Search Results (51 total)

Possible decay pathways associated with three candidates for triaxial strongly deformed (TSD) bands in ¹⁶⁸Hf have been investigated. The spin and excitation energy of the strongest band, TSD1, were determined approximately based on γ-ray coincidence relationships. Discrete links were established for the second band. The overall agreement between the observed properties of the bands and cranking calculations using the ULTIMATE CRANKER code provides strong support for an interpretation where band TSD1 is associated with a TSD minimum, (ɛ₂,γ)~(0.43,20^°), involving the π(i_13/2)² and the ν(j_15/2) high-j orbitals. This constitutes the first identification of a TSD band in Hf isotopes, which has been long-predicted by theoretical studies. The second band is understood as being associated with a near-prolate shape and a deformation enhanced with respect to the normal deformed bands. It is proposed to be built on the π(i_13/2h_9/2)⊗ν(i_13/2)² configuration.

Background: Experimental information on the structure of low-lying levels with isospin 3/2 in systems with 13 nucleons can be used to evaluate the quality of modern theoretical predictions for the light exotic systems. This level structure is poorly known at present. Purpose: Search for T=3/2 states in ¹³C was performed in this work. Method: These states were observed in the resonance elastic scattering of radioactive beam ¹²B on protons using the thick target inverse kinematics technique. Results: Six new states in ¹³C were identified as T=3/2 states. Tentative spin-parity assignments are suggested. Comparison to the previous knowledge of the level structure of T=3/2 A=13 system and to shell-model predictions is given. Conclusion: The elastic scattering of neutron-rich beams on proton target is a convenient tool for isobaric analog states search. The observed ¹²B+p excitation function is determined by the T=3/2 states and no features associated with T=1/2 states were found. Although the level scheme of T=3/2 states in A=13 systems is still far from being complete the emerging picture shows significant disagreements with predictions of contemporary shell models.

Angular distributions for the ⁹Be(⁸Li,⁹Be)⁸Li elastic-transfer reaction have been measured with a 27-MeV ⁸Li radioactive nuclear beam. Spectroscopic factors for the 〈⁹Be|⁸Li+p〉 bound system were obtained from the comparison between the experimental differential cross sections and finite-range distorted-wave Born approximation calculations made with the code FRESCO. The spectroscopic factors so obtained are compared with shell-model calculations and other experimental values. Using the present value for the spectroscopic factors, cross sections and reaction rates for the ⁸Li(p,γ)⁹Be direct proton-capture reaction of astrophysical interest were calculated in the framework of the potential model.

Evaporation residue and fission cross sections of radioactive ¹³²Sn on ⁶⁴Ni were measured near the Coulomb barrier. A large subbarrier fusion enhancement was observed. Coupled-channel calculations, including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, there is no extra fusion enhancement in ¹³²Sn+⁶⁴Ni with respect to stable Sn+⁶⁴Ni. A systematic comparison of evaporation residue cross sections for the fusion of even ^112-124Sn and ¹³²Sn with ⁶⁴Ni is presented.

Angular distributions for the elastic scattering of ⁸Li on ⁹Be and the neutron transfer reactions ⁹Be(⁸Li,⁷Li)¹⁰Be and ⁹Be(⁸Li,⁹Li)⁸Be were measured with a 27 MeV ⁸Li radioactive nuclear beam. Spectr- oscopic factors for ⁸Li ⊗n= ⁹Li and ⁷Li ⊗n= ⁸Li bound systems were obtained from the comparison between the experimental differential cross section and finite-range distorted-wave Born approximation calculations with the code FRESCO. The spectroscopic factors obtained were compared to shell model calculations and to other experimental values from (d,p) reactions. Using the present values for the spectroscopic factor, cross sections for the direct neutron-capture reactions ⁷Li(n,γ)⁸Li and ⁸Li(n,γ)⁹Li were calculated in the framework of a potential model.

A neutron-α-particle coincidence experiment was performed to study breakup of ⁶He on a ²⁰⁹Bi target at an incident energy of 22.5 MeV. Direct breakup, as well as breakup through the 2⁺ excited state of ⁶He and also the ⁵He+n channel, could be distinguished. A total breakup cross section of 205±65 mb was measured. The data compare well with a coupled-channels calculation that assumes a 50% reduction in the B(E1) computed from a pure ⁴He+2n cluster configuration, in agreement with results obtained at much higher energies. A value of B(E2⇑)=5.4±0.7e²fm⁴ is deduced for the 2⁺ state in ⁶He.

