Calibration of the isomer shift of129I

Radioactive^129mTe was implanted in 6 different transition metals (V, Fe, W, Au, Pt, Pb) and internal conversion electron spectra as well as Mössbauer spectra of the daughter nucleus¹²⁹I were measured. A value (r ²)=+1.8(4)×10^–2 fm² (corresponding to R/R=+4.1(10)×10^–4) was derived for the change of the mean square charge radius during the 27.8 keV transition in¹²⁹I. The consequences of this measurement on the calibration constants of the Mössbauer isomer shifts of other isotopes of the 5sp elements are discussed.     

Study of electric quadrupole interactions of111In and111Cd following ion implantation of111In into graphite

The nuclear electric quadrupole interaction of¹¹¹In ion-implanted in hightly oriented pyrolytic graphite has been observed by means of low-temperature nuclear orientation and by means of perturbed angular correlations. From the first kind of experiment, it is concluded that a relatively large number of indium nuclei experience a well-defined macroscopic orientation, which is partly lost after the radioactive decay to cadmium. Indeed, the second kind of experiment revealed a broad distribution of electrid field gradients interacting with the 245 keV Cd excited state, as well as a small faction experiencing a unique electric field gradient. The experimental results are compared with theoretical calculations of the electric field gradient at various lattice positions, in which carbon and indium electronic wavefunctions are allowed to hybridize. Lattice positions of the covalent indium atom between the graphite layers can explain the measured electric field gradient ofV ₂₂=+1.47(11)·10²² V/m², directed parallel to the graphitec-axis.     

Generalized parton distributions from lattice QCD

We perform a quenched lattice calculation of the first moment of twist-two generalized parton distribution functions of the proton, and assess the total quark (spin and orbital angular momentum) contribution to the spin of the proton.     

On the quadrupole moments of the 300 ps, 5/2− states in75,77As

The DPAC technique was applied to measure the quadrupole moments of the 300 ps, 5/2⁻ states in^75,77As. The isotopes were implanted in α-Ga. The following results were obtained: Q(5/2⁻)=30(10) fm² for⁷⁵As, and Q(5/2⁻)<75 fm² for⁷⁷As.     

Transverse spin structure of the nucleon from lattice-QCD simulations

We present the first calculation in lattice QCD of the lowest two moments of transverse spin densities of quarks in the nucleon. They encode correlations between quark spin and orbital angular momentum. Our dynamical simulations are based on two flavors of clover-improved Wilson fermions and Wilson gluons. We find significant contributions from certain quark helicity flip generalized parton distributions, leading to strongly distorted densities of transversely polarized quarks in the nucleon. In particular, based on our results and recent arguments by Burkardt [Phys. Rev. D 72, 094020 (2005)], we predict that the Boer-Mulders function h(1/1), describing correlations of transverse quark spin and intrinsic transverse momentum of quarks, is large and negative for both up and down quarks.     

Hydrogen-vacancy interaction in W and Mo

The hydrogen-vacancy interaction in W and Mo was investigated by applying the perturbed angular correlation technique, using the isotope¹¹¹In as a probe. Hydrogen trapping at InV₂ manifests itself as a change of the vacancy-induced quadrupole frequency. We have observed trapping of up to two H atoms. The binding energies for the first H atom are 1.07(2) eV in Mo and 1.16(2) eV in W. These results are close to the values calculated with the effective-medium theory. The frequency shifts are both negative and amount to a few Mrad/s per trapped H atom. Contrary to the theoretical predictions, the binding energies for the second H atom differ strongly: 0.44(3) eV in Mo and 0.99(2) eV in W. The corresponding frequency shifts have opposite signs. Therefore, the lattice positions of the first H atom in Mo and W are similar, while those of the second H atom must be completely different.