Showing papers by "SERC Reliability Corporation published in 1999"

Epidemiology of primary liver cancer.

Abstract: Liver cancer (LC) ranks fifth in frequency in the world with an estimated number of 437,000 new cases in 1990. In developing countries, incidence rates are two- to three-fold higher than in developed countries. The geographic areas at highest risk are located in Eastern Asia, with age-adjusted incidence rates (AAIRs) ranking from 27.6 to 36.6 per 100,000 in men; Middle Africa, with AAIRs ranking from 20.8 to 38.1 per 100,000 in men; and some countries of Western Africa, with AAIRs ranking from 30 to 48 per 100,000 in men. The geographic areas at lowest LC risk are Northern Europe, Australia, New Zealand, and the Caucasian populations in North and Latin America, with AAIRs below 5.0 per 100,000 in men. Excess of LC incidence among men compared to women is universal, with sex ratios between 1.5 and 3.0. Significant variations in LC incidence among different ethnic groups living in the same geographical area and among migrants of the same ethnic groups living in different areas have been extensively described. The variability of LC incidence rates between countries and within countries, strongly suggests differences in exposure to risk factors. The role of chronic infection with the Hepatitis B and hepatitis C viruses (HBV and HCV) in the etiology of LC is well established. The attributable risk estimates for LC for each of these hepatotropic viruses vary among countries but the combined effects of persistent HBV or HCV infections account for well over 80% of LC cases worldwide. Other documented risk factors such as aflatoxin exposure in diets, cigarette smoking, alcohol consumption, and oral contraceptives may explain the residual variation between and within countries. Interactions between some risk factors have been postulated, and are subject of active research. New laboratory techniques and biological markers such as polymerase chain reaction detection of HBV DNA and HCV RNA, as well as specific mutations related to aflatoxin exposure may help to provide quantitative estimates of the risk related to each these factors.

Morphology and composition of the helix nebula

Abstract: We present new narrowband filter imagery in Hα and [N II] λ6584, along with UV and optical spectrophotometry measurements from 1200 to 9600 A of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter, with an inner region containing hot, highly ionized gas that is generally invisible in narrowband images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000 K and 100 L☉, and we estimate a lower limit to the nebular mass to be 0.30 M☉. Abundance measurements indicate the following values: He/H=0.12 (± 0.017), O/H=4.60×10-4 (± 0.18), C/O=0.87 (± 0.12), N/O=0.54 (± 0.14), Ne/O=0.33 (± 0.04), S/O=3.22 × 10-3 (± 0.26), and Ar/O=6.74 × 10-3 (± 0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio that we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the supersolar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5 M☉ progenitor that underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.