Monday, November 10, 2008

Half-life (more Or Less)

Science News: "Physicists are stirred by claims that the sun may change what’s unchangeable—the rate of radioactive decayTHE SOURCEENLARGE Physicists have responded with curiosity and skepticism to reports that the sun causes variations in the decay rates of some isotopes.SOHO (ESA & NASA)
It’s nuclear physics 101: Radioactivity proceeds at its own pace. Each type of radioactive isotope, be it plutonium-238 or carbon-14, changes into another isotope or element at a specific, universal, immutable rate. This much has been known for more than a century, since Ernest Rutherford defined the notion of half-life—the time it takes for half of the atoms in a radioactive sample to transmute into something else. So when researchers suggested in August that the sun causes variations in the decay rates of isotopes of silicon, chlorine, radium and manganese, the physics community reacted with curiosity, but mostly with skepticism.
In one experiment, a team at Purdue University in West Lafayette, Ind., was monitoring a chunk of manganese-54 inside a radiation detector box to precisely measure the isotope’s half-life. At 9:37 p.m. on December 12, 2006, the instruments recorded a dip in radioactivity. At the same time, satellites on the day side of the Earth detected X-rays coming from the sun, signaling the beginning of a solar flare.
The sun’s atmosphere was spewing out matter, some of which would reach Earth the day after. Charged particles would contort the planet’s magnetic field, disrupt satellite communications and pose a threat to astronauts on the International Space Station.
But that dip in the manganese-54 radioactivity was not a coincidental experimental fluke, nor was it the solar flare discombobulating the measurements, the Purdue researchers claim in a paper posted online (arxiv.org/abs/0808.3156). In West Lafayette the sun had set while X-rays were hitting the atmosphere on the other side of the globe, and the electrically charged matter that created electromagnetic disturbances worldwide was still in transit. After a solar flare has begun, “the charged particles arrive several hours later,” points out theorist Ephraim Fischbach, coauthor of the paper with his Purdue colleague Jere Jenkins."

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