Search

Excited states have been observed for the first time in the neutron-deficient nucleus Ba117 using the recoil-decay tagging technique following the heavy-ion fusion-evaporation reaction Zn64(Ni58, 2p3n)Ba117. Prompt γ rays have been assigned to Ba117 through correlations with β-delayed protons following the decay of A=117 recoils. Through the analysis of the γ-γ coincidence relationships, a high-spin level scheme consisting of two bands has been established in Ba117. Based on the systematics of the level spacings in the neighboring barium isotopes, the two bands are proposed to have νh11/2[532]5/2- and νd5/2[413]5/2+ configurations, respectively. The observed band-crossing properties are interpreted in the framework of cranked shell model.

Excited states have been identified in the very neutron-deficient Z=58 nucleus 123Ce. This is the most neutron-deficient odd-A cerium isotope in which excited states have been identified. The states have been unambiguously assigned to 123Ce by detecting de-excitation γ rays in coincidence with evaporated charged particles and neutrons. Three rotational bands, each consisting of at least eight E2 transitions, have been observed. The bands have tentatively been assigned to be based on neutrons in g7/2 and h11/2 orbitals. Two of the bands have been assigned to be signature partners, although no interband transitions have been observed. The aligned angular momenta of the bands have been studied in comparison with neighboring nuclei and with the results of Woods-Saxon cranking calculations. Observation of the deformation-dependent π(h11/2)2 quasiparticle alignment at 0.36MeV/ in each of the bands suggests a quadrupole deformation of β2 0.3, in good agreement with theoretical predictions for the suggested possible configuration assignments.

Gamma-ray transitions have been observed in the proton-emitting N=Z+2 (Tz=1) isotope 112Cs. The transitions have been unambiguously assigned to 112Cs by correlation with the characteristic proton decay, using the method of recoil-decay tagging with mass selection. The measured proton-decay energy and half-life are Ep=810(5) keV and T1/2=470(50) μs, respectively, which are consistent with previous measurements. Five γ-ray transitions have been observed which appear to form a rotational sequence. The energy differences between excited states in the sequence are consistent with an assignment as the favored signature of the ν(h 11/2) π(h11/2) structure. Tentative evidence for fine structure in the 112Cs proton decay is also observed.

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in Rf256, which suggest that a long-lived, low-energy 8- isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a Rf256 nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-μs isomer was identified. However, its low population, ~5(2)% that of the ground state instead of the expected ~30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z=104, which describes nuclei with Z=92-103 very well.

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in (256)Rf, which suggest that a long-lived, low-energy 8(-) isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a (256)Rf nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-mu s isomer was identified. However, its low population, similar to 5(2)% that of the ground state instead of the expected similar to 30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z = 104, which describes nuclei with Z = 92-103 very well.

Isomers have been populated in Cm246 and No252 with quantum numbers Kπ=8-, which decay through Kπ=2- rotational bands built on octupole vibrational states. For N=150 isotones with (even) atomic number Z=94-102, the Kπ=8- and 2- states have remarkably stable energies, indicating neutron excitations. An exception is a singular minimum in the 2- energy at Z=98, due to the additional role of proton configurations. The nearly constant energies, in isotones spanning an 18% increase in Coulomb energy near the Coulomb limit, provide a test for theory. The two-quasiparticle Kπ=8- energies are described with single-particle energies given by the Woods-Saxon potential and the Kπ=2- vibrational energies by quasiparticle random-phase approximation calculations. Ramifications for self-consistent mean-field theory are discussed.

Gamma rays deexciting states in the proton emitter Tm145 were observed using the recoil-decay tagging method. The Tm145 ground-state rotational band was found to exhibit the properties expected for an h11/2 proton decoupled band. In addition, coincidences between protons feeding the 2+ state in Er144 and the 2+→0+ γ-ray transition were detected, the first measurement of this kind, leading to a more precise value for the 2+ excitation energy of 329(1) keV. Calculations with the particle-rotor model and the core quasiparticle coupling model indicate that the properties of the πh11/2 band and the proton-decay rates in Tm145 are consistent with the presence of triaxiality with an asymmetry parameter γ≈20°.

