Noise limitations of multiplier phototubes in the radiation environment of space

The contributions of Cerenkov emission, luminescence, secondary electron emission, and bremsstrahlung to radiation-induced data current and noise of multiplier phototubes were analyzed quantitatively. Fluorescence and Cerenkov emission in the tube window are the major contributors and can quantitatively account for dark count levels observed in orbit. Radiation-induced noise can be minimized by shielding, tube selection, and mode of operation. Optical decoupling of windows and cathode (side-window tubes) leads to further reduction of radiation-induced dark counts, as does reducing the window thickness and effective cathode area, and selection of window/cathode combinations of low fluorescence efficiency. In trapped radiation-free regions of near-earth orbits and in free space, Cerenkov emission by relativistic particles contributes predominantly to the photoelectron yield per event. Operating multiplier phototubes in the photon (pulse) counting mode will discriminate against these large pulses and substantially reduce the dark count and noise to levels determined by fluorescence

Ancestral Genetic Resources Provide an Alternative to GMO Crops

Concern about the effects of pesticides on human health and the environment, has been a major rationale for promoting transgenic crops, often referred to as genetically modified organisms (GMOs), or as genetically enhanced (GE) crops. Companies that sell genetically engineered crop plants claim that biotechnology offers a safe alternative to agricultural chemicals and is necessary to feed the world’s expanding human population. However, there are still many unknowns about the safety of GMOs for human health and the environment, and virtually nothing is known about how the genomes of organisms may be affected by horizontal transfer of alien genes into plants, animals, and even humans. An alternative approach to transgenic technology is the exploitation of beneficial genes from wild relatives of crop plants using conventional breeding methods. This paper describes how genetic engineering differs from conventional plant breeding, then compares and contrasts benefits from transgenic engineering with traditional methods of crop improvement. An example of how the ancestral genes model has been employed to impart an insect resistance trait to corn based on native resistance from a wild relative is compared to transgenic corn with resistance to the same insect engineered with a transgene from a bacterium. Using the ancestral genes approach, harmful chemicals used to control the worst insect pest of corn can be eliminated with no consequences to human health or the environment; whereas with the transgenic approach, there are many safety concerns in both arenas

Fluorescence and phosphorescence of photomultiplier window materials under electron irradiation

The fluorescence and phosphorescence of photomultiplier window materials under electron irradiation were investigated using a Sr-90/Y-90 beta emitter as the electron source. Spectral emission curves of UV grade, optical grade, and electron-irradiated samples of MGF2 and LiF, CaF2, BaF2, sapphire, fused silica, and UV transmitting glasses were obtained over the spectral range of 200 nm to 650 nm. Fluorescence yields, expressed as the number of counts in a solid angle of 2 pi steradian per 1MeV of incident electron energy deposited, were determined on these materials utilizing photomultiplier tubes with cesium telluride, bialkali, and trialkali (S-20) photocathodes, respectively

ISDN at NASA Lewis Research Center

An expository investigation of the potential impact of the Integrated Services Digital Network (ISDN) at NASA Lewis Research Center is described. To properly frame the subject, the paper contains a detailed survey of the components of Narrowband ISDN. The principles and objectives are presented as decreed by the Consultative Committee for International Telephone and Telegraph (CCITT). The various channel types are delineated and their associated service combinations are described. The subscriber-access network functions are explained pictorially via the ISDN reference configuration. A section on switching techniques is presented to enable the reader to understand the emergence of the concept of fast packet switching. This new technology is designed to operate over the high bandwidth, low error rate transmission media that characterizes the LeRC environment. A brief introduction to the next generation of networks is covered with sections on Broadband ISDM (B-ISDN), Asynchronous Transfer Mode (ATM), and Synchronous Optical Networks (SONET). Applications at LeRC are presented, first in terms of targets of opportunity, then in light of compatibility constraints. In-place pilot projects and testing are described that demonstrate actual usage at LeRC

The Spiritual South

Dennis Covington in conversation with Ralph Eubanks