Proceedings of the Third Cryocooler Conference

Mechanical and non-mechanical types of small cryocoolers in the temperature range of 4 to 80 K are discussed. Applications of these small cryocoolers include the cooling of infrared detectors, cryopumps, small superconducting devices and magnets, and electronic devices

The Integration of Positron Emission Tomography With Magnetic Resonance Imaging

A number of laboratories and companies are currently exploring the development of integrated imaging systems for magnetic resonance imaging (MRI) and positron emission tomography (PET). Scanners for both preclinical and human research applications are being pursued. In contrast to the widely distributed and now quite mature PET/computed tomography technology, most PET/MRI designs allow for simultaneous rather than sequential acquisition of PET and MRI data. While this offers the possibility of novel imaging strategies, it also creates considerable challenges for acquiring artifact-free images from both modalities. This paper discusses the motivation for developing combined PET/MRI technology, outlines the obstacles in realizing such an integrated instrument, and presents recent progress in the development of both the instrumentation and of novel imaging agents for combined PET/MRI studies. The performance of the first-generation PET/MRI systems is described. Finally, a range of possible biomedical applications for PET/MRI are outlined

Mobile telephony through LEO satellites: To OBP or not

GLOBALSTAR is a satellite-based mobile communications system that is interoperable with the current and future Public Land Mobile Network (PLMN) and Public Switched Telephone Network (PSTN). The selection of the transponder type, bent-pipe, or onboard processing (OBP), for GLOBALSTAR is based on many criteria, each of which is essential to the commercial and technological feasibility of GLOBALSTAR. The trade study that was done to determine the pros and cons of a bent-pipe transponder or an onboard processing transponder is described. The design of GLOBALSTAR's telecommunications system is a multi-variable cost optimization between the cost and complexity of individual satellites, the number of satellites required to provide coverage to the service areas, the cost of launching the satellites into their selected orbits, the ground segment cost, user equipment cost, satellite voice channel capacity, and other issues. Emphasis is on the cost and complexity of the individual satellites, specifically the transponder type and the impact of the transponder type on satellite and ground segment cost, satellite power and weight, and satellite voice channel capacity

Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by ~ 1 eV in the semiconducting (8,0) tube and by ~ 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.Comment: 6 pages, 3 figures, 2 table

Open-Loop Spatial Multiplexing and Diversity Communications in Ad Hoc Networks

This paper investigates the performance of open-loop multi-antenna point-to-point links in ad hoc networks with slotted ALOHA medium access control (MAC). We consider spatial multiplexing transmission with linear maximum ratio combining and zero forcing receivers, as well as orthogonal space time block coded transmission. New closed-form expressions are derived for the outage probability, throughput and transmission capacity. Our results demonstrate that both the best performing scheme and the optimum number of transmit antennas depend on different network parameters, such as the node intensity and the signal-to-interference-and-noise ratio operating value. We then compare the performance to a network consisting of single-antenna devices and an idealized fully centrally coordinated MAC. These results show that multi-antenna schemes with a simple decentralized slotted ALOHA MAC can outperform even idealized single-antenna networks in various practical scenarios.Comment: 51 pages, 19 figures, submitted to IEEE Transactions on Information Theor

The variables which effect research productivity and extension publications and their relationships to performance of land-grant university extension specialists

The purpose of this study was to investigate the relationships among demographic variables, research productivity. Extension publications, and job performance evaluations for Extension specialists in the 1862 southern land-grant universities. The final sample was composed of 241 agricultural Extension specialists in the southeast. The study revealed that the average specialist was between the ages of 30 and 59, was located on the university campus, had an academic rank of full, had a terminal educational doctoral degree, and their major audiences were adults. Faculty average department size was almost 17 staff members. Faculty had memberships in 4.5 professional organizations, but they held an average of less than one office in these organizations on any level. However, they averaged serving on more than 1.5 committees on the state level. The average number of journal subscriptions was more than 3.5 and they read less than .5 journals from cover to cover per month, but when just reading articles of interest, they average more than 3 journals per month. The respondents averaged planning 82 percent of their time in Extension, however only actually spent 76 percent conducting Extension work. Agricultural Extension specialists participated in more county teaching events than at any other level with an average of 36, however, when organizing teaching events, they averaged conducting more on the state level with almost 9. The results of this study indicated that research productivity and Extension publications were affected differently by the demographic characteristics. Research productivity for agricultural Extension specialists was significantly affected by many of the same demographic characteristics that significantly affected collegiate faculty as shown in the literature. However, Extension publications were only significantly affected by job responsibility (both planned and actual) and teaching event scores. The results of this study showed that agricultural Extension specialists who were actively teaching were also publishing more articles and publications. Agricultural Extension specialists\u27 attitudes were not significantly affected by many of the demographic characteristics. However, rank was significantly related to three of the four types of attitudes measured. Those at the assistant rank perceived they had more time for conducting and writing research than those at the associate and full ranks. Those at the associate rank perceived that the organization placed higher emphasis on refereed journal articles than did those at the ranks of assistant and full. Those who held the rank of full perceived that promotion and evaluation were conducted more fairly by the organization. However, rank was one of the few demographic variables which did not have a significant relationship with Extension publication scores and research productivity. Agricultural Extension specialists over the age of 59 were more positive about the fairness of promotion and evaluation, and felt that they had adequate time to conduct and write research. However the variable of age had no significant relationships with research productivity or Extension publication scores. Self-reported job performances was only slightly affected by research productivity and Extension publication scores. Extension publication scores had more influence on job performance in both department ratings and how they rated their own performance. However, research productivity correlation with departmental performance rating was very close with that of the Extension publication score (a differences of .011) and this correlated more than Extension productivity with self-evaluations ratings. However, both analyses indicated that Extension publication were more important to agricultural Extension specialists job evaluation than research productivity

LANDSAT D local user terminal study

The effect of the changes incorporated in the LANDSAT D system on the ability of a local user terminal to receive, record and process data in real time was studied. Alternate solutions to the problems raised by these changes were evaluated. A loading analysis was performed in order to determine the quantities of data that a local user terminal (LUT) would be interested in receiving and processing. The number of bits in an MSS and a TM scene were calculated along with the number of scenes per day that an LUT might require for processing. These then combined to a total number of processed bits/day for an LUT as a function of sensor and coverage circle radius

Analysis and Design of Multiple-Antenna Cognitive Radios with Multiple Primary User Signals

We consider multiple-antenna signal detection of primary user transmission signals by a secondary user receiver in cognitive radio networks. The optimal detector is analyzed for the scenario where the number of primary user signals is no less than the number of receive antennas at the secondary user. We first derive exact expressions for the moments of the generalized likelihood ratio test (GLRT) statistic, yielding approximations for the false alarm and detection probabilities. We then show that the normalized GLRT statistic converges in distribution to a Gaussian random variable when the number of antennas and observations grow large at the same rate. Further, using results from large random matrix theory, we derive expressions to compute the detection probability without explicit knowledge of the channel, and then particularize these expressions for two scenarios of practical interest: 1) a single primary user sending spatially multiplexed signals, and 2) multiple spatially distributed primary users. Our analytical results are finally used to obtain simple design rules for the signal detection threshold.Comment: Revised version (14 pages). Change in titl