A comparative evaluation of time-of-event counting for low-level radioactivity measurements

A comparison is made in this paper of the number of radioactive atoms which can be detected by means of two data analysis methods. One method is the conventional data acquisition and reduction method and the other measures and records the time of each detected decay event. By use of simulation methods it is shown that under appropriate conditions the latter method can detect a smaller number of atoms of a decaying radionuclide in the presence of a constant background component.This paper is based on work sponsored by the Division of Chemical Sciences of the Department of Energy and performed under DOE Contract DE-AC06-76RLO-1830.     

Evidence of complex positron-ammonia interaction in ammonia gas

Positron annihilation in ammonia gas at temperatures of ?19°C, 22,5°C, and 95°C in the density range 1.76 × 10^?4 g/cm³ to 6.63 × 10^?3 g/cm³ is investigated. ¹Z_eff for orthopositronium annihilation is 0.58 ± 0.04 Z_eff/ Z for positrons not forming positronium varies from about 117 to 1329.     

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg⁰ and its oxidized forms Hg^I and Hg^II. The efficient gas‐phase photolysis of Hg^II and Hg^I has recently been reported. However, whether the photolysis of Hg^II leads to other stable Hg^II species, to Hg^I, or to Hg⁰ and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg⁰ by photolysis. Results are based on non‐adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric Hg^II, Hg^I, and Hg⁰ compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg⁰ being the main photoproduct of Hg^II photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.