Text 207, 79 rader
Skriven 2004-09-29 20:58:16 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 702
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 702 September 28, 2004
by Phillip F. Schewe and Ben Stein

TWENTY MILLION AMPS OF CURRENT, released from a bank of capacitors over 100
nsec and sent into a cage of wires, is converted at Sandia's Z facility into
1.8 mega-joules of soft-x-ray energy, with a peak power of 200 tera-watts. 
Thus the Z machine is the highest peak-current pulsed-power device in the world
(over nanosecond timescales), and the most potent source of soft x rays
(radiation in the 100-10,000 eV range).  The total x-ray energy conversion
fraction---utility power turned into x rays---is 10-15%, much higher than for
any other x-ray source.  This makes the Z machine potentially useful for
studying two important transactions: nuclear fusion reactions, maybe for
producing commercial power; and the radiation spewing out of nuclear bombs. 
Owing to treaties, the physics of nuclear weapons cannot be studied directly by
explosions but only indirectly by tests such as those at Sandia National Lab
with its Z machine. The newest development in this subject is Sandia's ability
to photograph the sequence in which the tiny array of wires carrying the
stupendous mega-amp current implodes (the vaporizing wires are pinched inwards
by a huge magnetic field) and forms an x-ray-emitting plasma.  The first
surprise, once the dynamics of the event could be unfolded from data recorded
with special crystals, was how long the pinched wires survived the ordeal.  The
series of photos, taken using a separate (weaker) x-ray source to backlight the
interaction zone, should allow the Sandia researchers to optimize their
wire-array design in order to produce even greater x-ray yields.  (Sinars et
al., Physical Review Letters, 1 October 2004; contact Daniel Sinars,
dbsinar@sandia.gov, 505-284-4809; website
www.opp.sandia.gov/pbfaz.html)

RED NUCLEI.   Experiments conducted in Oslo and Budapest have determined that
the gamma rays streaming out of excited iron nuclei come in all different
energies---relatively low energy (3 MeV) as well as the expected higher energy
(10 MeV).  In other words, the nuclei proved to be (if one can impute colors to
the gamma spectrum equivalent to the visible spectrum) "redder" than thought. 
Why is this a surprise?  First of all, knowledge of energy levels in the
nuclear realm is not nearly as detailed as it is for atoms.  Quantum
electrodynamics (QED), the theory which rules the atomic world, can specify
energy levels with uncertainties in parts per trillion.  By contrast, quantum
chromodynamics (QCD), the theory that attempts to grapple with the strong
nuclear force, is rather vague, a shortcoming owing chiefly to the strength of
the nuclear force.  The best predictions of energy levels, in some nuclei, are
only good to about 10%.  Not only that, but when a nucleus such as iron is
"heated" (via particle interactions) through a "temperature" corresponding to 1
MeV, thousands of higher energy levels can be populated.  When researchers
observe the subsequent cooling of such nuclei what they see is not the spectrum
of discrete lines one gets with atoms but instead a quasi-continuum of gamma
lines.  According to Andreas Schiller of Michigan State University
(schiller@nscl.msu.edu, 517-324-8142), the unexpected red gamma rays might
correspond to the excitation energy of some new robust, collective,
low-frequency oscillation in the iron nucleus.  The collaboration includes
scientists from the Joint Institute of Nuclear Research (Russia), the
University of Oslo (Norway), Chemical Research Centre (Hungary), Osmangazi
University (Turkey), and several US institutions---Ohio University, Lawrence
Livermore National Lab, North Carolina State, and MSU.  (Voinov et al.,
Physical Review Letters,1 October 2004)
                                                        
THE HELIUM-SIX NUCLEUS consists of a He-4 nucleus (two protons plus two
neutrons) surrounded by a halo cloud consisting of two more neutrons. The
charge radius for He-6 has been now measured for the first time.  The
experimental value, 2.1 fm (2.1 x 10^-15 m), is larger than the radius for
He-4, 1.7 fm, the reason being that the halo neutrons in He-6 cause the core
portion of the nucleus to inflate somewhat (see figure at
http://www.aip.org/png/2004/222.htm).  The He-6 nuclei are made at a special
beamline at Argonne National Lab by smashing a beam of lithium ions into a
target.  The stray He-6 atoms made in the process (about a million per second)
are drawn into and lodged within a trap at a rate of about one a minute.  This
is sufficient to do laser spectroscopy on the atoms.  The charge radius of the
nucleus can be deduced from the way in which the frequency of the light
corresponding to an internal atomic transition from one quantum state to
another in the atoms is shifted in going from He-6 to He-4.
 Zheng-Tian Lu of Argonne (lu@anl.gov, 630-252-0583) says that He-6 is the
lightest known nucleus to have a neutron halo, and that the collaboration's
next experimental quarry, He-8, represents the most neutron-rich (highest
neutron-to-proton ratio) nuclear matter in the world.  (Wang et al., Physical
Review Letters, 1 October 2004; lab website at www-mep.phy.anl.gov/atta/)

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