A Gamma-Ray and Neutron Spectrometer (GRNS) instrument has been developed as part of the science payload for NASA's Discovery Program mission to the planet Mercury. Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSEN-GER) launched successfully in 2004 and will journey more than six years before entering
The authors briefly review the development of the virtual impactor which, as an inertial particle separator according to aerodynamic sues, has played a unique role in particle sampling, concentration, classification, and generation. Its performance characteristics in sue separation are predictable by theoretical model calculations. However, its behavior in terms of internal wall losses has thus far defied quantitative analysis, and its ultimate control has eluded most practitioners in virtual impactor design. Through experimentation, the authors identify the relevant parameters in a virtual impactor and indicate their relative sensitivity and acceptable ranges of variability. With the detailed illustration of specific highefficiency virtual impactor design, which has a cutpoint of 2.5 pm and wall losses of under 1%, it is demonstrated the underlying principles cited are crucial to minimizing losses and may be generally applicable to future developments.
A new particle, the cosmion, has been proposed to be the dark matter of the Universe and to explain the solar v deficit by cooling the solar core to reduce 8 B v production. Such cosmions in the galactic halo would scatter from nuclei in terrestrial detectors. Measurements were made in Si ionization detectors in a very-low-background environment down to energies of 1.1 keV. These results exclude nearly all of the mass range possible for cosmions with coherent nuclear interactions.PACS numbers: 95.30. Cq, 14.60.Gh, 14.80.Pb, 96.60.Kx Cosmions, weakly interacting massive particles (or WIMPs), have been proposed 1 to solve simultaneously the problem of dark matter, which makes up perhaps 90% of the mass of the Universe, and the deficit of 8 B neutrinos from the Sun. If they exist, they would also speed up the evolution of stars in globular clusters 2 and modify the sound speed near the solar center. 3 There is controversy over whether the cosmion hypothesis is compatible with observations, but these potentially important particles can be searched for directly. As the Earth moves through the pervading sea of dark matter, cosmions would scatter 4 from the nuclei of ionization detectors, giving a detectable signal if the energy threshold and backgrounds are sufficiently low. Ge detectors provide some limits 5 on cosmions, but Si detectors should have greater sensitivity. 6 The lighter nucleus gives a larger recoil energy, more of that energy goes into ionization, and Si detectors have lower energy thresholds. These advantages were such that a small quantity of unselected Si with relatively large backgrounds produced the very useful results reported here.In this experiment an array of four planar Si detectors was mounted on a single-crystal Si cold finger (4 mm thick) to maintain the detectors near liquid-nitrogen temperature. The detectors and cold finger were enclosed in an electroformed Cu (0.25-mm wall thickness) vacuum cryostat evacuated with ion pumps. Each Lidrifted Si detector had a sensitive region with a diameter of 34 and 8-mm thickness, giving an active mass of 17 g. This array was mounted inside the cavity formed by ten blocks of Nal of 15-cm thickness initially used for a search for neutrinoless double-/
We report on the development of high-sensitivity and compact Compton imaging systems built of large and position-sensitive Si(Li) and HPGe detectors. The primary goal of this effort is to provide improved capabilities in the passive detection of nuclear materials for homeland security. Our detectors are implemented in double-sided strip configuration which -along with digital signal processing -provides energies and three-dimensional position information of individual gamma-ray interactions. Gamma-ray tracking algorithms are then determining the scattering sequence of the gamma ray which in turn allows us -employing the Compton scattering formula -to reconstruct a cone of possible incident angles and ultimately an image. This Compton imaging concept enables large-field-of-view gamma-ray imaging without the use of a heavy collimator or aperture. The intrinsically high energy resolution of the detectors used, the excellent position resolution we have demonstrated, both combined with the high efficiency of large-volume detectors is the basis for high Compton imaging sensitivity. These capabilities are being developed to identify and localize potential threat sources and to potentially increase the sensitivity in detecting weak sources out of the midst of natural, medical, or commercial sources. Gamma-ray imaging provides a new degree of freedom to distinguish between spatial and temporal background fluctuations and compact threat sources.
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