Search

Isomers with three- and five-nucleon-hole configurations have been established in $^{203}$Tl. These include newly identified levels with a three-nucleon structure: {\it I}$^{\pi }$ = (15/2$^{-}$) with {\it T}$_{1/2}$ = 7.9(5) ns, and {\it I}$^{\pi }$ = (35/2$^{-}$) with {\it T}$_{1/2}$ = 4.0(5) ns. In addition, five-quasiparticle states: {\it I}$^{\pi }$ = (39/2$^{-}$) with {\it T}$_{1/2}$ = 1.9(2) ns, and {\it I}$^{\pi }$ = (49/2$^{+}$) with {\it T}$_{1/2}$ = 3.4(4) ns have also been established. The previously determined long-lived decay [{\it T}$_{1/2}$ = 6.6(3) $\mu $s from this work] is associated with isomerism of the {\it I}$^{\pi }$ = (29/2$^{+}$) state. Levels above this long-lived isomer have been identified through a delayed-prompt coincidence measurement. Five-nucleon-hole states with excitation energies {\it E}$_{x}$ $\approx $ 7 MeV have been established as well as possible octupole excitations of the $^{208}$Pb core built on these levels. The level scheme of $^{203}$Tl is extended up to {\it E}$_{x}$ $\approx $ 11 MeV with the inclusion of 25 new transitions. Empirical and shell-model calculations have been performed to aid in the description of the observed states which are found to be predominantly of intrinsic character., Comment: 14 pages, 10 figures

The negative-parity band structure built on the proton h9/2 state in 199Tl has been established up to an excitation energy of 7 MeV and spin of (51/2) ¯h. The level scheme has been extended with the inclusion of fourteen new transitions de-exciting levels at high spin. The I = 1 γ rays are found to be more prominently visible in the spectra when comparing to I = 2 transitions. Rotation alignments are evident at frequencies of 0.22 and 0.30 MeV with both being attributed to the breaking of pairs of neutrons in the i13/2 subshell. Since 199Tl is expected to have weak oblate deformation, and lies in a transitional region where competing contributions to the spin are expected from collective rotation and the angular momentum of high-j nucleons, calculations using both the principal axis cranking (PAC) and tilted axis cranking (TAC) formalisms have been performed to understand its structure. Both the PAC and TAC calculations are found to provide a satisfactory description of the evolution of excitation energies with spin. The PAC calculations give a good account of the experimental crossing frequencies and associated spins. The structure of the yrast, negative-parity band in 199Tl may be primarily understood in terms of the collective rotation of a moderately deformed oblate nucleus, along with contributions to the angular momentum from two pairs of rotation-aligned i13/2 neutrons.

Two long-standing puzzles in the decay of ^{185}Bi, the heaviest known proton-emitting nucleus are revisited. These are the nonobservation of the 9/2^{-} state, which is the ground state of all heavier odd-A Bi isotopes, and the hindered nature of proton and α decays of its presumed 60-μs 1/2^{+} ground state. The ^{185}Bi nucleus has now been studied with the ^{95}Mo(^{93}Nb,3n) reaction in complementary experiments using the Fragment Mass Analyzer and Argonne Gas-Filled Analyzer at Argonne National Laboratory's ATLAS facility. The experiments have established the existence of two states in ^{185}Bi; the short-lived T_{1/2}=2.8_{-1.0}^{+2.3} μs, proton- and α-decaying ground state, and a 58(2)-μs γ-decaying isomer, the half-life of which was previously attributed to the ground state. The reassignment of the ground-state lifetime results in a proton-decay spectroscopic factor close to unity and represents the only known example of a ground-state proton decay to a daughter nucleus (^{184}Pb) with a major shell closure. The data also demonstrate that the ordering of low- and high-spin states in ^{185}Bi is reversed relative to the heavier odd-A Bi isotopes, with the intruder-based 1/2^{+} configuration becoming the ground, similar to the lightest At nuclides.

