Consider a cosmic ray muon, generated high in the atmosphere from the collision of an extremely high energy terrestrial bound proton with the rather sparse constituents of the upper atmosphere (ie. nitrogen or oxygen), on a collision course with the deep underground Soudan 2 detector. Upon traversing the atmosphere, and then the dense earth rock, the muon suffers energy losses induced by the electromagnetic interactions between it and the medium it crosses. If the muon originally resulted from a collision that imparted to it an energy of about 1 trillion electron volts, then on average, it possesses enough energy to penetrate to the depth of the Soudan 2 detector. As it turns out, we see muons in the detector at a rate of about 1 every 2 seconds.
The Soudan 2 detector, is comprised of 224 modules each weighing on average 4.5 tons, for a total weight of approximately 1000 tons. Each module has dimensions 2.7 meters tall by 1 meter wide and 1.1 meters deep. These dimensions were fundamentally dictated by two factors, (1) the expectation of the breadth of the "typical" proton decay event and (2) and more practically, the limitations on size and weight of an object that could be hoisted in the "cage", an elevator like contraption resembling a railroad car that transfers man and material down the one half mile shaft to the 27th level of the Soudan mine.
Each Soudan 2 module contains a 241 layer stack of 6 millimeter thick corrugated steel. Between each layer of steel are sheets of an insulating mylar ("bandolier") that have inserted, 14mm diameter hytrel tubes. The tubes are layed laterally, alternating between 31 or 32 per layer, and act as manifolds for a distribution of a pure concentration of Argon and CO2 within the entire free volume of the module. A plane of current sensitive anode wires and cathode pads (wireplane) are mounted at each end of the array of hytrel tubes. As the high energy muon penetrates the module, it ionizes the Argon-CO2 gas atoms that are in the proximity of the traversing muon. As a result of the ionization, electrons are freed. Once the electrons are free, they are forced to drift under the influence of high voltage copper electrode strips extruded into the bandolier. Their orientation relative to the drift tubes is perpendicular and vary in voltage (0 to -9000 volts) so that the electrons drift at a constant velocity down the hytrel tubes towards the sides of the module where the plane of 50 micro-meter anode wires are held at a large positive voltage (2300 volts). The current on the anode wires is monitored via sensitive electronic circuitry. As the cascade of negative charge is absorbed onto a particular positively charged anode wire, a mirrored charge is induced on a horizontal cathode pad that also has sensitive current monitoring devices connected. These two signals constitute two of the three spatial dimensions required for computer reconstructions. The third spatial dimension is provided by a calculation of drift time that is dependent on variables such as the concentration of gas and the ambient atmospheric pressure. When sufficient activity occurs on a contiguous set of anode wires and cathode pads (trigger), the activity in the entire detector is captured in the buffers of a large array of computers and passed down a "CAMAC highway" (physical connections to larger process oriented computers) for online processing. Upon a computer assisted reconstruction program (STING), the characteristic rectilinear trajectory of a high energy muon can be seen.