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{at}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{at}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{at}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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