Search

β-delayed γ-ray measurements have been part of an experimental program at the NSCL to understand the role of the πf7/2 - νf5/2 proton-neutron monopole interaction in neutron-rich pf-shell nuclei above 48Ca. Central to this study has been an attempt to observe the development of new shell structure at N = 32, 34 through the systematic observation of E(2+1 ) as a function of neutron number. Additionally, the ground state spin and parity of odd-odd and odd-A nuclei were interpreted in an extreme single-particle model to follow the monopole migration of the νf5/2 as protons are removed from the πf7/2state.

A new experiment devoted to the fission of 252Cf is described. It continued a series of our experiments based on correlation measurements of γ rays emitted by fission fragment pairs. The measurements of γ-γ and γ-γ-γ coincidences were done at Gammasphere with closed 252Cf sources. The open source was used for the first time in the last experiment. Fission fragment detectors were arranged in the center hole of Gammasphere. Correlations between fission fragment masses, total kinetic energy, and γ rays were observed. The first, preliminary results of data analysis are discussed.

The excitation function for the fusion–evaporation reaction Si 28 + Ni 64 has been measured down to a cross section of 25 nb. This is the first observation of fusion hindrance at extreme sub-barrier energies for a system with a small, negative Q-value ( − 1.78 MeV ). This result is further proof that heavy-ion fusion hindrance, reported earlier only for systems with large, negative Q-values, is a general phenomenon. The measured behavior can be reproduced by coupled-channels calculations with a modified ion–ion potential incorporating the effects of nuclear incompressibility.

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work, the high-spin structure of Er-156,Er-157,Er-158 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently four strongly or superdeformed (SD) sequences have been observed in Hf-174, and cranking calculations using the Ultimate Cranker code predict that such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment took place to search for linking transitions between the SD bands, possible wobbling modes, and new SD band structures.

Conventionally, reaction rates relevant to nova nucleosynthesis are determined by performing the relevant proton capture reactions directly for stable species, or as has become possible more recently in inverse kinematics using short-lived accelerated radioactive beams with recoil separators. A secondary approach is to compile information on the properties of levels in the Gamow window using transfer reactions. We present a complementary technique where the states of interest are populated in a heavy-ion fusion reaction and their gamma decay studied with a state-of-the-art array of high-purity germanium detectors. The advantages of this approach, including the ability to determine resonance energies with high precision and the possibility of determining spins and parities from gamma-ray angular distributions are discussed. Two specific examples related to the 22Na(p, γ) and 30P(p, γ) reactions are presented.

The doubly magic N = Z nucleus 56Ni has been investigated with two fusion-evaporation reactions; 40Ca(28Si, 3α)56Ni at a beam energy of 122MeV and 28Si(32S, 2p2n)56Ni at 130MeV. To detect γ-rays in coincidence with evaporated particles the Ge-detector array Gammasphere was used in conjunction with the charged-particle detector system, Microball and a 1π neutron detector array. Results include a significantly extended level scheme of 56Ni, which is compared to large-scale shell model calculations in the fp shell. The experimental and theoretical results agree to a large extent, with one notable exception; the theoretical model fails to predict the proper sequence of the yrast and yrare 8+ states.

A new focal-plane detector system has been designed and constructed for measuring very small heavy-ion induced fusion-evaporation cross sections with the Fragment Mass Analyzer (FMA) at Argonne National Laboratory. The system has been tested and successfully used in a fusion-evaporation excitation function measurement for 64 Ni + 100 Mo down to the 5 nb level. The design and performance of the detector system are described in detail with some examples from the latest data. Besides excellent tracking capabilities for each event measured in the detector, suppression of scattered beam background of the order of 4 × 10 - 17 has been demonstrated.

