Photodissociation of p-process nuclei studied by bremsstrahlung induced activation

A research program has been started to study experimentally the near-threshold photodissociation of nuclides in the chain of cosmic heavy element production with bremsstrahlung from the ELBE accelerator. An important prerequisite for such studies is good knowledge of the bremsstrahlung distribution which was determined by measuring the photodissociation of the deuteron and by comparison with model calculations. First data were obtained for the astrophysically important target nucleus 92-Mo by observing the radioactive decay of the nuclides produced by bremsstrahlung irradiation at end-point energies between 11.8 MeV and 14.0 MeV. The results are compared to recent statistical model calculations.Comment: 6 pages, 8 figures, Proceedings Nuclear Physics in Astrophysics II, May 16-20, 2005, Debrecen, Hungary. The original publication is available at www.eurphysj.or

Pygmy dipole strength close to particle-separation energies - the case of the Mo isotopes

The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for well separated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4 MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum - here applied to nuclear resonance fluorescence in a novel way - delivers dipole strength functions, which are combining smoothly to those obtained from (g,n)-data. Enhancements at 6.5 MeV and at ~9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.Comment: 6 pages, 5 figures, proceedings Nuclear Physics in Astrophysics II, May 16-20, Debrecen, Hungary. The original publication is available at www.eurphysj.or

Istraživanje dipolnih uzbuda u 89Y ispod gigantske dipolne rezonancije u pogonu ELBE

We have investigated the dipole response of the stable N = 50 nucleus 89Y up to the excitation energy of the giant dipole resonance (GDR) using the bremsstrahlung facility at the ELBE accelerator at the research center Helmholtz-Zentrum DresdenRossendorf. After eliminating background radiation from atomic processes as well as inelastic transitions in the spectra by using statistical methods, the photoabsorption cross section could be deduced from the photon-scattering data. This cross section was combined with that known from (γ, n) experiments. An excess of the cross section relative to a simple Lorentz approximation of the tail of the GDR is found in the energy range from about 6 MeV up to 11 MeV. The comparison with predictions of a quasiparticle-random-phase approximation (QRPA) showed that the corresponding excitations may be caused by an oscillation of the excessive neutrons forming a so-called pygmy dipole resonance (PDR) as it was found for the neighbouring isotones 88Sr and 90Zr.Istraživali smo dipolne rezonancije u stabilnoj jezgri 89Y s N = 50 do energije uzbude giganske dipolne rezonancije (GDR) rabeći sustav za kočno zračenje pri ubrzivaču ELBE u istraživačkom središtu Dresden-Rosendorf. Nakon uklanjanja pozadinskog zračenja od atomskih procesa kao i od neelastičnih prijelaza u spektrima, mogli smo, primjenom statističkih metoda, odrediti fotoapsorpcijske udarne presjeke za podatke o fotonskom raspršenju. Ti su udarni presjeci povezani s onima iz mjerenja (γ, n) reakcija. Našli smo višak u udarnim presjecima u odnosu na jednostavno Lorentzovo približenje u repu GDR, u području energije od oko 6 MeV do 11 MeV. Usporedba s približenjem kvazičestica i slučajnih faza pokazuje na mogućnost da su odgovorne uzbude uzrokovane oscilacijama viška neutrona koji načine tzv. “pigmy” dipolnu rezonanciju, nađenu u susjednim izotonima 88Sr i 90Zr

Istraživanje dipolnih uzbuda u 89Y ispod gigantske dipolne rezonancije u pogonu ELBE

