Physicists Strive To Create Atoms Of Antihydrogen From Antimatter
By MALCOLM W. BROWNE
Published: November 15, 1994
ANTIMATTER: the super fuel for driving spacecraft to the stars, the wonder annihilator that could make hydrogen bombs seem like firecrackers, the antistuff of antigalaxies inhabited by antipeople . . ..
In the 66 years since the British physicist Paul A. M. Dirac theorized the existence of antimatter, the wilder notions about this fascinating but elusive substance have remained closer to fiction than to physics. Nevertheless, the creation of antimatter in high-energy laboratories has become routine, bringing exciting new possibilities within reach.
One is the likelihood that portable containers may soon be capable of storing and transporting billions of antimatter particles for months at a time, freeing scientists from the need to remain close to one of the handful of particle accelerators capable of making antimatter.
Another possibility that now seems feasible is the assembly of complete antiatoms from the subatomic antiparticles physicists have already learned to create. The simplest and easiest to make of these antiatoms would be atomic antihydrogen, which would consist of a single negatively charged antiproton at its nucleus, with a single positively charged antielectron, or positron, in orbit around it.
(Particles of antimatter have electric charges equal but opposite to those of their ordinary-matter counterparts.)
If physicists were to succeed in making antihydrogen, and if enough of it could be frozen into a ball of antihydrogen ice, and if some way could be found to store this exotic and dangerous ice without letting it come into contact with ordinary matter (thereby causing mutual annihilation and a titanic explosion), the gateway to interstellar rocket fuels might begin to open.
Although scientists predict that antihydrogen may be created in the next two years by one of several planned experiments, they regard the prospect of storing it in the form of antihydrogen ice as little more than a pipe dream at this point.
Progress in the exploration of antimatter is accelerating and has yielded some spectacular recent successes.
One scientific triumph made possible by the use of antimatter was the discovery of the long-sought "top" quark -- an elementary building block of matter whose existence was predicted by the highly successful theory of matter known as the Standard Model. Physicists at the Fermi National Accelerator Laboratory in Batavia, Ill., used beams of matter and antimatter particles (protons and antiprotons) colliding at combined energies near two trillion electron-volts to search for the top quark. From the microscopic fireballs caused by these collisions and their resulting annihilations, sprays of new particles were created, of which a half dozen were deemed to contain evidence of top quarks.
With that achievement, announced in April, physicists had discovered all six of the quarks predicted by the Standard Model -- the "up" and "down" quarks inside the protons and neutrons of ordinary atomic nuclei, as well as the "charm," "strange," "top" and "bottom" quarks that can exist only in ultrahot conditions like those that existed right after the Big Bang, the event believed to have created the universe.
Another form of antimatter -- the positron, or antielectron -- has found many practical uses, one of which is in Positron Emission Tomography, or PET. PET scanning allows doctors not only to make detailed images of soft tissue inside patients, but to observe chemical processes in their bodies, including the varying rates of fuel consumption by different regions of the brain as it tackles cognitive tasks. The positrons used for such scanning are spontaneously emitted by short-lived radioactive isotopes like oxygen-15 and fluorine-18, which can be ducted into the air a patient breathes. Since these isotopes survive for only a few minutes or hours after they are made, a patient undergoing a PET scan must be somewhere near the cyclotron particle accelerator that makes the isotopes. These large, expensive machines are typically set up in hospital basements.
Among the physicists working on new ways to make, handle and store antimatter are Dr. Gerald Gabrielse of Harvard University, Dr. Gerald A. Smith of Pennsylvania State University, Dr. Michael Holzsheiter of Los Alamos National Laboratory and Dr. F. J. Hartmann of the Technical University of Munich, Germany.
A major objective of several research teams is to develop portable thermos jugs in which millions or billions of antiprotons can be stored or carried around for months at a time.
Making such a container is no easy task, because it must prevent the antiprotons it contains from coming into contact with any atom of ordinary matter.
The basic scheme for such a container, called a Penning trap, was devised in 1936 by Frans Michel Penning, a Dutch scientist. The empty core of a Penning trap is kept at a temperature of about minus 452 degrees Fahrenheit by a chamber containing liquid helium, which in turn is surrounded by a chamber filled with liquid nitrogen.
Nearly all the air must be pumped out of the core of the trap before use, and the few remaining molecules of atmospheric gases then condense and stick to its cold walls, leaving an almost perfect vacuum. Negatively charged particles --ordinary electrons -- are then injected into the core for storage, where an array of coils and electrodes creates a powerful magnetic field that squeezes them into a space running along the trap's central axis. By manipulating the electric and magnetic fields confining the electrons, the trap slows them down, or "cools" them, almost to a standstill. Then antiprotons are ducted into the trap, where their movements through a mass of cold electrons slows them down, in somewhat the way a bowling ball passing through a pile of stationary Ping-Pong balls would be gradually slowed and stopped.