The combined data of two Coulomb excitation experiments has verified the purely electromagnetic population of the K^π=4⁺,6⁺,8^-, and 16⁺ rotational bands in ¹⁷⁸Hf via 2≤ν≤14 K-forbidden transitions, quantifying the breakdown of the K-selection rule with increasing spin in the low-K bands. The γ-, 4⁺, and 6⁺ bands were extended, and four new states in a rotational band were tentatively assigned to a previously known K^π=0⁺ band. The quasiparticle structure of the 6⁺ (t_{1 / 2}=77 ns) and 8^- (t_{1 / 2}=4 s) isomer bands were evaluated, showing that the gyromagnetic ratios of the 6⁺ isomer band are consistent with a pure π7 / 2⁺[404],π5 / 2⁺[402] structure. The 8^- isomer band at 1147 keV and the second 8^- band at 1479 keV, thought to be predominantly ν7 / 2^-[514],ν9 / 2⁺[624] and π9 / 2^-[514],π7 / 2⁺[404], respectively, are mixed to a degree approaching the strong-mixing limit. Based on measured 〈K^π=16⁺‖E2‖K^π=0⁺〉 matrix elements, it was shown that heavy-ion bombardment could depopulate the 16⁺ isomer at the ~1% level, although no states were found that would mediate photodeexcitation of the isomer via low-energy x-ray absorption.

An excitation function for resonance elastic scattering of p+¹²N was measured in the center of mass energy range of 0.8–2.7 MeV. Measurements were performed using inverse kinematics and the thick-target techniques. The data were analyzed in the framework of the R-matrix formalism. A definitive spin and parity assignment was possible for the first excited state of ¹³O at an excitation energy of 2.69 MeV as J^π=1 / 2⁺. An indication of the presence of a new level in ¹³O at an excitation energy of 3.29 MeV with tentative spin-parity assignment J^π=1 / 2^- or 3 / 2^- was obtained. We evaluate the impact of this measurement on the reaction rate of ¹²N(p,γ)¹³O for nucleosynthesis in low-metallicity stars.

The Doppler Shift Attenuation Method was used to measure lifetimes for levels in ³⁸K at excitation energies of 1698, 2404, 2830, 2996, and 3671 keV, populated using the ⁴⁰Ca(d, α)³⁸K reaction at a beam energy of 4.5 MeV. Values of 109(29), 95(22), 457(63), 130(40), and 160(50) fs, respectively, were measured and are compared with previous values obtained using different stopping powers. The matrix element for the transition between the J^π=2_T=1⁺ and 0_T=1⁺ states in this T_z=0 nucleus is compared with the analogous transition in the other nuclei in the T=1 triplet, ³⁸Ca (T_z=-1) and ³⁸Ar (T_z=+1), and with the results of shell-model calculations.

High-spin states in ¹²⁴Ba were investigated in two experiments using the ⁶⁴Ni(⁶⁴Ni, 4n)¹²⁴Ba reaction at three different beam energies. In-beam γ-ray coincidences were measured with the Euroball and Gammasphere detector arrays. In the experiment with Euroball, the CsI detector array Diamant was employed to discriminate against charged-particle channels. Six new rotational bands were observed in ¹²⁴Ba, and previously known bands were extended to higher spins. One of the bands shows a transition from collective to noncollective behavior at high spins. Configuration assignments are suggested on the basis of comparison with cranked shell model and cranked Nilsson-Strutinsky calculations.

Lifetime measurements of yrast levels in ¹⁶²Yb and ¹⁶⁶Hf were performed using the recoil distance Doppler-shift method in coincidence mode. Excited states in ¹⁶²Yb and ¹⁶⁶Hf were populated via the reactions ¹¹⁶Cd(⁵⁰Ti, 4n) and ¹²²Sn(⁴⁸Ti, 4n), respectively. The resulting B(E2) values are compared with the X(5) critical point model predictions and interacting boson approximation (IBA) model calculations. The X(5) model provides a reasonable description of the yrast B(E2) values in ¹⁶⁶Hf, whereas the IBA fails to reproduce the transition strengths from the higher spin levels. In ¹⁶²Yb, some transitions agree with the X(5) predictions while others are more consistent with the predictions of the IBA or a deformed symmetric rotor.

Excited states in ²⁰⁹Fr were produced following the ¹⁷⁶Yb(³⁷Cl, 4n) reaction. An excitation function was measured with data taken at 173, 179, and 185 MeV. Recoiling fusion-evaporation products were separated using the SASSYER gas-filled spectrometer. HPGe clover detectors of the YRAST Ball array at the target position and at the focal plane were used to detect prompt and delayed gamma ray decays, providing the first spectroscopic information about prompt and delayed transitions in ²⁰⁹Fr. The decay from a proposed πi_13/2 configuration isomer was observed in ²⁰⁹Fr, and its half-life was measured to be 446(14)ns. States above the isomer were also observed.

Coulomb activation of the four quasiparticle K^π=16^{+ 178}Hf isomer (t_1/2=31  y) has led to the measurement of a set of Eλ matrix elements coupling the isomer band to the ground band. The present data combined with earlier ¹⁷⁸Hf Coulomb excitation data have probed the K components in the wave functions and revealed the onset and saturation of K mixing in low-K bands, whereas the mixing is negligible in the high-K bands. The implications can be applied to other quadrupole-deformed nuclei.