Excited states have been identified in the very neutron-deficient 122Ce nucleus. This is the first observation of this nucleus and its excited states. The ground-state rotational band has been observed up to spin 14 ℏ. The band has been assigned to 122Ce by detecting gamma rays in coincidence with evaporated charged particles and neutrons. The E ( 2 1 + ) value suggests a rather large ground-state deformation of β 2 = 0.35 , in good agreement with Hartree–Fock–Bogoliubov (HFB) mean-field calculations. The aligned angular momentum of the band has been studied and is compared with those of the neighboring even–even cerium isotopes, and to Woods–Saxon cranking calculations. The non-observation of the π ( h 11 / 2 ) 2 alignment until at least 0.4 MeV/ℏ is consistent with the extracted value of β 2 .

The low-lying levels in 59,60Cr have been populated with 13,14C(48Ca, 2p) reactions using a beam energy of 130 MeV. Prompt electromagnetic radiation was detected using the Gammasphere array, in coincidence with recoiling ions measured with the Fragment Mass Analyzer. The residues were selected and identified on the basis of charge-to-mass ratio, energy-loss and time-of-flight measurements. Preliminary results for 60Cr, when compared to lighter even-even isotopes, indicate a softness in shape which increases with neutron number. The low-lying structure of 59Cr is clearly inconsistent with the results of a spherical shell-model calculation and requires the inclusion of the νg9/2 orbital. The sequence of states can be understood within the Nilsson model assuming a moderate oblate ground-state deformation. This is in contrast to lighter odd-Cr nuclei where there is evidence for prolate deformation after excitation of g9/2 neutrons.

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of special terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work the high-spin structure of Er-157 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently, four strongly or superdeformed (SD) sequences have been observed in Hf-174 and ultimate cranker calculations predict, such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment was run to search for linking transitions between the SD bands and possible wobbling modes.

Excited states in 57 Cr have been populated to high spin in the C 14 ( Ca 48 , α n ) reaction at a beam energy of 130 MeV. A regular sequence of stretched quadrupole transitions has been established above the yrast 9 / 2 + level. This sequence is interpreted as a rotational band associated with prolate deformation induced by the excitation of the odd neutron into the 1 / 2 + [ 440 ] orbital. Total Routhian surface calculations, which follow this configuration to high spin, reproduce the observed features of the band. They are also able to account for a similar, but less well developed structure in 55 Cr. In contrast, the isomeric yrast 9 / 2 + state in 59 Cr appears to be a band-head state dominated by the 9 / 2 + [ 404 ] configuration, which is favoured at oblate deformations. Such a marked difference presents a significant challenge for theoretical descriptions, but is consistent with features exhibited by the low-lying negative-parity states in 59 Cr.

Understanding the processes which create and destroy 22Na is important for diagnosing classical nova outbursts. Conventional 22Na(p,γ) studies are complicated by the need to employ radioactive targets. In contrast, we have formed the particle-unbound states of interest through the heavy-ion fusion reaction, 12C(12C,n)23Mg and used the Gamma-sphere array to investigate their radiative decay branches. Detailed spectroscopy was possible and the 22Na(p,γ) reaction rate has been re-evaluated. Hydrodynamical calculations incorporating the revised rate indicate a reduction in the yield of 22Na by a factor of four from previous estimates implying a reduction by a factor of two for the maximum detectability distance for 22Na γ rays from novae.

A study of high spin states in the odd-neutron isotones 239 Pu and 237 U is reported. Striking differences were found in the high-spin properties of rotational bands built on the 1 /2 + [631] ground states in these two nuclei. These differences mirror those observed in the even–even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations.

More than a decade after the large γ -ray detector arrays came into operation, their impact is as large as ever. A few examples from Gammasphere are discussed here. They illustrate that the arrays make substantial contributions, not only in areas where they were expected to do so such as studies of nuclei at high angular momentum, but also in other domains such as nucleosynthesis, for example.

