what makes AMANDA unique?
Well, for over 400 years, ever since the time that Galileo manufactured
the first useful telescope, astronomers have relied on light to
convey information from the deep recesses of space.
Neutrinos are new messengers from distant objects
- we have to exploit every one of the precious few information
carriers that nature affords us.
All telescopes, whether they are radio telescopes, microwave
telescopes, infrared telescopes, x-ray telescopes, gamma-ray
telescopes, and of course optical telescopes like the 10m Keck
Telescope in Hawaii collect electromagnetic waves (or "light").
AMANDA breaks from this honorable tradition. It is designed
to detect high energy NEUTRINOS,
a ghostly particle with little mass and even less inclination
to socialize. The latter property makes it almost ideal
as a particle messenger. Once produced, the travel path
of a neutrino will be unaffected by galactic magnetic fields,
intergalactic material such as dust , or cosmic microwave background
light. In fact, virtually nothing can prevent a neutrino
from traveling to the AMANDA detector at the South Pole. This
is quite a contrast with electromagnetic waves such a light.
At high enough energies, the universe is opaque to light!
For example, a particle of light with energies in excess of
100 TeV cannot reach us from even the nearest galaxies outside
the Milky Way.
Neutrinos carray information from the central engines of spectacularly
powerful objects in space. For example, enormous black holes are
thought to power the output of active galactic nuclei (AGN), but precise
knowledge of the physics mechanisms remain elusive. One concept is shown
next. A supermassive black hole is surrounded by a donut of material
which feeds the powerful jets of accelerated particles (yellow cones).
So how does
The neutrino cannot be detected directly by AMANDA (nor any other
detector for that matter). Rather we rely on a surrogate particle
called a "muon".
- As mentioned above neutrinos
do not interact much with ordinary materials like rock or
ice. But every once in while, they will collide with
the nucleus of an oxygen atom in the ice. The collision
will not only destroy the oxygen nucleus (much like a car
crash), but transform the neutrino into another particle,
called a muon. The muon is like a heavy electron.
It carries electric charge, which makes it relatively easy
to detect. Since it is heavy, it can travel for a long
distance before it runs out of energy. At the energies
relevant to AMANDA, the muon can travel 5-10 miles before
it "runs out of gas". This gives AMANDA an enormous advantage
- the size of the detector is not limited to the instrumented
volume of the ice because the neutrino can be detected even
if it interacts 5-10 miles away!
- The amazing thing about the collision
is that the direction of the muon will be nearly the same
as the neutrino that created it (to within a half degree).
Therefore, by measuring the direction of the muon, which is
relatively easy to do, we know the direction of the neutrino.
- Because the muon is electrically
charged, it will generate a special kind of purplish light
called "Cherenkov radiation". This light is only observable
in transparent materials when a charged particle is traveling
in excess of the speed of light (in that material,not vacuum
of course!). Therefore, it is the optical equivalent
of a sonic boom. It is this feable light that is detected
by AMANDA sensors.
The next figure shows the process in a bit more detail. The Cherekov
light is emitted at about 45 degrees to the direction of travel.
By measuring the time that the Cherenkov light arrives at each sensor (called
an optical module, or OM), we can deduce the direction of the muon. The
the graphic below, the middle OM of the rightmost string will see light
first, then the bottom OM, and so on. The OMs in AMANDA measure the
time that the photon strikes the OM with great precision.
Hi Resolution of AMANDA OM
|A few more bits of information are required to fully understand
the design. First, a neutrino is not the only particle that
can create a muon. In fact, it is downright easy for ordinary
cosmic rays to make muons when they collide with the atmosphere
of the earth. The muons from cosmic ray collisions are are
more common than muons from neutrinos, so how do we tell the difference?
We use the trick of looking through the center of the earth (in
other words, AMANDA looks down instead of up like ordinary telescopes!).
Neutrinos are the only particles we know that can penetrate the
earth. In astronomy terms, our "dark sky" is beneath AMANDA.
Anyway, neutrinos are the only particle capable of producing a
muon with a direction that comes from below the horizon of the
detector. So if the direction of the particle is upward,
we know it was produced by a neutrino that interacted within a
few miles of the detector.
The blizzard of muons from cosmic ray collisions with the atmosphere
can be reduced in intensity by going underground. That
is one of the reasons
why AMANDA is buried so deep (the other reason? Antarctic
ice only becomes transparent to Cherenkov light at depths greater
than 1400 meters).
The architecture of AMANDA was selected because of its simplicity
and reliability. In fact, the technologies used in the
AMANDA string, optical modules, and surface electronics were
chosen for their robustness against harsh environmental conditions.
The basic physics principles that guide the design of AMANDA
are not so different from those employed by many detectors in
high energy astrophysics, as the schematic shows
consists of 19 strings of optical modules
buried between 1 and 1.5 miles beneath the snow surface of the geographic
south pole. The total number of OMs in the array is 680, although
only about 600 are fully operational at any given moment. Cherenkov
light is converted to electrical signals by the photomultiplier tubes
within each OM.
4 strings buried at depths between 800-1000m. Ice is
filled with air bubbles, so only suitable for cascade detection
and ice property studies.
Inner 10 strings of 302 sensors. Completed in January, 1997.
Sensors buried at depths of 1.0-1.5 miles where the optic
properties of ice are suitable for muon detection and direction
reconstruction. The strings were inserted in the ice to form
a cylindrical shell of radius 60 meters.
The famous Superkamiokande detector
(Super-K) is shown on the same scale.
The composite of AMANDA-B10 plus an addition nine strings
(for a total of 19) of sensors. Three of the strings
have sensors that extend over 1000 meters in depth, from 1300m
to a depth of 2350m.
If you wish to learn more on how the AMANDA detector was constructed
at the geographic south pole and a walking tour of the Amundsen-Scott South
Pole station, then go here.
Physicists on AMANDA worked closely with the Polar Ice Coring Office (PICO)
to develop the machinery to drill holes in the ice to the necessary depths
and diameter. They had to brave bone-chilling conditions to
drill the holes and deploy the strings of sensors.
FUTURE: Who knows what
secrets will be uncovered as the wealth of data from AMANDA-II is processed
and analyzed? And the
of AMANDA-II looks even more exciting. The efficiency
of the detector will be improved by modifying the data
acquisition system to record the complete waveform from each optical
module. We plan to take steps to increase the on-time fraction to
90%, so data can be collected at an even more prodigious rate. These changes
will help AMANDA remain the pre-eminent detector for astrophysical neutrino
detection at the energy frontier.
Full waveform captured by prototype DAQ. The source of light
was an in situ nitrogen laser. The red vertical lines indicate the
photon arrival times collected by the standard AMANDA DAQ.
Notice that the waveforms contain far more information.
Electronics used to capture waveforms. This picture was taken at the