The Soudan Underground Laboratory is a
general-purpose science facility, which provides the deep
underground environment required by a variety of sensitive
experiments. The Lab currently hosts two large projects: MINOS,
which investigates elusive and poorly understood particles called
neutrinos; and CDMS II, a "dark-matter" experiment
which may help explain how galaxies are formed. Both were built
for basic research - exploring how the universe
works - but similar efforts have spawned practical (if
unforeseen) byproducts, including the world-wide web and even
advanced medical imaging techniques.
What is a neutrino?
A neutrino is a tiny particle similar to an electron but without
its electric charge. Neutrinos are produced by natural radioactive
decay and inside the sun and other stars. They
don't interact very often with normal matter, but
MINOS can still teach us how they behave, and how important they are
to the rest of the universe.
What is "dark matter"?
Actually, no one really knows! What we do
astronomical observations) is that there is a lot of dark matter
around - perhaps 90% or more of the material in the universe. We
can't see it, but we know it's there from the gravitational
force it exerts. Neutrinos may make up part of the dark matter, but
another possibility is very heavy (so-far undiscovered) particles
nicknamed "WIMPs" (Weakly-Interacting Massive Particles).
CDMS II searches for WIMPs that may have been produced shortly after
the,"Big Bang," the cataclysmic explosion that formed the universe
some 15 billion years ago.
Why is the Laboratory so far underground?
MINOS and CDMS II are extremely sensitive instruments searching for
particles that (at best) are very seldom seen. At ground level,
naturally occurring cosmic
rays strike the surface of the earth so often
they could completely mask the rare effects these experiments seek.
The half-mile or so of earth above the Lab blocks almost all these
cosmic rays, providing a much quieter research environment.
How big is the Laboratory?
The MINOS cavern is 82 meters (270') long, 15 meters
(50') wide, and 13 meters (40') high. The Soudan 2/CDMS II
cavern is similar in shape but only 70 meters (230') long. The
surrounding rock formation is not iron ore but actually Ely
Greenstone, about 2.7 billion years old. Nearly 100,000 tons were
excavated to build the lab - all hoisted to the surface, six
tons at a time. Some was used beneath the parking area west of the
Engine House, while the rest can be seen piled southeast of the
Where do neutrinos come from?
Most are made naturally, by cosmic rays and the sun, but MINOS
looks primarily for neutrinos from the Fermi National Accelerator
Laboratory (Fermilab). There, the "Main Injector" accelerator
can direct an intense beam of protons onto a special graphite target,
where they produce new particles called "pions" The pions are
pointed toward Soudan along a 675-meter (2,200') vacuum pipe (top
illustration), where they decay to make neutrinos. Other particles
are also created, but they are stopped at the end of the pipe by a
10-meter (33') thick steel "absorber" and 240 meters
(800') of solid rock. Because neutrinos don't interact very much,
virtually all simply pass straight through the absorber and rock, as
well as the 735 km (455 miles!) of earth between Fermilab and
What and where is Fermilab?
Fermilab was established in 1970 to house the highest-energy
particle accelerator ("atom smasher") in the world. Located in Batavia,
Illinois (40 miles west of Chicago), it is operated by a consortium
of more than ninety major universities who guide its mission to probe
the fundamental nature of matter and energy. In addition to MINOS,
Fermilab directs a major effort to discover the Higgs. Perhaps the last
missing piece in the particle physics puzzle, the Higgs is a key to
the secret of why some particles have mass, while others do not.
How big is MINOS?
The MINOS "Far Detector" at Soudan consists of two
identical "super-modules." Each is an octagon (a neutrino stop sign!)
8 meters (26') across, and 15 meters (48') long. Together they
will weigh more than 6,000 tons (about the same as a medium-sized
destroyer). The MINOS "Near Detector" at Fermilab is similar
but smaller, about 1,000 tons. The Near Detector samples the
beam at its source, providing a comparison for Far Detector results
How does the detector "see" neutrinos?
Most of the time, in fact, it doesn't! Neutrinos interact only
very weakly with ordinary matter, which is why they don't need a
tunnel to get from Fermilab to Soudan. The number of neutrinos in the
beam is very
large, however, so about once every two hours one
is unlucky enough to actually hit something as it passes through the
detector. Even then we don't see the neutrino
itself, but only the charged particles ejected from its
How does it all work?
When charged particles travel through the detector they create
light in the plastic scintillator. This is collected by special
optical fibers, which also change the color to green. The light
travels down the fiber to the edge of the detector, where it passes
to another fiber, and then to sensitive devices called
photomultipliers. These convert the light to electrical signals, from
which the original tracks can be reconstructed using sophisticated
What will MINOS tell us about neutrinos?
Neutrinos come in at least three different types: one paired with
the electron, another with its heavier cousin the "muon" and
the last with the even heavier "tau" Quantum mechanics, the physics
of particles, tells us that mass allows neutrinos to spontaneously
change from one type to another and back again (that is, to
"oscillate"). MINOS is designed to search for these
oscillations. Because neutrinos were long thought to be massless and
independent, a positive signal would be tremendously important. Any
masses are already known to be quite small, however, so the beam must
travel a long way (from Fermilab to Soudan) to give MINOS a chance to
see anything. Nevertheless previous experiments, studying
naturally-occurring neutrinos in Italy, Japan, and right next door in
the original Soudan 2 cavern, indicate that neutrino oscillations may
well occur, and have told MINOS how to look for them.
