showered by x rays traversing the universe from our Sun and other
hot stellar objects. A major branch of astronomy is devoted to detecting
and studying x rays from distant stars and galaxies, and Lawrence
Livermore scientists have long studied the x rays that are produced
from nuclear detonations.
However, it came as quite
a surprise to scientists in 1996 when the Rosat X-Ray Satellite
detected low-energy (less than 1 kiloelectronvolt) x rays streaming
from the comet Hyakutake. X-ray emission is usually associated with
hot plasmas from stars, nuclear reactions, and black holes, not
from ice-cold objects such as comets. Since the original discovery,
other x-ray satellites have established that several other comets
traveling through our solar system emit x rays with fluxes, or intensities,
as high as 1025 photons per second.
To help resolve the apparent
contradiction, a national team of scientists headed by Lawrence
Livermore physicist Peter Beiersdorfer is working on the laboratory
production of low-energy x rays identical to those produced by comets
traveling near the Sun. The team is using Livermores electron
beam ion trap (EBIT) to produce the x rays and an x-ray spectrometer
(XRS) designed by the National Aeronautics and Space Administration
(NASA) to detect them.
The research is providing
much greater understanding about the x rays that are emitted by
comets as they pass the Sun. The effort is also providing scientists
with valuable information that will help them interpret data to
be collected by a joint U.S.Japan x-ray satellite mission
scheduled for launch in 2005.
current explanation for comet x rays is called charge exchange.
This process is believed to occur when heavy ions (Aq+) from
the solar wind flowing from the Sun (right) collide with electrically
neutral atoms and molecules (B) in the comets atmosphere
(left). During a collision, a heavy ion captures one or more
electrons from a comets atmospheric atom, ionizing it
to B(qp)+. The solar wind ion, now Ap+, momentarily
enters an excited state and kicks out an x ray as the electrons
return to a low-energy state. The x ray can be detected by
a spacecraft (lower left).
Chunks of Ice
are odd-shaped chunks of ice (water and frozen gas) and dust a few
kilometers to a few tens of kilometers in diameter. They are the
oldest, most primordial objects in the solar system. X rays emanate
from a comets nebulous atmosphere called a coma, which can
stretch tens of thousands of kilometers in front of or behind the
comet. The coma is formed when the comet gets close enough to the
Sun so that some of the ice is vaporized.
More than a dozen theoretical
models were first proposed to explain why comets give off x rays.
Some models predicted that comet x rays are reradiated x rays from
the Sun. Other models were based on some kind of interaction between
the molecules in the comets thin atmosphere and ions or electrons
from the Suns solar wind, the stream of particles that blow
off the Suns corona at 400 kilometers per second.
The current leading explanation
is called charge exchange. This process is believed to occur when
solar wind forces heavy ions of carbon, nitrogen, oxygen, and other
elements to collide with the electrically neutral atoms and molecules
found in a comets atmosphere. During a collision, a heavy
ion from the solar wind captures an electron from a comets
atmospheric atom or molecule and momentarily enters an excited state.
The ion immediately kicks out an x ray as the electron returns to
a low-energy state.
Very little experimental
data are available on charge-exchange-induced x rays and what the
spectrum emission lines look like, says Beiersdorfer. The
goal of our research is to re-create, in the laboratory, the same
x-ray emissions that are produced when the solar wind and comets
interact. In this way, we can better understand the nature of charge
exchange and help other scientists interpret data taken by x-ray
research, supported by Laboratory Directed Research and Development
funding and NASA, is a collaboration between scientists from Livermore,
NASAs Goddard Space Flight Center, and Columbia University.
The investigators include Daniel Thorn, Mark May, and Hui Chen from
the Laboratory; Richard Kelley, Scott Porter, Caroline Stahle, Keith
Gendreau, Gregory Brown, Andy Szymkowiak, and Kevin Boyce from Goddard;
and Steven Kahn from Colombia. In addition, space researchers Casey
Lisse from the University of Maryland and Bradford Wargelin from
the Harvard Smithsonian Observatory are aiding the research effort.
The team is using Livermores
EBIT, which produces and traps highly charged ions by means of a
high-current-density electron beam instead of traditional high-energy
particle accelerators. The instrument was developed in 1985 by Laboratory
physicists Mort Levine and Ross Marrs. Other electron beam ion traps,
most of which are based on Livermores design, are used at
research centers in the U.S., Europe, and Japan.
EBITs electron beam
collides with selected ions to strip them of one or more electrons,
depending on the beams energy. The current version, named
SuperEBIT, can produce an electron beam energy of up to 250 kiloelectronvolts,
enough to make uranium (U92+) ions. SuperEBIT can produce
virtually any ion, x ray, or visible photon desired, says
Livermores electron beam ion trap (EBIT) facility, scientists
study the charge-exchange process and the effects of different
ions and interaction gas molecules on the x-ray emission patterns
recorded by the x-ray spectrometer (XRS) (front). The EBIT (in
back) provides a source of ions to re-create solar wind particles.
At the left is an old-style spectrometer. Also shown is the
intersection of the XRS and EBIT.
Generation of Spectrometer
The XRS was designed by NASA
for Japans Astro-E X-Ray Satellite, but a failed rocket launch
in February 2000 means a wait of five years before its replacement,
the Astro-E2 Satellite, can be placed in orbit. Fortunately, the
Astro-Es engineering spare XRS was still available for laboratory
x-ray astrophysics measurements. It was sent to Livermore after
the failed launch of Astro-E. The LivermoreGoddard team adapted
the instrument to fit on EBIT and uses it to study x-ray detection.
