PrimaryE2 transitions observed following neutron capture for the mass region 144≦A≦180

Values or upper limits for the radiative widths of 12 primaryE2 transitions observed in thermal neutron capture for nuclei with 144≦A≦180 are presented. The results are compared with the predictions of the Axel-Brink hypothesis using the systematics of the isoscalar and isovector quadrupole resonances. The strengths are on the average lower than expected. Analysis of previously reported average resonance data results in the same conclusion.     

Limiting excitation energies in fusion evaporation reactions

The mass dependence of the critical excitation energy above which a hot nucleus does not decay any more by particle evaporation, has been studied in the mass range 75≦A≦130. The experimental values of the critical excitation energy have been obtained by means of a new integral method based on the analysis of the widths of the evaporation residue mass distributions as a function of the residue velocity. The obtained mass dependence appears to be stronger than expected by various models and a predicted dependence on theZ/A ratio of the decaying nucleus seems to be confirmed.     

Studies on the goodness of IBA group symmetries. I. Centroids,widths and partial widths for irreducible representations of IBA group symmetries

We present explicit analytic expressions for the centroids, widths and partial widths of the eigenvalue distribution over an irreducible representation corresponding to various group symmetries of the interacting boson approximation model of Arima and Iachello. As an example we present a study of the goodness of IBA group symmetries for Sm isotopes.     

Investigation of the 4f-electron-phonon-coupling in the ferromagnet LiTbF4 by inelastic scattering of light

The polarized Raman spectra of LiTbF₄ in an external magnetic field (B≦8 T) have been recorded in the wavenumber interval 0≦500 cm^?1 and at temperatures of 1.8 K and 4.2 K. We have studied effects of the 4f electron-phonon (magnetoelastic) coupling manifesting itself in the splitting of a twofold degenerate phonon mode and in anticrossing effects between phonons and electronic transitions. In the spectra probing the scattering tensor elements (xz) and (zy) this anticrossing shows an asymmetry with respect to the frequencies and scattering intensities of the quasi-degenerate components. These effects are discussed in detail and can be interpreted by the theory of magnetoelastic interaction and by taking into consideration the finite widths of the electronic and phonon components (“critical coupling”). Brillouin scattering has been used to determine the sound velocities in thea-direction. No effect of the magneto-elastic interaction could be detected in this case.     

Shell model calculations on natural parity states in 10≦ A≦14 nuclei

Shell model calculations of natural parity states in the 10≦A≦14 mass region have been performed by assuming an inert⁴He core with the residual interaction in the 1p shell only. The modified surface delta interaction (MSDI) has been used as an effective two-body interaction. The MSDI parameters as well as the single-particle binding energies have been deduced from a least-squares fit to experimentally known levels in, firstly, the seperate¹⁰B,¹¹B-C,¹²C,¹³C-N and¹⁴N nuclei, and secondly, the whole mass region 10≦A≦14. Multipole moments for ground states and M1 and E2 radiative widths for excited states have been calculated with the resultant wave functions.     

Der Einfluß der Kopplung von Barkhausen-Sprüngen auf deren Größenverteilung

The size distribution of Barkhausen discontinuities was investigated in poly-crystalline nickel as a function of coercive force (2.1≦H _c ≦17.0 Oe), demagnetizing factor (0.0316≦N≦0.405) and counting rate (5≦dn/dt≦500 pulses/sec). The comparison by means of the factor (1 +N·χ) of size distributions from specimens with differentN gives much better results, if the distributions are measured at constant counting rate. This rate should be as large as possible because of the blocking mechanism of the demagnetizing field.     

Spectral distributions of mixed configurations of identical nucleons in the seniority scheme I. Generalized seniority scheme

The procedure for calculating the centroid energies as well as the total and partial widths of the generalized seniority distributions of mixed configurations of identical nucleons is described. An extensive use of the quasispin formalism is made. In some particular cases, analytical formulae, some of which are new, are derived for the centroids and widths. Numerical calculations are performed in the nickel and tin isotopes. They show that although the admixtures with |Δv| = 4 should in many cases be relatively small, those with |Δv| = 2 should be very large, owing to the important contribution of the single-particle potential.     

The ionic charge distribution of fission products and the influence of internal conversion on highly preionized heavy ions

At the mass spectrometer LOHENGRIN of the Institut Laue-Langevin the ionic charge state distributions of²³⁵U(n _th,f)-fission products separated according to their mass, nuclear charge and kinetic energy were determined. The mean values and the widths of the ionic charge state distributions are compared with semiempirical predictions. Deviations between the experimental data and the estimation of Nicolaev and Dmitriev are found. Furthermore, the influence of the internal conversion process on the ionic charge state distribution of highly ionized fission products was established. Internal conversion is observed mainly for odd-odd nuclei and nuclei with 59 neutrons. The Auger cascade following the internal conversion process shifts the ionic charge state distribution by about 3 charge units. The yield of conversion electrons per fragment was determined in the mass range 85≦A≦103.     

Four-momentum dependence of inelastic scattering of cosmic ray muons on nucleons

Data taken by a combination of a horizontal solid iron muon spectrograph and an interaction calorimeter have been analysed with respect to the process of inelasticμN-scattering. The muon momentum range covered by the spectrograph extended from 7 GeV/c≦p _μ ≦1000 GeV/c. Energy transfers to hadrons between 0.3 GeV≦v≦10 GeV were recorded. The results at low four-momentum transfers are consistent with precocious scaling.     

