Visualization of Muon tracks in the AMANDA detector

Jodi Lamoureux (LBNL)
&
Chris Spitzer (UCB)

April 2001

THE END OF THE STORY FIRST:
movie of light emitted from a muon traveling near the speed of light and passing through the AMANDA detector. The data is real and the reconstruction is from our model.
movie An exceptionally bright event from the AMANDA data, reconstructed from our model.

SOURCES OF LIGHT:

AMANDA uses the 2.8 kilometer thick South Pole icepack as a transparent detection medium for high energy particles. Down-going muon tracks are commonly the result of cosmic ray interactions in the atmosphere. Up-going muon tracks are much less common because the earth shields the detector from most particles. These up-going tracks are usually the result of cosmic ray interactions that produce neutrinos that subsequently decay to muons. At these energies muons can travel up to a few 10s of kilometers through ice.

LIGHT TRANSPORT:

Muons traveling faster than the speed of light in a medium produce a cone of radiation, known as Cerenkov radiation. This phenomenon is similar to the two dimensional wave from the front of a boat in water. Boats can easily travel faster than the natural speed of a wave in water, and when they do, they create a V-shaped bow wave. In a three dimensional medium, such as ice, this wave front is a cone.

movie of cerenkov cone.
In this movie, time is represented by color. Early times are yellow, and later times are blue. As the muon travels from the bottom of the screen to the top, the light radiated from the muon forms a light cone that travels with the muon. The AMANDA detector consists of about 300 photomultiplier tubes that sense the light at various positions. From the time that the sensor sees the light, the light cone of the muon can be reconstructed, and the track inferred.

Ice, however, is rarely a completely simple medium. Dust and bubbles can absorb or scatter light as it travels along its path. At the AMANDA depths, bubbles are pretty much compressed out, and dust is the main contributor. Kurt Woschnagg has used LEDs and laser light at different wavelengths to measure the amount of scattering and absorption in the icecap. These measurements are interesting because they map out volcanic activity in earth's history. The effect of scattering is to slow down the progress of the light cone, and to isotropize the light in the cone. We don't visualize the light isotropy, but the next movie shows what happens to the mean photon transport. The cone becomes bullet shaped as the light isotropizes..
movie of cerenkov cone + scattering

Absorption also changes the mean photon arrival time. The photomultipliers respond to the first photon that converts into an electron. If there are many photons, the probability that this happens is earlier than if there are just a few photons. As the light travels further, some photons become distributed over a larger area and some are absorbed. The parts of the light cone far from the muon track are thus further slowed by the counting statistics of the detector.
movie of cerenkov cone + scattering + npe

As mentioned earlier, volcanic activity produced dust that now resides in the ice. Ever since the last ice age, about 4 million years ago, small amounts of volcanic ash were leyered as the ice pack grew. The dust is modulated over 20-50 meter layers as the rate of volcanic activity changed on earth. The following movie shows the modulation of the light cone from the dust distribution measured at the South Pole.
movie of cerenkov cone + scattering + npe + depth

Finally, we modify the brightness of the cone to indicate visually the number of photons. Bright and opaque areas have more photons, and dim regions have 1 or fewer per meter. As the movie shows, photons do not travel forever but disappear as they are absorbed as they travel away from the track.
movie of cerenkov cone + scattering + npe + depth dependence+ attenuation

AMANDA DATA:

This movie shows the simulated light cone along with data from the AMANDA sensors. Sensors that detected photons are colored wth the time of the detection and a size proportional to the number of photons. The detector instruments 1 vertical kilometer per "string" of detectors. There are 10 strings separated by 60 meters. Detectors are separated by 15-20 meters. To set the scale, think BIG; the simulated volume is on order 100 million cubic meters.
movie of cerenkov cone + scattering + npe + depth dependence + attenuation + AMANDA data

The science from this movie is summarized in the following plot. The y-axis is the difference between the simulated time and the measured time. The x-axis is the distance traveled by the photons. The scatter is expected to be wider for 1 photo-electrons, and the more narrow for multiple photo-electrons. The pulses can be seen to congregate within the triangular regions defined by the RMS of simulated photons.
screen shot jpg: eview screen shot with residuals.

HIGH-MULTIPLICITY AMANDA EVENT:

One very large event was observed in the 1997 data sample. This particle traveled horizontally through the detector and produced photo-electrons in 152 phototubes, covering about 60 million cubic meters. This is the first analyzed high energy event.
movie of a high-multiplicity event.