Non-yrast, excited states in neutron-rich $^{186}$W were populated via inelastic-scattering reactions using beams of $^{136}$Xe nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particle-$ \gamma $ coincidence techniques using the Gammasphere array of High-Purity Germanium detectors and the compact heavy-ion counter, CHICO2. The $ K^{\pi} = 2 ^{+} $ ($\gamma$), $ K^{\pi} = 0^{+}$ and $ K^{\pi} = 2^{-} $ (octupole) rotational side bands were extended to spins $ 14\hbar $, $ 12\hbar $, and $ 13\hbar $, respectively. A staggering pattern observed in the energies of levels in the $ K^{\pi} = 2^{+} $ band was found to be consistent with a potential that gets softer to vibration in the $ \gamma $ degree of freedom with increasing spin. The odd-even staggering of states in the $ K^{\pi} = 2^{-}$ band was found to exhibit a phase opposite to that seen in the $ \gamma $ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in $^{186}$W., Comment: 14 pages, 16 figures

The astrophysical $^{25}\mathrm{Al}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{26}\mathrm{Si}$ reaction represents one of the key remaining uncertainties in accurately modeling the abundance of radiogenic $^{26}\mathrm{Al}$ ejected from classical novae. Specifically, the strengths of key proton-unbound resonances in $^{26}\mathrm{Si}$, that govern the rate of the $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ reaction under explosive astrophysical conditions, remain unsettled. Here, we present a detailed spectroscopy study of the $^{26}\mathrm{Si}$ mirror nucleus $^{26}\mathrm{Mg}$. We have measured the lifetime of the ${3}^{+}$, 6.125-MeV state in $^{26}\mathrm{Mg}$ to be $19(3)\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ and provide compelling evidence for the existence of a ${1}^{\ensuremath{-}}$ state in the $T=1,\phantom{\rule{0.28em}{0ex}}A=26$ system, indicating a previously unaccounted for $\ensuremath{\ell}=1$ resonance in the $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ reaction. Using the presently measured lifetime, together with the assumption that the likely ${1}^{\ensuremath{-}}$ state corresponds to a resonance in the $^{25}\mathrm{Al}+p$ system at 435.7(53) keV, we find considerable differences in the $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ reaction rate compared to previous works. Based on current nova models, we estimate that classical novae may be responsible for up to $\ensuremath{\approx}15%$ of the observed galactic abundance of $^{26}\mathrm{Al}$.

Excited states in the neutron-rich nucleus Fe 68 were populated using a Be 9 ( Co 68 , 68 Fe + γ ) X charge-exchange reaction at 95 MeV/u. The new γ -ray transitions reported here for the first time complement data from β -decay studies and nucleon knockout reactions. In comparison to shell-model calculations with the LNPS effective interaction, two candidate states for the 6 1 + level emerge. The distinct population pattern of excited states and the magnitude of the cross section, σ inc = 0.51 ( 6 ) mb, make this reaction a promising one for future in-beam γ -ray spectroscopy. Reaction calculations with nuclear structure input from a new, locally optimized Hamiltonian, f 7 j 4 a , together with general considerations for heavy-ion-induced charge-exchange reactions appear consistent with most of the observations, although challenges remain.

Employing the Argonne Fragment Mass Analyzer and the implantation-decay-decay correlation technique, a weak 0.50(21)% proton decay branch was identified in 108I for the first time. The 108I proton-decay width is consistent with a hindered l = 2 emission, suggesting a d 5 2 origin. Using the extracted 108I proton-decay Q value of 597(13) keV, and the Q α values of the 108I and 107Te isotopes, a proton-decay Q value of 510(20) keV for 104Sb was deduced. Similarly to the 112,113Cs proton-emitter pair, the Q p ( I 108 ) value is lower than that for the less-exotic neighbor 109I, possibly due to enhanced proton-neutron interactions in N ≈ Z nuclei. In contrast, the present Q p ( Sb 104 ) is higher than that of 105Sb, suggesting a weaker interaction energy. For the present Q p ( Sb 104 ) value, network calculations with the one-zone X-ray burst model Mazzocchi et al. (2007) [18] predict no significant branching into the Sn-Sb-Te cycle at 103Sn.