Beta-decay energies and nuclear masses of very neutron-rich Rb and Cs isotopes

Theβ-endpoint energies of very neutron-rich Rb and Cs isotopes with mass numbers 94≦A≦98 and 142≦A≦146, respectively, have been measured with a plastic scintillatorβ-telescope at the on-line mass separator OSTIS. From these,Q _β-values and mass excesses are calculated and compared with the results obtained in direct mass determinations.     

An electronically-driven volume transition in CeSi2−xGax

The pseudobinary compounds CeSi_2?xGa_x were prepared, and the lattice parameters and the susceptibilities were measured. At the Si-rich end, 0≦x<0.2, the system behaves as one with a high (～200K) Kondo temperature exhibiting no magnetic order. For 0.5<x≦1.3, the system shows the unit-cell volume 3% larger and orders ferromagnetically around 10K. The α-ThSi₂ structure is retained up to x=1.3, but one observes a two-phase region for 0.2≦x≦0.5, consisting of small- and large-volume phases. This volume transition is magnetically one of the most drastic compared to previously studied Ce-based pseudobinaries.     

Collective mass parameters and linear response techniques in three-dimensional grids

The²⁴Mg(α,γ ₀)²⁸Si capture reaction has been studied for bombarding energies 4.4 ≦E _α ≦9.6 MeV. Through angular distributions taken in steps of 60 keV the groundstate yield has been decomposed into its contributingE1 andE2 components. Excitation energies and widths of several narrow 1^? and 2⁺ levels were determined. Significant deviations of the phase factor cosδ from the statistical expectation value were analysed in terms of a two-state interfering process and yielded as much as (40 ±10)% semidirect contributions to the α-capture reaction.     

Backward electroproduction of π+ mesons in the second and third nucleon resonance region

The reactionep→enπ ⁺ has been studied in the invariant mass range of 1.44≦W≦1.83 GeV. The outgoing pion was detected in the backward direction (cosλ _π ^* ≦-0.94) with respect to the virtual photon at four-momentum transfers of 0.4≦q ²≦0.7 (GeV/c)². The values of the helicity-1/2-partial wave amplitudesA _2?(D ₁₃(1520)) andA _3?(F ₁₅(1688)) were determined by a phenomenological analysis and compared with single quark transition models (S.Q.T.M.).     

Direct photon production from pions and protons at 200 GeV/c

Direct photon production has been studied by an experiment performed with the NA3 spectrometer at CERN, using incident negative and positive beams at 200 GeV/c interacting with an isoscalar Carbon target. Two different triggers have been used; one of them requires the photon conversion. The experiment is sensitive to direct photons produced with 3.0≦P _T ≦6 GeV/c and center-of-mass rapidity ?0.4≦y ^*≦1.2. Inclusive cross sections are given for incident π^± and protons, and compared with second order QCD predictions; finally an estimation of the gluon structure function of the nucleon is given.     

New instanton and meron solutions of a generally covariant four-dimensional sigma model

We construct new solutions of the fourdimensional sigma model coupled to the metric tensor field and having an internalO invariance. Our solutions interpolate continuously between the known instanton and meron solutions depending upon a parameterf. We find that the typical domain for the instantons is 2<f≦3 while for the merons is 0≦f≦2.     

Spontaneous fission half-life-times of double-odd nuclei (Z≧97)

Spontaneous fission half life times of odd-odd and even-even nuclei in the region 97≦Z≦109 and 140≦N≦ 160 are calculated. New Nilsson potential parameters (κ, μ) generating the proper single particle spectrum are estimated. Angular momentum projections of odd-odd particles are properly taken into account. Hindrance factors 5–7 orders of magnitude are found forZ≦101 nuclei and 1–3 orders of magnitude for nucleiZ≧ 103.     

Isotope shifts of perturbed 6sns 1 S 0 and3 S 1 Rydberg states of odd barium isotopes

Isotope shifts of the pairs Ba 135–138 and 137–138 have been measured for the 6sns ¹ S ₀ (11≦n≦23) and 6sns ³ S ₁ (14≦n≦23) Rydberg states. A quantitative analysis of the data in terms of hyperfine-induced singlet-triplet mixing was carried out. Admixtures of local perturbers belonging to the 5d 7d configuration were easily identified and admixture coefficients were derived.     

The influence of pairing and nuclear structure on the thermal-neutron-induced fission of235U

The mass separated fission product beam provided by the mass separator “Lohengrin” has been used to determine the nuclear charge distribution for the thermal-neutron-induced fission of²³⁵U for all light fission products in the region 80≦A≦107. The measurements were performed at the most probable kinetic energy of the fission products. By using the known fission product mass yields, the independent yields for a total number of 100 nuclides were obtained under the condition of the most probable kinetic energy. The proton pairing effect modulates the average nuclear charge of the fission fragments and the isobaric charge distribution widths in a regular fashion. The probabilities of breaking a pair and of forming fragments with an energetically unfavourable neutron-to-proton ratio are found to compete with each other. Both probabilities depend on the mass split and reach their maximum values in the region of the most probable masses. The odd-even-proton effect is found to vary smoothly between 16% for the most abundant mass splits and 40% for the rare mass splits. The odd-even-neutron effect exhibits maxima nearN=50 andN=60, where it reaches 16%. These maxima and the extremely low Tcyield (0.13±0.05%) are discussed with regard to fragment shell effects.     

Scattering of fast muons by copper nuclei

The relative probability forK-capturePK ₉₃₆/PK ₅₃₉≦0.05 and forL-capturePL ₉₃₆/PL ₅₃₉=4.4±0.7 yield a total transition energyQ _EC in the energy Intervall 981≦Q _EC ≦1,021keV. The high-energy γ-spectrum and the half-life of¹⁹¹Pt were remeasured.