Wednesday, December 9, 2009
Why are we doing this?
Since I have a few quiet moments now, I thought it would be a good idea to explain what we are trying to do with ARIANNA.
In the 19th century, scientists observed that the earths surface was permeated with a mysterious radiation which did things like discharge electrometers and cause certain materials (scintillators) to occasionally give off flashes of light. Noone knew where this radiation came from, but the prevailing assumption was that it came from the Earth. To test this, in 1911-1913, physicist Victor Hess made a series of balloon flights, eventually reaching 5300 m (about 17,000 feet). To his surprise, he found that the radiation increased as he ascended: the mysterious radiation came from space.
By the 1930's, scientist had learned that the radiation could cause Geiger Counters (simple electronic radiation detectors) to occasionally fire simultaneously, even if they are separated by distances of many miles; these simultaneous hits are called cosmic-ray air showers. We now know that these showers occur when an ultra-high energy cosmic-ray particle (a proton (hydrogen ion) or heavier nucleus, like iron) hits the top of the atmosphere and interacts with an oxygen or nitrogen atom. It converts it's kinetic energy into a large number of particles (remember E=mc^2); showers containing trillions of particles have been observed; This corresponds to an initial energy of about 3*10^20 electron Volts, roughly the energy of a well-hit tennis ball, or a boxers punch. The Auger collaboration has produced a nice animation of a shower being created, here. To put things in perspective, the highest energy cosmic rays have about 40 million times the energy of the protons accelerated at the LHC (and more if the cosmic rays are heavy ions), and it would require an accelerator 40 million times as large as the LHC to accelerate them. With current technology, one would need to build an accelerator around the sun to produce these particles. We would very much like to know where these cosmic accelerators are, and how they work.
However, despite decades of study, we do not know where these particles come from. They are electrically charged, so are bent in interstellar magnetic fields; even when we record their arrival direction, they do not point back to their sources. We don't even know if they are protons or heavier ions. Furthermore, the most energetic cosmic rays lose energy in transit, so the ones that we observe must come from the 'local' universe, within about 75 million parsecs (225 million light years) of Earth. This sounds like a long distance, but, on cosmic scales, it isn't very far.
Because of the bending and limited range, to learn more, we need another probe. Neutrinos are attractive, for several reasons. First, being electrically neutral, they are not deflected in-flight. Second, they interact weakly, so can easily escape from dense sources that would contain other cosmic rays.
The flip side of the weak interactions is a huge detector is needed to observe cosmic neutrinos. IceCube, shown below, now being built at the South Pole will be 1 cubic kilometer (about 0.6 miles on a side) in volume. This should be (but no guarantees) big enough for moderate energy neutrinos (from 10^8 to 10^17 electron volts). At higher energies, we do not expect many neutrinos, and need a larger detector. Probably, a volume of 100 cubic kilometers is required; this requires a new technology. Our goal for this winter/summer is to demonstrate this technology: radio detection of neutrinos.
In 1962, the Soviet-Armenian physicist Gurgan Askaryan pointed out that the particle showers produced by neutrino interactions will contain more electrons (which are negatively charged) than positrons (positively charged). These particles are all moving in a almost the same direction, producing an electrical current; in a dense medium, this current will emit radio waves in a cone. The strength of the radio waves scales as the square of the neutrino energy (the process is coherent, for physics experts), so we expect good signals for neutrinos with energies above about 10^17 electron volts.
A number of groups have worked on detecting neutrinos via the Askaryan effect. Collaborations have looked for neutrino interactions in the moon (using radio telescopes), the Greenland ice cap (looking down from a satellite), at the South Pole, and in underground "Salt Domes." Groups are also looking for acoustic radiation produced by neutrino interactions; the shower deposits energy in the target medium, producing a sound wave. These diverse methods have their plusses and minuses. The big advantage of ARIANNA (shared with the South Pole effort) is that it can detect neutrinos that are less energetic than the other approaches. Also, the ice-water interface below the 650 meter thick Ross Ice Shelf reflects radio waves. Because of this ARIANNA will be able to detect downward going neutrinos.
If our prototype works well, this will give us the basis to propose a larger detector, composed of many stations, which would be big enough to detect a good sample (maybe 100 events) of cosmic neutrinos, thereby solving at least some of the mysteries of ultra-high energy cosmic rays.
Posted by Spencer Klein at 7:57 PM