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by Dan Stober
Stanford Report
August 23, 2010
from
Stanford Website
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Chantal Jolagh, a
science-writing intern at the Stanford News Service,
contributed to this story.
When researchers found an
unusual linkage between solar flares and the inner life
of radioactive elements on Earth, it touched off a
scientific detective investigation that could end up
protecting the lives of space-walking astronauts and
maybe rewriting some of
the assumptions of physics. |
Peter Sturrock
professor emeritus of
applied physics
It's a mystery that presented itself
unexpectedly:
The radioactive decay of some
elements sitting quietly in laboratories on Earth seemed to be
influenced by activities inside the sun, 93 million miles away.
Is this possible? Researchers from
Stanford and Purdue University believe it is.
But their explanation of how it happens
opens the door to yet another mystery.
There is even an outside chance that this unexpected effect is
brought about by a previously unknown particle emitted by the sun.
"That would be truly remarkable,"
said Peter Sturrock, Stanford professor emeritus of applied
physics and an expert on the inner workings of the sun.
The story begins, in a sense, in
classrooms around the world, where students are taught that the rate
of decay of a specific radioactive material is a constant.
This concept is relied upon, for
example, when anthropologists use
carbon-14 to date ancient artifacts
and when doctors determine the proper dose of radioactivity to treat
a cancer patient.
Random numbers
But that assumption was challenged in an unexpected way by a group
of researchers from Purdue University who at the time were more
interested in
random numbers than nuclear decay.
(Scientists use long strings of random
numbers for a variety of calculations, but they are difficult to
produce, since the process used to produce the numbers has an
influence on the outcome.)
Ephraim Fischbach, a physics professor at Purdue, was looking
into the rate of radioactive decay of several isotopes as a possible
source of random numbers generated without any human input.
(A lump of radioactive cesium-137, for
example, may decay at a steady rate overall, but individual atoms
within the lump will decay in an unpredictable, random pattern. Thus
the timing of the random ticks of a Geiger counter placed near the
cesium might be used to generate random numbers.)
As the researchers pored through published data on specific
isotopes, they found disagreement in the measured decay rates - odd
for supposed physical constants.
Checking data collected at Brookhaven National Laboratory on
Long Island and the Federal Physical and Technical Institute
in Germany, they came across something even more surprising:
long-term observation of the decay rate of silicon-32 and radium-226
seemed to show a small seasonal variation. The decay rate was ever
so slightly faster in winter than in summer.
Was this fluctuation real, or was it merely a glitch in the
equipment used to measure the decay, induced by the change of
seasons, with the accompanying changes in temperature and humidity?
"Everyone thought it must be due to
experimental mistakes, because we're all brought up to
believe that decay rates are constant," Sturrock said.
The sun speaks
On Dec 13, 2006, the sun itself provided a crucial clue, when a
solar flare sent a stream of particles and radiation toward Earth.
Purdue nuclear engineer Jere Jenkins,
while measuring the decay rate of
manganese-54, a short-lived isotope
used in medical diagnostics, noticed that the rate dropped slightly
during the flare, a decrease that started about a day and a half
before the flare.
If this apparent relationship between flares and decay rates proves
true, it could lead to a method of predicting solar flares prior to
their occurrence, which could help
prevent damage to satellites and
electric grids, as well as save the lives of astronauts in space.
The decay-rate aberrations that Jenkins noticed occurred during the
middle of the night in Indiana - meaning that something produced by
the sun had traveled all the way through the Earth to reach Jenkins'
detectors.
What could the flare send forth that
could have such an effect?
Jenkins and Fischbach guessed that the culprits in this bit of
decay-rate mischief were probably
solar neutrinos, the almost
weightless particles famous for flying at almost the speed of light
through the physical world - humans, rocks, oceans or planets - with
virtually no interaction with anything.
Then, in a series of papers published in Astroparticle Physics,
Nuclear Instruments and Methods in Physics Research and Space
Science Reviews, Jenkins, Fischbach and their colleagues showed
that the observed variations in decay rates were highly unlikely to
have come from environmental influences on the detection systems.
Reason for
suspicion
Their findings strengthened the argument that the strange swings in
decay rates were caused by
neutrinos from the sun.
The swings seemed to be in synch with
the Earth's elliptical orbit, with the decay rates oscillating as
the Earth came closer to the sun (where it would be exposed to more
neutrinos) and then moving away.
So there was good reason to suspect the sun, but could it be proved?
Enter Peter Sturrock, Stanford professor emeritus of applied
physics and an expert on the inner workings of the sun. While on a
visit to the National Solar Observatory in Arizona, Sturrock was
handed copies of the scientific journal articles written by the
Purdue researchers.
Sturrock knew from long experience that the intensity of the barrage
of neutrinos the sun continuously sends racing toward Earth varies
on a regular basis as the sun itself revolves and shows a different
face, like a slower version of the revolving light on a police car.
His advice to Purdue: Look for
evidence that the changes in radioactive decay on Earth vary with
the rotation of the sun.
"That's what I suggested. And that's
what we have done."
A surprise
Going back to take another look at the decay data from the
Brookhaven lab, the researchers found a recurring pattern of 33
days. It was a bit of a surprise, given that most solar observations
show a pattern of about 28 days - the rotation rate of the surface
of the sun.
The explanation?
The core of the sun - where nuclear
reactions produce neutrinos - apparently spins more slowly than the
surface we see.
"It may seem counter-intuitive, but
it looks as if the core rotates more slowly than the rest of the
sun," Sturrock said.
All of the evidence points toward a
conclusion that the sun is "communicating" with radioactive isotopes
on Earth, said Fischbach.
But there's one rather large question left unanswered. No one knows
how neutrinos could interact with radioactive materials to
change their rate of decay.
"It doesn't make sense according to
conventional ideas," Fischbach said.
Jenkins whimsically added,
"What we're suggesting is that
something that doesn't really interact with anything is changing
something that can't be changed."
"It's an effect that no one yet understands," agreed Sturrock.
"Theorists are starting to say,
'What's going on?' But that's what the evidence points to. It's
a challenge for the physicists and a challenge for the solar
people too."
If the mystery particle is not a
neutrino,
"It would have to be something we
don't know about, an unknown particle that is also emitted by
the sun and has this effect, and that would be even more
remarkable," Sturrock said.
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