TEMPE, Ariz. - In searching science books for help with homeland
security, you wouldn't be likely to linger over the chapter on neutrinos.
Elementary particle physics seems as far from the war on terrorism as
Afghanistan is from Albuquerque. What neutrinos are and what they do
rarely makes the Top 10 list of things every public policy-maker should
know.
Yet these subatomic particles may offer diplomats a new device for
strengthening treaties to counter one of terrorism's most fearsome
threats - nuclear explosives.
Building a nuclear bomb isn't all that easy, but the blueprints aren't
exactly secret anymore. Experts agree that technically trained
terrorists could construct a crude nuclear explosive device. The main
obstacle to creating such a nuclear nightmare would be acquiring the raw
materials - substances capable of rapid nuclear fission.
Fission, or "splitting the atom," was discovered in 1938, originally in
uranium. Ordinary uranium poses no explosive danger, though; for a bomb
you need a highly-enriched recipe containing mostly a form found only in
traces in uranium ore. Or you could use plutonium, an element rare in
nature but copiously produced in nuclear reactors.
Worldwide, about 1,000 reactors operate nowadays, more than 400 to
produce electric power, with most of the others used for research. It's
possible to extract the plutonium produced in either reactor type and
use it to build a nuclear explosive.
International treaties have attempted to prohibit any such plutonium
diversion. Those treaties call for safeguards including video monitoring
and occasional inspections. But nobody really thinks the system is
foolproof.
"I think it's well-known in that community that you can't trust the
monitoring very well," says Giorgio Gratta, a physicist at Stanford
University.
But you can trust neutrinos.
Seven decades ago, the Austrian physicist Wolfgang Pauli suspected that
some radioactive substances emitted an invisible particle along with
other particles easier to detect. By the 1950s, other physicists figured
out how to record the presence of Pauli's particle, by then known as the
neutrino.
Neutrinos are famous for their ghostly properties - able to zip through
a wall of lead a light-year thick, so light as to be nearly massless, so
fast as to be nearly as fast as light itself. But scientific
neutrinobusters can apprehend the elusive particles anyway - if they are
present in sufficient abundance.
And nuclear reactors are the most prolific neutrino factories on the
planet. When a uranium or plutonium atom splits, the fragments are just
the sort of radioactive substances that emit neutrinos like crazy.
(Actually, we're talking about antineutrinos here, but let's keep it
simple.)
What's more, the neutrinos zip out of the reactor core carrying
different amounts of energy, depending on what atoms made them.
Measuring the energy of neutrinos flying from a reactor, therefore, can
tell scientists what substances the reactor is hiding inside, Dr. Gratta
explained last week at a seminar sponsored by the Council for the
Advancement of Science Writing.
Dr. Gratta and Yifang Wang of Stanford have collaborated with Adam
Bernstein and Todd West of Sandia National Laboratories in Livermore,
Calif., to calculate just how effective neutrino monitoring of a reactor
could be. (Don't tell O.B.L., but you can read their paper at
xxx.lanl.gov/abs/nucl-ex/0108001.)
In most cases, it would be possible to detect the extraction of
plutonium from a reactor or even to tell whether the reactor fuel had
been altered to produce plutonium more rapidly.
Unlike ordinary inspections, which might take place only twice a year,
when the reactor was turned off, neutrino monitoring could go on
constantly.
"You could have it on the Internet," says Dr. Gratta. "You're looking in
real time at what's happening inside the core of the reactor."
All it takes is connecting computers to a cube about 3 feet on each side
filled with the proper organic solvents, along with detectors to record
the flashes of light created by neutrinos striking hydrogen atoms in the
solvent. (These collisions also produce neutrons, so you have to measure
them, too.) Computers can analyze the collisions to determine the
neutrinos' energy.
This plan is about to be tested at the San Onofre reactor in California,
where the detecting cube will be positioned in a service area about 80
feet from the core. The cube should record nearly 3,000 neutrino hits a
day.
Of course, to know whether anybody is messing with the reactor, you have
to know what the energy of those neutrinos is supposed to be in the
first place. So this system works only because scientists have been
playing around with neutrinos for decades - not for homeland security,
but for understanding the particles that nature is made of, how the sun
shines, and how the universe works. It's the kind of physics that
politicians often criticize as not sufficiently relevant to daily life
to be worth a lot of funding. Knowledge and security, though, are both
precious to society, and it's unwise to demand one without the other.