Two Gammasphere experiments have been performed in order to establish the possible triaxial nature of strongly deformed (SD) bands in 174Hf. A lifetime measurement, using the Doppler-shift attenuation method, confirmed the large deformation of the four previously observed bands in this nucleus with transition quadrupole moments ranging from 12.6 to 13.8 b. These values are significantly larger than those predicted for triaxial minima by ultimate cranker (UC) calculations. A thin-target, high-statistics experiment was also carried out to search for linking transitions between the SD bands. No such transitions, which represent an experimental signature for wobbling modes, were observed. Four additional SD bands were found in 174Hf together with a single SD band in 173Hf. These results indicate that the strongly deformed sequences of N ≈ 102 Hf isotopes behave differently than the triaxial strongly deformed (TSD) bands found in Lu nuclei near N = 92 . The interpretation of these bands in terms of possible stable triaxial deformation is confronted with the experimental findings and UC predictions.

Deep-inelastic reactions between a beam of 1360 MeV 208Pb ions and a thick 238U target have been used to populate the neutron-rich nucleus 211Pb. The observation of its γ decay has allowed identification of excited states up to the highest spin which can be formed from the three valence neutrons, including identification of three high-spin isomers. Level energies and transition strengths are compared to shell-model calculations with empirical interactions and predictions are made for the expected behaviour of more neutron-rich lead isotopes. The evidence for a possible increase in the neutron effective charge moving away from the N = 126 shell gap is evaluated.

The r-process nuclide 244Pu (t1/2 = 80.8 Myr) was extant in the Early Solar System and is presently considered as extinct. However, fresh 244Pu produced in the Galaxy, may reach the Solar System and Earth through influx of interstellar dust or as direct ejecta from supernovae. Detection of such traces requires high sensitivity, discrimination power and efficiency. Accelerator mass spectrometry (AMS), despite low efficiency, is the method of choice. We report on the successful detection of 244Pu by AMS, using highly-charged positive ions produced in an Electron Cyclotron Resonance (ECR) ion source and accelerated with the superconducting heavy-ion linear accelerator ATLAS at Argonne National Laboratory. 244Pu34+,35+,36+ ions with a final energy of 0.7 MeV/u were dispersed in m/q using the Argonne Fragment Mass Analyzer (FMA) and are well discriminated from parasitic ions. A count rate of 0.7 244Pu counts/min was observed for a sample containing 2.7 × 108 244Pu atoms/mg. The technique described here is presently used, with improved setup and accelerator conditions, for the measurement of neutron capture cross sections in the actinide region.

The (8)B(alpha,p)(11)C reaction is part of the network that can bypass the triple a process leading to the production of (12)C. We have measured the astrophysical reaction rate for this reaction by studying the inverse (11)C(p,alpha)(8)B process. The radioactive (11)C beam was produced via the p((11)B,(11)C)n reaction using the in-flight facility at the ATLAS accelerator. The astrophysical reaction rate obtained from the excitation function measured in the energy range E(x)=8.8-10 MeV was found to be a factor of 10-50 higher than previous estimates.

The β decay of neutron-rich nuclei in the π 1 f 7/2 and νpf shells has been studied at the NSCL to track the development of new proposed shell closures at N=32 and 34 due to the monopole migration of the ν 1 f 5/2 orbital. From a measurement of absolute γ -ray intensities following the β decay of odd-odd nuclei, the β -decay branching ratios to levels in the even-even daughters can be deduced. The branching ratios, especially to the 0 + daughter ground state, can be used to assign spins and parities to the parent ground states. The interpretation of these assignments for odd-odd nuclei in a single-particle limit can be used to track the migration of the ν 1 f 5/2 single-particle state.

Precise mass measurements of nuclei along the N = Z line are important input parameters for simulations of the rp-process. Of particular interest is the mass of the 6834Se34 rp-process waiting point nucleus for determining the possibility of a two-proton capture branch bypassing its slow β-decay. The mass of 68Se was measured via the β-decay endpoint. 68Se was produced by the 12C(58Ni, 2n)68Se reaction and subsequently implanted onto a moving tape system using the fragment mass analyzer at the ATLAS facility of Argonne National Laboratory. A mass excess value of ( − 54189 ± 240 ) keV was determined from the β-endpoint measurement of Q EC = 4710 ( 200 ) keV . Evaluations of proton separation energies based on the measured mass were used in a one zone type I X-ray burst model. It is concluded that the N = Z = 34 isotope 68Se is a waiting point for the rp-process.

