Text 408, 77 rader
Skriven 2005-04-27 20:46:22 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 729
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 729 April 27, 2005
by Phillip F. Schewe, Ben Stein
                                                                
PYROFUSION: A ROOM-TEMPERATURE, PALM-SIZED NUCLEAR FUSION DEVICE has been
reported by a UCLA collaboration, potentially leading to new kinds of fusion
devices and other novel applications such as microthrusters for MEMS
spaceships.  The key component of the UCLA device is a pyroelectric crystal, a
class of materials that includes lithium niobate, an inexpensive solid that is
used to filter signals in cell phones.  When heated a pyroelectric crystal
polarizes charge, segregating a significant amount of electric charge near a
surface, leading to a very large electric field there.  In turn, this effect
can accelerate electrons to relatively high (keV) energies (see Update 564,
http://www.aip.org/pnu/2001/split/564-2.html).  The UCLA researchers (Seth
Putterman, 310-825-2269) take this idea and add a few other elements to it.  In
a vacuum chamber containing deuterium gas, they place a lithium tantalate
(LiTaO3) pyroelectric crystal so that one of its faces touches a copper disc
which itself is surmounted by a tungsten probe.  They cool and then heat the
crystal, which creates an electric potential energy  of about 120 kilovolts at
its surface.  The electric field at the end of the tungsten probe tip is so
high (25 V/nm) that it strips electrons from nearby deuterium atoms. Repelled
by the negatively charged tip, and crystal field, the resulting deuterium ions
then accelerate towards a solid target of erbium deuteride (ErD2), slamming
into it so hard that some of the deuterium ions fuse with deuterium in the
target.  Each deuterium-deuterium fusion reaction creates a helium-3 nucleus
and a 2.45 MeV neutron, the latter being collected as evidence for nuclear
fusion.  In a typical heating cycle, the researchers measure a peak of about
900 neutrons per second, about 400 times the "background" of naturally
occurring neutrons.   During a heating cycle, which could last from 5 minutes
to 8 hours depending on how fast they heat the crystal, the researchers
estimate that they create approximately 10^-8 joules of fusion energy.  (To
provide some perspective, it takes about 1,000 joules to heat an 8-oz (237 ml)
cup of coffee one degree Celsius.)  By using a larger tungsten tip, cooling the
crystal to cryogenic temperatures, and constructing a target containing
tritium, the researchers believe they can scale up the observed neutron
production 1000 times, to more than 10^6 neutrons per second.  (Naranjo,
Gimzewski, Putterman, Nature, 28 April 2005).  The experimental setup is
strikingly simple: "We can build a tiny self-contained handheld object which
when plunged into ice water creates fusion," Putterman says. 
(http://rodan.physics.ucla.edu/pyrofusion )

NICKEL-78, THE MOST NEUTRON-RICH OF THE DOUBLY-MAGIC NUCLEI, has had its
lifetime measured for the first time, which will help us better understand how
heavy elements are made.  Indeed, where do gold atoms come from?  Physicists
believe gold and other heavy elements (beyond iron) were built from lighter
atoms inside star explosions billions of years ago.  In the "r-process" (r
standing for rapid) unfolding inside the explosion, a succession of nuclei bulk
up on the many available neutrons.  This evolutionary buildup is nicely
captured in a movie simulation showing all the species in the chart of the
nuclides being made one after the other
(http://www.jinaweb.org/html/movies.html).  In some models the buildup can slow
down at certain strategic bottlenecks.   Nickel-78 is one such roadblock.  This
is because Ni-78 is a "doubly magic" nucleus.  It has both closed neutron and
proton shells; it is "noble" in a nuclear sense in the way that a noble gas
atom is noble in the chemical sense owing to its completely filled electron
shell.   Knowing more about this crucial nuclide is made difficult by the fact
that it is, in our modern era, very rare, and hard to make artificially. 
Nevertheless, scientists at the National Superconducting Cyclotron (NCSL) at
Michigan State University have now culled 11 specimens of Ni-78 from among
billions of high-energy collision events recorded.  In effect, the NCSL is a
factory for reproducing supernova conditions here on Earth.  Hendrik Schatz
(schatz@ncsl.msu.edu, 517-333-6397), speaking at last week's American Physical
Society meeting in Tampa, reported that from the available Ni-78 decays
recorded, a lifetime of 110 milliseconds could be deduced.  This is some 4
times shorter than previous theoretical estimates, meaning that the bottleneck
nucleus lived shorter than was thought, which in turn means that the obstacle
to making heavier elements was that much less.  So far the exact conditions and
site for the r-process are still unknown. With the new measurement model
conditions have to be readjusted to produce the observed amounts of precious
metals in the universe. This will provide a better idea of what to look for 
when searching for the
site of the r-process.   (See also Hosmer et al., Physical Review Letters, 25
March 2005)

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 * Origin: Big Bang (1:106/2000.7)