Efficiencies of the drift chambers in the EMMA experiment

Cosmic rays are high-energy subatomic particles which travel almost at the speed of light all over the space. The shape of the cosmic ray energy spectrum is measured experimentally, but it is not perfectly understood. The slope of the spectrum at high energies is constant up to the knee energy (about 10^15 eV) where the slope steepens. The knee has been tried to be explained by several models which aim to describe the origin and the acceleration mechanisms of the cosmic ray. The stars and the shockwaves from supernova explosions are believed to be at least a part of cosmic ray evolution. The cosmic rays were found in the early 20th century and they have been studied with several methods. When a primary cosmic particle collides with Earth atmosphere, different reactions create a cascade of secondary particles (air shower) which may be detected on Earth. The EMMA experiment studies the cosmic ray with the knee energy by detecting the muons of air shower. EMMA is operating underground at the depth of 75 meters in Pyhäsalmi mine. The measurement stations are reached only by the muons with 50 GeV thresold energy. The stations are consisted of drift chambers, scintillation detectors and limited streamer tubes. The gas-filled drift chambers form the basis of the experiment. Their operation is based on the gas ionization which causes signals on the electrical wires. One plank is formed of seven drift chambers attached together. In this work I study the efficiencies of the drift chambers of EMMA. My C++ program evaluates the efficiencies as a function of time and as a function of position by using data measured in calibration runs in the surface laboratory. According to my results the average ”mean top efficiency” (excluding the largest efficiency peaks) of the drift chambers is 76.5 %. The efficiency of the worst plank is 65.4 % and the best 88.1 %. The results’ systematical inaccuracy may be from the method of determining the efficiency and possible fault in data. The random inaccuracy may be from the problems of the measurement system and the method of calculating the mean top efficiency by excluding the efficiency peaks. My results seem to be slightly lower than the others’ results, thus my method of determining the efficiency may be stricter. Actually some efficiency peaks may be explained by the external factors like pressure changes and problems in electronics. The differencies of left and right part of a chamber may be due to the possible problems in grading lines.