A seven-quasiparticle isomer with $K^{\pi} = (49 /2^ + )$ and T 1/2 = 53 ( + 3 -7) ns has been identified in 179Ta. By comparing its excitation energy with results from multi-quasiparticle calculations that include the effects of blocking and residual nucleon-nucleon interactions, the isomer is assigned the $\pi^{3}$ (5/2 + [402], 7/2 + [404], $9/2^{-}[514]) \otimes \nu^{4}$ (5/2-[512], 7/2-[514], 7/2-[503], 9/2 + [624]) configuration. The decay of this isomer is found to be unusually fast, a feature that is attributed to a mixing with a specific collective level. The interaction strength is found to be orders of magnitude lower than that observed between interacting collective levels.

Superdeformed bands in 88Mo and 89Tc were populated using 40Ca-induced fusion-evaporation reactions on 58Ni at a beam energy of 185MeV. Gamma-rays emitted in the reactions were detected using the Gammasphere spectrometer, in coincidence with charged particles detected by the Microball array. A new superdeformed band was assigned to the nucleus 88Mo, leading to a revisit of earlier configuration assignments for superdeformed structures in this nucleus. In particular, the theoretical interpretation of a pair of identical (isospectral) superdeformed bands in 88Mo/89Tc is discussed. The configurations that are assigned to the four SD bands belonging to 88Mo have properties that are predicted to be significantly affected by pair correlations.

The fusion–evaporation reaction 10 B( 12 C, 2 n ) was used to make the first observation of in-beam γ decays from the astrophysically important nucleus 20 Na, lying adjacent to the proton drip-line. All states below the proton threshold in 20 Na were populated and identified in the experiment. These include new levels, previously unresolved levels, and states located with improved energy precision. The level structure of 20 Na, and its γ transitions, are compared to the mirror partner 20 F measured simultaneously in this experiment. In particular, a high degree of energy stability is found for all negative parity states. These results are discussed in the context of the nuclear shell model.

Long-lived high- K states have been identified in 177 Lu, including an isomer with τ >10 μs, placed at 3530 keV and associated with the yrast K π =39/2 − , 5-quasiparticle state predicted in multi-quasiparticle-state calculations. The γ -decay of the isomer exposes the rotational band based on the 160-day 23/2 − isomer, 177m Lu , with band properties which support its proposed configuration. A 90-ns isomer at 1325 keV is associated with the predicted 3-quasiparticle K π =25/2 + state while a 5-quasiparticle 33/2 + , 902-ns isomer with highly-hindered decays is identified at 2771 keV. Both exhibit rotational bands whose properties are used to support the assigned configurations. The possibility that the K π =39/2 − isomer found in this work should be identified with a β -decaying isomer, proposed recently to populate the 51-min 37/2 − isomer in 177 Hf, is discussed in terms of the implied log ft values, configuration changes, and hindrances for K -forbidden γ -decays.

