25% above the Coulomb barrier. The level scheme was established on the basis of multifold prompt and delayed coincidence relationships measured with the Gammasphere array. In addition to the previously known states, many new levels were identified. For most of the strongly populated states, spin-parity assignments are proposed on the basis of angular distributions. The reinvestigation of the ν(g9/2)2, 8+ isomeric decay results in the firm identification of the low-energy E2 transitions involved in the 8+→6+→4+ cascade, and in a revised 6+ level half-life of 92(10) ns, nearly a factor of 2 longer than previously measured. Among the newly identified states figure spin I=4–10ℏ levels associated with the νg9/2i11/2 multiplet, as well as yrast states involving νg9/2j15/2, νi11/2j15/2, and ν(j15/2)2 neutron couplings. The highest-spin excitations are understood as 1p−1h core excitations and the yrast population is found to be fragmented to the extent that levels of spin higher than I=17ℏ could not be reached. Four E3 transitions are present in the 210Pb yrast decay; three of these involve the g9/2→j15/2 octupole component, as reflected in the 21(2) and >10 Weisskopf unit enhancements of the B(E3) rates of the first two. The fourth, 16+→13−E3 transition corresponds to the 3− core octupole excitation built on the νi11/2j15/2 state, in analogy to a similar E3 coupling to the νj15/2 level in 209Pb. Shell-model calculations performed for two-neutron states and 1p−1h 208Pb core excitations are in good agreement with the data. Evidence was found for the existence of a hitherto unknown high-spin β-decaying isomer in 210Tl. Shell-model calculations of the 210Tl levels suggest the possibility of a 11+ long-lived, β-decaying state, and the delayed yields observed in various reactions fit rather well with a 210Tl assignment.

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus As67 have been studied by using the Ca40(Ar36,2αp)As67 reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states.

Excited states in $^{137}\mathrm{Cs}$ have been studied by using the near-barrier, single-proton transfer reaction $^{10}\mathrm{B}(^{136}\mathrm{Xe},\phantom{\rule{4pt}{0ex}}^{9}\mathrm{Be}\ensuremath{\gamma})^{137}\mathrm{Cs}$ in inverse kinematics and performing particle-$\ensuremath{\gamma}$ and particle-$\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence measurements. The low-spin level scheme of $^{137}\mathrm{Cs}$ has been augmented: (i) The expected set of single-proton, seniority $\ensuremath{\upsilon}=1$ states has been delineated in its entirety; (ii) Another member of the $\ensuremath{\upsilon}=3$ proton $0{g}_{7/2}$ multiplet, a low-lying $9/{2}^{+}$ level, has been identified and this set of states now approaches completion; (iii) A second $9/{2}^{+}$ and a candidate $13/{2}^{+}$ level have been observed. The results are discussed in the framework of shell-model calculations in the proton $2s1d0g0h$ shell.

A sequence of low-energy levels in _{32}^{78}Ge_{46} has been identified with spins and parity of 2^{+}, 3^{+}, 4^{+}, 5^{+}, and 6^{+}. Decays within this band proceed strictly through ΔJ=1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 2^{+} level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a γ-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the ΔJ=2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.

The structure of deformed neutron-rich nuclei in the rare-earth region is of significant interest for both the astrophysics and nuclear structure fields. At present, a complete explanation for the observed peak in the elemental abundances at $A\ensuremath{\sim}160$ eludes astrophysicists, and models depend on accurate quantities, such as masses, lifetimes, and branching ratios of deformed neutron-rich nuclei in this region. Unusual nuclear structure effects are also observed, such as the unexpectedly low energies of the first ${2}^{+}$ levels in some even-even nuclei at $N=98$. In order to address these issues, mass and $\ensuremath{\beta}$-decay spectroscopy measurements of the $^{160}{\mathrm{Eu}}_{97}$ and $^{162}{\mathrm{Eu}}_{99}$ nuclei were performed at the Californium Rare Isotope Breeder Upgrade radioactive beam facility at Argonne National Laboratory. Evidence for a gap in the single-particle neutron energies at $N=98$ and for large deformation (${\ensuremath{\beta}}_{2}\ensuremath{\sim}0.3$) is discussed in relation to the unusual phenomena observed at this neutron number.

