Ionien takaisinvirtauksen variaation mittaus GEM-pohjautuvissa mittareissa.

In high energy physics the microscopic nature of our universe is studied. A common experimental way is to collide particles such as protons with almost the speed of light and study the fragments that fly away from the interaction point. The results are compared to existing theories and help to modify or create knowledge of our universe. However, our senses are not capable of directly measuring those tiny and fast fragments from the collisions. Therefore, we need dedicated devices called particle detectors. Several types of radiation detectors are known to us. Most of those utilize the fact that particles in such experiments are ionizing when traversing matter. The detectors we are interested in contain gas as the main interacting material for the ionizing radiation, called gaseous detectors. One of these is the Gas Electron Multiplier (GEM), which uses electric fields to make electrons drift through the holes of one or several foils producing a signal that can be detected. Gain and ion backflow are two properties that can be used to evaluate the effectiveness of the detector. The effect of the hole diameters of the foil on the gain and on the ion backflow is insufficiently known, however. In this thesis such measurements have been performed. For this purpose a GEM detector operating in proportional region was constructed. The detector contained a special foil with four quadrants in such a way that the diameters of the holes were different in each quadrant. The functionality of the detector was verified by measuring the leakage current, by using a multichannel analyzer, and by calculating the sum of all currents. The detector constructed passed all the tests. The diameters of the holes in the foil quadrants were estimated by calculating the amount of pixels from the foil image taken. The outer hole diameters were around 50 µm and inner hole diameters 40 µm in the quadrant with smallest holes. The corresponding diameters in the quadrant with largest holes were around 70 µm and 66 µm. The measurements were started by searching the proper values for the drift and the induction fields and for the GEM voltage range. A drift field of 2 kV/cm, an induction field of 7 kV/cm, and a voltage range from 400 V to 500 V were chosen. The results from the actual measurements were similar on both sides of the foil. The effective gain increased steadily along with the voltage being around 10-fold at 500 V compared to that at 400 V . The ion backflow, on the other hand, stayed constant or even slightly decreased. The results measured from the four quadrants differed clearly from each other. In the quadrant with the smallest holes the effective gain was about twice as high as in the quadrant with the largest holes. Respectively the ion backflow was about 30 % higher in the quadrant with small holes than in the quadrant with large holes