A new measurement of fusion reactions down to 10 nb has been performed for the system 64 Ni + 64 Ni. Coupled-channels analyses have been carried out for the systems 60 Ni + 89 Ya nd 64 Ni + 64 Ni. They demonstrate that coupled-channels calculations are unable to reproduce the unexpected steep falloff of the cross sections at extreme sub-barrier energies. Heavy-ion fusion excitation functions are also analyzed in terms of the S factor, as this offers a pragmatic way to study fusion behavior in the energy regime of interest. It is shown that the steep falloff in cross section observed in several heavy-ion systems translates into a maximum of the S factor. The energies where the maximum occurs can be parameterized with a simple empirical formula. The parameterization, which is derived here for rather stiff heavy-ion systems, provides also an upper limit for reactions involving softer nuclei. The asymptotic behavior of reaction cross sections at very low energies is a critical issue for calculating reaction rates of astrophysical interest. Recently, it was pointed out that fusion cross sections for several heavy-ion systems show an unexpected behavior at very low energies, 1) with a much steeper falloff than predicted by coupled-channels calculations, 2) or from Wong’s formula. 3) In the present work, we first present a new result for the excitation function in an open-shell system, 64 Ni + 64 Ni down to the 10 nb level, we then analyse in a coupled-channels description the fusion of 60 Ni + 89 Ya nd 64 Ni + 64 Ni, i.e. the systems which have been measured to extreme sub-barrier energies. Fusion reactions at extreme sub-barrier energies are then studied in a representation in terms of the S factor. This parametrization, which in the past has been successfully applied to fusion reactions with lighter ions at low energies, proves to be an alternative and pragmatic way of characterizing and exploring the behavior of heavy-ion fusion cross sections in the energy domain of interest. To our surprise, a maximum appears systematically in the evolution of S with decreasing energy for all systems that exhibit the steep falloff in cross section described above. This maximum occurs at rather high excitation energies in the compound nucleus.

Particle gamma-ray angular correlation measurements have been used to study the spin alignment and magnetic-substate population parameters for the 2 + 1 (4.443 MeV) state in 12 C, populated in the 12 C ( 12 C , 12 C [0 + 2 ]) 12 C (2 + 1 ) inelastic scattering reaction in the vicinity of a prominent, narrow peak in the scattering excitation function. The data show a strong alignment of the spin with the orbital angular momentum, and suggest that the cross section peak corresponds to a spin 14 + resonance at E c.m. =28.0 MeV. This energy is close to that where a strong peak is also observed in the 0 + 1 +0 + 2 excitation function. A comparison between the data for these two channels lends some support to recent theoretical calculations of resonance behavior for angular-momentum-mismatched channels in 12 C + 12 C inelastic scattering.

Isomer decays, β–γ and electron–γ spectroscopy, α–γ fine structure studies, and Coulomb excitation are cases of nuclear structure experiments where the ideal detector is a compact, highly segmented, very efficient germanium (Ge) detector box. In order to develop such a structure, we are working on the R&D of large, segmented, high-purity planar Ge strip detectors (HPGeDSSD) which form the walls of such a box. We have developed a 92 mm×92 mm×20 mm HPGeDSSD, which has 16×16 orthogonal strips of 5 mm width. We are in the process of designing a focal plane detector for the Argonne fragment mass analyzer which consists of a five-sided box, each side having a HPGeDSSD backed by a large segmented clover HPGe detector. MCNP simulations indicate this detector would have an efficiency of ∼60% for 122 keV gamma rays and ∼15% for 1.33 MeV radiation, which is ideal for studying the decays of nuclei far from stability that are usually produced with very low cross-sections.

We report on the progress towards measuring the charge radius of 6 He and 8 He nuclei by performing laser spectroscopy on these helium atoms in a magneto-optical trap (MOT). First tests to produce neutral 6 He atoms via the 12 C( 7 Li, 6 He) 13 N reaction at the ATLAS accelerator have been successfully conducted. The MOT apparatus including the laser system and the discharge source to populate the metastable level are currently being set up.

Among the many production and destruction nuclear reactions that bear on the net 44 Ti yield in the supernova environment, recently highlighted by γ-ray astronomy observations, the α capture on 40 Ca stands out in the α-rich freezeout phase conjectured by astrophysical models. We activate a He-gas target with a 40 Ca beam and implant recoiling 44 Ti nuclei (∼10 6 ) in a Cu catcher. The 44 Ti atoms chemicallyextracted with a nat Ti carrier are counted by accelerator mass spectrometry (AMS). The measurement of the 44 Ti/Ti ratio, together with the amount of Ti carrier added, determine the number of 44 Ti nuclei produced in the activation, independent of chemical yield or transmission efficiency. The resonance strength (7.4±2.5 eV) at E cm ∼1.1 MeV/n determined for two close-by levels of 44 Ti is in excellent agreement with previous prompt-γ measurements. We plan to extend our experiments to the lower energy range.

