Investigation of the Frisch-grid inefficiency by means of wave-form digitization

Abstract: Frisch grid ionization chambers are commonly used experimental tools for charged particle spectroscopy. In the ideal Frisch grid chamber the anode pulse height is independent of where inside the sensitive volume the charge has been created. This ideal cannot be realized because of imperfect shielding of the anode by the grid. The effect of the imperfect shielding is generally referred to as grid inefficiency. For accurate energy determination the anode pulse height needs to be corrected for this. At present two opposing explanation for grid inefficiency exist. The first explanation suggests that there is a reduction of the anode pulse height. This is said to arise from positive ions inducing charge on the anode as the electrons are collected. The second explanation suggests that there is a too large anode signal. The addition to the anode signal is said to arise from the drift of electrons. In this thesis the concept of grid inefficiency is investigated by means of wave form digitization. The use of digital signal processing makes it possible to maintain information on the drift of electrons. This information is lost in charged particle spectroscopy experiments using electronic signal processing networks. A series of experiments is described in this thesis. The first experiment was performed to find good measuring conditions for the following experiments. For this purpose the drift velocity of electrons was measured in two chamber filling gases, P-10 and CF4. The measured drift velocities are presented for the two gases. Finally, P-10 was chosen as filling gas for the following experiments. In the second experiment the grid inefficiency was measured for two different types of shielding electrodes. The method of determining the grid inefficiency is based on the analysis of the shape of digitized charge signals. The measured values are shown to be in good agreement with calculated values. In the final experiment the effects of grid inefficiency on alpha particle spectroscopy is investigated. It is shown how the correction for grid inefficiency by the two existing models yield equivalent results for energy determination. An attempt to separate the two models is also presented indicating that there is in fact a reduction of the anode pulse height because of grid inefficiency. The thesis is concluded with a theoretical discussion of the anode pulse shape. There grid inefficiency is explained by the drift of electrons. It is shown in this section how explaining grid inefficiency by the drift of electrons should yield the same result as explaining it by the effect of positive ions.