Deformed states of high spin in42Ca

The⁴²Ca levels at 4,715 and 6,633 keV excitation energy have been investigated using the³⁹ K(α,pγ reaction atE _α=14 and 15 MeV. From particle-γ-ray angular correlations the spin assignmentsJ(4,715)=6, 4 andJ(6,633)=8, 6, 4 have been obtained. Lifetime measurements using the Doppler-shift attenuation method yieldedτ (4,715)=120±46 fs andτ(6,633)=52±21 fs. Both levels have positive parity and decay by enhancedE2 transitions. They are interpreted as theJ ^π=6⁺ and 8⁺ members, respectively, of theK ^π=0⁺ rotational band which has theE _x =1,837, 2,423 and 3,250 keV states as further members. The enhancement of inbandE2 transitions is 50 _?16 ⁺³⁵ W.u. (6→4) and 63 W.u. (8→6) respectively. The intrinsic quadrupole moments which have been derived on the basis of the coexistence model, areQ ₀=1.13?0.16/+0.37b(8→6) andQ ₀=1.36±0.25b(6→4), respectively. TheJ ^π=10⁺ member of the rotational band has possibly been observed as a level at 8,856±5 keV excitation energy.     

Lifetimes of a dipole cascade in192Hg

Lifetimes of ten states with spins between 23 and 32 have been measured in a cascade of dipole transitions in¹⁹²Hg using the Doppler-shift recoil-distance method. Average reduced transition probabilities, deduced from the lifetimes, are B(M1)≈0.01 and 1.1µ _n ² in the lower and higher spin region of the cascade, respectively, and B(E2)≈0.13 (eb)². The jump in the B(M1) values can be explained by different multi-quasiparticle configurations in the lower and higher spin regions of the band.     

Evidence for the coexistence of spherical and deformed states in38Ar

The³⁸Ar levels at E_x=5350, 7289 and 9341 keV have been investigated using the³⁵Cl(α,pγ) reaction at E_α=l4 and 14.5 MeV. From particle-γ-ray angular correlations the spin assignments J(5350)=4, J(7289)=6,4 and J(9341)=8,6,4 have been obtained. Life-time measurements using the Doppler-shift attenuation method yielded τ(9341)=106±25 fs and τ(7289)=77±30 fs, while the lifetime τ(5350)=200±50 fs was known previously. All levels have positive parity and decay by enhanced E2 transitions. Hence we propose that they are the J^π=4⁺, 6⁺ and 8⁺ members, respectively, of a K^π=0⁺ rotational band which has the E_x=3377keV, J^π=0⁺ and the 3937 keV, J^π=2⁺ levels as further members. The enhancement of inband E2 transitions is 30 _?6 ⁺¹⁰ W.u. (4→2), 35 _?14 ⁺³⁰ W.u. (6→4) and 20–36 W.u. (8 → 6), respectively, yielding an average intrinsic quadrupole moment Q₀=850 _?125 ⁺²⁰⁰ mb. The moment of inertia is given by h²2θ=92 keV. The present data are in good agreement with the predictions of a deformed state in³⁸Ar that coexists with the spherical states.     

Lifetime measurements of a triaxial band in133Ce

The Doppler shift attenuation method has been used to determined the transition quadrupole moment of a triaxial band belonging to the γ-soft nucleus,¹³³Ce. Doppler broadened lineshape (DBLS) analysis has revealed the magnitude of theQ _t value, to be ～ 2.4 eb. This contribution provides new information of (i) the transition quadrupole moment of this band (ii) the life-times of in-band and sidefeeding transition and (iii) the relative deformations of coexisting nuclear shapes.     