Author(s): Lubna, RS; Tripathi, V; Tabor, SL; Tai, PL; Kravvaris, K; Bender, PC; Volya, A; Bouhelal, M; Chiara, CJ; Carpenter, MP; Janssens, RVF; Lauritsen, T; McCutchan, EA; Zhu, S; Clark, RM; Fallon, P; MacChiavelli, AO; Paschalis, S; Petri, M; Reviol, W; Sarantites, DG | Abstract: Excited states in the neutron-rich isotopes P33 and P34 were populated by the O18+O18 fusion-evaporation reaction at Elab=24 MeV. The Gammasphere array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus S36. The use of Microball enabled the selection of the proton emission channel. It also helped in determining the exact position and energy of the emitted proton; this was later employed in kinematic Doppler corrections. 16 new transitions and 13 new states were observed in P33 and 21 γ rays and 20 energy levels were observed in P34 for the first time. The nearly 4π geometry of Gammasphere allowed the measurement of γ-ray angular distributions leading to spin assignments for many states. The experimental observations for both isotopes were interpreted with the help of shell-model calculations using the (0+1)ω PSDPF interaction. The calculations accounted for both the 0p-0h and 1p-1h states reasonably well and indicated that 2p-2h excitations might dominate the higher-spin configurations in both P33 and P34.

A sequence of low-energy levels in _{32}^{78}Ge_{46} has been identified with spins and parity of 2^{+}, 3^{+}, 4^{+}, 5^{+}, and 6^{+}. Decays within this band proceed strictly through ΔJ=1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 2^{+} level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a γ-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the ΔJ=2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.

The structure of deformed neutron-rich nuclei in the rare-earth region is of significant interest for both the astrophysics and nuclear structure fields. At present, a complete explanation for the observed peak in the elemental abundances at A∼160 eludes astrophysicists, and models depend on accurate quantities, such as masses, lifetimes, and branching ratios of deformed neutron-rich nuclei in this region. Unusual nuclear structure effects are also observed, such as the unexpectedly low energies of the first 2^{+} levels in some even-even nuclei at N=98. In order to address these issues, mass and β-decay spectroscopy measurements of the ^{160}Eu_{97} and ^{162}Eu_{99} nuclei were performed at the Californium Rare Isotope Breeder Upgrade radioactive beam facility at Argonne National Laboratory. Evidence for a gap in the single-particle neutron energies at N=98 and for large deformation (β_{2}∼0.3) is discussed in relation to the unusual phenomena observed at this neutron number.

A beam containing a substantial component of both the J^{π}=5^{+}, T_{1/2}=162 ns isomeric state of ^{18}F and its 1^{+}, 109.77-min ground state is utilized to study members of the ground-state rotational band in ^{19}F through the neutron transfer reaction (d,p) in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2^{+} band-terminating state. The agreement between shell-model calculations using an interaction constructed within the sd shell, and our experimental results reinforces the idea of a single-particle-collective duality in the descriptions of the structure of atomic nuclei.

Author(s): Jones, MD; MacChiavelli, AO; Wiedeking, M; Bernstein, LA; Crawford, HL; Campbell, CM; Clark, RM; Cromaz, M; Fallon, P; Lee, IY; Salathe, M; Wiens, A; Ayangeakaa, AD; Bleuel, DL; Bottoni, S; Carpenter, MP; Davids, HM; Elson, J; Gorgen, A; Guttormsen, M; Janssens, RVF; Kinnison, JE; Kirsch, L; Larsen, AC; Lauritsen, T; Reviol, W; Sarantites, DG; Siem, S; Voinov, AV; Zhu, S | Abstract: A model-independent technique was used to determine the γ-ray strength function (γSF) of Fe56 down to γ-ray energies less than 1 MeV for the first time with GRETINA using the (p,p′) reaction at 16 MeV. No difference was observed in the energy dependence of the γSF built on 2+ and 4+ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

A beam containing a substantial component of both the $J^{\pi}=5^+$, $T_{1/2}=162$ ns isomeric state of $^{18}$F and its $1^+$, 109.77-min ground state has been utilized to study members of the ground-state rotational band in $^{19}$F through the neutron transfer reaction $(d$,$p)$ in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2$^+$ band-terminating state. The agreement between shell-model calculations, using an interaction constructed within the $sd$ shell, and our experimental results reinforces the idea of a single-particle/collective duality in the descriptions of the structure of atomic nuclei.