The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via Pb204(Ca48,xn)No252-x reactions. Isotopically pure (>99.7%) Pb204 targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t1/2=3.7-0.8+1.1 and 43-15+22 μs) were deduced from a total of 158 fission events. Both decays originate from No250 rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.

Excited states have been observed for the first time in the very neutron-deficient 55114Cs59 nucleus. The assignment to Cs114 was made by detecting γ rays in coincidence with evaporated charged particles and with recoiling evaporation residues. A rotational band has been observed up to spin 21 (tentatively 25). Excitation-energy systematics and a study of quasiparticle alignments suggest that the band is based on the ν(h11/2)π(h11/2) configuration. Unlike bands based on this configuration in heavier cesium isotopes, only one signature partner is observed, and there is no evidence for signature inversion. These observations agree with theoretical calculations which predict that Coriolis-induced signature splitting should dominate at N=59.

Excited states have been observed for the first time in the very neutron-deficient, odd-odd nucleus, 55116Cs61. The assignment to Cs116 has been made by the detection of γ rays in coincidence with evaporated charged particles and with evaporation residues. The observed states form a rotational band which has been assigned to the ν(h11/2)π(h11/2) configuration. Tentative spin assignments have been made on the basis of systematic comparisons with neighboring cesium isotopes. A low-spin signature inversion is observed in the band at a rotational frequency of about 0.23 MeV/. The observed signature inversions in the odd-odd Cs116-126 isotopes have been compared with the results of extended total Routhian surface calculations, in which signature inversion arises as a consequence of quadrupole-pairing correlations and triaxial deformation. As previously shown for some of the odd-odd 120 isotopes, the calculations reproduce the signature inversions reasonably well.

The excitation function for the fusion-evaporation reaction 28Si + 64Ni 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.

We have identified two isomers in No254, built on two- and four-quasiparticle excitations, with quantum numbers Kπ=8- and (14+), as well as a low-energy 2-quasiparticle Kπ=3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps.

Excited states in the proton-rich N=84 isotones 72156Hf84,73157Ta84, and 74158W84 were observed using Pd102(Ni58,xp2n) reactions at 270 MeV. γ rays were detected with the Gammasphere array of Compton-suppressed Ge detectors coupled with the Argonne fragment mass analyzer and were assigned to individual reaction channels using the recoil-decay tagging method. Prompt γ-ray cascades were associated with the α decay of both the ground state and the 8+ isomeric state in Hf156, the h11/2 state in Ta157, and the 8+ isomeric state in W158. The level schemes constructed for Hf156,Ta157, and W158 are compared with these of lighter N=84 isotones and are discussed within the framework of the shell model.

In-beam γ-ray spectroscopic measurements have been made on No102253. A single rotational band was identified up to a probable spin of 39/2, which is assigned to the 7/2+[624] Nilsson configuration. The bandhead energy and the moment of inertia provide discriminating tests of contemporary models of the heaviest nuclei. Novel methods were required to interprete the sparse data set associated with cross sections of around 50nb. These methods included comparisons of experimental and simulated spectra, as well as testing for evidence of a rotational band in the γγ matrix.

γ-ray spectroscopy of high-spin states of the neutron-deficient nucleus Pb189 has been conducted with the Gd158(Ar36,5n) and Er164(Si29,4n) reactions. With the first of these, detection of evaporation residues and mass gating were used to unambiguously assign a number of prompt γ-ray transitions to Pb189. With the second reaction and a pulsed beam, an isomer with a mean life of 32 μs was found. Although inconclusive, the available evidence favors identification of the isomer with the 332+ state of the ν(i13/2)-3 configuration. The levels observed below the isomer can be identified with states involving three different structures: the neutron (i13/2)-3 multiplet in the spherical well; a prolate-deformed band involving mixed i13/2 neutron orbitals; and a state with the oblate π(2p-2h)0+ν(i13/ 2)-1 configuration. The evidence for structures associated with different shapes is supported by the observation of E0 components in some of the Jπ→Jπ transitions linking them.

