Delbrück scattering of 2.754 MeV photons by Bi and Th

Differential cross sections for the elastic scattering of 2.754 MeV photons by Bi and Th have been measured for angles ranging from 60° to 120°. The results deviate from the predictions of lowest-order Delbrück theory by factors between 1.1 and 1.4. The strong increase of these deviations with charge number, previously reported for Pb and U, has been confirmed. An interpretation in terms of the effect of the Coulomb correction is given.     

Elastic scattering of 2.754 MeV photons by Ta and interference between Rayleigh,Thomson and Delbrück scattering

Differential cross sections for the elastic scattering of 2.754 MeV photons by Ta have been measured for angles ranging from 30° to 150°. A comparison with lowest-order Delbrück theory reveals discrepancies of the same size as previously observed for Pb and Bi. Consideration is given to interference phenomena between Delbrück, Rayleigh and nuclear Thomson scattering.     

Nuclear resonance fluorescence in124Te and126Te: Vacancy cascades in molecules

Using gaseous sources of I₂ and HI containing the radioactive isotopes¹²⁴I and¹²⁶I effective cross sections for nuclear resonance scattering from the first 2⁺ states of¹²⁴Te and¹²⁶Te were measured. Information is obtained about the charging of the nonradioactive molecular partner afterEC decay, the interaction of highly charged ions with neutral molecules, and the slowing down of atoms in a Xe buffer gas. Indications of an iodine complex being stable in the gaseous phase are found.     

The energy dependence of Delbrück scattering investigated at Z = 73, 82 and 92

Using neutron capture γ-rays from a ¹⁴⁰CeO₂ source installed in the Grenoble high-flux reactor, differential cross sections for the elastic scattering of photons by Ta, Pb and U through θ = 120° have been measured for E = 4.291 and 4.767 MeV. These data have been supplemented by measuring elastic differential cross sections for U, θ = 120° and energies ranging from 0.279 to 1.332 MeV, using radioactive sources. The experimental differential cross sections below 1 MeV confirm the predicted Rayleigh amplitudes based on the second-order S-matrix within 3%. An excellent agreement between experiment and lowest-order Delbrück theory is observed between 1.0 and 1.4 MeV, showing that Coulomb corrections are small close to the threshold for pair production. At 4.291 and 4.767 MeV experiment and lowest-order Delbrück theory agree within ~12%     

Dependence of the nuclear recoil motion on the phonon spectrum of solids investigated by nuclear resonance fluorescence

The time dependence of the recoil velocity of a¹⁴⁰Ce nucleus in a La and a LaF₃ lattice after emission is studied via the Doppler shift of the 1.596 MeV transition in¹⁴⁰Ce. Evidence is obtained that the nucleus does not leave the lattice site even at a recoil energy of about 20 eV but carries out a damped oscillation. The quantitative comparison of the recoil velocity with the predictions of lattice theory leads to partial agreement.Supported by Deutsche Forschungsgemeinschaft, contract Schu 222     

The Z-dependence of the elastic scattering of γ-rays

The differential cross sections for elastic scattering of 662 keV and 279 keV γ-rays from Pb, Ta, Nd, Sn, Mo, and Zn have been measured at angles ranging from 45° to 135°. The results are compared with theoretical predictions taking into account Rayleigh scattering and nuclear Thomson scattering. The theoretical Rayleigh amplitudes are based on second order perturbation theory according to Brown et al. and on form factors calculated from relativistic HFS wave functions. A semi-empirical method of correcting the form factors is developed, valid for energies between 150 keV and 750 keV and charge numbers up to Z = 82. The average difference between calculated and measured differential cross sections was found to be 6%.     

Die Konversion des Hg199m

Hg^199m was produced from natural HgO by fastn irradiation. The cascade decay was investigated with scintillation spectrometers by application of the summing technique. The following results were obtained: half-lifeT _1/2=(43±0,5) min; energy of the isomeric transitionE _γ1=(375±3)keV; conversion coefficients α _K375=3,05±0,25; α_total=5,45±0,25; yielding the multipole order of the isomeric transition to beM4+(25±15)%E5; energy of the second transitionE _γ2=(159±2)keV; conversion coefficients α _K159=0,30±0,03; α_total=0,9±0,1, confirming the multipolarityE2 of this transition.     

Zur Isomerie des Tl197

Tl^197m was produced by the (α, 4n) reaction on Au¹⁹⁷ using 49 MeVα particles. The following results were obtained: Half-lifeT _1/2=(0.55±0.02) sec; energy of the isomeric level (607±4) keV, determined directly by summing up the cascade transitions in a well-type scintillator; energy of the isomeric transition (222±2) keV; conversion coefficients of this transitionα _K222=0.41±0.05 andα _tot222=2.1±0.2, indicating anE3 multipolarity; energy of the second transition (385±3)keV; conversion coefficientsα _K385=0.09±0.03 andα _tot385<0.1. This transition was identified asE2 with aM1 admixture of 20 to 30%. Spins and parities are 1/2⁺, 3/2⁺ and 9/2^? for the ground state, the 385 keV state and the 607 keV isomeric state, respectively, in disagreement with the extreme single-particle model. If this model were correct, anotherM1 transition should appear. No furtherM1 transition having an energy greater than theL-shell binding energy of T1 was observed.     

Nuclear resonance fluorescence in238U and a new approach to Doppler-shift-attenuation using high speed rotation

Using⁵⁶Co sources, a 3.254 MeV level in²³⁸U was photoexcited by aγ-line emitted in the decay of the 4.100 MeV level in⁵⁶Fe. 10γ-transitions deexciting this 3.254 MeV level were identified. Spin and parity were determined to be 1⁻. By moving the⁵⁶Co source in a high speed centrifuge the profile of the 3.254 MeV emission line was investigated. By applying a new variety of Doppler shift analysis a lifetime ofτ _Fe=55±25fs was obtained for the 4.100 MeV level in⁵⁶Fe. The cross section for nuclear resonance fluorescence yields a lifetime ofτ _U =0.33±0.12 ps for the 3.254 MeV level in²³⁸U.

     

Zur Isomerie des Au197

Crystals in strong electric fields may conduct currents which are inversely proportional to the field strength. The influence of the electric field on the electron-phonon interaction is investigated.