We report the first observation of the 108Xe→104Te→100Sn α-decay chain. The α emitters, 108Xe [Eα=4.4(2) MeV, T1/2=58+106−23 μs] and 104Te [Eα=4.9(2) MeV, T1/2100Sn were produced using a fusion-evaporation reaction 54Fe(58Ni,4n)108Xe, and identified with a recoil mass separator and an implantation-decay correlation technique. This is the first time α radioactivity has been observed to a heavy self-conjugate nucleus. A previous benchmark for study of this fundamental decay mode has been the decay of 212Po into doubly magic 208Pb. Enhanced proton-neutron interactions in the N=Z parent nuclei may result in superallowed α decays with reduced α-decay widths significantly greater than that for 212Po. From the decay chain, we deduce that the α-reduced width for 108Xe or 104Te is more than a factor of 5 larger than that for 212Po.© 2018 American Physical Society

International audience; Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

Cross sections and corresponding momentum distributions have been measured for the first time at zero degrees for the exotic nuclei obtained from a beam of 18O at 8.5 MeV/A impinging on a 1 mg/cm2 238U target. Sizable cross sections were found for the production of exotic species arising from the neutron transfer and proton removal from the projectile. Comparisons of experimental results with calculations based on deep-inelastic reaction models, taking into account the particle evaporation process, indicate that zero degree is a scattering angle at which the differential reaction cross section for production of exotic nuclei is at its maximum. This result is important in view of the new generation of zero degrees spectrometers under construction, such as the S3 separator at GANIL, for example. Keywords: Nuclear reactions, Deep-inelastic collisions, Exotic nuclei

Rotation-aligned isomeric states and associated oblate collective sequences are established in even Pt isotopes. Reduced E 2 transition probabilities for the deexcitation of the 12 + isomers indicate an abrupt and unexpected quenching of oblate collectivity around neutron number N = 120 . Structure and shape evolution at high spin in the heaviest stable isotopes is found to be markedly different from observations in the lighter ones.

Fast-rotating N = 151 isotones 245 Pu, 247 Cm and 249 Cf have been studied through inelastic excitation and transfer reactions with radioactive targets. While all have a ground-state band built on a ν j 15 / 2 [ 734 ] 9 / 2 − Nilsson configuration, new excited bands have also been observed in each isotone. These odd- N excited bands allow a comparison of the alignment behavior for two different configurations, where the ν j 15 / 2 alignment is either blocked or allowed. The effect of higher order deformations is explored through cranking calculations, which help clarify the elusive nature of ν j 15 / 2 alignments.

Background: The $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction becomes important for sulfur production in novae if the $^{31}\mathrm{P}(p,\ensuremath{\alpha})^{28}\mathrm{Si}$ reaction rate is somewhat greater than currently accepted. The rate of the $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction is uncertain, primarily due to the properties of resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=156$ and 549 keV.Purpose: We precisely determined the excitation energies of states in $^{32}\mathrm{Cl}$ through high-resolution $\ensuremath{\gamma}$ spectroscopy including the two states most important for the $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction at nova temperatures.Method: Excited states in $^{32}\mathrm{Cl}$ were populated using the $^{10}\mathrm{B}(^{24}\mathrm{Mg},2n)\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction with a $^{24}\mathrm{Mg}$ beam from the ATLAS facility at Argonne National Laboratory. The reaction channel of interest was selected using recoils in the Fragment Mass Analyzer, and precise level energies were determined by detecting $\ensuremath{\gamma}$ rays with Gammasphere.Results: We observed $\ensuremath{\gamma}$ rays from the decay of six excited states in $^{32}\mathrm{Cl}$. The excitation energies for two unbound levels at ${E}_{x}=1738.1$ (6) keV and 2130.5 (10) keV were determined and found to be in agreement with a previous high-precision measurement of the $^{32}\mathrm{S}(^{3}\mathrm{He},t)^{32}\mathrm{Cl}$ reaction [1].Conclusions: An updated $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction rate is presented. With the excitation energies of important levels firmly established, the dominant uncertainty in the reaction rate at nova temperatures is due to the strength of the resonance corresponding to the 2131-keV state in $^{32}\mathrm{Cl}$.