Precision infrared spectroscopy in CF379Br by means of infrared-microwave double resonance

From the observation of double resonance effects on the microwave spectrum two coincidences between 9.4 μm CO₂ laser lines and infrared transitions of the ν₆ → (ν₆ + ν₁) band of CF₃⁷⁹Br have been determined: R(30) laser line coincident with ^qR₂(7), F = 17/2→17/2 transition, R(28) laser line coincident with all four ΔF = 0 hyperfine components of the ^qQ₈(13) transition. In both cases other infrared transitions lay within the tuning range of the laser. The frequencies of these two laser lines allowed calculations of the band center frequency ν₀ = 1083.530 ± 0.001cm^?1 and α_A = 11.93 ± 0.3MHz, for the ν₆ → (ν₆ + ν₁) band.α_B constants were determined for the vibrational states v₆, (v₆ + v₁), v₁, and v₃.     

Conversion--electron gamma--ray coincidence spectroscopy of superdeformed 135Nd

Three mini-orange conversion–electron spectrometers and four Euroball Ge Cluster detectors have been used for γ–e^- coincidence spectroscopy of superdeformed ¹³⁵Nd. Transitions within the superdeformed band are shown to have the expected E2 multipolarity. The 766.5–keV transition which links the band to a positive-parity state has a conversion coefficient consistent with M1 multipolarity. Consequently, positive parity is deduced for the superdeformed band. No evidence for E0 transitions was found. Received: 22 December 1997 / Revised version: 19 January 1998     

The ground-state rotational band in 159Tb

162 MeV ⁴⁰Ca ions have been used to Coulomb excite the ground-state band of ¹⁵⁹Tb up to spin $\text{25}\text{2}$. Lifetimes for levels up to spin $\text{25}\text{2}$ have been determined with DSA and recoil distance methods. Multipole mixing ratios for several cascade transitions were extracted from an analysis of γ-ray angular correlation data. Reduced transition probabilities thus deduced in a model-independent way were found to be in agreement with the rotational model with Q₀ = 7.41 ± 0.06 e · b and g_K?g_R = 1.377 ± 0.010.     

Observation of superdeformed bands in194Hg

Two rotational bands, with energy spacings characteristic of superdeformed shapes, have been observed following bombardment of¹⁵⁰Nd with⁴⁸Ca. The more intensively populated band consists of 18 transitions and is assigned to¹⁹⁴Hg. The depopulation of this band occurs around spin 10. The second band, consisting of at least 16 transitions, was populated less strongly and is tentatively assigned to¹⁹⁴Hg also. The lowest level in this band is assigned spin 8. The energy differences between transitions for both bands decrease from ～40 keV at low rotational frequencies to ～30 keV at the highest observed frequencies. The moments of inertia of the bands are similar to those of the two previously observed superdeformed bands in^191,192Hg. The similarities and differences of the four known bands in the mercury region are discussed.     

Conversion-electron spectroscopy in the decay of superdeformed 135Nd

We have investigated the structure of the superdeformed band in ¹³⁵Nd using conversion-electron spectroscopy. The transitions within the superdeformed band show conversion coefficients which are consistent with an E2 multipolarity. From the conversion coefficients of transitions linking this band with known low-lying states its positive parity was established. Hence, a direct determination of the parity of a high-spin superdeformed band could be performed for the first time.     

A precision determination of the radial charge parameters and the quadrupole moment of 181Ta using muonic X-rays

We report the measurement of the energies of the 4f → 3d, 3d → 2p and 2p → 1s atomic transitions in muonic ¹⁸¹Ta. Using transitions to and from the 1s and 2p levels, as well as the hyperfine splittings of the 2p and 3d states, we obtained the radial charge parameters assuming deformed Fermi distributions. We found the intrinsic static quadrupole moment Q₀ = 6.82 ± 0.06 e · b, in excellent agreement with Coulomb excitation results. We verified that the dynamic (corresponding to transitions to the first excited state) and the static (ground state) E2 moments were equal to within 1.1 %. The intrinsic hexadecapole moment was determined to be Π₀ = ?0.12 ± 0.40 e · b².     

