Numerical study of axisymmetric vortex breakdowns

Axisymmetric flow of rotating stream analyzed to determine conditions allowing isolated vortex breakdown to develo

Photon strength distributions in stable even-even molybdenum isotopes

Electromagnetic dipole-strength distributions up to the particle separation energies are studied for the stable even-even nuclides $^{92,94,96,98,100}$Mo in photon scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf. The influence of inelastic transitions to low-lying excited states has been corrected by a simulation of $\gamma$ cascades using a statistical model. After corrections for branching ratios of ground-state transitions, the photon-scattering cross-sections smoothly connect to data obtained from $(\gamma,n)$-reactions. With the newly determined electromagnetic dipole response of nuclei well below the particle separation energies the parametrisation of the isovector giant-dipole resonance is done with improved precision.Comment: Proceedings Nuclear Physics in Astrophysics 3, March 2007, Dresden Journal of Physics G, IOP Publishin

Neutron charge radius and the Dirac equation

We consider the Dirac equation for a finite-size neutron in an external electric field. We explicitly incorporate Dirac-Pauli form factors into the Dirac equation. After a non-relativistic reduction, the Darwin-Foldy term is cancelled by a contribution from the Dirac form factor, so that the only coefficient of the external field charge density is $e/6 r^2_{En}$, i. e. the root mean square radius associated with the electric Sachs form factor . Our result is similar to a recent result of Isgur, and reconciles two apparently conflicting viewpoints about the use of the Dirac equation for the description of nucleons.Comment: 7 pages, no figures, to appear in Physical Review

Nuclear level densities and gamma-ray strength functions in 44,45Sc

The scandium isotopes 44,45Sc have been studied with the 45Sc(3He,alpha gamma)44Sc and 45Sc(3He,3He' gamma)45Sc reactions, respectively. The nuclear level densities and gamma-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental gamma-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (gamma,n) and (gamma,p) experiments. The strength functions show an enhancement at low gamma energies that cannot be explained by the present, standard models.Comment: 21 pages, 13 figures. Published versio

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

Neutron electric dipole moment: Constituent-dressing and compositeness

Contributions to the neutron's EDM, are calculated using a well-constrained Ansatz for the nucleon's Poincare' covariant Fadde'ev amplitude. The momentum-dependent quark dressing amplifies the contribution from the current-quarks' EDMs; and dressed-quark confinement and binding make distinguishable the effect of the two CP and T violating interactions: i gamma_5 sigma_{mu nu} (p_1-p_2)_nu and gamma_5 (p_1+p_2)_mu, where p_{1,2} are the current-quarks' momenta. The value of |d_n| obtained using the current-quark EDMs generated by a minimal three Higgs doublet model of spontaneous CP violation is close to the current experimental upper bound.Comment: 9 pages, LaTeX2e. Errors in Table 1 corrected; five references added. To appear in Phys. Rev.

Neutron capture cross section measurements for 197Au from 3.5 to 84 keV at GELINA

Cross section measurements have been performed at the time-of-flight facility GELINA to determine the average capture cross section for 197Au in the energy region between 3.5 keV and 84 keV. Prompt gamma-rays, originating from neutron induced capture events, were detected by two C6D6 liquid scintillators. The sample was placed at about 13 m distance from the neutron source. The total energy detection principle in combination with the pulse height weighting technique was applied. The energy dependence of the neutron flux was measured with a double Frisch-gridded ionization chamber based on the 10B(n,alpha)reaction. The data have been normalized to the well-isolated and saturated 197Au resonance at 4.9 eV. Special care was taken to reduce bias effects due to the weighting function, normalization, dead time and background corrections. The total uncertainty due to normalization, neutron flux and weighting function is 1.0%. An additional uncertainty of 0.5% results from the correction for self-shielding and multiple interaction events. Fluctuations due to resonance structures have been studied by complementary measurements at a 30 m flight path station. The results reported in this work deviate systematically by more than 5% from the cross section that is recommended as a reference for astrophysical applications. They are about 2% lower compared to an evaluation of the 197Au(n,gamma) cross section, which was based on a least squares fit of experimental data available in the literature prior to this work. The average capture cross section as a function of neutron energy has been parameterized in terms of average resonance parameters. Maxwellian average cross sections at different temperatures have been calculated.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Large enhancement of radiative strength for soft transisitons in the quasicontinuum

Radiative strength functions (RSFs) for the 56,57-Fe nuclei below the separation energy are obtained from the 57-Fe(3-He,alpha gamma)56-Fe and 57-Fe(3-He,3-He' gamma)57-Fe reactions, respectively. An enhancement of more than a factor of ten over common theoretical models of the soft (E_gamma ~< 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56-Fe(n,2gamma)57-Fe reaction confirm the enhancement.Comment: 4 pages including 3 figure

Monte-Carlo Simulations of Radiation-Induced Activation in a Fast-Neutron and Gamma- Based Cargo Inspection System

An air cargo inspection system combining two nuclear reaction based techniques, namely Fast-Neutron Resonance Radiography and Dual-Discrete-Energy Gamma Radiography is currently being developed. This system is expected to allow detection of standard and improvised explosives as well as special nuclear materials. An important aspect for the applicability of nuclear techniques in an airport inspection facility is the inventory and lifetimes of radioactive isotopes produced by the neutron and gamma radiation inside the cargo, as well as the dose delivered by these isotopes to people in contact with the cargo during and following the interrogation procedure. Using MCNPX and CINDER90 we have calculated the activation levels for several typical inspection scenarios. One example is the activation of various metal samples embedded in a cotton-filled container. To validate the simulation results, a benchmark experiment was performed, in which metal samples were activated by fast-neutrons in a water-filled glass jar. The induced activity was determined by analyzing the gamma spectra. Based on the calculated radioactive inventory in the container, the dose levels due to the induced gamma radiation were calculated at several distances from the container and in relevant time windows after the irradiation, in order to evaluate the radiation exposure of the cargo handling staff, air crew and passengers during flight. The possibility of remanent long-lived radioactive inventory after cargo is delivered to the client is also of concern and was evaluated.Comment: Proceedings of FNDA 201