The Si31 nucleus was produced through the O18(O18, αn) fusion-evaporation reaction at Elab=24MeV. Evaporated α particles from the reaction were detected and identified in the Microball detector array for channel selection. Multiple γ-ray coincidence events were detected in Gammasphere. The energy and angle information for the α particles was used to determine the Si31 recoil kinematics on an event-by-event basis for a more accurate Doppler correction. A total of 22 new states and 52 new γ transitions were observed, including 14 from states above the neutron separation energy. The positive-parity states predicted by the shell-model calculations in the sd model space agree well with experiment. The negative-parity states were compared with shell-model calculations in the psdpf model space with some variations in the N=20 shell gap. The best agreement was found with a shell gap intermediate between that originally used for A≈20 nuclei and that previously adapted for P32,34. This variation suggests the need for a more universal cross-shell interaction.

Excited states in Re186 with spins up to J=12 were investigated in two separate experiments using W186(d,2n) reactions at beam energies of 12.5 and 14.5 MeV. Two- and threefold γ-ray coincidence data were collected using the CAESAR and CAGRA spectrometers, respectively, each composed of Compton-suppressed high-purity germanium detectors. Analysis of the data revealed rotational bands built on several two-quasiparticle intrinsic states, including a long-lived Kπ=(8+) isomer. Configuration assignments were supported by an analysis of in-band properties, such as gK-gR values. The excitation energies of the observed intrinsic states were compared with results from multi-quasiparticle blocking calculations, based on the Lipkin-Nogami pairing approach, that included contributions from the residual proton-neutron interactions.

The calcium isotopes have emerged as a critical testing ground for new microscopically derived shell-model interactions, and a great deal of experimental and theoretical focus has been directed toward this region. We investigate the relative spectroscopic strengths associated with 1f7/2 neutron hole states in Ca47,49 following one-neutron knockout reactions from Ca48,50. The observed reduction of strength populating the 7/21- state in Ca49, as compared to Ca47, is inconsistent with shell-model calculations using both phenomenological interactions such as GXPF1, and interactions derived from microscopically based two- and three-nucleon forces. The result suggests a fragmentation of the l=3 strength to higher-lying states as suggested by the microscopic calculations, but the observed magnitude of the reduction is not reproduced in any shell-model description.

International audience; The 12C+12C fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle coincidence technique to measure the 12C+12C S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below Ec.m. = 3 MeV.

The nucleus ${}^{208}$Pb provides an iconic test of the nuclear shell model because its 82 protons and 126 neutrons are both magic numbers. When excited levels are discovered and characterized, this nucleus provides a means to investigate the mechanisms responsible for the generation of angular momentum and, ultimately, for the development of collectivity. This experiment is a tour de force that reveals extensive new information on ${}^{208}$Pb about high-angular-momentum states, isomeric states, and collective states of octupole nature; thus it serves as a showcase for shell-model descriptions. Many of the states explored show good overall agreement with theory, but others at the highest energies reveal discrepancies, pointing to the need for improved calculations. Above the highest-lying ${28}^{-}$ isomer $\ensuremath{\gamma}$ transitions arising from yet higher spin levels could be identified in two separate experiments as displayed in the figure.

International audience; Neutron-rich isotopes are an important source of new information on nuclear physics. Specifically, the spin-isospin components in the nucleon-nucleon (NN) interaction, e.g., the proton-neutron tensor force, are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest. The study of the excitation energy of states corresponding to specific configurations in even-even isotopes, together with the single-particle character of the first excited states of odd-A, neutron-rich Ni isotopes, probes the evolution of the neutron orbitals around the Fermi surface as a function of the neutron number a step forward in the understanding of the region and the nature of the NN interaction at large N/Z ratios. In an experiment carried out at the National Superconducting Cyclotron Laboratory [1], new spectroscopic information was obtained for 68Ni and the distribution of single-particle strengths in 67,69Ni was characterized by means of single-neutron knockout from 68,70Ni secondary beams. The spectroscopic strengths, deduced from the measured partial cross sections to the individual states tagged by their de-exciting gamma rays, is used to identify and quantify configurations that involve neutron excitations across the N = 40 harmonic oscillator shell closure. The de-excitation γ rays were measured with the GRETINA tracking array [2]. The results challenge the validity of the most current shell-model Hamiltonians and effective interactions, highlighting shortcomings that cannot yet be explained. These results suggest that our understanding of the low-energy states in such nuclei is not complete and requires further investigation.

Despite the more than 1 order of magnitude difference between the measured dipole moments in ^{144}Ba and ^{146}Ba, the octupole correlations in ^{146}Ba are found to be as strong as those in ^{144}Ba with a similarly large value of B(E3;3^{-}→0^{+}) determined as 48(+21-29) W.u. The new results not only establish unambiguously the presence of a region of octupole deformation centered on these neutron-rich Ba isotopes, but also manifest the dependence of the electric dipole moments on the occupancy of different neutron orbitals in nuclei with enhanced octupole strength, as revealed by fully microscopic calculations.