EquilibriumK-shell excitation of highly ionized neon

Reorientation effect and multiple Coulomb excitation measurements have been performed for^{198, 200, 202, 1204}Hg. By measuring particle spectra after Coulomb excitation of an enriched¹⁹⁸Hg target with⁴He and¹⁶O beams, a positive quadrupole moment of Q₂ = 0.66±0.18 eb (for negative interference) or Q₂=0.80±0.18 eb (for positive interference) andB(E2,0→2)=0.991±0.006 e²b² were obtained. All otherB(E2) values and quadrupole moments were determined relative to these numbers from preciseγ-ray yields obtained from Coulomb excitation of a frozen^natHg target with⁴He and¹⁶O projectiles. Oblate shapes of all Hg isotopes were found. The data have been interpreted in terms of triaxial rotation and anharmonic vibrations.     

Lifetimes of excited states in164Hf

Lifetimes of excited states in¹⁶⁴Hf were measured using the Doppler-shift recoil-distance method. Gamma-gamma coincidences at eleven target-stopper distances were measured using the twelve Compton-suppressed Ge-detectors of the OSIRIS spectrometer. From the coincidence spectra the intensities of the Doppler-shifted and unshiftedγ-lines were determined. The quadrupole momentsQ _tof transitions within the rotational bands derived from the lifetimes are constant within the experimental uncertainties up to the 18⁺ state. TheQ _tvalue for the transition between the ground- and the neutroni ₁₃ ^2/2 band (s-band) which is strongly reduced compared to the values of the in-band transitions is used to deduce a band interaction energy of 34 keV ≦V≦57 keV.     

A superdeformed band in131Ce

A rotational band of 16γ-rays has been found in¹³¹Ce with a high moment of inertia indicating a deformation ε₂～0.38 and extending to spin ～40?. Its intensity is ～～5% of the total in¹³¹Ce, confirming a difference in the intensity systematics for superdeformed states in Ce nuclei compared with Nd.     

Lifetime measurements of levels in 34Ar: Isoscalar and isovector matrix elements for E2 analogue transitions

Mean lifetimes have been measured for the low-lying levels in ³⁴Ar(T_z = ? 1) excited by the bombardment of ³He-implanted Au targets with a beam of 80 MeV ³²S. The lifetimes determined by fitting Doppler-broadened γ-ray lineshapes are as follows (E_x in keV, τ in fs): 2091, 460 ± 60; 3288, 280 ± 50; 3871, > 270 and 4128, 1301⁺¹⁷⁰_?130. The resulting transition matrix elements for the 2⁺₁ → 0⁺₁ and 2⁺₂ → 0⁺₁ transitions are compared with those for the analogue transitions in ³⁴S (T_z = + 1) and ³⁴Cl(T_z = 0) to determine the isoscalar and isovector components. The component values are compared with theoretical calculations and with values of the ratio of neutron to proton matrix elements determined by inelastic scattering with strongly interacting probes.     

In-beam γ-ray spectroscopy with GASP

The performance of the multi-detector array GASP is described in detail with emphasis on the improvement due to utilization of ancillary detectors. GASP has been running for three years producing interesting new data on different nuclear structure topics. The study of the decay-out of superdeformed bands in the A=130 mass region is presented. The sudden disappearance of the superdeformed bands in the odd^{133, 135, 137}Nd isotopes is explained by the Total Routhian Surface calculations through a change of the nuclear shape which is microscopically related to the transfer of the valence neutron from a N=6 to a N=4 Nilsson orbital. A transient fieldg-factor measurement in the superdeformed band of¹³³Nd will also be presented. A meang-factor of $\bar g = 0.31(8)$ has been determined at the I*=41/2⁺ superdeformed state. The experimental value is compared with theoretical predictions and supports the assignment of the superdeformed band to thev[660]1/2⁺ Nilsson intruder orbital.     