The 15 O(α,γ) 19 Ne reaction is an important pathway from the hot CNO cycle to the rp process that leads to the synthesis of heavier nuclei. At lower temperatures, the reaction rate is dominated by the transition to the 4.033 MeV 3/2 + state in 19 Ne, the first level above the 15 O(α,γ) threshold at 3.534 MeV. We have performed an indirect measurement of the ratio Λ α Λ γ by populating the 3/2 + state via the 3 He( 20 Ne,α) 19 Ne reaction in inverse kinematics. The α particles were detected in a double-sided Si strip detector telescope while the heavy reaction products, 19 Ne and 15 O, were identified in the focal plane of a magnetic spectrograph. For the 4.033 MeV state in 19 Ne an upper limit Λ α Λ γ of ≤ 7×10 −4 has been obtained.

We report on four recent results concerning the population and the decay of superdeformed states, namely the structure of excited superdeformed states in 194Hg, the search for fine structure of the last superdeformed transitions in 194Pb, the primary decay-out strength analysis in 194Hg and, as a consequence of this, the possibility of using the decay-out as a tool to study order-to-chaos properties of normally deformed states.

Entry distributions in angular momentum and excitation energy have been measured for several very proton-rich isotopes of Pt and Au. This is the first systematic study of the energy-spin phase space for nuclei near and beyond the proton drip line. Comparisons are made between the distributions associated with proton-unbound Au nuclei and more stable Pt isobars. In 173 Au the first evidence is seen for the limits of excitation energy and angular momentum which a nucleus beyond the proton drip line can sustain.

With new high statistic data, new isotopes and new high-spin structures are observed in neutron-rich nuclei populated in the spontaneous fission of 252Cf. The 135Te levels are extended, and many new levels in 139,141Ba observed. The coexistence of collective and single particle-hole states is found in 135Te. The N = 83 135Te and 139Ba show marked differences associated with differences in their particle and hole states. New levels in 141Ba complete evidence for two opposite-parity doublets characteristic of stable octupole deformation. In 114,116Pd a second backbend is observed for the first time in this mass region and the backbend in 118Pd occurs earlier than in 112-116Pd because of a reduction in pairing. Gamma-type vibrational bands are seen up to 13+ to 15+ in 104,106Mo, 108-112Ru, and 112-116Pd. Their behavior reflects prolate to triaxial shapes in these nuclei. The levels of 162,164Gd are observed for the first time. As N increases toward mid-shell at 104, the moments of inertia in N = 100 164Gd show an unexpected decrease compared to N = 98 162Gd. The levels in 162,164Gd form remarkable shifted identical bands with nuclei separated by 2n, 2p, α, and 2α.

High-spin states in 59Cu were populated using the fusion-evaporation reaction 28Si + 40Ca at a beam energy of 122 MeV. The Gammasphere Ge-detector array in conjunction with the 4π charged-particle detector array Microball allowed for the detection of γ-rays in coincidence with evaporated light particles. The resulting extensive high-spin decay scheme of 59Cu is presented, which comprises more than 320 γ-ray transitions connecting about 150 excited states. Their spins and parities have been assigned via directional correlations of γ-rays emitted from oriented states. Average quadrupole moments of rotational bands have been determined from the analysis of residual Doppler shifts. Shell model calculations in the fp shell are invoked to study some of the low-spin states, while the experimental characteristics of the rotational bands are analyzed in the configuration-dependent cranked Nilsson-Strutinsky (CNS) approach.

The spontaneous fission (SF) of 252Cf has been studied via γ-γ-γ coincidence and light charged particle—γ-γ coincidence with Gammasphere. The yields of correlated Mo-Ba pairs in binary fission with 0–10 neutron emission have been remeasured with an uncompressed cube. The previous hot fission mode with 8–10 neutron emission seen in the Mo-Ba split is found to be smaller than earlier results but still present. New 0n binary SF yields are reported. By gating on the light charged particles detected in ΔE-E detectors and γ-γ coincidence with Gammasphere, the relative yields of correlated pairs in alpha ternary fission with zero to 6n emission are observed for the first time. The peak occurs around the α2n channel. A number of correlated pairs are identified in ternary fission with 10Be as the light charged particle. We observed only cold, 0n10Be and little, if any, hot, xn10Be channels.