The second-phonon ( n ω = 2 ) wobbling band has been established in the nucleus 135Pr. Conclusive evidence for its wobbling nature comes from the Δ I = 1 , E2 character of the transitions between the new band and the previously identified transverse wobbler band ( n ω = 1 ) in this nucleus. Theoretical calculations in the framework of the quasiparticle triaxial rotor and triaxial projected shell models are found to be in good agreement with the experimental results.

The B(E3) transition strength from the 1897-keV, 3− level in 96Zr has been reevaluated from six high-statistics, independent data sets. The measured value of 42(3) W.u. is significantly lower than that adopted in recent compilations. It is, however, in line with the global systematics of collective B(E3) rates found throughout the periodic table. Thus, the “exceptional” character of this transition, that challenged theory, no longer applies. Monte Carlo shell-model calculations indicate that the collectivity of the octupole vibration arises from both proton and neutron excitations involving a large number of orbitals. Keywords: 90≤A≤149, 3− level, Octupole excitation, Lifetimes and branching ratios, B(E3) strength, Monte Carlo shell model calculations

The $^{24}\mathrm{Mg}+^{12}\mathrm{C}$ fusion reaction was used to perform a detailed $\ensuremath{\gamma}$-ray spectroscopy study of the astrophysically important nucleus $^{34}\mathrm{Ar}$. In particular, an experimental setup, coupling the advanced $\ensuremath{\gamma}$-ray tracking array GRETINA with the well-established Argonne fragment mass analyzer (FMA), was employed to obtain excitation energies and spin-parity assignments for excited states in $^{34}\mathrm{Ar}$, both above and below the proton separation energy. For the first time, an angular distribution analysis of in-beam $\ensuremath{\gamma}$ rays from fusion-evaporation reactions, using a tracking array, has been performed and Coulomb energy differences of analog states in the $T=1,$ $A=34$ mirror system, explored from 0 to 6 MeV. Furthermore, we present a comprehensive discussion of the astrophysical $^{33}\mathrm{Cl}(p,\ensuremath{\gamma})$ stellar reaction rate, together with implications for the identification of nova presolar grains from sulfur isotopic abundances.

The ground-state properties of neutron-rich $^{106}\mathrm{Nb}$ and its $\ensuremath{\beta}$ decay into $^{106}\mathrm{Mo}$ have been studied using the CARIBU radioactive-ion-beam facility at Argonne National Laboratory. Niobium-106 ions were extracted from a $^{252}\mathrm{Cf}$ fission source and mass separated before being delivered as low-energy beams to the Canadian Penning Trap, as well as the X-Array and SATURN $\ensuremath{\beta}$-decay-spectroscopy station. The measured $^{106}\mathrm{Nb}$ ground-state mass excess of $\ensuremath{-}66202.0(13)$ keV is consistent with a recent measurement but has three times better precision; this work also rules out the existence of a second long-lived, $\ensuremath{\beta}$-decaying state in $^{106}\mathrm{Nb}$ above 5 keV in excitation energy. The decay half-life of $^{106}\mathrm{Nb}$ was measured to be 1.097(21) s, which is $8%$ longer than the adopted value. The level scheme of the decay progeny, $^{106}\mathrm{Mo}$, has been expanded up to $\ensuremath{\approx}4\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The distribution of decay strength and considerable population of excited states in $^{106}\mathrm{Mo}$ of $J\ensuremath{\ge}3$ emphasizes the need to revise the adopted ${J}^{\ensuremath{\pi}}={1}_{}^{\ensuremath{-}}$ ground-state spin-parity assignment of $^{106}\mathrm{Nb}$; it is more likely to be $J\ensuremath{\ge}3$.

The dipole strength of the $N=28$ closed-shell nuclide $^{54}\mathrm{Fe}$ was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of kinetic energies of 7.5 and 13.9 MeV at the $\ensuremath{\gamma}\mathrm{ELBE}$ facility as well as using quasimonoenergetic and linearly polarized photon beams of 26 different energies within the range from 5.5 to 11.4 MeV at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility. About 100 $J=1$ states were newly identified, out of them 19 with ${1}^{+}$ and 30 with ${1}^{\ensuremath{-}}$ assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $\ensuremath{\gamma}$-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental $M1$ strengths from resolved ${1}^{+}$ states are compared with results of large-scale shell-model calculations.

