PHYSICS NEWS UPDATE -- Number 702 September 28, 2004 by Phillip F. Schewe and Ben Stein

From: Sam Wormley (swormley1_at_mchsi.com)
Date: 09/28/04 

Date: Tue, 28 Sep 2004 16:45:18 GMT

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
http://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
http://www-mep.phy.anl.gov/atta/)

***********
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