R. Löhken

Elastische und inelastische Streuung von 104 MeV Alpha-Teilchen an58,60,62,64Ni

Differential cross sections are measured for the elastic and inelastic scattering of 104 MeV α-particles from58, 60, 62, 64Ni. The experimental results are analyzed in terms of coupled channels on the basis of an anharmonic vibrational model and prove to be quite sensitive to the values of the deformation parameters. For forward angles the 4+ angular distributions of58,60Ni are dominated by aL=4 single excitation mechanism. This is in keeping with theE4-transitions observed in (e, e′) scattering. From the 21+- and 31+-results transition rates are derived which can be compared to results of electromagnetic methods and of inelastic proton scattering studies. The comparison indicates that the transition rates differ and are generally higher for the proton scattering. Especially in the case of the 3− states of58,60Ni the differences are obvious.The elastic cross sections are analyzed both on the basis of the usual phenomeno-logical model and in terms of a semimicroscopic folding model resulting in values of rms-radii for the nuclear matter distribution.

Quasirotational groundstate bands of even-even nuclei with 50≦N, Z≦82

The quasirotational groundstate bands of140Sm,142Sm,144Gd and136Nd have been determined by studying the prompt γ-decay after (α,xn+yp+zα.) reactions using α-particles of up to 104 MeV and targets of144Sm,142Nd,141Pr,140Ce,136Ce and139La. The new levels continue the trends of the neighbouring even-even nuclides smoothly. These systematics of the groundstate bands in the region 50<N,Z<82 are not disturbed by expected subshell closure effects.

(α, τ) and (α, t) reactions with 104 MeV α-particles

Abstract The differential cross sections for the one-nucleon stripping reactions and the elastic scattering of 104 MeV α-particles on 12 C, 27 Al and 40 Ca were measured. The experimental results show that the (α, τ) cross sections are larger than those obtained from (α, t) reactions on the same target nuclei. The reaction data were analyzed in the framework of the DWBA. The calculations using the zero-range approximation are able to reproduce the measured angular distributions. In addition the effect of finite-range corrections were studied in the local energy approximation. From the analysis it becomes evident, that the inner part of the target nucleus contributes significantly to the reaction cross section. The larger yield of 3 He particles compared to that of tritons can be explained mainly by different normalization factors for the (α, τ) and (α, t) reactions, respectively.

Evidence for oblate shape of 12C from inelastic α-particle scattering

Abstract The differential cross sections for the scattering of 104 MeV α-particles from the ground and first excited states of 12 C were measured in the angular range of 5 to 176 ° . The differential cross sections were analysed with the theory of Austern and Blair assuming the rotational model and taking into account multiple excitations for both the elastic and inelastic scattering. This technique proves to be sensitive to the sign of the quadrupole deformation. A negative sign leads to the best fit of the data. A hexadecapole deformation could not be detected. Electric multipole moments and deformation parameters were deduced taking into account electron scattering results. These values are compared with Hartree-Fock calculations.

The spherical to asymmetric shape transition in the mass region with 50<(N, Z)<82

Collective potential energy surfaces were determined in the mass region with 50<(N, Z)<82 by fitting experimental level spectra andB(E 2)-values on the basis of the generalized collective model of Gneuss and Greiner. While the nuclides withN=80 and withZ=52 are rather soft vibrators we find an abrupt transition to asymmetric rotators forN=78 and forZ=54 tending to more symmetric shapes further away from the closed shells. The results predict the position of levels not yet observed andB(E 2)-values.

139Sm, a new nuclide of relevance for the systematics of theN=77 isomers

The ground state and an isomeric state of139Sm have been identified witht1/2m=9.5 ± 1.0 s andt1/2g=2.6 ± 0.3 min. The isomeric decay scheme is presented. It extends the chain of isomers withN=77. The distance between thed3/2 andh11/2-levels reaches a maximum forZ=60. This maximum is much more pronounced than expected from previous theoretical considerations and from systematic trends in theN=81 and 79 chains.

Spectral shape and nuclear matrix elements in the β-decay of 206Tl

Abstract The spectral shape, endpoint energy and half-life of the β-decay of 206 Tl were measured. The β-particles were analysed in a 4π Si(Li) sandwich spectrometer. The sources used were isotopically separated and therefore extremely thin. The measured spectrum was corrected for the escape of bremsstrahlung. The extracted shape factor shows only small deviations from the statistical form, the endpoint energy of the decay is 1527 ± 4 keV, its half-life 4.27 ± 0.05 min. The nuclear matrix elements of this 0 − → 0 + decay have been calculated from single-particle wave functions. The resulting theoretical spectral form describes the experimental data very well, whereas the total transition probability is underestimated theoretically by a factor of 7.

A 4πβ-γ-coincidence spectrometer using Si(Li) and NaI(Tl) detectors

Abstract The well-tested method of measuring β-spectra with Si(Li) semiconductor detectors in 4 π geometry is extended to inner β − -spectra and positron decays by applying coincidence techniques. For this purpose a 4 π β - γ -coincidence spectrometer has been constructed. The radioactive source and the two 2 π Si(Li) β-detectors are put together in a sandwichlike arrangement. The γ-quanta, emitted in coincidence with the β-particles, are detected by two large NaI(Tl) scintillation counters, which cover a solid angle of 0.6 × 4 π with respect to the source. A sluice enables the source to be quickly brought into the vacuum chamber, so that short-lived decays with halflives down to one minute can be measured. The characteristics of the coincidence spectrometer are checked by measuring the β − -spectrum of 24 Na and the positron spectrum of 22 Na. The results are in good agreement with those obtained by other methods and show the reliability of the apparatus.

Beta Spectrum of Tl 206

The spectrum shape of the

Spectral shape of the (7/2−→5/2+)-transition in the β-decay of139Ba

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