Neutron-induced reaction cross sections on unstable nuclei are inherently difficult to measure due to target activity and the low intensity of neutron beams. In an alternative approach, named the “surrogate” technique, one measures the decay probability of the same compound nucleus produced using a stable beam on a stable target to estimate the neutron-induced reaction cross section. As an extension of the surrogate method, in this paper we introduce a new technique of measuring the fission probabilities of two different compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. This method was benchmarked in this report by measuring the probability of deuteron-induced fission events in coincidence with protons, and forming the ratio P[²³⁶U(d,pf)]/P[²³⁸U(d,pf)], which serves as a surrogate for the known cross section ratio of ²³⁶U(n,f)/²³⁸U(n,f). In addition, the P[²³⁸U(d,d^'f)]/P[²³⁶U(d,d^'f)] ratio as a surrogate for the ²³⁷U(n,f)/²³⁵U(n,f) cross section ratio was measured for the first time in an unprecedented range of excitation energies.

Excited states in ¹⁶⁶Hf were populated in the β⁺/ε decay of ¹⁶⁶Ta and studied through off-beam γ-ray spectroscopy at the Yale moving tape collector. New coincidence data found no support for two previously reported excited 0⁺ states and led to a substantially revised level scheme. Similarities between the revised level scheme of ¹⁶⁶Hf and the X(5) critical point symmetry are discussed, and the extent of X(5) behavior in this mass region is explored through the W and Os isotopes. Among X(5) candidates with N>90, good agreement is observed for most energies and interband B(E2) strengths, while all exhibit similar disagreements with other key observables, in particular, yrast B(E2) values and spacing in the excited K  =  0⁺ sequence.

An unexpected strong interaction between normal deformed and triaxial strongly deformed levels at high spin, ∼32ℏ, is observed in ¹⁶⁷Lu. This constitutes the first observation of accidental degeneracy, at such high spins, causing cross talk between levels in the normal deformed and in the second potential well in any known mass region. Furthermore, evidence of quasi-particle excitations highly competing with the wobbling excitation mode in a triaxial superdeformed well has been established in ¹⁶⁷Lu.

Low-energy excited states of ²⁰⁸Ra were investigated using the ¹⁸²W(³⁰Si, 4n) reaction at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer SASSYER. Delayed γ rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of three clover Ge detectors. Transitions following a proposed J^π=8⁺ isomer were observed, and the half-life was measured.

The nucleus ⁹⁸Ru, the lightest collective Ru isotope with N>50, has been studied employing the (α,2n) reaction using YRAST Ball detectors. The new data, as well as a systematic set of limits on the intensities of unobserved transitions, reveal a structure that resembles a near harmonic vibrator spectrum up through the two-phonon triplet but show an almost complete breakdown of vibrator structure thereafter. These results are not understood theoretically and are at variance with previous interpretations of this nucleus. A rare anomaly in yrast B(E2) values is also pointed out.

High-spin states in ¹²³Cs were populated in the ⁶⁴Ni(⁶⁴Ni,p4n) reaction at a beam energy of 265  MeV. Gamma-ray coincidences were measured using the Gammasphere spectrometer. Two additional bands have been placed in the level scheme and the four previously known bands have been extended to higher spin. At the highest spins, two of the bands show irregular level sequences. These structures of noncollective excitations, which compete with collective rotation, are interpreted as band-terminating states. The results are compared to cranked Nilsson-Strutinsky calculations, and configuration assignments to the bands and to the terminating states are discussed.

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 i_13∕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 excitation energies and spins of the levels in the yrast superdeformed band of ¹⁹¹Hg have been determined from two single-step γ transitions and the quasicontinuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included.

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 i_13∕2 midshell (N<102) are discussed.

Excited states in ²¹⁰Ra and ²⁰⁹Ra were populated using the ¹⁸⁴W(³⁰Si,xn) reaction at 148  MeV beam energy. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Prompt γ rays were detected using Compton-suppressed Ge detectors from the YRAST Ball array surrounding the target. Delayed γ rays, following isomeric decays, were detected at the focal plane of SASSYER with a smaller array of Ge detectors. The decay energy and lifetime for the 8⁺(πh_9∕2)ⁿ isomer of ²¹⁰Ra were determined; values for the yrast B(E2;8⁺→6⁺) in ²¹⁰Ra and neighboring nuclei are interpreted within the seniority scheme. This isomer was also used to select γ-rays deexciting levels above the isomeric state in ²¹⁰Ra. In addition, transitions in ²⁰⁹Ra were identified for the first time. Two high-spin isomers are suggested to exist in this isotope.

B(E2) values from isomeric states near closed shells are discussed in the context of the behavior of seniority conserving transitions induced by even tensor operators. This result provides a signature for the transition from the seniority regime to collective motion that can be of use in identifying shell structure in exotic nuclei.