The 92-keV resonance in the 25Mg ( p , γ ) 26 Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26 g Al, or isomeric state, 26 m Al. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f 0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f 0 = 0.52 ( 2 ) s t a t ( 6 ) s y s t . The f 0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26 g Al and its cosmic 1809-keV γ rays.

Isomeric states and associated collective structures have been studied up to high spin in $^{198,200,202}\mathrm{Hg}$ using multinucleon transfer reactions and the Gammasphere array. A coupled rotational band, with possible four-quasiparticle character, is established in $^{198}\mathrm{Hg}$. Sequences built on two-quasiparticle, positive- and negative-parity levels are assigned to $^{202}\mathrm{Hg}$. New isomers in $^{202}\mathrm{Hg}$ with ${I}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}})$ and $({9}^{\ensuremath{-}})$, and ${T}_{1/2}$ = 10.4(4) ns and 1.4(3) ns, respectively, have been identified. A half-life of 1.0(3) ns is established for the ${I}^{\ensuremath{\pi}}={12}^{+}$ state in $^{200}\mathrm{Hg}$. $B(E2)$ values deduced from isomeric transitions in Hg isotopes indicate that, while collectivity near the ground state gradually diminishes from $N$ = 112 to $N$ = 124, it is found to increase for the ${12}^{+}$ and ${9}^{\ensuremath{-}}$ states up to $N$ = 118, followed by a reduction for higher neutron numbers. Calculations using the ultimate cranker code provide insight into the variation of deformation with spin and allow for an understanding of observed band crossings. The evolution of collectivity with spin, and along the isotopic chain, is described.

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number $N=40$. We report here on the first $\ensuremath{\gamma}$-ray spectroscopy beyond the $({4}_{1}^{+})$ state in $^{62,64}\mathrm{Cr}$, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the ${6}^{+}$ state in $^{64}\mathrm{Cr}$ is presented as well as one for, possibly, the second excited ${0}^{+}$ state in $^{62}\mathrm{Cr}$. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the $N=40$ harmonic oscillator gap into the ${g}_{9/2}$ and ${d}_{5/2}$ orbitals. The calculated level schemes for $^{62,64}\mathrm{Cr}$ reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, $^{62}\mathrm{Cr}$ emerges as a transitional system on the path to the center of the $N=40$ island of inversion.

High-spin states in Xe-123 were populated in the Se-80(Ca-48, 5n) Xe-123 reaction at a beam energy of 207 MeV. gamma-ray coincidence events were recorded with the Gammasphere spectrometer. Four new high-spin bands have been discovered in this nucleus. The bands are compared with those calculated within the framework of cranked Nilsson-Strutinsky and cranked Nilsson-Strutinsky-Bogoliubov models. It is concluded that the configurations of the bands involve two-proton excitations across the Z = 50 as well as excitation of neutrons across the N = 82 shell gaps resulting in a large deformation, epsilon(2) approximate to 0.30 and gamma approximate to 5 degrees.

Author(s): Morse, C; MacChiavelli, AO; Crawford, HL; Zhu, S; Wu, CY; Wang, YY; Meng, J; Back, BB; Bucher, B; Campbell, CM; Carpenter, MP; Chen, J; Clark, RM; Cromaz, M; Fallon, P; Henderson, J; Janssens, RVF; Jones, MD; Khoo, TL; Kondev, FG; Lauritsen, T; Lee, IY; Li, J; Potterveld, D; Santamaria, C; Savard, G; Seweryniak, D; Stolze, S; Weisshaar, D | Abstract: The neutron-rich barium nuclei have been the subject of intense interest due to the enhanced octupole correlations they are predicted to exhibit. The observation of enhanced octupole collectivity in Ba144,146 as measured in sub-barrier Coulomb excitation, consistent with static octupole deformation, has further heightened this interest. In the present work, these studies are extended to the neighboring odd-mass Ba143 to investigate the interplay between single-particle and collective octupole degrees of freedom. A new measurement of the first 92 - state lifetime is also presented. Reflection-Asymmetric Triaxial Particle Rotor Model calculations indicate that the negative-parity bands in Ba143 can be understood as a decoupled structure of νh9/2 parentage, while the positive-parity bands are built on a decoupled octupole phonon. No evidence for E3 excitation is observed in this work, but an upper limit is placed on the E3 matrix element to the lowest octupole band.