The performance of new scintillator detectors based on LaBr{sub 3}(Ce) technology was evaluated in conjunction with the Gammasphere spectrometer. Specifically, the lifetimes of states between 50 ps and 1 ns, populated in the decay of the {sup 177m}Lu isomeric state (T{sub 1/2} = 160 d), have been measured. Even though the decay scheme is rather complex, it is possible to perform precise measurements because of the superior energy resolution of the LaBr{sub 3}(Ce) detectors, compared to that of BaF{sub 2} scintillators, when used in combination with the power of the Gammasphere array to isolate a specific {gamma} cascade

A detailed level scheme for $^{221}\mathrm{Th}$ has been established in an experiment using the $^{18}\mathrm{O}+{}^{207}\mathrm{Pb}$ reaction at 96 MeV. The evaporation residues from this fissile system were selected with the HERCULES detector array and residue-gated $\ensuremath{\gamma}$ rays were measured with Gammasphere. Three band structures of interlinked, alternating-parity levels are observed, two of which are non-yrast. In addition, several high-lying excitations are found. The yrast band is seen up to spin-parity $37/{2}^{\ensuremath{-}}$ and $39/{2}^{+}$, beyond which a high-spin feeding transition is observed. The non-yrast sequences are interpreted as parity-doublet structures, based on a configuration similar to that of the yrast band in $^{223}\mathrm{Th}$ $(K=5/2)$. The key properties of even-odd nuclei in this mass region $[B(E1)/B(E2)$ and $B(M1)/B(E2)$ ratios, spin alignments, parity splittings] are reviewed.

International audience; The level scheme of the nucleus Te-120, populated in the reaction Se-80(Ca-48, alpha 4n), was reinvestigated using gamma-ray coincidence data measured with the Gammasphere spectrometer. Previously, five high-spin rotational bands were discovered in this nucleus. The present reinvestigation revealed that the decay of band b1 is more complex than suggested in the earlier work and that it cannot be uniquely determined. Furthermore, a number of new transitions are added to the level scheme. The implications for the spin assignments and excitation energies of the five bands and for comparisons with cranked Nilsson-Strutinsky calculations are discussed.

A significant extension of the level scheme for the odd-odd nucleus $^{172}\mathrm{Re}$ was accomplished through the use of the Gammasphere spectrometer. States up to a tentative spin assignment of 39 were observed and two new structures were identified. Configuration assignments are proposed based on alignment properties and observed band crossings.

