Investigation of lifetimes in quadrupole bands of 142Gd

For a level scheme investigation of ¹⁴²Gd an experiment with the γ -spectrometer EUROBALLIII has been carried out and lifetimes have been measured with EUROBALL IV using the Doppler-shift attenuation method. The high-spin states have been populated in these experiments by means of the ⁹⁹Ru(⁴⁸Ti, 2p3n) reaction at a beam energy of 240MeV and the ¹¹⁴Sn(³²S, 2p2n) reaction at 160MeV, respectively. Reduced E2 transition probabilities B(E2) were determined for 15 members of four quadrupole bands. For the interpretation of the positive-parity even-spin quadrupole bands, calculations in the cranked Nilsson-Strutinsky as well as interacting boson models have been performed. From the former calculations it was concluded that the high-spin states of the (+ , 0)₁ band in ¹⁴²Gd represent a triaxial nucleus rotating around the longest principal axis.     

Moderately deformed bands in the continuum of144Gd

The lifetimes of the gamma-ray continuum in¹⁴⁴Gd were measured up to spin ～ 50? via the DSAM technique following the⁷⁴Ge(⁷²Ge, 2n) reaction at 270 MeV beam energy. The average Doppler Shift of theE2 bump corresponds mostly to a structure with a deformation (β～0.2–0.3), consistent with the triaxial structure predicted to lie close to the yrast line at spins 30–50? in this nucleus.     

Investigation of lifetimes in dipole bands of 142Gd

Lifetimes have been measured for dipole bands in ¹⁴²Gd using DSAM. The deduced B(M1) and B(E2) values as well as B(M1)/B(E2) ratios are compared with calculations in the framework of the TAC (Tilted Axis Cranking) and SPAC (Shears mechanism with Principal Axis Cranking) models. The dipole bands DB1 to DB4 can be interpreted as magnetic rotational bands.     

Hyperfine structure in the Gd II spectrum and the nuclear electric quadrupole moment of157Gd

The hyperfine structure of¹⁵⁷Gd was investigated in 20 Gd II lines by means of a photoelectric recording Fabry-Perot interferometer with digital data processing. The hyperfine splitting factors,A andB, were obtained by computer fits to the observed line structures. Using a multiconfigurational set of wave functions in intermediate coupling derived by Wyart, mono-electronic hyperfine parameters were deduced by a parametric treatment. The nuclear electric quadrupole moment of¹⁵⁷Gd was evaluated from the quadrupole interaction of the 5d electron in 4f ⁷ 5d6s, the 5d electron in 4f ⁷ 5d6p, and the 6p electron in 4f ⁷ 5d6p. The three values obtained for the quadrupole moment agree very well. The final result, corrected for Sternheimer shielding, isQ(¹⁵⁷Gd) =1.34(7)·10^?24 cm².     

Investigation of lifetimes in dipole bands of 141Eu

Lifetimes have been measured for dipole bands in ¹⁴¹Eu using DSAM. The deduced B(M1) and B(E2) values as well as B(M1)/B(E2) ratios are compared with calculations in the framework of the TAC (Tilted Axis Cranking) and SPAC (Shears mechanism with Principal Axis Cranking) models. The dipole bands can be interpreted as magnetic rotational bands.Received: 6 April 2004, Revised: 14 May 2004, Published online: 13 July 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 21.10.Re Collective levels - 21.10.Tg Lifetimes - 27.60. + j      

Dipole and quadrupole cascades in the yrast region of 143Gd

The high spin states of ¹⁴³Gd have been studied via the ¹¹¹Cd(³⁵Cl,1p2n) reaction at 170 MeV. One dipole cascade has been newly found and two dipole cascades have been extended to higher spin states. A relatively long E2 cascade consisting of irregular transition energies has been found, which has only weak connections to the other dipole and quadrupole cascades. This resembles the ones recently found in ^142,143,144Eu and ¹⁴⁴Gd. The dipole and quadrupole cascades have been found to appear by turns in an energy increment of about 0.2 MeV above the yrast line. Received: 1 November 1997     

