High-spin states in 208Pb

High-spin states in ²⁰⁸Pb have been studied by γ-ray-spectroscopy methods in deep inelastic reactions induced by beams of ²⁰⁸Pb, ¹³⁶Xe and ⁷⁶Ge beams on a thick ²⁰⁸Pb target. The 11⁺ ₂ state and new γ-transitions between the one-particle one-hole states of highest spin have been found and electromagnetic matrix elements verified. High-spin states of two-particle two-hole structure have been detected for the first time. The results are compared to shell model calculations with realistic interactions in the complete Kuo-Herling space. Received: 31 October 2000 / Accepted: 26 March 2001     

Characterization of quasiparticle states at and beyond stability in ytterbium isotopes: Spectroscopy of 175Yb, 176Yb and 177Yb

Excited states in ¹⁷⁵Yb, ¹⁷⁶Yb and ¹⁷⁷Yb were populated via the bombardment of a ¹⁷⁶Yb target with a 750 MeV ¹³⁶Xe beam. Gamma-ray decays from these states were measured with the AFRODITE multi-detector spectrometer. The rotational band previously assigned to the ground state of ¹⁷⁷Yb has been reassigned to the first-excited state of ¹⁷⁵Yb. A new rotational band based on the ground state of ¹⁷⁷Yb is presented, and the band based on the K^π = 4^- two-quasiparticle state in ¹⁷⁶Yb has been identified. Also a candidate for the rotational band based on the K^π = 8^-, T_1/2 = 11.4(3) s two-quasiparticle state in ¹⁷⁶Yb has been found. Comparisons of g_K values derived from in-band branching ratios are consistent with the ν9/2⁺[624] assignment to the ground state of ¹⁷⁷Yb, the ν²{9/2⁺[624] ⊗ 1/2^-[510]} assignment to the K^π = 4^- state and with the ν²{9/2⁺[624] ⊗ 7/2^-[514]} assignment to the K^π = 8^- metastable excited state in ¹⁷⁶Yb.     

Reduced transition probabilities for high-spin states in 232Th

The lifetimes and reduced transition probabilities for spins 6⁺ ? 18⁺ in the ground-state rotational band of ²³²Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using ⁸⁴Kr and ¹³⁶Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6⁺ ? 18⁺. The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.     

High spin states in 210Pb

High spin states in nuclei in the ²⁰⁸Pb region were populated in deep inelastic collisions of ¹³⁶Xe and ²⁰⁸Pb projectiles with thick ²⁰⁸Pb targets at about 50 MeV above the Coulomb barrier. Prompt and prompt-delayed γ-ray coincidences were measured. Four new states above the 8⁺ isomer in ²¹⁰Pb were found and spins and parities assigned from theoretical expectations. The results are discussed in the frame of shell model theory.     

Non yrast spectroscopy with the (α, n) reaction at the coulomb barrier

About 50 states with spins ≦8? in¹²⁸Xe have been populated by means of the¹²⁵Te (α, n) reaction at projectile energies near the Coulomb barrier. In particular eight 2⁺ or (1,2⁺) states have been observed. The (α, n) reaction at particle energies near the Coulomb barrier seems to be a powerful tool to by-pass the strong selectivity for the yrast states in normal (HI, xn) reactions.     

The (p,t) reactions on nuclei in the rare earth region

The (p, t) reactions on isotopic targets of ^{178, 180}Hf and all the stable isotopes of Yb and on natural targets of Gd, Dy, Er, Hf, Ta, W, Os and Au were studied at a beam energy of 19 MeV with an average resolution of 12 keV. A split-pole magnetic spectrometer was used to measure (p, t) Q-values and absolute differential cross sections. On the basis of angular distribution shapes definite 0⁺ and tentative 2⁺ assignments were made. Rotational bands were identified assuming an I(I+1) spacing. The (p, t) reaction populates excited 0⁺ states strongly in ¹⁷⁴Yb, ¹⁷⁶Hf, ¹⁶⁶Yb and several Gd, Dy and Er isotopes. The ¹⁷⁴Yb and ¹⁷⁶Hf 0⁺ states are discussed in terms of the pairing phase transition and in terms of Nilsson orbitals with unequal (p, t) reaction amplitudes. Members of gamma and octupole vibrational bands were observed in the even-N nuclei. The lowest L = 0 transfers to states in ^{169, 171}Yb were found to have less than 55% of the strength to ground states in adjacent even-N nuclei. A strong L = 0 transfer to a state at 1513 keV in ¹⁷¹Yb indicates the presence of a possible K = 0 core vibration coupled to the unpaired $\text{5}\text{2}$[512] neutron. The natural targets have furnished information on trends in cross sections for members of ground bands, gamma bandheads, 3^? octupole states, and strongly excited 0⁺ states.     

