Number 232, July 6, 1995 by Phillip F. Schewe and Ben Stein
ONE STEP CLOSER TO PRODUCING ANTI-ATOMS , Gerald Gabrielse and his colleagues
at Harvard have devised a method for trapping and cooling positrons, the
antimatter version of electrons, in the proper environment needed for producing
anti-hydrogen, an atom consisting of an antiproton orbited by a positron.
Over the past decade, Gabrielse (617-495-4381) and his colleagues have
honed techniques for trapping and cooling antiprotons in a tabletop device
connected to an antiproton source at CERN in Geneva. Combining antiprotons
and positrons to make the world's first anti-atom, a major pursuit in physics,
will not be a trivial feat. The trick is to get them to slow down enough
so that the positron is gently ensnared by the antiproton. To do this,
you need to cool each type of antiparticle and store them in separate traps.
The traps must exist in an environment as free as possible from background
gas atoms, so that the antiparticles are not knocked out of the traps.
The researchers have now stored approximately 35,000 positrons in an ultra-high
vacuum environment having a pressure estimated to be better than 5x10**-17
torr, corresponding to fewer than 100 background gas atoms per cubic centimeter.
Released from the radioactive decay of a sodium-22 source, 3 million positrons
per second strike a tungsten crystal, which acts as a "moderator"
to slow down the particles. Some of these positrons then enter a Penning
trap, a device which uses a combination of electric and magnetic fields
to prevent sluggish positrons from escaping. The circulating particles
induce a current in an electrical circuit, which further reduces the positrons'
energy. At a temperature of 4.2 K, the trapped positrons are an ultracold
plasma, a frigid collection of charged particles. Incidentally, this cold
bath of positrons can be used to significantly cool down highly charged
ions, such as the recently produced bare uranium ion, perhaps the most
highly reactive form of matter in nature. Based on studies of how electrons
and protons combine to form hydrogen atoms, Gabrielse believes that having
10-100 times the present number of trapped positrons is desirable for successful
anti-hydrogen production. The necessary increase may come through the use
of a more efficient crystal moderator and a more intense radioactive source.
The researchers plan to refine their device at Harvard and test it at CERN
as early as next summer. In future attempts to produce anti-hydrogen, the
researchers would probably combine the positron and anti-proton traps into
a single device. (L.H. Haarsma et al., 31 July 95, Phys Rev. Lett. Reporters:
For a preprint of the article and some accompanying graphics, please contact
AIP Public Information at physnews@aip.org or 301-209-3091).
GRAVITATIONALLY REDSHIFTED LIGHT suggests the existence of a black hole
in the active galaxy MCG-6-30-15. According to theories of black holes,
matter drawn in toward a black hole should accelerate to high speeds. Providing
the matter has not yet crossed the point- of-no-return known as the Schwarzschild
radius, radiation emitted by the matter can still escape from the vicinity,
albeit in an altered form. Just such a relativistic gravitational-redshift
effect has now been observed by astronomers using the ASCA x-ray satellite.
The spectrum of ionized iron atoms tells the story: from the broadening
and asymmetry of specific emission lines, the ASCA scientists deduce that
the luminous matter in question was traveling at a velocity of 100,000
km/sec and was at a distance of only about 3-10 Schwarzschild radii from
the center of what is probably a supermassive black hole. (Y. Tanaka et
al., Nature, 22 June 1995.)
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