Excited states above the seniority $\ensuremath{\nu}=2$ isomers have been investigated in even neutron-rich ${}^{118--128}$Sn isotopes produced by fusion-fission of 6.9 MeV/$A$ $^{48}\mathrm{Ca}$ beams with $^{208}\mathrm{Pb}$ and $^{238}\mathrm{U}$ targets and by fission of 6.7 MeV/$A$ $^{64}\mathrm{Ni}$ beams on a $^{238}\mathrm{U}$ target. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold $\ensuremath{\gamma}$-ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques, including prompt and delayed cross-coincidence methods. As a result, seniority $\ensuremath{\nu}=4$, 15${}^{\ensuremath{-}}$, and 13${}^{\ensuremath{-}}$ isomers were observed and their half-lives determined. These long-lived states in turn served as steppingstones to delineate the isomeric decays and to locate higher-lying states with good sensitivity. As the observed isomeric decays feed down to 10${}^{+}$ and 7${}^{\ensuremath{-}}$ isomers, firm spin-parity assignments could be proposed for most of the seniority $\ensuremath{\nu}=4$ states. Higher-lying, seniority $\ensuremath{\nu}=6$ levels were assigned tentatively on the basis of the observed deexcitation paths as well as of general yrast population arguments. Shell-model calculations were carried out down to $^{122}\mathrm{Sn}$ in the ${g}_{7/2}$, ${d}_{5/2}$, ${d}_{3/2}$, ${s}_{1/2}$, and ${h}_{11/2}$ model space of neutron holes with respect to a $^{132}\mathrm{Sn}$ core. Effective two-body interactions were adjusted such that satisfactory agreement with data was achieved for $^{130}\mathrm{Sn}$. The results reproduce the experimental level energies and spin-parity assignments rather well. The intrinsic structure of the states is discussed on the basis of the calculated wave functions which, in many instances, point to complex configurations. In a few cases, the proposed assignments lead to unresolved issues. The smooth, systematic decrease of the level energies with mass $A$ is accompanied by the similarly regular behavior with $A$ of the reduced transition probabilities extracted from the isomeric half-lives. This $A$ dependence is discussed for the $E1$ and $E2$ transitions in the decay of the seniority $\ensuremath{\nu}=4$ isomers and is compared to that determined in earlier work for the $E2$ transition rates from the $\ensuremath{\nu}=2,3$ isomers.

Comprehensive measurements of the excitation energy and spin-parity assignments for states in S31 are presented, from the first excited state, up to energies relevant for the P30(p,γ)S31 reaction in ONe novae. This reaction rate strongly influences heavy element abundances in novae ejecta. States in S31 are paired with their P31 analogues using γ rays detected with the Gammasphere detector array following the Si28(He4, n) fusion-evaporation reaction. The evolution of mirror energy differences is explored and the results are compared with new shell-model calculations. The excellent agreement observed in this work between experimental data and shell-model calculations provides confidence in using computed estimates in situations where experimental data are unavailable. © 2014 American Physical Society.

Recent results on high-spin isomers populated in deep-inelastic reactions in the transitional tungsten-osmium region are outlined with a focus on Os-190, Os-192 and Os-194. As well as the characterization of several two-quasinutron isomers, the 12(+) and 20(+) isomers in Os-192 are interpreted as manifestations of maximal rotation alignment within the neutron i(13/2) and possibly proton k(11/2) shells at oblate deformation.

The neutron-deficient mercury isotopes, $^{184,186}$Hg, were studied with the Recoil Distance Doppler Shift (RDDS) method using the Gammasphere array and the Koln Plunger device. The Differential Decay Curve Method (DDCM) was employed to determine the lifetimes of the yrast states in $^{184,186}$Hg. An improvement on previously measured values of yrast states up to $8^{+}$ is presented as well as first values for the $9_{3}$ state in $^{184}$Hg and $10^{+}$ state in $^{186}$Hg. $B(E2)$ values are calculated and compared to a two-state mixing model which utilizes the variable moment of inertia (VMI) model, allowing for extraction of spin-dependent mixing strengths and amplitudes.

The β −decay branching ratio of 12 B to the Hoyle state in 12 C was measured by detection of γ rays. 12 B nuclei were produced via the 11 B(d,p)12 B reaction in inverse kinematics on a TiD2 target. The present results corroborate those obtained recently for the β branch by implantation. The value from both experiments is inconsistent with that accepted in the literature.

We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of 254No is measured to be Bf=6.0±0.5 MeV at spin 15ℏ and, by extrapolation, Bf=6.6±0.9 MeV at spin 0ℏ. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for 220Th and only a lower limit of the fission barrier height can be determined: Bf(I)8 MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements.

The influence of inelastic excitations on heavy-ion fusion is well established and can be quantitativly described by coupled-channels calculations. The influence of transfer channels, however, is still under debate. We have analyzed a large set of heavy-ion-induced fusion excitation functions involving nuclei with similar structures and show that there is a universal correlation between the shape (and enhancement) of the excitation function and the strength of the total neutron-transfer cross sections for systems ranging from light to heavy masses.