Quadrupole collective properties of 114Cd

The nucleus ¹¹⁴Cd has been Coulomb excited using beams of ¹⁶O, ⁴⁰Ca, ⁵⁸Ni and ²⁰⁸Pb. Several new states have been observed and an almost complete set of reduced E2 matrix elements for the lowest-lying positive-parity states in ¹¹⁴Cd have been measured. In total, about 40 E2 matrix elements have been determined in a model-independent way, including the static quadrupole moments of the 4⁺₁, 6⁺₁, 2⁺₂ and 2⁺₃ states. Large negative static quadrupole moments were found for the 4⁺₁ and 6⁺₁ states and a large positive quadrupole moment for the 2⁺₂ state. All E2 matrix elements involved in the deexcitation of the quintuplet of states at an energy of about 1.2 Me V were determined. The data are compared with predictions by various models, including a configuration mixing calculation within the framework of the IBA model, the harmonic vibrator and a model in which the states of a near-harmonic vibrator are mixed with those of a rotational intruder band with a large deformation. Although no perfect match is obtained, the data favour a vibration-like structure involving levels up to the four-quadrupole phonon multiplet.     

Fission of 232Th, 238U, 244Pu, 248Cm induced by Xe and Kr ions at coulomb barrier energies

Excitation functions and angular distributions have been measured for the fission of ²³²Th, ²³⁸U, ²⁴⁴Pu and ²⁴⁸Cm induced by ⁸⁶Kr and ¹³⁶Xe ions at energies in the vicinity of the Coulomb barrier. No large differences as a function of either target or projectile were found. These results suggest that the main process occuring is not Coulomb fission.     

Investigation of the (d,p)-reaction on136, 132Xe in inverse kinematics

The one-neutron transfer reactions d(^132,136 Xe, p)^133,137 Xe have been investigated in inverse kinematics with xenon beams incident on deuterium loaded titanium targets. The angular distributions of the protons, measured with a detector array of 100 PIN-photodiodes, have been analyzed using standard DWBA. Generally, good agreement is obtained with results previously obtained in reactions induced by light-ion beams.     

Physics with heavy neutron-rich RIBs at the HRIBF

The Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory has recently produced the world's first post-accelerated beams of heavy neutron-rich nuclei. The first experiments with these beam are described, and the results discussed. B(E2;0⁺ → 2⁺) values for neutron-rich ^126,128Sn and ^132,134,136Te isotopes have been measured by Coulomb excitation in inverse kinematics. The results for ¹³²Te and ¹³⁴Te (N = 80, 82) show excellent agreement with systematics of lighter Te isotopes, but the B(E2) value for ¹³⁶Te (N = 84) is unexpectedly small. Single-neutron transfer reactions with a ¹³⁴Te beam on ^natBe and ¹³C targets at energies just above the Coulomb barrier have also been studied. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: radford@mail.phy.ornl.gov     

New neutron-rich 179Yb and 181,182Lu isotopes produced in reactions of 9 MeV/u 136Xe ions on tantalum and tungsten targets

The new neutron-rich isotopes ¹⁷⁹Yb and ^181,182Lu were produced in multinucleon transfer reactions by irradiating ^natW/Ta targets with 9 MeV/u ¹³⁶Xe ions, and identified by mass separation and decay spectroscopy. The measured half-lives of ¹⁷⁹Yb, ¹⁸¹Lu and ¹⁸²Lu are 8.1±0.8, 3.5±0.3 and 2.0±0.2 min, respectively. The properties of the excited states of ^181,182Hf are discussed. The possibility of studying neutron-rich nuclei outside the classical fission-product regions is demonstrated.     

Coulomb excitation of radioactive 132,134,136Te beams and the low B(E2) of 136Te

The B(E2;0(+)-->2+) values for the first 2+ excited states of neutron-rich 132,134,136Te have been measured using Coulomb excitation of radioactive ion beams. The B(E2) values obtained for 132,134Te are in excellent agreement with expectations based on the systematics of heavy stable Te isotopes, while that for 136Te is unexpectedly small. These results are discussed in terms of proton-neutron configuration mixing and shell-model calculations using realistic effective interactions.     

Fission of238U induced by136Xe for energies close to the Coulomb barrier

Excitation functions and angular distributions have been measured for fission of²³⁸U induced by¹³⁶Xe ions with bombarding energies between 4.5 and 5.9 MeV/N. Structures expected theoretically as characteristic for Coulomb fission have not been observed. The Q-value of ?(7.5±1.0) MeV measured for bombarding energies below 4.7MeV/N, however, appears to be compatible with inelastic scattering (e.g. Coulomb excitation) rather than subcoulomb transfer followed by fission. The total kinetic energy of deep inelastic events at 5.9 MeV/N is consistent with the current knowledge about mass diffusion, but also (for the highest excitation energies) with a fast fission process in the presence of the projectile.     

