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By Steven Profaizer
Sun Staff
The dream of unlocking neutrinos’ secrets has kept
scientists tossing and turning since the subatomic particles
were first theorized in the 1930s. The vision has been
there, but the technology to materialize that vision has
not.
Three teams of scientists are now preparing to use Antarctic
ice as the key to open the strongbox that has contained
extraterrestrial neutrinos’ mysteries since the early
universe — even though no one is sure what they will
reveal.
Strange but true
Neutrinos are smaller than a single atom. They have no
charge and very little mass. Magnetic fields have no effect
on them. Gravity’s influence on them is almost nonexistent.
Neutrinos fly across the universe, slipping between gaps
in the atoms of all matter, indifferent to the worlds
around them.
A star, a distant planet, a human — the particles
pass right through matter as they continue in virtually
straight lines from their points of creation. Neutrinos
saturate our universe, flying in all directions to no
particular destination, and the more scientists learn
about them, the stranger they seem to be.
“They exist in a way that’s almost otherworldly,”
said Peter Gorham, principal investigator of the neutrino
detector ANITA (ANtarctic Impulsive Transient Antennae).
“They have very little interaction with matter as
we know it. They don’t affect anything that we do
day to day. In some ways, they are the particle of the
mystic. Physicists know they exist and can make precision
measurements of their characteristics, but everything
about them is strange and unexpected.”
One of neutrinos’ bizarre truths is that despite
the lack of effect that matter has on them, they exert
an enormous amount of influence on matter.
“In the early universe, before there were any elements,
neutrinos dominated for some time,” Gorham said.
“During that time is when all the elements were formed.
And without that neutrino soup in the very early part
of the universe, we would not have the elements we have
now. We could not. They are absolutely, inextricably tied
into chemistry, physics and the physical elements. Those
things could not exist without neutrinos.”
To see the unseen
Neutrinos’ huge importance and vast numbers have
not, however, provided an easy way to study the particles.
But there is one interaction that has given scientists
an opportunity to observe them indirectly.
A neutrino is so small that it slips through the gaps
in matter, but no one is at the wheel. The fact that they
don’t often run into other subatomic particles shows
how small they are, not that they somehow zigzag through
an atomic obstacle course.
Every once in a while, a neutrino happens to miss the
gap and plows directly into a piece of an atom, destroying
the neutrino and producing a reaction that scientists
can detect. The collision creates a cone of visible light
and a radio pulse that expands from the point of collision
and continues in the direction the neutrino was headed.
Scientists can use the information gathered by the observation
of these reactions to learn neutrinos’ direction
of origin, speed and energy.
Researchers need to make their observations in a medium
that is effective at transmitting the radio and light,
and they have found the best substance to be ice —
something of which Antarctica happens to have the world’s
largest supply.
The three current neutrino projects plan to look at either
the radio or light signature to study neutrinos’
origins and physics. But despite studying the same particle,
the groups go about it in very different ways.
In the ice
The IceCube neutrino observatory at the South Pole is
about to enter its third season of construction. Eighty
strands of sensors buried vertically in the ice will make
up the array. The sensors are called photomultipliers
and detect the light created by the neutrino collisions.
When a neutrino hits an atom that makes up the ice, it
destroys itself, but the collision creates a negatively
charged particle called a muon. The muon then continues
in the same direction the neutrino was headed. And as
it travels through the ice, faster than light would through
the same substance, it produces a cone of blue light,
which is called Cherenkov radiation.
Scientists have found the ice under the South Pole to
be the ideal location to observe the effect. The South
Pole lies on the Polar Plateau, which is covered by three
kilometers of ice. The landscape of snow is flat, like
an icy, white sheet pulled taut against the Earth. The
ice that lies below the surface provides the vast, dark
and transparent background that scientists need to study
the neutrinos’ light show.
Construction crews have installed nine strands so far.
Each string of 60 sensors requires a hole be drilled so
crews can lower it almost two kilometers down into the
ice. When complete, the array will consist of 4,800 sensors
and have a volume of one cubic kilometer. The infrequency
of the events IceCube scientists study means the array
must be that large in order to be truly effective, said
Francis Halzen, principal investigator for the project.
IceCube works by registering the exact time each equally
spaced sensor gets hit by the collision-produced light
— down to three billionths of a second. The scientists
can then combine the data from each sensor to create the
data set they need.
“Now it’s up to nature to deliver; it’s
out of our hands,” Halzen said.
The team expects to install 12 more strands this season
and will have the material on hand to complete two additional
strands if they get ahead of schedule, Halzen said.
Next year, the group expects to finish 14 to 16 strands.
Construction estimates are more conservative this year
due to the effort to move IceCube operations out of its
current, temporary laboratory, he said. The new facility
will serve as IceCube’s permanent nerve center, where
scientists can monitor data and calibrate the array. The
relocation effort will require the nine previously installed
strands to be disconnected from the system and powered
down.
“[Moving to the new lab] is something we’re
very concerned about, but it has to happen,” Halzen
said. “The experimentalists tell me that this is
OK, that there’s nothing to worry about. We’ve
turned off strings before and brought them back to life,
but some of these strings will be off for more than a
week, so it’s a bit scary.”
He said the key to completing this season’s aggressive
agenda is to start on time. Last season, the team lost
several drilling weeks while making improvements to the
drill system.
“This year we just have some fine tuning,” Halzen
said. “Last season we were doing major revamping
to some of the equipment.
“After this [season], we hope it just becomes routine
construction. We’ll have to wait and see. I guess
nothing is ever routine in Antarctica.”
On the ice
About 80 kilometers away from McMurdo Station this season,
another science team will test its theory for a massive
neutrino detector near Minna Bluff on the Ross Ice Shelf.
Unlike IceCube, this array would try to detect the radio
pulse, instead of the light, created by neutrino collisions
with the ice.
- Antarctic
Sun -
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