Gamma-ray spectra and positive parity bands in 158Gd

A Ge(Li)-NaI(Tl) pair spectrometer and a Ge(Li)-NaI(Tl) anti-Compton spectrometer were used to measure γ-rays from the reaction ¹⁵⁷Gd(n, γ)¹⁵⁸Gd. A detailed decay scheme for levels of ¹⁵⁸Gd was constructed up to an excitation energy of 2504 keV. Branching ratios for transitions from members of the positive parity bands to members of the ground state band were determined. It is shown that a single band-mixing parameter is not sufficient to explain the experimental results, and that even a four-band-mixing calculation does not provide satisfactory agreement. The binding energy of the last neutron is determined to be 7937.4±0.7 keV.     

Ground state bands in142Gd and140Sm

The ground state bands of theN=78 isotones¹⁴² Gd and¹⁴⁰Sm were observed up to the 8⁺ state by bombarding¹⁴⁴Sm and¹⁴²Nd withα-particles of energies between 80 and 130 MeV. Excitation functions,γ-γ coincidences, lifetimes and angular distributions were measured. The ground state band in¹⁴⁰Sm is partially fed by an isomeric state ofT _1/2?17ns. No such isomerism is observed for¹⁴²Gd. The level energies are very similar in both cases and agree well with the predictions of the VMI model.     

Beam-foil level lifetimes in Krypton III

The radiative lifetimes of levels in doubly-ionized Krypton have been measured after foil excitation of a beam of Krypton particles in the wavelength range 120nm–500nm. The results are compared with the experimental values obtained by Fink et al. [1], as well as theoretical values resulting from Coulomb approximation (C.A.) and single configuration Hartree-Fock (H.F.) calculations.     

Determination of landau level lifetimes in n-GaAs

The lifetime of electrons in the first excited Landau level of n-GaAs is determined from a combination of measurements of far infrared cyclotron resonance induced absorption and conductivity change. Values of T₁ of the order of 10^?8s for densities of excited electrons of 10¹¹ cm^?3 and a temperature dependence of T^?2.7 are found. An upper limit for the N = 0 Landau level to donor recombination time of the order of 10^?9s was derived from pulsed conductivity measurements.     

Structure of N=81144Eu from the decay of144Gd

The structure of¹⁴⁴Eu is investigated through the beta decay of¹⁴⁴Gd, whose decay half-life is measured to be (4.47±0.06) m. A level scheme comprising 67 transitions depopulating 24 excited states is constructed. With the help of the deducedf t values and measured internal conversion coefficients, spin-parity assignments are proposed for several states in¹⁴⁴Eu. This level structure is discussed using the systematics of the odd-A N=81 and N=82 isotones which are investigated by a spherical BCS calculation.     

Observation of intermediate bands feeding the positive-parity yrast band in 155Gd

High-spin states of ¹⁵⁵Gd were populated by using the ¹⁵⁴Sm(α,3nγ)¹⁵⁵Gd reaction at E _α= 33 MeV. γ-γ coincidence, E _γ singles, excitation function, and the DCO ratios were measured. we have identified three intermediate bands with ΔI= 2 feeding the positive yrast band. The bands are interpreted as such candidate bands that are mixed with the negative-parity ground state band. This observation can provide a plausible explanation for unusually large population of the positive-parity yrast band observed in a recent Coulomb excitation. Received: 2 November 1999     