How do you tell the neutrinos apart?
Neutrino types can be determined from the particles they produce.
Electron-type neutrinos tend to make electrons, which create short
shower-like patterns in the
detector. Muon-type neutrinos make muons, which exhibit long straight
tracks. The tau-type is
difficult to observe directly, but could be seen statistically
as "too many of the wrong sort" of events. The beam contains
primarily one type of neutrino (muon-type). If neutrinos have no mass
it should look essentially the same here at Soudan as it did when it
left Fermilab. Any significant difference between the "Near" and
"Far" Detectors is the signature of neutrino oscillations
Why are neutrinos important?
Neutrino mass and neutrino oscillations could add considerably to our
knowledge of the fundamental interactions that govern the universe.
From the "Big Bang" some 15 billion years ago to the present, and at
all scales from the sub-atomic to the astrophysical and cosmological,
these interactions determine the fate of our universe. There are so
many neutrinos that even a small mass could make them an important
part of the "dark matter" not all of it, or even most of it, but
significant relative to the normal matter that stars and planets (and
people) are made of.
What does MINOS mean?
MINOS is the Main Injector Neutrino Oscillation Search: "Main
Injector" for the Fermilab accelerator, and "Oscillation Search" for
the theory that links neutrino mass to oscillations. In Greek
mythology Minos was the son of Zeus and Europa, King of Crete, and
builder of the labyrinth.
What's Next Door?
The other cavern is the original Soudan 2 Laboratory. The Soudan
2 detector is about the same size as the MINOS Near Detector (1,000
tons) and is also built primarily of steel, but employs a different
(gas-based) detector technology. Soudan 2 was built to test the
ultimate stability of matter by looking for
proton decay, and operated
between 1989 and 2001. While proton decay has not yet been observed,
Soudan 2 contributed to the initial evidence for neutrino
oscillations on which MINOS is based.
What's CDMS II?
The Soudan 2 cavern also houses the Cryogenic Dark Matter
Search (CDMS II). This experiment looks for the main component
of dark matter, which may be in the form of WIMPs (Weakly Interacting
Massive Particles). In contrast to neutrinos, which are light, fast,
and plentiful, WIMPs would be heavy, slow, and less common, and could
be even more difficult to detect.
How does CDMS II work?
The CDMS II detectors are hockey puck-sized disks of silicon and
germanium. A special cryogenic apparatus cools them to less than a
of a degree above absolute zero (-460 F, the coldest
place in Minnesota!). A WIMP passing through would deposit only a
tiny amount of energy in the detector, but it should be enough to
raise its temperature very slightly. The detector signals are
carefully recorded and analyzed by computers to distinguish trueWIMP
signals from random noise. Despite being under a half mile of rock,
scientists also use intricate shielding, carefully selected
materials, and a special clean room to further reduce unwanted
How was all this equipment brought underground?
Much like the proverbial ship in a bottle. Everything you see in
the Lab came down the same narrow mine shaft that brought you
underground. The hoist can handle equipment 1.3 m by 2 m by 10 m
long (a little over 4' by 6' by 33'), weighing up to six tons. Each
item in the laboratory was carefully designed to fit within these
limits. During excavation of the cavern a full-sized front-end loader
was brought underground in pieces, assembled, used for a year, and
brought back out again.
How are the detector planes put together?
Eight sheets of 0.5" steel are needed for each 1" thick octagon.
They are plug welded in the
open area directly below the Visitors's Gallery,
where the scintillator modules are also attached and tested. The
crane uses a special fixture called a strongback to lift the
assembled plane to vertical, and to its proper location in the
How long did it take to build the Laboratory?
The "Soudan 1" experiment began in 1981, using an existing cavern
on the 23rd
level of the mine. The Soudan 2 Detector
Lab was completed 1986, and MINOS Far Detector excavation began in
1999. The ceiling, walls, and floor were completed in 2000, with
outfitting--steel supports, electrical, communications, and other
systems--finished in July 2001. The first MINOS Far Detector plane
was installed at the end the same month. The CDMS II enclosures were
completed in spring 2002.
How much do the projects cost?
The overall cost of MINOS over a period of several years is about
$174 million, most spent on neutrino production facilities at
Fermilab. The MINOS Far Detector cost is about $32 million. The
cavern was approximately $7 million to excavate plus a similar amount
for steel supports and other outfitting. CDMS II will cost about $16
Who funds the projects?
The U.S. Department of Energy provides primary MINOS
support, with additional major contributions from the science funding
agency of the United Kingdom, the National Science Foundation, the
State of Minnesota, Research Corporation, and the member
institutions. CDMS II is funded by the National Science Foundation
and the Department of Energy.
How is the Laboratory related to the State Park?
U.S. Steel donated the Soudan site to the people of Minnesota in
1962. The Department of Natural Resources has administered it as a
State Park since then, operating and maintaining the hoist, pumps,
and electrical and other systems, and escorting approximately 40,000
visitors each year underground to the last working areas of the mine.
The University of Minnesota leases the Soudan Underground Laboratory
from the State, and operates it under contract with the Department of
Are there other labs like this?
During the past fifty years more than a dozen underground mines
and tunnels have been used for physics experiments. Major active
underground laboratories include Homestake (Lead, SD) Creighton
(Sudbury, Canada), Boulby (northeastern England), Gran Sasso (Italy),
Frejus (between Italy and France), Baksan (Russia), Kamioka (Japan),