ISSN:0031-9228    

DOI:10.1063/1.2808047

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808048

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808049

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808051

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Since the late 1970s, Princeton physicist William Happer had been investigating spin‐exchange optical pumping as a means of polarizing nuclei. Along the way, he had given thought to practical things one might do with large collections of nuclei thus polarized, among them the enhancement of fusion in tokamaks, the creation of new kinds of polarized targets for high‐energy physics and the improvement of clinical magnetic resonance imaging. “But I was always too busy to think seriously about imaging,” Happer told us. “Then, when I came to Washington in 1991 [to become, for a time, director of energy research at DOE] I ruptured a disk and they did an mri scan of my spine. I was in great pain, and that concentrates the mind wonderfully.”

ISSN:0031-9228    

DOI:10.1063/1.2808052

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The question of how to quantize a system whose classical dynamics is chaotic has long been a puzzle. The hallmark of classical chaos is sensitive dependence on initial conditions: A small perturbation of the system changes its evolution in a way that grows exponentially with time. But in quantum mechanics a small perturbation of the initial state will in general be mapped into a small perturbation of the final state. In reeveral cent years the study of quantum systems whose underlying classical dynamics is chaotic has been an active area of research.

ISSN:0031-9228    

DOI:10.1063/1.2808053

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Over the last few decades, astronomical image processing has become extremely sophisticated, encompassing image reconstruction and restoration, image filtering, object detection and classification. A collaboration from the Space Telescope Science Institute, in Baltimore; Johns Hopkins University; and the Lombardi Cancer Research Center at the Georgetown University Medical Center, in Washington, DC, is hoping to apply some of these methods to detect telltale signs of breast cancer in a digitized mammogram. The project was catalyzed over a year ago by Benjamin Snavely, program director for advanced technologies and instrumentation in the NSF division of astronomical sciences, which recently awarded the collaboration a $50 000 grant from the Small Grants for Exploratory Research Program.

ISSN:0031-9228    

DOI:10.1063/1.2808054

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

When a beam from a laser or some other coherent light source scatters off a random distribution of matter, it produces an interference pattern of light and dark “peckles” that is uniquely determined by the instantaneous matter distribution at the scale of the light's wavelength. And if changes occur in the sample as a function of time, the speckle pattern evolves to reflect those changes. Physicists exploit this phenomenon when they investigate the diffusion of micron‐sized particles in liquids, for example, with techniques like photoncorrelation spectroscopy and dynamical light scattering.

ISSN:0031-9228    

DOI:10.1063/1.2808055

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

During the cold war, the US and the Soviet Union each produced enough fissile material to make tens of thousands of nuclear weapons—between 100 and 200 metric tons of plutonium and about 1000 tonnes of highly enriched uranium (uranium enriched to more than 20&percent; in U‐235). This weapons‐usable fissile material seemed invulnerable to theft until the collapse of the Soviet Union in 1991. Since then, subkilogram quantities of plutonium and multikilogram quantities of highly‐enriched uranium have been intercepted in Russia, the Czech Republic and Germany—apparently stolen from Russian nuclear facilities and intended for sale. the old Soviet security system for fissile material, which focused on the surveillance and control of those in contact with such material, has been largely swept away. Gone too is the economic security of nuclear workers, who may now be tempted or threatened by predatory criminal groups. the situation poses a grave risk to global security, because the biggest obstacle facing non‐nuclear‐weapons states or even terrorist groups interested in acquiring nuclear weapons is lack of access to the bomb material.

ISSN:0031-9228    

DOI:10.1063/1.881463

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Atheory is usually expected to explain existing experimental results and to predict new results, while an experiment is usually expected to check the validity of existing theories and to gather data for modifying them. This approach is normally presented to students as foolproof as if it were one of the basic laws of “good” science. In practice these goals are achieved in some cases, but sometimes the comparison between a theory and an experiment can be very misleading. Here I am going to discuss these unusual cases, to warn against the possible pitfalls. They may or may not be rare, but in any event it is important to bear in mind that they do exist.

ISSN:0031-9228    

DOI:10.1063/1.881464

出版商:AIP    

年代:1995

数据来源: AIP