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).

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 Hi Resolution of OM strings

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

AMANDA terminology AMANDA-A:  4 strings buried at depths between 800-1000m.  Ice is filled with air bubbles, so only suitable for cascade detection and ice property studies. AMANDA-B10:  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. AMANDA-II:  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. The famous Superkamiokande detector (Super-K) is shown on the same scale.

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.

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 South Pole.

Barwick Group
School of Physical Sciences
University of California Irvine