A. P. Tonchev

Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.

Population of isomers in the decay of the giant dipole resonance

Abstract The value of an isomeric ratio (IR) in N=81 isotones ( 137 Ba, 139 Ce, 141 Ndand 143 Sm) is studied by means of the (γ,n) reaction. This quantity mea-sures a probability to populate the isomeric state in respect to the groundstate population. In (γ,n) reactions, the giant dipole resonance (GDR) isexcited and after its decay by a neutron emission, the nucleus has an exci-tation energy of a few MeV. The forthcoming γ decay by direct or cascadetransitions deexcites the nucleus into an isomeric or ground state. It has beenobserved experimentally that the IR for 137 Ba and 139 Ce equals about 0.13while in two heavier isotones it is even less than half the size. To explain thiseffect, the structure of the excited states in the energy region up to 6.5 MeVhas been calculated within the Quasiparticle Phonon Model. Many states arefound connected to the ground and isomeric states by E1, E2 and M1 tran-sitions. The single-particle component of the wave function is responsible forthe large values of the transitions. The calculated value of the isomeric ratio isin very good agreement with the experimental data for all isotones. A slightlydifferent value of maximum energy with which the nuclei rest after neutrondecay of the GDR is responsible for the reported effect of the A-dependenceof the IR.PACS numbers: 21.60.-n, 23.20.-g, 27.60.+j

Nuclear deformation and neutron excess as competing effects for dipole strength in the pygmy region.

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A=124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the γELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIγS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

Calculation of bremsstrahlung spectra from a thick tungsten radiator as a function of photon energy and angle

Abstract A method for the calculation of bremsstrahlung spectra produced by 5–30 MeV electrons is described for the case of a thick tungsten radiator at observation angles from 0° to 20°. The computation is based on Shiff integrated-over-angle bremsstrahlung cross-section and includes energy losses, absorption and multiple scattering of the electrons in the radiator, the variation of the bremsstrahlung cross-section angular distribution with photon energy, and the absorption of radiation in the target material. The results obtained by the offered method were compared with known experimental data and Monte Carlo calculations of thick target bremsstrahlung spectra. A basic advantage of this method is that it needs only a small memory and short computing time.

The high-efficiency γ-ray spectroscopy setup γ3 at HIγS

The existing Nuclear Resonance Fluorescence (NRF) setup at the HI{\gamma}S facility at the Triangle Universities Nuclear Laboratory at Duke University has been extended in order to perform {\gamma}-{\gamma} coincidence experiments. The new setup combines large volume LaBr3:Ce detectors and high resolution HPGe detectors in a very close geometry to offer high efficiency, high energy resolution as well as high count rate capabilities at the same time. The combination of a highly efficient {\gamma}-ray spectroscopy setup with the mono-energetic high-intensity photon beam of HI{\gamma}S provides a worldwide unique experimental facility to investigate the {\gamma}-decay pattern of dipole excitations in atomic nuclei. The performance of the new setup has been assessed by studying the nucleus \sulfur at 8.125 MeV beam energy. The {\gamma}-decay branching ratio from the

Parity measurements of nuclear dipole excitations using FEL-generated γ-rays at HIγS☆

1^+

Excitation of the high-spin 180Hf isomerand de-excitation of the 180Ta isomer in(γ,γ') reactions

level at 8125.4 keV to the first excited

Isomeric yield ratio of134I in photofission of232Th and238U

2^+

Pygmy and core polarization dipole modes in 206Pb: Connecting nuclear structure to stellar nucleosynthesis

state was determined to 15.7(3)%.

Measurements of Thermal Neutron Diffraction and Inelastic Scattering in Reactor-Grade Graphite

First Nuclear Resonance Fluorescence (NRF) experiments were performed at the storage ring FEL-driven High Intensity Gamma Source (HIgS) at the DFELL. Azimuthal NRF intensity ratios were measured around the polarized HIgS beam. Electric character was deduced for 18 dipole excitations in 138 Ba [1]. The measurements demonstrate the superior performance of the HIgS facility in making such measurements. We report here on the performance of this setup. r 2002 Elsevier Science B.V. All rights reserved.