RESUMO
Neutron howitzers are routinely used in universities to activate samples for instructional laboratory experiments on radioactivity. They are also a convenient source of neutrons and gammas for research purposes, but they must be used with caution. This paper describes the modeling, design, construction, and testing of a portable, economical shield for a 1.0 Curie neutron howitzer. The Monte Carlo N Particle Transport Code (MCNP5) has been used to model the (239)PuBe source and the howitzer and to design the external neutron and gamma shield.
RESUMO
The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data, accounting for the neutron background, give limits on the spin-independent WIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV/c2 WIMP mass and, at >75% C.L., the entire 3sigma allowed region for the WIMP signal reported by the DAMA experiment.
RESUMO
During February and March 1985, nitrite levels along the northern (approximately 7 degrees to 10 degrees S) Peruvian coast were unusually high. These accumulations occurred in oxygen-deficient waters, suggesting intensified denitrification. In a shallow offshore nitrite maximum, concentrations were as high as 23 micromoles per liter (a record high). Causes for the unusual conditions may include a cold anomaly that followed the 1982-83 El Niño. The removal of combined nitrogen (approximately 3 to 10 trillion grams of nitrogen per year) within zones of new or enhanced denitrification observed between 7 degrees to 16 degrees S suggests a significant increase in oceanic denitrification.