Stability of Ge-related point defects and complexes in Ge-doped SiO_2

We analyze Ge-related defects in Ge-doped SiO_2 using first-principles density functional techniques. Ge is incorporated at the level of ~ 1 mol % and above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency, with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and O vacancies binding strongly to Ge impurities. All the centers considered exhibit potentially EPR-active states, candidates for the identification of the Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure

Safety system function trend indicator: Theory and test application

Methods for formulation, interpretation, and validation of dynamic risk and reliability indicators are studied. The use of these indicators for monitoring various levels of safety performance in nuclear power plants, as identified by probabilistic risk assessments (PRAs), such as safety system unavailability, safety system failure frequency, and core-damage frequency, are explored. Simplified indicators for detecting trends in the unavailability of safety systems in nuclear power plants not requiring PRA models and extensive data collection effort are being developed for possible NRC use. These indicators, called safety system function trend indicators (SSFT), are designed because they are easy to implement to display trends in system unavailability which can be detected through their visual inspections. The characteristics of these indicators are studied and optimized through simulation studies to assure sufficient capability in displaying a trend. To further substantiate the findings from visual examination of these indicators, statistical trend tests for sparse data are developed to aid the interpretation of these indicators. To evaluate the indicators from a safety point of view (rather than pure statistical sense) methods for comparing these indicators to a given alert level are being explored. Finally, approaches for engineering interpretation of these indicators are being studied by identifying the major contributors and causes for anomalies in indicator behavior. A preliminary pilot application/validation of these indicators, based on data from a sample of eight units, was completed in FY89. The findings are promising and warrant further refinement in the methodology. 5 refs., 3 figs

Contrast-enhanced breast MR imaging of claustrophobic or oversized patients using an open low-field magnet

PURPOSE: A number of women who should undergo magnetic resonance (MR) imaging of the breast cannot use this diagnostic tool due to claustrophobia or excessive body size for the restricted confines of standard closed MR systems. Our aim was to evaluate the performance of open low-field magnet breast MR imaging in such patients using a high-relaxivity contrast agent. MATERIALS AND METHODS: Of 397 consecutive patients undergoing breast MR imaging, 379 (95.5%) were studied at 1.5 T. Due to claustrophobia (n=15) or large body size (n=3), 18 patients (4.5%) were studied on a 0.2-T open magnet using a body coil. A 3D dynamic T1-weighted gradient-echo 94-s sequence was acquired with intravenous injection of gadobenate dimeglumine (0.1 mmol/kg). The standard of reference was pathological examination for 16 lesions classified with a maximal Breast Imaging Reporting and Data System (BI-RADS) score from 3 to 5, fine-needle aspiration cytology and >or=2-year follow-up for two lesions classified as BI-RADS 3, and >or=2-years follow-up for five lesions classified as BI-RADS 2. RESULTS: Diagnostic MR image quality was achieved for 20/23 lesions in 15/18 patients. Three lesions (two invasive cancers and a cyst) were not assessed due to patient movement and considered as two false negatives and one false positive. Thus, an 86% sensitivity [13/15; 95% confidence interval (CI): 70%-100%], an 87% specificity (7/8; 95% CI: 65%-100%) and an 87% accuracy (20/23; 95% CI: 73%-100%) were obtained. The intraclass correlation coefficient between MR and pathologic lesion size was 0.845. CONCLUSION: In claustrophobic or oversized patients, open low-field breast MR with gadobenate dimeglumine yields good diagnostic performance