ISSN:0031-9228    

DOI:10.1063/1.2808660

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808661

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808662

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The most challenging paradox of the magnitude‐8.2 earthquake that struck the Bolivian Alteplano (literally, “high plane”) on 9 June is not that it caused minimal damage while still being strong enough to be felt throughout most of the Western Hemisphere, to excite normal modes of vibration of the Earth never before seen and potentially to enable geophysicists to map the Earth's interior with 50 km resolution. Rather it is that the earthquake could occur at all. That is because the depth of the Bolivian quake (640 km) puts it in a rare class of earthquakes called “deep focus” quakes, whose foci lie between 75 km and 670 km beneath the Earth's surface. How such deep‐focus quakes occur in the mantle, where high pressures and temperatures cause rock under stress to flow rather than fracture, has been a subject of intense interest and controversy ever since their existence was established in the late 1920s. The importance of the Bolivian quake is further increased because it is the first really large, very deep event to occur since the deployment of modern global networks of digital seismographs. Three networks of portable broadband seismographs recently deployed in South America promise to provide an unprecedented close‐up view of the quake. (The color relief map on the cover of this issue shows the positions of the stations in these networks in relation to the quake's epicenter.) Researchers hope that data from the earthquake will yield a wealth of information on the Earth's interior as well as shed light on how deep‐focus events occur.

ISSN:0031-9228    

DOI:10.1063/1.2808663

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Optical‐wavelength lasers are ubiquitous, from the checkout scanner at the supermarket to the CD player in your Discman. They come in all sizes, all the way down to microelectronic scales. Lasers operating at wavelengths shorter than 100 nanometers, however, have been rare and expensive behemoths ever since their first demonstration a decade ago; they rely on bulky, high‐powered optical lasers to produce the plasma conditions needed for x‐ray lasing.

ISSN:0031-9228    

DOI:10.1063/1.2808664

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

When high‐energy cosmic ray protons and nuclei hit the top of the atmosphere, they generate showers of mesons that quickly decay to muons, electrons, positrons and neutrinos. A few kilometers below the surface of the Earth, nothing remains but the most energetic muons and almost all of the neutrinos. Irrespective of the complicated details, unimpeachable particle theory dictates that these hadronic showers produce, on average, twice as many muon‐flavored neutrinos (&ngr;&mgr;and&ngr;¯&mgr;) as electron‐flavored neutrinos (&ngr;eand&ngr;¯e). The details leave wiggle room for only a few percent departure from this predicted 2:1 ratio.

ISSN:0031-9228    

DOI:10.1063/1.2808665

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The mention of few‐electron atoms usually brings to mind hydrogen, helium or other light elements in neutral form. However, these simple atoms are part of a sequence of ions having the same number of electrons but different nuclear charges. For example, the hydrogen‐like sequence spans neutral hydrogen through hydrogen‐like uranium,U91+.Both the atomic physics and the applications of the most highly charged ions in such isoelectronic sequences are receiving increasing attention. Recently the electronbeam ion trap has made it possible to produce and study any such ion in a modest‐sized apparatus (figure 1).

ISSN:0031-9228    

DOI:10.1063/1.881419

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

More than two decades ago a laser‐based spectroscopic technique was developed that has since proved effective in solving problems no other spectroscopic method, conventional or laser based, can tackle. Known as resonance ionization spectroscopy, it has become a very useful tool for pure and applied research in physics and chemistry (see an example in figure 1), and it has found application in many other fields, including microelectronics, the Earth sciences and biomedicine.

ISSN:0031-9228    

DOI:10.1063/1.881420

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The future of education, research and employment in physics has been under much discussion lately. Contrary to many glowing predictions in the mid‐1980s, physicists graduating today face a severe shortage of jobs. Recalling comments such as one made by Malcolm Beasley and Lawrence Jones (PHYSICS TODAY, June 1986, page 36) that “It appears that the overall demand will more and more exceed the supply in the coming decade” reminds us just how much the situation has changed. Many solutions are being proposed. Articles relating to jobs and offering employment advice proliferate in the publications of our scientific societies. The situation is similar in other countries.

ISSN:0031-9228    

DOI:10.1063/1.881421

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808667

出版商:AIP    

年代:1994

数据来源: AIP