Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World‘s team of editors and reporters
Exotic cosmic rays have mundane origins
Measurements made by the Alpha Magnetic Spectrometer (AMS) on the International Space Station suggest that exotic cosmic rays comprising boron nuclei have rather mundane origins. Astrophysicists divide cosmic rays into two categories: primary and secondary. Primary cosmic rays are produced in supernovae and other violent astrophysical processes, whereas secondary cosmic rays are created when their primary cousins collide with gas atoms in the interstellar medium. The vast majority of carbon-nuclei cosmic rays are thought to be primary in origin, whereas all boron cosmic rays are thought to be secondary in nature. As a result, the ratio of boron-to-carbon cosmic rays (B/C) reaching the AMS should provide a measure of the average amount of interstellar matter that the cosmic rays have passed through. There are several models that predict the shape of the B/C spectrum as a function of energy, but previous balloon-borne measurements of the B/C were not precise enough to decide which model is best. After analysing 80 billion cosmic rays collected over five years, AMS physicists have concluded that a relatively simple model developed in 1941 by the Russian mathematician Andrey Kolmogrov best describes the data. The result is of great interest to physicists studying the apparent excess of cosmic-ray positrons that reach the Earth. These particles were expected to have been created by similar secondary processes as the boron nuclei, but the AMS results could mean that there are hitherto unknown additional astrophysics sources of positrons in the universe. The research is described in Physical Review Letters.
Swiss reject nuclear phase-out
Switzerland has voted to reject an early shutdown of the country’s five ageing nuclear reactors in a referendum held yesterday. Some 54.2% of people voted “No” – on a turn-out of 45% – to phasing out the country’s nuclear plants by 2030. In Switzerland, nuclear power provides around a third of electricity – the second largest source behind hydro. Yet a few months after the Fukushima nuclear disaster in Japan in March 2011, the Swiss government abandoned plans to build new nuclear power plants and reactors. The referendum held yesterday was to decide whether those existing plants should be closed before their expected lifetime comes to an end. The plants are now likely to continue operating well into the 2030s, subject to approval from safety regulators.
Defects allow material properties to be programmed
Researchers at Purdue University in Indiana have unveiled a new type of cellular material with physical properties that can be “programmed” after manufacture. The honeycomb-like structures are made from shape-memory polymers and contain engineered defects that make the materials respond in certain ways to external forces. The programming can be done by heating the material and then applying a force to change its shape. The new shape is then retained when the material cools down. The stiffness of one material, for example, increases by 55% when it is compressed by 5%. “That is pretty impressive because ordinarily you would have to fabricate a new material with at least twice the thickness of the walls to obtain a material with a 50% increase in stiffness,” says Purdue’s David Restrepo. Possible applications of the new materials include acoustic metamaterials that can be tuned to absorb sound at specific frequencies and “stealthy” surfaces that do not reflect radar waves. Other uses, according to the researchers, include protective helmets and car seats that adjust to a driver’s weight. The materials are described in two papers in the International Journal of Solids and Structures.
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