Precise nuclear measurements give clues to astronomical X-ray bursts(Forwarded)

Argonne National Laboratory

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July 11, 2003

Precise nuclear measurements give clues to astronomical X-ray bursts

ARGONNE, Ill. -- Argonne physicists have precisely measured the masses of
nuclear isotopes that exist for only fractions of a second or can only be
produced in such tiny amounts as to be almost nonexistent in the laboratory.
Some isotopes had their masses accurately measured for the first time.

The results help explain the characteristic X-ray spectrum and luminosities of
strange astronomical objects called "X-ray bursters."

X-ray bursters comprise a normal star and a neutron star. Neutron stars are as
massive as our sun but collapsed to 10 miles across. The neutron star's
ferocious gravitational field pulls gas from its companion until the neutron
star's surface ignites in a runaway fusion reaction. For a few tens of seconds,
the light from the explosion may be the most brilliant source of X-rays in the sky.

The rapid proton capture process, or "rp-process," is the dominant source of
energy in a common type of X-ray bursters. In this nuclear fusion reaction,
nuclei capture protons and transmute into a heavier element, releasing energy in
the process. For example, arsenic-67 can capture a proton to become selenium-68.

The rp-process proceeds in fits and starts, due to what physicists call
"waiting-point nuclei." Some nuclides, like selenium-68, can't absorb an
incoming proton as quickly as others can. The reaction must "wait" for the
nucleus to absorb a proton -- which may take up to 30 minutes, a relative
eternity -- or for the neutron to decay to a proton, called beta decay, to
convert the nuclide into one with a more favorable capture rate. A beta-decay,
for example, converts the selenium-68 nucleus into arsenic-68. Arsenic-68
readily captures a proton, changing to selenium-69, and so on.

"How long the nova or X-ray burst lasts, and how far the rp-process reactions
proceed, is determined by the properties of these few waiting-point nuclei,"
said physicist Guy Savard, principal investigator. "Although there are hundreds
of nuclei in an X-ray burst, the properties of half a dozen of them make all the
difference."

Accurate measurements of waiting-point nuclei masses explain the astronomical
observations of X-ray bursts and confirm theories of how they are produced. But
measuring their masses is difficult. Some decay in fractions of a second; others
can only be produced in such small amounts that standard spectrometry techniques
give imprecise results.

Argonne's Unique ATLAS

Highly accurate mass measurements required the unique facilities available in
Argonne's Physics Division. The nuclei to be studied were created using the
Argonne Tandem Linac Accelerator System (ATLAS). For example, selenium-68 was
created by accelerating beams of nickel-58 to 220 million electron volts and
slamming them into a carbon target. Some of the ions in the beam combine with
nuclei in the target to create the ions of interest.

The created ions are slowed to a crawl in a "gas catcher" -- a tube filled with
pressurized helium. A gentle electric gradient pulls ions into a Canadian
Penning Trap Spectrometer developed by Savard and other scientists at Argonne,
the University of Manitoba and McGill University, Montreal, Texas A&M University
and the State University of New York.

The Penning trap confines ions using magnetic and electric fields. A measurement
may involve perhaps only a dozen individual ions, which can stay suspended in
the trap for many seconds. Their masses can then be measured using
radio-frequency (RF) fields.

"The ions will accept energy from the RF field only at certain frequencies,"
Savard said. "These frequencies are related to properties of the ion,
particularly the mass. By looking at what energies they accept, you can
precisely determine the mass."

Ions with previously unknown masses included antimony 107 and 108. The mass of
selenium-68 was determined with 30 times more precision than previous, and
contradictory, measurements.

"This is a unique system, because with the new gas catcher, we can inject any
species that can be produced here at ATLAS," Savard said. "Research is ongoing.
We're now exploring around the tin region, where the rp-process is expected to
terminate."

Mass measurement experiments crucial to RIA development

The proposed Rare Isotope Accelerator (RIA), an ambitious physics facility
concept now being designed, is in some ways an outgrowth of the mass-measurement
experiments at Argonne. The gas catcher cell that slows nuclei to a near-stop
for analysis is a crucial RIA technology.

RIA will enable physicists to explore the nature of nuclei -- the clusters of
particles that occupy the center of every atom by producing beams of short-lived
nuclei 10,000 times more intense than any now available. These beams will
provide insight into the origin of the elements and will test current physics
models. RIA also holds promise for important applications to medicine, industry
and other applied physics research.

The Argonne-developed concept has been approved by a U.S. Department of Energy
advisory committee. Michigan State University and other institutions are
involved with Argonne in the design and prototyping work.

Argonne is well positioned to be the host site for RIA, based on the
laboratory's pathbreaking expertise in advanced accelerator technology.

The nation’s first national laboratory, Argonne National Laboratory conducts
basic and applied scientific research across a wide spectrum of disciplines,
ranging from high-energy physics to climatology and biotechnology. Since 1990,
Argonne has worked with more than 600 companies and numerous federal agencies
and other organizations to help advance America's scientific leadership and
prepare the nation for the future. Argonne is operated by the University of
Chicago as part of the U.S. Department of Energy's national laboratory system.

IMAGE CAPTION:
[http://www.anl.gov/OPA/whatsnew/pix/...rayburster.jpg (7KB)]
This double-star system, located approximately 28,000 light-years away in the
constellation Sagittarius, is a source of powerful bursts of X-ray emission.
Argonne physicists have made precise measurements of exotic isotopes that
explain the characteristic X-ray spectrum and luminosities of such "X-ray
bursters." Illustration courtesy Dana Berry, Space Telescope Science Institute.