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 β2 = 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(h11/2)2 alignment until at least 0.4 MeV/ℏ is consistent with the extracted value of β2.

Multiple band structures in 179Au are established from γ-ray spectroscopic measurements with Gammasphere at the Argonne Fragment Mass Analyzer. The yrast band, based on the 13/2+ proton, confirms the predicted drop in excitation energy of the prolate deformed band head as compared to the heavier isotopes. The implications for the prolate energy minimum in odd-mass Au nuclei beyond the neutron i13/2 midshell (N

High-spin states in 174Pt were populated via the 92Mo(84Sr, 2p) and 92Mo(84Sr, 2p2n) reactions. The ground-state band has been extended from I= 14 to 24 (tentatively 26) and a new side band is observed up to a spin of 21 (tentatively 23). A low-frequency crossing is observed in the latter band at a rotational frequency that is similar to that seen in the ground-state band. The first and second i13/2 neutron alignments are also observed in 174Pt. Surprisingly, these crossings occur at approximately the same frequency. Total Routhian surface and cranked shell model calculations are used in an attempt to understand this behavior.

The fusion excitation function for 64Ni + 64Ni down to a cross section level of 10 nb and have observed a strong fusion hindrance at extreme sub-barrier energies. The onset of the sub-barrier fusion hindrance in 64Ni + 64Ni was found to occur 8.4 MeV lower in center-of-mass energy, whereas a 2 MeV higher energy was expected on the basis of systematics. The effect was found to appear to be associated with the nuclear structure of the interacting nuclei. A strong dependence of the energy where the hindrance was also found to occur on the stiffness of the interacting nuclei.

Rotational bands have been studied to high spins in the neutron-deficient 124,125Ce nuclei. The nuclei were populated using the 64Zn(64Zn,2p2n)124Ce and 64Zn( 64Zn,2pn)125Ce reactions, with a beam energy of 260 MeV. High-fold γ-ray coincidence data were collected using the Gammasphere germanium-detector array. The Microball charged-particle detector array was used to provide channel selection. The previously established level structures of both 124,125Ce have been extended to high spins (∼30ℏ), In addition, several new bands have been identified. The alignments of pairs of h11/0/2 neutrons and protons are observed in both nuclei. The alignments are compared to the predictions of Woods-Saxon cranked shell model calculations and to the systematics of h11/2 quasiparticle alignments in neighboring nuclei. The apparent ability of the cranked shell model to explain the h11/2 neutron alignments in 124,125Ce highlights the previously reported discrepancies between experiment and theory for the 128Ce isotope and, to a lesser extent, 126Ce.

High-spin states in the odd-odd N = Z nucleus 2550Mn have been investigated. A sequence of states up to jπ = 6+ has been assigned as the T = 1 analogue of the yrast band in 2450Cr for the first time. The differences in energy between levels in these bands are interpreted in terms of rotational alignments and the effect they have on the Coulomb energy of the nucleus. Comparisons with shell model calculations show that the Coulomb energy difference between the T = 1 analogue structures is an important indicator of the competition between isovector pairing modes in N = Z nuclei and their isobars.

