NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/physics/fusion/1830

Text File | 1992-07-25 | 5.8 KB | 122 lines

Newsgroups: sci.physics.fusion
Path: sparky!uunet!sun-barr!ames!riacs!danforth
From: [e-mail] (Douglas G. Danforth)
Subject: Radiation induced phase change in Helium-3
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: RIACS, NASA Ames Research Center
Date: Sun, 26 Jul 92 05:56:44 GMT
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I post the following article with some trepidation since the effect
mentioned is in Helium-3, acts at the atomic level and may only tangentially
be related to Ying's model. There are enough similarities, however, and
the last paragraph is most provocative ...

-- Doug Danforth

SCIENCE NEWS
July 18, 1992
Vol. 142, No. 3
Pages 33-48

          'BAKED ALASKA' COOKED UP IN LIQUID HELIUM

Baked Alaska seems an unlikely term to encounter in physics, but this
culinary surprise, consisting of meringue baked around ice cream, serves as
an apt description of an exotic, theoretical model accounting for a curious
aspect of liquid helium behavior. The model, proposed in 1984 by Anthony J.
Leggett of the University of Illinois at Urbana-Champaign, suggests that
high-energy particles produced by cosmic rays can trigger the otherwise
inexplicable formation of one form of superfluid helium-3 at the expense of
another.

In the baked Alaska scenario, high-energy electrons, created by the passage
of cosmic-ray-generated muons through the supercooled liquid, deposit
significant amounts of energy in spots less than a micron in diameter. Each
intensely heated microball of liquid helium expands into a hot shell, leaving
behind a pocket of cold superfluid helium. Isolated from the rest of the
liquid, this cold core provides a protected environment in which a bubble of
a different type of superfluid helium-3 can nucleate and start to grow.

Now researchers have obtained experimental evidence establishing the
plausibility of Leggett's scenario. "Our results are certainly consistent
with the [baked Alaska] model, though there are still unanswered questions,"
says peter E. Schiffer of Stanford University.

"The Stanford results show that at least the idea of nucleation by
high-energy particles isn't totally crazy," Leggett notes.

Schiffer, Douglas D. Osheroff, and coworkers report their findings in the
July 6 PHYSICAL REVIEW LETTERS.

Helium-3, a rare isotope of helium, becomes a super fluid -- a liquid that
flows without friction -- at temperatures below 2.5 millikelvins. In this
chilly state, helium atoms tend to form pairs. Because these pairs can
arrange themselves in two different ways, helium-3 has two distinct
superfluid states. Depending on the pressure and the magnetic field applied
to a sample, the so-called A phase is more stable than the B phase at higher
temperatures, whereas the B phase takes over at lower temperatures.

In 1977, Osheroff (then at AT&T Bell Laboratories) and co-worker Michael
Cross showed that the superfluids had characteristics implying that the A
phase, even when supercooled well below the temperature at which a transition
from the A to the B phase should occur, cannot by itself spontaneously make
the change. Because such phase transitions actually do occur, this puzzling
feature led to a search for a mechanism that would explain how the transition
happens.

"I played around with various ideas, and it eventually sank into my mind that
no mechanism based on a thermal equilibrium distribution of energy was going
to explain this," Leggett recalls. His baked Alaska model emerged out of this
line of reasoning.

"I had to convince myself you couldn't apply the normal laws of hydrodynamics
or thermal transport under these conditions because you're so far from
equilibrium," he says. "It really matters how the heat spreads out."

To check whether radiation can indeed trigger the nucleation of the B phase
within the A phase of superfluid helium-3, the Stanford group used a
specially designed, long, thin, silica glass tube with microscopically smooth
surfaces. Within this tube, the team discovered it could dramatically
supercool samples of the A phase to temperatures as low as 0.37 millikelvins,
much lower than temperatures achieved by other groups.

In addition, by placing sources of radiation near the sample cell, they
discovered that they could greatly reduce the length of time before
nucleation occurs in the supercooled A phase. Both gamma rays and neutrons
produced comparable effects.

"It's clear that radiation does play a part," Osheroff says.

These findings indirectly suggest that the presence of surface irregularities
or defects also has a strong influence on the nucleation of phase B. This
factor may have thwarted previous attempts to detect radiation-induced
nucleation.

Moreover, the Stanford experiment demonstrates the conditions necessary for
observing the A phase at lower temperatures and lower magnetic fields than
previously possible. "Now that we've got it pinned down, I think there's
going to be a burst of activity," Leggett says. "A lot of people would love
to have [A-phase] helium-3 in low magnetic fields at low temperatures. There
are all sorts of things you can do with it."

Precisely how surface roughness and the presence of minute traces of such
impurities as radioactive tritium contribute to the nucleation of phase B
remains unclear. Osheroff and his team are now discussing the design of
sample containers specially fabricated to have a certain roughness. The
researchers would also like to observe nucleation at different pressures and
magnetic fields.

"Helium-3 is an ideal system for understanding physics that would be
completely masked in any other system," Osheroff says.

To leggett, the A-B transition in superfluid helium-3 represents a
particularly clear example of how locally concentrated energy that can't
dissipate through normal channels can induce events that by any other,
reasonable, statistical measure would seem astronomically improbable.

                        -- I. Peterson