Photoelectron holography studies of Bi on Si(111)
Photoelectron holography is applied to Bi atoms adsorbed on Si(111) for a determination of the three-dimensional atomic structure. Instead of measuring the absolute intensity as a function of photon energy as is traditionally done, a differential measurement is carried out to yield the logarithmic derivative of the intensity. This leads to intensity self-normalization, and significantly improves the accuracy. The resulting images show a well-resolved trimer structure for the adsorption geometry.
Differential photoelectron holography
Atoms near a selected surface atomic site can be imaged in three dimensions by photoelectron holography. The experiment involves acquiring hundreds or thousands of angle-resolved photoemission spectra from a core level of the selected atomic site over a wide range of photon energy and for a large number of emission directions. For good image quality, the precision of the intensity measurement must be better than ~1% over the entire data set, but this is difficult to achieve. Two methods based on differential measurements that allow intensity self-normalization are presented here as a solution. One method is based on measurements of intensity branching ratios between spin–orbit-split core level peaks, and the other is based on measurements of logarithmic derivatives of the intensity function. These methods are illustrated by two examples, As-terminated Si(1 1 1) and Bi-terminated Si(1 1 1). The As/Si(1 1 1) results are in excellent agreement with the accepted structure of the system, thus verifying the efficacy of the methods. The Bi/Si(1 1 1) results resolve a long-standing controversy regarding the correct structure of this system.
Electron dose-rate conversion factors for external exposure of the skin from uniformly deposited activity on the body surface.
Dose-rate conversion factors have been calculated for external exposure of the skin from electrons emitted by sources that are deposited uniformly on the body surface. The dose-rate factors are obtained from electron scaled point kernels developed by Berger (Be71; Be73; Be74). The dose-rate factors are calculated at depths of 4, 8, and 40 mg cm-2 below the body surface as recommended by Whitton (Wh73), and at a depth of 7 mg cm-2 as recommended in ICRP Publication 26 (ICRP77). The dependence of the dose-rate factors at selected depths on the energy of the emitted electrons is displayed. The dose-rate factors for selected radionuclides of potential importance in radiological assessments are tabulated.