Excited states have been identified in the very neutron-deficient 11755Cs62 nucleus. High-spin spectroscopy has been performed using the Gammasphere array, and the assignment of gamma-ray transitions to 117Cs has been made in a separate experiment in which gamma rays were detected in coincidence with x rays and with recoiling evaporation residues. A previously observed sequence of five gamma rays has been extended by 11 transitions, to high spin, and has been identified as the yrast π(h11/2)[550]1/2- band of 117Cs. Two additional bands have been observed and are tentatively assigned to be based on protons in the [404]9/2+ and [422]3/2+ orbitals. Alignments of pairs of h11/2 neutrons and protons are observed in all of the bands. The alignments are compared to cranked Woods-Saxon calculations, and are discussed with respect to the effects of a neutron-proton interaction. Of particular interest are the features of the ν(h11/2)2 alignment in the π[550]1/2- band and of the π(h11/2)2 alignment in the [422]3/2+ band. The frequencies of these alignments can be qualitatively explained only if a neutron-proton interaction is taken into account.

Excited states in 126Ce have been observed with the GAMMASPHERE γ-ray detector array, used in conjunction with the MICROBALL charged-particle detector. The reaction 64Zn(64Zn, xpn α) (beam energy 260 MeV) was used to populate a wide range of nuclei in the neutron-deficient region with A ≈ 120. 126Ce was populated via the 2p evaporation channel. The three previously observed bands have been extended to higher spins and some other structures have been identified. The yrast band shows evidence of a delayed neutron alignment occurring at a rotational frequency ω≈0.5 MeV/ℏ as observed in the neighboring odd-/4 nucleus 127Pr. One of the two excited bands shows evidence for a similar crossing at a slightly lower frequency and also exhibits a sudden gain in alignment at ω=0.57 MeV/ℏ. The third band may involve the coupling of a γ-vibrational state. All three rotational bands are discussed in terms of standard and extended cranked shell model calculations.

Excited states in the proton-unbound 173,175,177Au nuclei were identified for the first time. Level structures associated with three different shapes were observed in 175Au. While the yrast lines of 175Au and 177Au consist of a prolate band built upon the intruder 1/2+[660] (i13/2) proton orbital, no sign of collectivity was observed in the lighter 173Au isotope. Implications for the deformation associated with these structures are discussed with a focus on shape co-existence in the vicinity of the proton-drip line.

The 270-MeV 58Ni+ 102Pd reaction was used for the first recoil-decay tagging measurement with Gamma-sphere coupled to the Fragment Mass Analyzer at Argonne National Laboratory. Level structures of 155Yb, 156Lu, and 157Lu, as well as the excited states associated with the 25/2- isomer in 155Lu, are identified for the first time. The systematical behavior of the energy levels is compared with that of neighboring isotones and isotopes. The attractive interaction between h11/2 protons and h9/2 neutrons plays an important role in the structure of 155Yb and 155,156Lu.

Proton and α emission from 185Bi have been confirmed and measured with improved statistics. The 185Bi nuclei were produced via the 95Mo(92Mo,pn) reaction at a bombarding energy of 420 MeV. The proton decay energy from the 1/2+ intruder state in 185Bi to the 184Pb ground state was measured to be 1.598(16) MeV with a proton branching ratio bp, = 0.85(6). An α decay branch from the same state was measured, bα = 0.15(6), with an energy of 8.08(3) MeV. The state has a half-life of 50(8) μs. In addition, the α branching ratio of the ground state of 184Pb was determined for the first time to be bα = 0.23(14).

Rotational bands feeding the ground state and the isomeric state in the proton emitter (141)Ho were observed using the recoil-decay tagging method. This constitutes direct evidence that (141)Ho is deformed. A quadrupole deformation of beta(2) = 0.25(4) was deduced for the ground state from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2(-)[523] and 1/2(+)[411] configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations for beta(2) = 0.25 indicates, however, that (141)Ho may have significant hexadecapole deformation and could be triaxial in the 7/2(-)[523] ground state.

The 270-MeV 58Ni+ APd reaction was used for the first recoil-decay tagging measurement with Gammasphere coupled to the Fragment Mass Analyzer at Argonne National Laboratory. Level structures of 158Hf and 159Hf are identified for the first time, and that of 160Hf is extended. The systematical behavior of the energy levels in neighboring isotones and isotopes, as well as the aligned angular momenta as a function of rotational frequency, are examined.

