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Image of tracks of two protons emitted in the decay
of iron-45; research appearing in the journal Physical Review
Letters the week of Nov. 5, 2007, represents the first-ever
description of the angular correlation between these protons.
Image � by
Marek Pfutzner, Warsaw University
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"We have proved in a direct and clear way that
this extremely neutron-deficient nucleus disintegrates by the
simultaneous emission of two protons," write the authors.
Pfutzner and his collaborators set out to better understand an exotic
form of radioactivity - two-proton emissions from iron-45, a nucleus
with 26 protons and 19 neutrons. The stable form of iron that is most
abundant on Earth has 26 protons and 30 neutrons. One possibility was
that the iron-45 isotope might occasionally release an energetically
linked two-proton pair, known as a diproton. Other possibilities were
that the protons, whether emitted in quick succession or
simultaneously, were unlinked.
The research was performed at Michigan State University's National
Superconducting Cyclotron Laboratory (NSCL), but the key device was a
detector built by Pfutzner and his Warsaw University colleagues.
Though nicknamed "the cannon" because of its vague resemblance to some
sort of space age military device, the detector didn't shoot anything
but rather was the target for the beam of rare isotopes produced at
the NSCL Coupled Cyclotron Facility.
The detector included a front-end gas chamber that accepted and then
slowed rare isotopes traveling at half the speed of light. The
back-end imaging system, built around a high-end digital camera with
standard charge-coupled device, or CCD, technology, recorded ghostly
images of trajectories of emitted protons from the decaying iron-45
nuclei shot into the cannon's mouth.
Analysis of these images ruled out the theorized diproton emission and
indicated that the observed correlations between emitted protons were
best described by a form of nuclear transformation known as three-body
decay. A theory of this process had previously been described by
Leonid Grigorenko, a physicist at the Joint Institute for Nuclear
Research in Dubna, Russia and a coauthor of the paper.
"There is amazing agreement between the experiment and Grigorenko's
theory, which takes into account the complex interplay between emitted
pairs of protons and the daughter nucleus," said Robert Grzywacz, a
physicist at the University of Tennessee and Oak Ridge National
Laboratory and a coauthor of the paper.
Besides shedding light on a novel form of radioactive decay, the
technique also could lead to additional discoveries about fleeting,
rare isotopes studied at accelerator facilities such as NSCL and Oak
Ridge National Laboratory. These isotopes may hold the key to
understanding processes inside neutron stars and determining the
limits of nuclear existence.
The experiment itself also harkens back to the early days of
experimental nuclear physics in which visual information served as the
raw data. Before the days of cameras, this information was usually
captured by scientists hunched over a microscope counting, for example,
tiny flashes as alpha particles struck a zinc sulfide screen under the
lens.
"It's perhaps the first time in modern nuclear physics that
fundamentally new information about radioactive decay was captured in
a picture taken by a digital camera," said Andreas Stolz, NSCL
assistant professor and a coauthor on the paper. "Usually, in nuclear
physics experiments you have digitized data and several channels of
information from electronics equipment, but never images."
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