|
Topic Name: Artificial atoms made of annihilating particles can pair up.
Category: Synchrotron Radiation
Research persons: Cassidy, David,Allen P. Mills, Jr.,Clifford M. Surko
Location: University of California, San Diego - Department of Physics, United States
Details
Two years after reporting the first tantalizing hints that matter might be
able to bind with antimatter, researchers in California have nailed convincing
evidence for the pairing.
David Cassidy and Allen Mills at the University of California at Riverside say
that they have found the signature of a molecule in which two positrons —
antimatter siblings of electrons — are bound together by electrons This hybrid
substance is called molecular
positronium,
denoted Ps2.
Because electrons and positrons have equal and opposite charges, they can become
bound together by their electrical attraction, just as a positively charged
proton is orbited by an electron in ordinary hydrogen atoms.
In theory, positronium atoms — electron-positron pairs — should also be able to
pair up to form molecules, just as two hydrogen atoms form H2. Because the mass
of a positron is just 1/1836 that of a proton, positronium molecules are much
lighter than hydrogen.
But Ps2 is peculiar stuff. Rather than being two well-defined atoms stuck
together, the four particles "do a merry dance around each other in a fuzzy,
lump-less soup", says physicist Clifford Surko of the University of California
in San Diego.
Muzzy molecules
These molecules are very hard to see because matter and antimatter annihilate
each other, releasing a burst of energy in the form of gamma-rays. When isolated
in a vacuum, positronium atoms typically survive for less than a millionth of a
second before they self-destruct. "Almost as soon as they are made, they
disappear again with a puff and a flash of light," says Surko.
But Cassidy and Mills found that if they could capture enough positronium, some
of the pseudo-atoms might combine before they vanish. If that happens, the
positronium molecules would then release a characteristic gamma-ray signature
when annihilation eventually occurs.
So the researchers fired a beam of positrons (made with a technique developed by
Surko) into porous silica glass, in an attempt to pick up electrons and make
Ps2. They estimated that they had a one-in-ten chance of two positronium atoms
combining.
Mills and his colleagues first reported that this might happen in 2005 (see 'Did
matter-antimatter mix yield molecules?' ). But it has taken until now to
firm up the evidence.
Cool stuff
The clinching data come from looking at how the intensity of the gamma-rays
changes as the temperature is altered.
Electron-positron annihilation should be more rapid in Ps2 than in lone
positronium atoms, because the binding increases the chance of collision. And
the positronium mix should have a greater proportion of molecules at lower
temperatures, since the cold makes molecules more stable. So the gamma-rays
should become more intense when the mixture is cooled.
Ultimately, says Surko, studying these blends of matter and antimatter could
help to answer one of the most perplexing questions in fundamental physics: why
is there so much more matter than antimatter in the Universe?
Mills also has a practical goal: to make Ps2 in sufficient quantities to create
a laser from the very-high-energy gamma-rays released in annihilation. To
achieve this, says Mills, "we have the ambitious goal of making not two but many
thousands of positronium atoms interact with each other."
About The Researchers:
Cassidy, David
Postdoctoral Scholar
Physics 1153
2919
david.cassidy@ucr.edu
Allen P. Mills, Jr.
Professor of Physics
Ph.D. 1967, Brandeis University
Experimental Solid State and Atomic Physics
E-mail: allen.mills@ucr.edu
Phone: (951) 827-6469
Fax: (951) 827-4529
Mills Group
Home Page
Goals: To make the first Bose-Einstein condensed positronium annihilation
gamma ray laser; To make a DNA neural network computer ten thousand times bigger
than a human brain
Prof. Mills has pioneered several techniques in the field of positron physics
including the single crystal negative affinity positron moderator (1978,9),
brightness enhancement of slow positron beams (1980) and the rare gas solid
moderator (1986). He is currently working on applying these techniques to the
problem of obtaining a Bose-Einstein condensed gas of positronium atoms. He is
also interested in defect spectroscopy, is preparing with Prof. Tom to study the
dynamics of laser exploded thin foils using a newly assembled microprobe funded
by the National Science Foundation under grant DMR-0216927.