Cross sections for the 44Ti(α, p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7MeV ≤ Ec.m. ≤ 9 MeV, using a beam of radioactive 44Ti. The cross sections and the deduced astrophysical reaction rates are larger than the results from theoretical calculations by about a factor of 2. The implications of this increase in the reaction rate for the search of supernovae using space-based gamma detectors are discussed.

Excited states have been identified in the very neutron-deficient 119Ba nucleus. Two bands have been observed, which are likely to be based on h11/2 and (g7/2d5/2) neutron orbitals. Despite this being the first observation of excited states in 119Ba, the bands extend to (75/2)ℏ and (79/2)ℏ, respectively. The bands have been assigned to 119Ba using gamma-recoil and gamma-x-ray coincidences. Several quasiparticle alignments have been identified, involving neutron (h11/2)2 and proton (h11/2)2 aligned configurations. Furthermore, the bands show features which are reasonably consistent with smooth band termination at high spin. Theoretical results for 119Ba are discussed within the framework of cranked Woods-Saxon and Nilsson-Strutinsky calculations.

Collective structures in 178Pt have been investigated with γ-ray spectroscopic techniques following mass selection. The present data expand considerably the level scheme of this nucleus. A new collective structure has been identified and the known bands have been extended to higher spins. Firm spin and parity assignments have been made to many levels. Of particular interest are the negative parity bands which are interpreted in terms of structures associated with octupole vibrations crossed at moderate spin by two-quasiparticle excitations. The systematics of the α-decay reduced widths in Pt nuclei has also been investigated. The differences of a factor of about 2 between the widths for the odd-and the even-A isotopes is attributed to changes in neutron pairing associated with the blocking effect. The trend is reproduced in calculations based on the Lipkin-Nogami pairing model.

Using the GAMMASPHERE array in conjunction with the Fragment Mass Analyzer, the level structure of the near drip-line nucleus 178Hg has been considerably expanded with the recoil-decay tagging technique. Of particular interest is a new rotational band which exhibits a complex decay towards the low spin states arising from both the prolate-deformed and the nearly spherical coexisting minima. It is proposed that this band is associated at low spin with an octupole vibration which is crossed at moderate frequency by a shape driving, two quasiproton excitation.

The entry distribution in angular momentum and excitation energy for the formation of 254No has been measured after the 208Pb\(48Ca,2n\) reaction at 215 and 219 MeV. This nucleus is populated up to spin 22h and excitation energy >˜6 MeV above the yrast line, with the half-maximum points of the energy distributions at ˜5 MeV for spins between 12h and 22h. This suggests that the fission barrier is >˜5 MeV and that the shell-correction energy persists to high spin.

High-spin positive-parity states in 72Br have been studied using the 16O+58Ni reaction. The πg9/2vg9/2 decoupled band in 72Br has been observed up to ∼10 MeV excitation energy and the expected odd-even staggering has been delineated. A larger signature splitting is observed for this band in 72Br than in the same collective structures in the heavier 74,76,78Br. No signature inversion at low spin is observed for this band in 72Br, in contrast to the heavier isotopes, 74,76,78Br, in which signature inversion is observed below ∼10(Latin small letter h with stroke sign). The observations are in general agreement with theoretical models in this mass region which predict no signature inversion for nuclei with less than 39 protons and neutrons.

High-spin states in the neutron-deficient 71As nucleus have been studied using in-beam g -ray spectroscopic techniques combined with mass identification. A new decay sequence of negative parity has been assigned to 71As and previously known sequences have been extended to higher spin and excitation. The new sequence has been identified as originating from the proton f 7/2 extruder orbital, which approaches the Fermi surface at large prolate deformations. Comparisons of experimental B(M1)/B(E2) ratios to theoretical expectations support this interpretation.