Excited states have been observed for the first time in the neutron-deficient nucleus $^{117}\mathrm{Ba}$ using the recoil-decay tagging technique following the heavy-ion fusion-evaporation reaction $^{64}\mathrm{Zn}$($^{58}\mathrm{Ni}$, 2p3n)$^{117}\mathrm{Ba}$. Prompt $\ensuremath{\gamma}$ rays have been assigned to $^{117}\mathrm{Ba}$ through correlations with $\ensuremath{\beta}$-delayed protons following the decay of $A=117$ recoils. Through the analysis of the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence relationships, a high-spin level scheme consisting of two bands has been established in $^{117}\mathrm{Ba}$. Based on the systematics of the level spacings in the neighboring barium isotopes, the two bands are proposed to have $\ensuremath{\nu}{h}_{11/2}[532]5/{2}^{\ensuremath{-}}$ and $\ensuremath{\nu}{d}_{5/2}[413]5/{2}^{+}$ configurations, respectively. The observed band-crossing properties are interpreted in the framework of cranked shell model.

States in $^{209}\mathrm{Tl}$ were populated using a multinucleon transfer reaction with a $^{136}\mathrm{Xe}$ beam impinging on a thick $^{208}\mathrm{Pb}$ target at $E=785$ MeV. The beam was pulsed at 825-ns intervals in order to perform isomer decay spectroscopy. The known ${J}^{\ensuremath{\pi}}=17/{2}^{+}$ isomer in $^{209}\mathrm{Tl}$ was located at 1228(4) keV and measured to have a half-life of ${T}_{1/2}=146(10)$ ns. A second isomer with ${J}^{\ensuremath{\pi}}=13/{2}^{+}$ was found to have ${T}_{1/2}=14(5)$ ns. The previously suggested low-energy X and Y transitions were found to have energies 57(2) and 47(2) keV respectively, while the measurement of conversion coefficients and a new decay path make the spin assignments below the isomers experimentally firm. Correlating the delayed $\ensuremath{\gamma}$ transitions with the prompt beam flash allowed the decay of states above the isomer to be found. The longer-lived isomer represents full alignment of the simplest two-particle, one-hole configuration and illuminates the remarkably weak coupling of the proton hole to the $^{210}\mathrm{Pb}$ core.

A fast-timing experiment was performed at the Argonne National Laboratory in December 2015 and January 2016, measuring decay radiation of fission products from a Cf-252 fission source. Details of the set-up, integration with Digital Gammasphere, and the data acquisition system are presented. The timing performance of the set-up, capable of measuring lifetimes from the nanosecond region down to tens of picoseconds, is discussed. First preliminary results from the fast-timing analysis of the fission fragment data are presented.

International audience; With the advent of γ-ray tracking detector arrays, it should be possible to measure entry distributions and the quasi-continuum of γ rays with better accuracy than was possible with the previous generation of escape—suppressed detector arrays. However, the techniques for doing so may represent more of a challenge for these new tracking arrays. Here, the techniques developed recently are being presented.

Carbon burning plays a crucial role in stellar evolution, where this reaction is an important route for the production of heavier elements. A particle-γ coincidence technique that minimizes the backgrounds to which this reaction is subject and provides reliable cross sections has been used at the Argonne National Laboratory to measure fusion cross-sections at deep sub-barrier energies in the 12 C+12 C system. The corresponding excitation function has been extracted down to a cross section of about 6 nb. This indicates the existence of a broad S-factor maximum for this system. Experimental results are presented and discussed.

Multi-quasiparticle high-K states, several of which are isomeric, were observed in Hf-180 with the Gammasphere array. We determined the lifetimes in the ns-μs range using centroid-shift and decay measurements within a mu s coincidence time window. The configurations of high-K states involve two and four quasiparticles, with states up to Kπ = (18-) established. High-K excitations are found to be progressively more favored with increasing excitation energy. The K quantum number is quite robust up to the highest spins observed, as evidenced by the large values of the reduced hindrance for isomeric decays. Furthermore, rotational bands built on three high-K states are identified, and the measured branching ratios in these sequences enable the assignment of underlying configurations. Multi-quasiparticle calculations using the Lipkin-Nogami approach for pairing, with blocking included, reproduce the observed high-K energies quite well.

