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10kBq jaro

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Nickel nuclei yield magic finding
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http://physicsweb.org/articles/news/9/4/6


Nickel nuclei yield magic finding


12 April 2005


Physicists in the US and Germany have measured the half-life of the "doubly-magic" nucleus nickel-78 for the first time and have found that it is shorter than expected (Phys. Rev. Lett. 94 112501). Nickel-78 is a highly stable nucleus that is thought to produce about half the elements heavier than iron in the universe. The finding, obtained by Paul Hosmer of Michigan State University and colleagues, could mean that supernova explosions produce gold and other heavy elements much faster than previously thought.


With 50 neutrons and 28 protons, nickel-78 has the largest excess of neutrons of any naturally occurring isotope. Nickel-78 is said to be a "doubly-magic" nucleus because it has closed shells of both protons and neutrons. It is as a result highly stable and one of only ten such nucleides that can be formed in nature. It is also very difficult to produce experimentally. Although physicists at the GSI lab in Darmstadt, Germany, had produced three nickel-78 nuclei before, they were not able to measure their properties.


Working at the National Superconducting Cyclotron Laboratory at Michigan State University, Hosmer and colleagues first fired a beam of stable krypton into a beryllium target. The krypton fragments to produce many exotic neutron-rich isotopes. About twice a day, one out of ten billion attempts per second produces a nickel-78 nucleus. This isotope was separated out and its decay half-life measured to be about 110 milliseconds, which is about four times shorter than nuclear theories predict.


According to some models, the decay of nickel-78 is part of the so-called "rapid neutron capture process" or r-process, which is thought to produce about half the elements heavier than iron in the universe. The r-process is the main source of elements such as gold, platinum and uranium and may take place in supernova explosions. More importantly, nickel-78 is one of the main bottlenecks in this process and acts like a valve for the build-up of heavier elements. The new shorter half-life for the isotope means that the r-process could be creating gold and other heavy elements much faster than previously thought. The finding could force existing models for the synthesis of heavy elements in the universe to be modified.


"Our result provides an important benchmark for nuclear theories attempting to extend our knowledge into the unknown domain of exotic neutron-rich nuclei," says team member Hendrik Schatz. "It is also crucial for the ongoing quest of finding the origin of the heavy elements in nature -- one of the most important unanswered questions in nuclear astrophysics today."


About the author


Belle Dumé is Science Writer at PhysicsWeb

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Dusty

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Am I the only one mildly ammused at the idea that the phrases "Highly stable" and "Half life of 110 milliseconds" can exist in the same article?



Dusty



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GoogleNaut

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Dusty, I know what your saying.

However, it is interesting that Nickel-78 may play an important part in the synthesis of heavier elements. I have a vague recollection of reading a Science article about the detection of radioactive cobalt, nickel and chrome nucleii in the debris cloud from Supernova 1987A. Almost all elements heavier than iron must have been forged in the extreme shocks of the dense interior cores of massive stars. As the interior iron core implodes to form neutronium (and releasing a torrent of neutrinos in the process) the outer core must be 'ablated' somewhat like in the radiation implosion mechanism of an H-Bomb secondary. The ablation driven shockwaves in the dense outer core material presumably forge heavier nucleii directly by collision. The energy/pressure regime must be awesome!

All of the gold, mercury, lead, thorium and uranium (and everything else heavier than iron) that exists must have been created in supernova explosions. Since these conditions of energy density and pressure presumably are transient (perhaps lasting no more than seconds or minutes as the shockwave travels through the core material at nearly the speed of light) then the presumed reaction rates must have been reasonably high. Plasma cooling by forced fusion of nucleii--the energy densities must have been stupendous! The energy densities must have approached or exceeded that of the matter in the core. No wonder heavier elements were synthesized!



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10kBq jaro

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quote:
Originally posted by: Dusty
"Am I the only one mildly ammused at the idea that the phrases "Highly stable" and "Half life of 110 milliseconds" can exist in the same article?"

Well, sort of.


I guess the guys who measured the half-life were surprised too, when it turned out to be "about four times shorter than nuclear theories predict."


Just think though -- in the nuclear world, a tenth of a second ( = 100 msec) is a very long time: For example, its a million times longer than the interval between uranium fissions in an A-bomb (the links in the chain-reaction -- which includes the time for the neutrons spit out in one fission to bounce around a bit and hit another U-nucleus in just the right way to cause another fission...).


You can also compare it to other neutron-heavy nuclei. For instance, Carbon-18 (6 protons + 12 neutrons) has a half-life of 90msec, while Carbon-22 (6P + 16N) has a half-life of 9msec. In the case of heavy Sodium, Na-35 (11P + 24N) the half-life is only 1.5msec. These last two are right on the neutron drip line -- add just one more neutron, and the half-life is zero (or something on the order of a nano-second, anyway -- essentially disintegration upon impact).


 



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