Favoured superdeformed states in 89Tc

A superdeformed band consisting of a cascade of ten γ-ray transitions has been identified and assigned to the nucleus ⁸⁹Tc, close to the proton dripline. The quadrupole moment of the band (Q _t= 6.7^+3.0 _−2.3 eb, as measured by the Residual Doppler Shift Method) as well as a large dynamic moment of inertia point to a highly elongated shape. With a relative population of approximately 15% of the γ-ray flux in the ⁸⁹Tc exit channel, the band is among the most intense superdeformed bands observed to date. Received: 6 July 1999     

On the coexistence of oblate and prolate deformed bands in83Zr

Deformation parameters of the positive parity yrast band and negative parity bands in⁸³Zr are deduced from lifetimes andE2/M1 mixing ratios. Lifetimes of high spin states have been determined from recoil distance Doppler shift and Doppler shift attenuation measurements using the⁵⁴Fe(³²S,2pnγ) ⁸³Zr reaction. Ten lifetimes and five lifetimes limits were determined. The positive parity band, built on theg _9/2 K=5/2 orbital has an average deformation ¦β ₂¦=0.28(2), and shows a reduction ofE2 transition strengths in the observed backbend region at I^π≈21/2⁺. In contrast, theE2 strengths in the negative parity states show a steady increase up to I^π≈=15/2^?. These states are more strongly deformed than the positive parity states (¦β ₂¦=0.33(3)). TheE2/M1 mixing ratios show that the negative parity band hasK=3/2 and is prolate, and favour oblate deformation for the positive parity yrast band. In theK=1/2^? band theE2 strength of the 7/2^?→3/2^? transition yields a deformation ¦β ₂¦=0.26(5). The band structure is compared with calculations within the Hartree-Fock Bogoliubov cranking model.     

High resolution spectroscopy of prompt and metastable decaying levels in highly ionized argon,especially of the metastable3 P 2-state of Ar16+ and the4 P 5/2-state of Ar15+

Besides the prompt x-ray emission of Ar-ions, the metastable decay of the heliumlike³ P ₂- and the lithiumlike⁴ P _5/2-levels of highly ionized Ar¹⁶⁺ and Ar¹⁵⁺ atoms has been observed in a foil-excited beam using a flat-crystal spectrometer. Due to the highx-ray energy resolution of the spectrometer the two³ P ₂ →¹ S ₀ and⁴ P _5/2 →² S _1/2 transitions could be resolved. The experiment yielded (3128 ±2) eV for the³P₂ → and (3091 ± 2)eV for the⁴ P _5/2 →² S _1/2 transition energy respectively. Using a time of flight technique the lifetimes of the³ P ₂- and the⁴ P _5/2-states were determined to be (1.44 ±0.09)ns and (0.66 ± 0.05) ns.     

Band head spin assignment of superdeformed bands in 86Zr

Two parameter expressions for rotational spectra viz. variable moment of inertia (VMI), ab formula and three parameter Harris ω² expansion are used to assign the band head spins (I₀) of four rotational superdeformed bands in ⁸⁶Zr. The least-squares fitting method is employed to obtain the band head spins of these four bands in the A~80 mass region. Model parameters are extracted by fitting of intraband γ-ray energies, so as to obtain a minimum root-mean-square (rms) deviation between the calculated and the observed transition energies. The calculated transition energies are found to depend sensitively on the assigned spins. Whenever an accurate band head spin is assigned, the calculated transition energies are in agreement with the experimental transition energies. The dynamic moment of inertia is also extracted and its variation with rotational frequency is investigated. Since a better agreement of band head spin with experimental results is found using the VMI model, it is a more powerful tool than the ab formula and Harris ω² expansion.     

Oblate shapes of200, 202, 204Hg

Measurements of the reorientation effect for the first excited 2⁺ states in^{200, 202, 204}Hg were performed by exploiting the dependence of theγ-ray yield on Q₂+ for different projectiles. For²⁰⁰Hg, a positive quadrupole moment of Q₂=0.96±0.11 eb (for negative interference) or Q₂=1.11±0.11 eb (for positive interference) was determined indicating an oblate shape. Small positive Q₂ values were also found for²⁰²Hg and²⁰⁴Hg. Nine B(E2) values for excitation of the 2⁺, 2⁺′ and 4⁺ states in^196–204Hg measured.