Electric-Field Gradients at Group-III Sites on GaAs and InAs (111)B Surfaces

We report a comparison of electric-field gradients (EFGs) measured at the group-III sites on epitaxially-grown surfaces of 2×2-reconstructed GaAs and InAs crystals. For this purpose, we used ¹¹¹In→¹¹¹Cd perturbed-angular-correlation (PAC) spectroscopy. Sharp spectral lines characterize the perturbation functions corresponding to both surfaces. On GaAs surface, we observe only one well-defined nuclear electric-quadrupole interaction (NQI); and on the InAs surface, we observe a primary and a secondary NQI. Very similar but experimentally-distinguishable values of the EFG-parameters characterize the primary interactions corresponding to the GaAs and InAs surfaces. Specifically, for the GaAs and InAs surfaces, ω _Q =28.0±0.2 Mrad s⁻¹ and η=0.43±0.02 and ω _Q =28.8±0.2 Mrad s⁻¹ and η=0.39±0.02, and the angles between EFG z-axis and the (111) direction are 65°±3° and β=53°±3°, respectively. These unexpected results indicate that the so-called lattice contribution to the EFG is not significant. Moreover, for the primary NQIs on both surfaces, the similar parameter values demonstrate that chemical differences between the ¹¹¹In probe and the indigenous Ga surface atoms cause no large quantitative effects. This information indicates that impurity probes and PAC spectroscopy can be used effectively to investigate III–V surfaces. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine interactions of111In→111Cd probe atoms on GaAs (111)B reconstructed surfaces

Hyperfine Interactions - We have used111In→111Cd perturbed-angular-correlation (PAC) spectroscopy to measure hyperfine interactions at surface sites on two molecular-beam-epitaxy-grown GaAs...     

Electric quadrupole interactions in non-axially symmetric electric field gradients of spinI=2 and 3 nuclei in polycrystalline sources

Calculations and formulae are presented for the purpose of understanding and analyzing Perturbed Angular Correlation (PAC) experiments that use spin-two and spin-three probe nuclei. For electric quadrupole interactions of probe nuclei in a polycrystalline source with non-axially symmetric electric field gradients, the interaction frequencies and perturbation functions for spin-two and spin-three nuclei differ qualitatively and quantitatively from those for spin-five-halves nuclei. These differences result primarily because them-states of integral-spin nuclei are nondegenerate in an asymmetric electric field gradient. To help the experimenter deal with the added complexity of the integral-spin casesI=2 and 3, closed-form expressions are provided for the energy eigenvalues and the eigenvectors as a function of the asymmetry parameter η. To deal with the problem of calculating the perturbation functionsG _kk (t) for η>0, the summation overm-state quantum numbers is formulated in terms ofa _n′,n ^(k) -coefficients. These coefficients are analogous to theS _kn -coefficients used in the case for η=0. To illustrate the differences between the half-integral-spin caseI=5/2 and the integral-spin casesI=2 and 3, energy difference diagrams anda _n′,n′ ⁽²⁾ diagrams are presented and discussed.     

Hyperfine Interactions of 181Ta in Zr2Ni Observed Using PAC Spectroscopy

We have measured nuclear electric–quadrupole interactions (EQI) at ¹⁸¹Ta impurities substituted as Hf atoms into the Zr site in Zr₂Ni. Using perturbed-angular-correlation (PAC) spectroscopy, we measured the EQI over temperatures ranging from 10 to 1200 K. Over the entire range of temperature, the Zr₂Ni crystal has a bct Al₂Cu structure that includes a single Zr site. The crystal field symmetry surrounding this site is rather low, giving rise to a highly asymmetric electric-field gradient tensor. At 10 K, the EQI is characterized by an angular frequency ω₀=601(3) Mrad s⁻¹, and an asymmetry parameter η=0.835(2). At 1200 K, ω₀ decreases to 516(3) Mrad s⁻¹, and η also decreases to 0.790(4). Although weak, the temperature dependence of ω₀ is consistent with a (1−BT ^3/2) power law, in which B=6×10⁻⁶ K^−3/2. The EQI also manifests a very narrow linewidth. We observed no evidence either for magnetic ordering or for structural phase transitions in the temperature range covered by this experiment. Moreover, the sharpness of the EQI indicates that the samples as prepared are remarkably free of strain and defects. These results indicate that the Zr₂Ni structure does not promote the formation of defects and that the power-law dependence of ω₀ on T is insensitive to the asymmetric nature of the crystal. This revised version was published online in September 2006 with corrections to the Cover Date.     

Sensitivity of spinI=5/2 perturbed-angular-correlation measurements to combined magnetic-dipole and electric-quadrupole hyperfine interactions

To design, analyze, and interpret perturbed-angular-correlation (PAC) experiments, the theory of the combined nuclear-magnetic-dipole and nuclear-electric-quadrupole interaction is applied. The applications involve polycrystalline sources that contain spinI=5/2 intermediate-quantum-level PAC probes. Frequenciesw _kk , amplitudesa _kk , and perturbation functionsG ₂₂ (t) are calculated for various values of the combined-interaction parameters, , and, wheny=w _L /W _Q is fixed at 30. For relatively large values ofy, the weak shift in them-state energies caused by the electric-quadrupole interaction generally produces a large change in the resulting perturbation function, and the effect is sensitive to the specific values of the parameters, , and. This sensitivity arises because the weak shift inm-state energies produces heterodyning. Moreover, this analysis indicates that PAC measurements of combined interactions in highly-distorted crystals (W _Q 0 and0) can provide potentially more information than previously thought. This analysis strongly suggests that more experimental investigations should be performed on highly-distorted crystals.     

Perspectives on teaching nuclear and radiochemistry at the Pennsylvania State University

I present some perspectives on teaching the subject of nuclear and radiochemistry in the form of a historical narrative. In addition, I briefly review a program, which we developed approximately seven years ago, to enchance learning in a specific course on the subject. This program involved developing illustrations, course notes, and video tutorials evaluating their effectiveness. After having used these aids extensively, I comment on their efficacy. Also, I briefly discuss the role of a nuclear science course in a nuclear engineering curriculum.