The fusion-evaporation reaction 28Si + 40Ca at 122 MeV beam energy was used to populate high-spin states in the odd-odd N = Z nucleus 62Ga. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array the decay scheme of 62Ga was extended beyond 10 MeV excitation energy. The onset of band structures was observed. These high-spin rotational states are interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357 and Grants No. DEFG02-94-ER40834, DEFG02-97-ER41041, DEFG02-97-ER41043, DEFG02-94-ER40848, and DESC0014231. UK personnel were supported by the Science and Technologies Facilities Council (STFC) and C. A. was supported by the Natural Sciences and Engineering Research Council of Canada. J. J. acknowledges support by the Spanish MINECO Grant No. AYA2017-86274-P, by the E. U. FEDER funds, and by the AGAUR/Generalitat de Catalunya Grant No. SGR661/2017. This research uses resources of ANL’s ATLAS facility, which is a DOE Office of Science User facility.

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

We extend previous measurements of the transfer product yields in the reaction of $$\hbox {E}_{c.m.} = 450\hbox { MeV} ^{~136}\hbox {Xe}$$ with $$^{198}\hbox {Pt}$$ by measurements of the product yields using Gammasphere. By recording events occurring in beam bursts and in between beam bursts, we are able to extend the number of measured product yields from 78 to 171 nuclides. Our new data span a much wider range of Z and A than observed in previous work and when compared to theoretical predictions, these new measurements provide a more stringent and thorough test of models of multi-nucleon transfer (MNT) reactions.

The European physical journal / A 56(4), 105 (2020). doi:10.1140/epja/s10050-020-00118-8, Published by Springer, Heidelberg

Excited states in $^{160}\mathrm{Gd}$ were populated via $\ensuremath{\beta}$ decay from the low- and high-spin isomers in $^{160}\mathrm{Eu}$. The high-spin, ${K}^{\ensuremath{\pi}}={5}^{\ensuremath{-}}$ state feeds several two-quasiparticle levels, as well as a sequence associated with a $\ensuremath{\gamma}$ vibration and a ${K}^{\ensuremath{\pi}}={4}^{+}$, hexadecapole vibrational structure. The decay scheme was significantly improved with the observation of new transitions and states when compared with the two competing level schemes from over four decades ago. Configuration assignments for some of the multiquasiparticle levels have been suggested, based upon decay properties, systematics from neighboring nuclei, and comparisons with theoretical calculations. In addition, 15 new low-spin states and approximately 60 new transitions were observed resulting from the decay of the low-spin $^{160}\mathrm{Eu}$ isomer.

A previous study of symmetric collisions of massive nuclei has shown that current models of multinucleon transfer (MNT) reactions do not adequately describe the transfer product yields. To gain further insight into this problem, we have measured the yields of MNT products in the interaction of 977 ($E/A=4.79$ MeV) and 1143 MeV ($E/A=5.60$ MeV) $^{204}\mathrm{Hg}$ with $^{208}\mathrm{Pb}$. We find that the yield of multinucleon transfer products are similar in these two reactions and are substantially lower than those observed in the reaction of 1257 MeV ($E/A=6.16$ MeV) $^{204}\mathrm{Hg}+^{198}\mathrm{Pt}$. We compare our measurements with the predictions of the GRAZING-F, dinuclear systems (DNS), and improved quantum molecular dynamics (ImQMD) models. For the observed isotopes of the elements Au, Hg, Tl, Pb, and Bi, the measured values of the MNT cross sections are orders of magnitude larger than the predicted values. Furthermore, the various models predict the formation of nuclides near the $N=126$ shell, which are not observed.

