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,
such as 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 uses magnetic and electric fields to confine ions. 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.”
Side
bar: Mass measurement experiments are crucial to
new accelerator development
For more information,
please contact David Jacqué.
Next: X-rays
reveal the secrets of diesel combustion
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