We have investigated the dipole response of the stable N = 50 nucleus 89Y up to the excitation energy of the giant dipole resonance (GDR) using the bremsstrahlung facility at the ELBE accelerator at the research center Helmholtz-Zentrum DresdenRossendorf. After eliminating background radiation from atomic processes as well as inelastic transitions in the spectra by using statistical methods, the photoabsorption cross section could be deduced from the photon-scattering data. This cross section was combined with that known from (γ, n) experiments. An excess of the cross section relative to a simple Lorentz approximation of the tail of the GDR is found in the energy range from about 6 MeV up to 11 MeV. The comparison with predictions of a quasiparticle-random-phase approximation (QRPA) showed that the corresponding excitations may be caused by an oscillation of the excessive neutrons forming a so-called pygmy dipole resonance (PDR) as it was found for the neighbouring isotones 88Sr and 90Zr.Istraživali smo dipolne rezonancije u stabilnoj jezgri 89Y s N = 50 do energije uzbude giganske dipolne rezonancije (GDR) rabeći sustav za kočno zračenje pri ubrzivaču ELBE u istraživačkom središtu Dresden-Rosendorf. Nakon uklanjanja pozadinskog zračenja od atomskih procesa kao i od neelastičnih prijelaza u spektrima, mogli smo, primjenom statističkih metoda, odrediti fotoapsorpcijske udarne presjeke za podatke o fotonskom raspršenju. Ti su udarni presjeci povezani s onima iz mjerenja (γ, n) reakcija. Našli smo višak u udarnim presjecima u odnosu na jednostavno Lorentzovo približenje u repu GDR, u području energije od oko 6 MeV do 11 MeV. Usporedba s približenjem kvazičestica i slučajnih faza pokazuje na mogućnost da su odgovorne uzbude uzrokovane oscilacijama viška neutrona koji načine tzv. “pigmy” dipolnu rezonanciju, nađenu u susjednim izotonima 88Sr i 90Zr

Experimental study of the electric dipole strength in the even Mo nuclei and its deformation dependence

Two methods based on bremsstrahlung were applied to the stable even Mo isotopes for the experimental determination of the photon strength function covering the high excitation energy range above 4 MeV with its increasing level density. Photon scattering was used up to the neutron separation energies Sn and data up to the maximum of the isovector giant resonance(GDR) were obtained by photo-activation. After a proper correction for multi-step processes the observed quasi-continuous spectra of scattered photons show a remarkably good match to the photon strengths derived from nuclear photo effect data obtained previously by neutron detection and corrected in absolute scale using the new activation results. The combined data form an excellent basis to derive a shape dependence of the E1 strength in the even Mo isotopes with increasing deviation from the N = 50 neutron shell, i.e. with the impact of quadrupole deformation and triaxiality. The wide energy coverage of the data allows for a stringent assessment of the dipole sum-rule, and a test of a novel parameterization developed previously which is based upon. This parameterization for the electric dipole strength function in nuclei with A>80 deviates significantly from prescriptions generally used previously. In astrophysical network calculations it may help to quantify the role the p-process plays in the cosmic nucleosynthesis. It also has impact on the accurate analysis of neutron capture data of importance for future nuclear energy systems and waste transmutation.Comment: 41 pages; accepted for publication in PR

Characterization of Plasmopara viticola isolates from Bulgaria with microsatellite markers

Abstract. The fungus Plasmopara viticola is the causal agent of downy mildew, one of the most severe grapevine diseases. The genetic diversity of this pathogen in Bulgaria has not yet been investigated. In order to assess the polymorphism among P. viticola isolates in Bulgaria six samples collected in different regions of the country were genotyped with 7 SSRs. The genetic polymorphism revealed by the used microsatelite markers allowed discrimination of P. viticola isolates originating from the different areas of Bulgaria. All samples were found to have a unique microsatellite profile. One sample was found to have a multiallelic profile at three out of seven loci and was excluded from the calculation of genetic diversity parameters. The number of alleles per locus varied between 2 and 6 with a mean value of 3.5 alleles per locus. The obtained values of observed heterozygosity ranged from 0.2 to 0.8 with an estimated average value of 0.46±0.13, which is a bit lower than the mean expected heterozygosity – 0.53±0.07. With this study the characterization of population structure of P. viticola pathogen in Bulgaria was initiated