Nuclear polarization and signs of quadrupole moments of high-spin Gd isomers

Time-differential perturbed-angular-distribution (TDPAD) experiments have been performed on polarized intermediate- and high-spin isomers ${}^{144}\text{Gd}\text{(10}{}^{\mathrm{+}}\text{)}\text{and}{}^{147}\text{Gd}\text{(}\text{13}\text{2}{}^{\mathrm{+}}\text{,}\text{27}\text{2}{}^{\mathrm{?}}\text{,}\text{49}\text{2}{}^{\mathrm{+}}\text{)}$. Excited Gd nuclei populated by $\text{(}{}^{28}\text{Si}\text{, x}\text{n}\text{)}$ reactions were polarized by the tilted-multifoil technique and implanted in a Gd single-crystal host. Observation of the subsequent electric quadrupole interaction with the known electric field gradient yielded negative values of the sign of the deformation for all isomers studied. The magnitude of the induced nuclear polarization P_I was also determined from the TDPAD data and compared to model calculations in order to deduce the average atomic angular momentum and polarization of Gd ions at v/c ～ 0.018.     

Nuclear structure of the heavyN=82 isotones:144Sm and146Gd

For single-closed shell nuclei with a large number of extra-protons, outside theZ=50,N=82 double-closed shell nucleus, a standard shell-model calculation yields dimensions of the Hamiltonian matrices beyond the scope of present computer technology. A quasi-particle (QP) calculation however, is able to describe the most important proton-excitations in a much restricted configuration space. Extended BCS+TDA calculations are performed on the single-closed shell¹⁴⁴Sm and¹⁴⁶Gd nuclei with a Gaussian force as effective two-body interaction. The influence of the nonconservation of the exact number of extra-protons in the BCS-approach is studied by performing a projection onto the states with an exact number of extra-protons (BCS+P). The first excited 2⁺, 4⁺ and 6⁺ states in¹⁴⁶Gd are described mainly by the (1g7/2) ₀ ⁸ (2d5/2) ₀ ⁴ \((1h11/2)_{J^\pi }^2 \) configuration.     

Experimental energy bands,exchange splittings,and lifetimes for Ni in the vicinity of the x-point

Angle-resolved photoelectron spectroscopy using synchrotron radiation has been used to determine energy band dispersions along the ΓKX and ΓX directions in nickel. A detailed picture of spin-dependent energy levels and band topology around the symmetry point X has been derived. We have measured the exchange splitting for different band symmetries and find the splitting for the X₂-S₄ band along (110) to be 0.17 eV. For the X₅-S₃ band we find a splitting of 0.33 eV which is in close agreement with the value found earlier for the Σ₂ band. This can be explained by different self-energy corrections for t_2g and e_g-type states respectively. Our values for the energy positions (inverse lifetimes) are the following: X_2↓ = ?0.04 eV (0.08 eV, X_2↑ = ?0.24 eV (0.19 eV), X_5↑ = ?0.11 eV.     

Coherent quadrupole states of three interacting bands and the solution of a Bohr-Mottelson Hamiltonian

The projected functions of the coherent state model (CSM) describing the ground, gamma and beta bands are studied in the intrinsic frame of reference. Several symmetry relations for the intrinsic functions are obtained. Analytical expressions for the matrix elements of the collective operatorβ ^m+n cos ^m γsin ⁿ γ are derived. Important selection rules are established for them. The probability distribution for the deformationsβ andγ are studied and compared to those corresponding to the solution of a microscopically determined Bohr-Mottelson Hamiltonian.     

The use of stability bands to improve the performance of quadrupole mass filters

The possibility of increasing the resolving power of quadrupole mass filters has been discussed. It has been shown that the limitations associated with the finite time of flight imposed by Von Zahn’s rule are modified while using the islands of stability that appear when quadrupole is excited by the additional signals. By calculation of the exponential increment of growth of the oscillation amplitude the effect of the acceleration of mass separation and improvement of the peak shape, when the islands of stability are used for ion filtering, is explained. The case of the excitation by two signals at different frequencies has been studied theoretically. The conditions under which suppression of the first order resonance for one of the directions of motion is obtained. The direct modeling of the peak shape of the mass filter shows the possibility of obtaining a resolution of 10,000 with a time of flight of ions through the quadrupole of 100 cycles of the main RF supply, and low sensitivity of the new operating mode to the nonlinear field distortions in the quadrupoles with rods of circular cross sections.