Reorientation precession measurements of quadrupole moments in103Rh

The quadrupole moments of the 3/2^? and 5/2^? states in¹⁰³Rh have been determined by measuring the precession of the gamma-ray angular distribution following Coulomb excitation;¹⁶O and³²S beams have been used. The structure of the negative-parity states in¹⁰³Rh is found to be in agreement with the model of Arima and Iachello.     

A measurement of B(E3;0+ → 31−) and some E2 transition probabilities in 132, 134, 136, 138Ba using Coulomb excitation

Reduced transition probabilities, B(E2) and B(E3), have been measured for low-lying 2⁺ and 3^? states in ^{132, 134, 136, 138}Ba using Coulomb excitation by 40 MeV ¹²C ions. The B(E2) values are in general consistent with previous measurements and the B(E3;0⁺ → 3₁^?) values are 0.176 ± 0.022, 0.148 ± 0.018, 0.155 ± 0.018 and 0.133 ± 0.013 e² · b³ for ^{132, 134, 136, 138}Ba respectively. These B(E3) values correspond to about 24 to 17 W.u. and such enhancements suggest that these 3^? states have an essentially collective character which may be attributed to octupole vibrations.     

Nuclear deorientation of W,Os and Pt recoiling in vacuum at 0.02c

The attenuations of γ-ray angular correlations have been measured for some 2⁺ and 4⁺ nuclear states in W. Os and Pt ions recoiling in vacuum with a velocity of 0.02c. The states were Coulomb excited by beams of 100 MeV³²S ions and the recoil-distance method was used to measure the time-dependence of the attenuations during the first 70 ps of the flight time. A simple parametrization was found to fit all the data and was used in a re-analysis of a published recoil-in-vacuum experiment on^{204, 206}Pb to give upper limits on theg factors of the 2 ₁ ⁺ states of |g(204)|<0.01 and |g(206)|<0.015.     

Electric moments of the first excited state of 18O

The static quadrupole moment Q_2⁺ and the B(E2; 0⁺ → 2⁺) value of the first excited state of ¹⁸O at e_x = 1.982 MeV have been determined using the reorientation effect in Coulomb excitation. Surface-barrier detectors at laboratory angles of 90° and 174° were used to detect ¹⁸O ions elastically and inelastically scattered from ²⁰⁸Pb. At both angles, we determined experimentally the maximum bombarding energies at which nuclear interference effects were negligible. It is found that Q_⁺ = ?0.023 ± 0.021 e · b (?0.052 ± 0.021 e · b) for destructive (constructive) interference from higher states. This result is in good agreement with theoretical expectation. For the transition moment we find B(E2; 0⁺ → 2⁺) = 0.00390 ± 0.00018 e² · b² (0.00371 ± 0.00018 e² · b²) for destructive (constructive) interference.     

Reorientation precession measurements of quadrupole moments in103Rh

The quadrupole moments of the 3/2^– and 5/2^– states in¹⁰³Rh have been determined by measuring the precession of the gamma-ray angular distribution following Coulomb excitation;¹⁶O and³²S beams have been used. The structure of the negative-parity states in¹⁰³Rh is found to be in agreement with the model of Arima and Iachello.     

In-beam spectroscopy of 120Xe

Excited states of ¹²⁰Xe have been studied by in-beam γ-ray spectroscopy using the ¹¹⁰Cd(¹³C, 3n) and ¹¹¹Cd(¹²C, 3n) reactions. The yrast band was observed up to the 14⁺ level, showing backbending at the 12⁺ state. A γ-band and two new negative-parity bands were found. The low-lying levels of ¹²⁰Xe could be described by the IBA-1 hamiltonian plus a triaxial degree of freedom.     

Microscopic and macroscopic aspects of 135 MeV proton scattering from 16O

Differential cross sections have been measured for the scattering of 135 MeV protons from ¹⁶O and data from the transitions to 13 states (up to 19.5 MeV excitation) have been analysed using microscopic and macroscopic nuclear reaction models. Extensive collective model calculations have been made of the transitions to all natural-parity states. The deformation parameters for the 4p4h rotational band are in good agreement with theoretical models. The inelastic scattering data from the excitation of the negative-parity states have also been analysed in the distorted-wave approximation using microscopic (shell and RPA) models of nuclear structure and with density-dependent two-nucleon t-matrices. For positive-parity states, we report the first shell-model calculation using the complete 2?ω basis space and find that the triplet of 2p2h states (4⁺, 2⁺, 0⁺) around 11 MeV excitation is quite well described by this model, as may be a 1⁺ state which is observed for the first time by proton scattering from ¹⁶O.