Seeing the Neutrinos
As mentioned before, there are 4800 Digital Optical Modules (DOMs) that will make up the optical array in IceCube. There are many components that make up the DOM.
The first is the pressure sphere, which contains all of the other components. This sphere must withstand tremendous pressure, and the similar spheres from AMANDA will be employed. By special request, the manufacturer was able to change the chemical content of the glass, since a normal isotope in glass produced Cherenkov light when it decays. These spheres are 13 inches in diameter, and while a larger sphere could hold a larger photosensor, there would also be a larger hold required in the ice to bury the equipment, and the cost of drilling and deployment would quickly rise. These spheres cost about $350 a piece, for a total estimated cost of $1,680,000.
The photomultiplier is housed inside of the pressure sphere and detects the Cherenkov light. This is a hemisphere about 10 inches in diameter. These highly sensitive detectors will amplify a signal about 300 times. So the sensor won't move, they are encased in a gel. The properties of the gel will protect the sensor, without appreciably interfering with the Cherenkov light.
An additional component is a metal cage or grate. The purpose of this is to shield the DOM components from the Earth's magnetic field. Although the DOM itself would not be damaged by it, the field could deflect or disrupt the process going on. In order to amplify the signal from the phototube, electrons are bounced through a series of dynodes, and it is critical that these electrons are moving together and in a very precise direction, hence he need for magnetic shielding.
The other components housed in the pressure sphere have the jobs of digitizing the signal, putting a time stamp on it, and sending the signal to the surface. Overall, the time accuracy is in the < 10 ns range, however, there is a slight time difference in transmitting the signal from the closest DOM to the furthest away. The signals are ultimately synchronized to a global time system, so they can be studied. The digitization is an improvement over the majority of the AMANDA modules. The digital signals provide a more accurate and reliable data stream.