Properties of neutron-rich nuclei in the A˜160 region are important for achieving a better understanding of the nuclear structure in this region where little is known owing to diffculties in the production of these nuclei at the present nuclear physics facilities. These properties are essential ingredients in the interpretation of the rareearth peak at A˜160 in the r process abundance distribution, since theoretical models are sensitive to nuclear structure input. Predicated on these ideas, we have initiated a new experimental program at Argonne National Laboratory. During the first experiment, beams from the Californium Rare Isotope Breeder Upgrade radioactive beam facility were used in conjunction with the SATURN decay station and the X-array. We focused initially on several odd-odd nuclei, where β decays of both the ground state and an excited isomer were investigated. Because of the spin difference, a variety of structures in the daughter nuclei were selectively populated and characterized based on their decay properties. Mass measurements using the Canadian Penning Trap aimed at establishing the excitation energy of the β-decaying isomers were also carried out. Evidence was found for a change in the single-particle structure, which in turn results in the formation of a sizable N=98 sub-shell gap at large deformation. Results from the first experimental campaign using the newly-commissioned β-decay station at Gammasphere are also presented.

New, low-lying levels in the odd-odd, N=Z nucleus 62Ga have been identified using a sensitive technique, where in-beam γ rays from short-lived nuclei are tagged with β decays following recoil mass identification. A comparison of the results with shell-model and IBM-4 calculations demonstrates good agreement between theory and experiment, with the majority of predicted low-lying, low-spin T=0 states now identified. There is a dramatic change in the level density at low excitation energies for the N=Z nucleus 62Ga when compared with neighbouring odd-odd Ga isotopes where, in contrast, the low-lying level structure is dominated by configurations with T=1 pairing interactions between excess neutrons. This illustrates the distinctively different aspects of nuclear structure exhibited by nuclei with N=Z. © 2013 Elsevier B.V.

New spectroscopic results for high-spin states in Os-192 populated in deep-inelastic reactions include the identification of a 2-ns, 12(+) isomeric state at 2865 key and a 295-ns, 20(+) state at 4580 keV and their associated Delta J = 2 sequences. The structures are interpreted as manifestations of maximal rotation alignment within the neutron i(13/2) and proton h(11/2) shells at oblate deformation. Rotational band members based on the long-lived, K-pi =10(-) isomer are also identified for the first time. Configuration-constrained, potential-energy-surface calculations predict that other prolate multi-quasiparticle high-K states should exist at low energy. (c) 2013 Elsevier B.V. All rights reserved.

The first rotational sequences have been assigned to the odd-odd nucleus $^{168}\mathrm{Re}$. Coincidence relationships of these structures with rhenium x rays confirm the isotopic assignment, while arguments based on the $\ensuremath{\gamma}$-ray multiplicity ($K$-fold) distributions observed with the new bands lead to the mass assignment. Configurations for the two bands were determined through analysis of the rotational alignments of the structures and a comparison of the experimental $B(M1)/B(E2)$ ratios with theory. Tentative spin assignments are proposed for the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\nu}{i}_{13/2}$ band, based on energy level systematics for other known sequences in neighboring odd-odd rhenium nuclei, as well as on systematics seen for the signature inversion feature that is well known in this region. The spin assignment for the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\nu}({h}_{9/2}/{f}_{7/2})$ structure provides additional validation of the proposed spins and configurations for isomers in the $^{176}\mathrm{Au} \ensuremath{\rightarrow} ^{172}\mathrm{Ir}\ensuremath{\rightarrow}^{168}\mathrm{Re} \ensuremath{\alpha}$-decay chain.

The distribution of single-particle strength in 67,69Ni was characterized with one-neutron knockout reactions from intermediate-energy 68,70Ni secondary beams, selectively populating neutron-hole configurations at N = 39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While 69Ni compares well to shell-model predictions, the results for 67Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semi-magic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

International audience; In this paper, we provide a formalism for the characterization of tracking arrays with emphasis on the proper corrections required to extract their photopeak efficiencies and peak-to-total ratios. The methods are first applied to Gammasphere, a well characterized 4π array based on the principle of Compton suppression, and subsequently to GRETINA. The tracking efficiencies are then discussed and some guidelines as to what clustering angle to use in the tracking algorithm are presented. It was possible, using GEANT4 simulations, to scale the measured efficiencies up to the expected values for the full 4π implementation of GRETA.