An experiment involving a heavy-ion-induced fusion-evaporation reaction was carried out where high-spin states of Xe123 were populated in the Se80(Ca48,5n)Xe123 reaction at 207 MeV beam energy. Gamma-ray coincidence events were recorded with the Gammasphere Ge detector array. The previously known level scheme was confirmed and enhanced with the addition of five new band structures and several interband transitions. Cranked Nilsson-Strutinsky (CNS) calculations were performed and compared with the experimental results in order to assign configurations to the bands.

The discovery of presolar grains in primitive meteorites has initiated a new era of research in the study of stellar nucleosynthesis. However, the accurate classification of presolar grains as being of specific stellar origins is particularly challenging. Recently, it has been suggested that sulfur isotopic abundances may hold the key to definitively identifying presolar grains with being of nova origins and, in this regard, the astrophysical 33Cl(p,γ)34Ar reaction is expected to play a decisive role. As such, we have performed a detailed γ-ray spectroscopy study of 34Ar. Excitation energies have been measured with high precision and spin-parity assignments for resonant states, located above the proton threshold in 34Ar, have been made for the first time. Uncertainties in the 33Cl(p,γ) reaction have been dramatically reduced and the results indicate that a newly identified ℓ=0 resonance at Er=396.9(13) keV dominates the entire rate for T=0.25–0.40GK. Furthermore, nova hydrodynamic simulations based on the present work indicate an ejected 32S/33S abundance ratio distinctive from type-II supernovae and potentially compatible with recent measurements of a presolar grain., This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357 and Grants No. DEFG02-94-ER40834, DEFG02-97-ER41041, DEFG02-97-ER41043, DEFG02-94-ER40848, and DESC0014231. UK personnel were supported by the Science and Technologies Facilities Council (STFC) and C. A. was supported by the Natural Sciences and Engineering Research Council of Canada. J. J. acknowledges support by the Spanish MINECO Grant No. AYA2017-86274-P, by the E. U. FEDER funds, and by the AGAUR/Generalitat de Catalunya Grant No. SGR661/2017. This research uses resources of ANL’s ATLAS facility, which is a DOE Office of Science User facility.

A recoil mass separator SECAR has been designed for the purpose of studying low-energy ( p , γ ) and ( α , γ ) reactions in inverse kinematics with radioactive beams for masses up to about A = 65. Their reaction rates are of importance for our understanding of the energy production and nucleosynthesis during explosive hydrogen and helium burning. The radiative capture reactions take place in a windowless hydrogen or He gas target at the entrance of the separator, which consists of four Sections . The first Section selects the charge state of the recoils. The second and third Sections contain Wien Filters providing high mass resolving power to separate efficiently the intense beam from the few reaction products. In the following fourth Section , the reaction products are guided into a detector system capable of position, angle and time-of-flight measurements. In order to accept the complete kinematic cone of recoil particles including multiple scattering in the target in the center of mass energy range of 0.2 MeV to 3.0 MeV, the system must have a large polar angle acceptance of ± 25 mrad. This requires a careful minimization of higher order aberrations. The present system will be installed at the NSCL ReA3 accelerator and will be used with the much higher beam intensities of the FRIB facility when it becomes available.

Journal of physics / G 49(1), 010502 (2022). doi:10.1088/1361-6471/ac2827, Published by IOP Publ., Bristol

The structure of 71Zn was investigated by one-neutron transfer and heavy-ion induced complex (deep-inelastic) reactions using the GRETINA-CHICO2 and the Gammasphere setups, respectively. The observed inversion between the 9/2+ and 1/2− states is explained in terms of the role of neutron pairing correlations. Non-collective sequences of levels were delineated above the 9/2+ isomeric state. These are interpreted as being associated with a modest oblate deformation in the framework of Monte-Carlo shell-model calculations carried out with the A3DA-m Hamiltonian in the p f g 9 / 2 d 5 / 2 valence space. Similarities with the structure of Ni 40 28 68 were observed and the shape-coexistence mechanism in the N = 40 region of neutron-rich nuclei is discussed in terms of the so-called Type-II shell evolution, with an emphasis on proton–neutron correlations between valence nucleons, especially those involving the shape-driving g 9 / 2 neutron orbital.