Prof. Mills has worked on various experimental aspects of DNA computation since
1996. Most recently he was part of the team lead by Bernard Yurke of Bell
Laboratories that developed a molecular-size machine made from DNA molecules
that operates using DNA as a fuel.
Prof Mills is also interested in certain aspects of semiconductor physics and is
currently involved with Prof. Yarmoff and Prof. Haddon in implementing the idea
of Contactless Organic Semiconductor devices for low-cost high speed laminated
electronics with support from UCR's Center for Nanoscale Innovation for Defense
(CNID) and DOD/DARPA/DMEA under Award No. DMEA90-02-2-0216.
Clifford M. Surko
Professor
Ph.D. UC Berkeley, 1968
Developing techniques to accumulate, store and manipulate large numbers of
positrons and to make state-of-the-art cold positron beams – in essence, to make
low-energy antimatter in the laboratory a reality. We are also interested in
using these collections of antimatter to study a number of scientific issues. We
conducted the first studies of electron-positron plasmas. We have also conducted
a number of precision, high-energy-resolution studies of the interaction of
positrons with atoms and molecules, including positron excitation of, and
annihilation with, atoms and molecules. Recent work provided the first
experimental evidence that positrons bind to neutral matter. Experiments on
positron studies of freely suspended atomic clusters are in the planning stages.
In the area of antimatter technology, we are developing a positron beam with 1
meV energy resolution and a new positron trap capable of storing > 10^10
positrons for days. A multicell trap, currently in the planning stages, is
expected to extend antimatter storage capabilities by additional orders of
magnitude.
SELECTED PUBLICATIONS
"An Electron-Positron Beam-Plasma Experiment," R. G. Greaves and C. M. Surko,
Phys. Rev. Lett. 75, 3846 (1995); also see S. J. Gilbert et al., Phys. Plasmas,
8, 4982 (2001).
"Creation of a Monoenergetic Pulsed Positron Beam," S. J. Gilbert, et al., Appl.
Phys. Lett. 70, 1944 (1997).
“Inward Transport and Compression of a Positron Plasma by a Rotating Electric
Field,” R. G. Greaves and C. M. Surko, Phys. Rev. Lett. 85, 1883 (2000); and
Phys. Plasmas 8, 1879 (2001).
“A Multicell Trap to Confine Large Numbers of Positrons,” C. M. Surko and R. G.
Greaves, Radiation Physics and Chemistry 68, 419 (2003).
“Emerging Physics and Technology of Antimatter Plasmas and Trap-Based Beams,” C.
M. Surko and R. G. Greaves, Phys. Plasmas 11, 2333 (2004).
“Torque-Balanced High-Density Steady States of Single-Component Plasmas,” J. R.
Danielson and C. M. Surko, Phys. Rev. Lett. 94, 035001 (2005).
“Excitation of Molecular Vibrations by Positron Impact,” J. P. Sullivan, et al.,
Phys. Rev. Lett. 86, 1494 (2001); also see J. P. Sullivan, et al., Phys. Rev. A
66, 042708 (2002).
“Excitation of Electronic States of Ar, H2, and N2 by Positron Impact,” J. P.
Sullivan, et al., Phys. Rev. Lett. 87, 073201 (2001).
“Vibrational-Resonance Enhancement of Positron Annihilation in Molecules,” S. J.
Gilbert et al., Phys. Rev. Lett. 88, 043201 (2002); also see L. D. Barnes, et
al., Phys. Rev. A 67, 032706 (2003).
“Low-Energy Positron Interactions with Atoms and Molecules,” C. M. Surko, G. F.
Gribakin, and S. J. Buckman, J. Phys. B 38, R57 (2005).
Some Recent Related Research :
Quantum Dot
Research
Fermionic atoms can pair up
Artificial
Radiation Sources
LEGAL
IMPLICATIONS of NANOSCIENCE
Exploration of the Earth's
Magnetosphere
Applications of Nuclear Physics
Atoms and
Electromagnetism
Connecting
Quarks with the Cosmos
|
