ABSTRACT
A challenge preventing successful inverse kinematics measurements with heavy nuclei that are not fully stripped is identifying and tagging the beam particles. For this purpose, the HEavy ISotope Tagger (HEIST) has been developed. HEIST utilizes two micro-channel plate timing detectors to measure the time-of-flight, a multi-sampling ion chamber to measure energy loss, and a high-purity germanium detector to identify isomer decays and calibrate the isotope identification system. HEIST has successfully identified 198Pb and other nearby nuclei at energies of about 75 MeV/A. In the experiment discussed, a typical cut containing 89% of all 198Pb80+ in the beam had a purity of 86%. We examine the issues of charge state contamination. The observed charge state populations of these ions are presented and, using an adjusted beam energy, are well described by the charge state model GLOBAL.
ABSTRACT
^{18}Mg was observed, for the first time, by the invariant-mass reconstruction of ^{14}O+4p events. The ground-state decay energy and width are E_{T}=4.865(34) MeV and Γ=115(100) keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of ^{16}Ne. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2^{+} state. As this energy exceeds that for the 2^{+} state in ^{20}Mg, this observation provides an argument for the demise of the N=8 shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.
ABSTRACT
The effects of methylmercury (MeHg) and selenium (Se) contamination on food webs in the San Francisco Estuary have received considerable attention during the past decade. However, knowledge of their effects on native fishes of California is lacking. This study investigated the interactive effects of dietary MeHg and seleno-methionine (SeMet) on Sacramento splittail (Pogonichthys macrolepidotus) larvae. Twelve diets containing increasing levels of SeMet (0.64, 8.2 and 35.0 microg Se g(-1) diet) and MeHg (0.01, 0.13, 4.7 and 11.7 microg Hg g(-1) diet) were fed to 21-day post-hatch larvae for 4 weeks in 2-L beakers at 25 degrees C. Fish were fed twice a day at a feeding rate of 40, 30, 25 and 20% of body weight during the 1st, 2nd, 3rd and 4th week, respectively. At the end of week 4, no significant difference (P>0.05) was observed among treatments for mortality, body length or weight, and condition factor. Bioaccumulation of Hg and Se responded positively and significantly (P<0.05) to their dietary concentrations. The molar ratio of Se/Hg in diets was linearly correlated to the ratio of Se/Hg in fish. Dietary Se inhibited Hg accumulation, which was negatively correlated to the dietary Se/Hg ratio. Histopathological examination revealed severe gill anomaly and liver glycogen depletion in fish fed the 11.7 microg Hg g(-1) diet. Liver glycogen depletion and kidney tubular dilation were found in larvae fed the 11.7 microg Hg and 11.7 microg Hg+35 microg Se g(-1) diets. In conclusion, dietary Hg enhanced Se accumulation but dietary Se inhibited Hg accumulation in splittail. Dietary Se showed a protective effect in fish fed the high MeHg diet. This protection was related to the dietary Se/Hg ratio, which is a more reliable criterion for evaluating the interactive effect between Se and Hg in splittail.
