Number 702, September 28, 2004
by Phil Schewe and Ben Stein
Twenty Million Amps of Current
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; lab website at 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-6 Nucleus
The helium-6 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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