One-phonon states of vibrational nuclei with mixed proton–neutron symmetry have been observed throughout the nuclear chart besides the mass A ≈ 200 region. Very recently, it has been proposed that the 2 2 + state of 212Po is of isovector nature. This nucleus has two valence protons and two valence neutrons outside the doubly-magic 208Pb nucleus. The stable isotope 204Hg, featuring two valence-proton and valence-neutron holes, with respect to 208Pb, is the particle-hole mirror of 212Po. In order to compare the properties of low-lying isovector excitations in these particle-hole mirror nuclei, we have studied 204Hg by using the projectile Coulomb-excitation technique. The measured absolute B ( M 1 ; 2 2 + → 2 1 + ) strength of 0.20 ( 2 ) μ N 2 indicates that the 2 2 + level of 204Hg is at least the main fragment of the 2 1 , m s + state. For the first time in this mass region, both lowest-lying, one-quadrupole phonon excitations are established together with the complete set of their decay strengths. This allows for a microscopic description of their structures, achieved in the framework of the Quasi-particle Phonon Model.

Author(s): Elman, B; Gade, A; Janssens, RVF; Ayangeakaa, AD; Bazin, D; Belarge, J; Bender, PC; Brown, BA; Campbell, CM; Carpenter, MP; Crawford, HL; Crider, BP; Fallon, P; Forney, AM; Harker, J; Liddick, SN; Longfellow, B; Lunderberg, E; Prokop, CJ; Sethi, J; Taniuchi, R; Walters, WB; Weisshaar, D; Zhu, S | Abstract: The neutron-rich Ni isotopes have attracted attention in recent years because of the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in Ni70 following γ-ray tagged one-proton, one-neutron, and two-proton knockout from Cu71, Ni71, and Zn72 rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of Ni70 is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z=28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for Ni70, 29 new transitions are reported, of which 15 are placed in its level scheme.

The decay of $K$ isomers in Cm ($Z$ = 96) isotopes has been studied using inelastic and transfer reactions. The half-life of a previously identified 2-quasiparticle (qp) ${K}^{\ensuremath{\pi}}={8}^{\ensuremath{-}}$ isomer in $^{246}\mathrm{Cm}$ has been measured. A new 2-qp isomer is observed in $^{248}\mathrm{Cm}$, its half-life measured and its decay scheme established. The reduced $K$ hindrances extracted for the decay transitions from the isomers in $^{246,248}\mathrm{Cm}$ indicate $K$ to be a robust quantum number and validate axial symmetry in these nuclei. The excitation energies of the 2-qp isomers in $^{246}\mathrm{Cm}$ ($N$ = 150) and $^{248}\mathrm{Cm}$ ($N$ = 152) support the persistence of a deformed subshell gap at $N$ = 152 in the $Z\ensuremath{\approx}100$ region down to $Z$ = 96 nuclei.

An extensive, model-independent analysis of the nature of triaxial deformation in ^{76}Ge, a candidate for neutrinoless double-beta (0νββ) decay, was carried out following multistep 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 γ bands. Both sequences were determined to be characterized by the same β and γ 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 0νββ.

The structure of decoupled positive-parity and semidecoupled negative-parity bands up to high spin in $^{197,199}\mathrm{Hg}$ has been studied using multinucleon transfer reactions with the Gammasphere array. A positive-parity band in $^{199}\mathrm{Hg}$ with the ${(\ensuremath{\nu}{i}_{13/2})}^{3}$ decoupled configuration that deexcites to levels in the negative-parity sequences is newly established. The semidecoupled sequences in both isotopes are extended to higher spin. Rotation-aligned angular momenta and crossing frequencies determined from experiment are in fair agreement with those from cranking calculations. A transition from oblate collective rotation at low spin to triaxial noncollective shapes at high spin is indicated.