The XRS, a new generation
of spectrometer, uses microcalorimeter detectors that are so sensitive
they can detect the heat produced by the energy of an individual
x-ray photon. To accomplish this, its microcalorimeter array is
cooled to an extremely low temperature of 460°F (0.060
kelvin). (The absence of all heat, called absolute zero, is 0.0
kelvin, and has never been achieved.) The XRS has an energy detection
range of 0.4 to 10 kiloelectronvolts with an energy resolution of
10 electronvolts, which allows scientists to see much finer detail
in the x-ray spectrum.
the Livermore EBIT facility, scientists study the details of the
charge-exchange process and the effects of different ions and interaction
gas molecules on the x-ray emission patterns recorded by the XRS.
The EBIT facility provides a source of ions to re-create solar wind
particles interacting with atoms and molecules in a comets
atmosphere. The XRS records the x-ray spectra, and diagnostic detectors
characterize the experimental conditions. In effect, says Beiersdorfer,
the EBIT facility has become a test stand for the Astro-E2 satellite,
which will carry an enhanced XRS.
on charge-exchange-induced x-ray emissions supplied by electron
beam ion trap (EBIT) experiments with the x-ray spectrometer
(XRS) are enormously more detailed than data supplied by old-style
detectors. This graph compares the data supplied by each instrument
in registering x-ray emissions caused by charge-exchange reactions
involving an ion of neon (Ne9+).
Mimic Space Interactions
The EBIT experiments begin
with the production of several million ions of either carbon, oxygen,
neon, magnesium, silicon, or iron. These ions are found in the solar
wind and are believed to be involved in charge-exchange reactions
with comets. The beam is then turned off, and the trap is operated
in the so-called magnetic mode, in which the ions are confined by
a magnetic field to a volume of about 2 cubic centimeters. At this
density, the physics is the same as that found in the vicinity of
a comet passing close by the Sun. (A greater density of ions would
introduce completely different physics regimes.) Next, neutral molecules
of water, methane, nitrogen, or carbon dioxide, all of which have
been identified in comets atmospheres, are injected into the
For a few hours, the XRS
records the x rays produced by charge-transfer collisions between
the ions and the neutral molecules. The result is a catalog of emission
lines that serve as tell-tale fingerprints of a particular ions
x-ray-producing collision. Beiersdorfer says that the experiments
are validating the hypothesis that charge exchange is a viable mechanism
for producing comet x rays, although the exact mechanics of the
process are probably more complex than is known.
The researchers discovered
that the x-ray emission pattern changes with the kinetic energy
of the ions. They found that the average x-ray energy emitted by
the ions shifts to higher values as the kinetic energy of the ions
is lowered. They also uncovered subtle changes in the x-ray emission
lines when different neutral gases collide with the heavy ions.
The composition of the interaction gas is another important
variable, says Beiersdorfer.
of x-ray emissions from an ion of oxygen (O7+) caused by interactions
of O8+ with methane (CH4, the green trace) or with nitrogen
(N2, the red trace) reveal subtle changes in the x-ray emission
lines when these different neutral gases collide with the
heavy ions. The changes are found near 850 electronvolts.
Beiersdorfer predicts that
careful detection and measurement of x rays produced by the interaction
between the solar wind and comets will one day provide a powerful
means to monitor space weather inside the solar system
without the need for spacecraft circling the Sun. In this way, he
says, comets could be used as probes to measure the intensity, speed,
and composition of the solar wind, its intermittent quiet
time, and the chemical composition of comet gases.
Given that more than
three bright comets with appreciable x-ray emissions enter the inner
solar system each year, their x rays can provide a valuable diagnostic
of the solar wind. This capability has opened up a whole new window
to our solar system; its a very rich field.
astronomers have conjectured that as the solar wind slows down throughout
the heliosphere, it may generate weak x rays through charge-exchange
reactions with natural gas streaming in from the interstellar medium
(mostly hydrogen atoms). If this hypothesis is borne out by x-ray
satellite data, astronomers will have to revise their assumption
that the soft x-ray background that seems to permeate the universe
may in fact be partly due to charge-exchange reactions from the
Handbook on Comet X Rays
The result of the EBIT experiments
will likely be a small handbook for scientists to guide their interpretation
and understanding of the comet x-ray data sent back by Astro-E2,
beginning in 2005. The scientific community will be well prepared
when Astro-E2 launches, says Beiersdorfer. In the meantime,
NASA has committed a second, advanced XRS for the EBIT teams
the EBIT experiments continue, other scientists are looking at the
theoretical model of charge exchange. Atomic theorists Ronald Olson
from the University of Missouri at Rolla; Jim Perez from Luther
College in Decorah, Iowa; Charles Weatherford from Florida A&M
University in Tallahassee; and Burke Ritchie from Livermore are
aiding the research effort. Lawrence Livermore researchers have
extensive experience in modeling short-wavelength radiation phenomena,
and physicist Ritchie is using high-performance supercomputers to
elucidate in greater detail charge-exchange reactions using the
quantum theory of atomic collisions.
primordial chunks of dirty ice still hold a few surprises for scientists.
Key Words: charge
exchange, comets, electron beam ion trap (EBIT), microcalorimeter,
solar wind, x rays, x-ray spectrometer (XRS).
information contact Peter Beiersdorfer (925) 423-3985 (firstname.lastname@example.org).