Abstract
The subject of space charge in ionization detectors is reviewed, showing how the observations and the formalism used to describe the effects have evolved, starting with applications to calorimeters and reaching recent, large time-projection chambers. General scaling laws, and different ways to present and model the effects are presented. The relations between space-charge effects and the boundary conditions imposed on the side faces of the detector are discussed, together with a design solution that mitigates some of the effects. The implications of the relative size of drift length and transverse detector size are illustrated. Calibration methods are briefly discussed.
Highlights
The term space charge refers to the underlying distribution of charge, usually positive ions, reaching a level capable of significantly affecting the electric field established by the voltage on the electrodes of an electronic device
Ionization detectors without gas multiplication can be affected by space charge as well, if the time necessary to dispose of the positive ions becomes long enough to cause significant accumulation of positive charge density
This situation occurs in detectors based on dense media, such as liquid argon or krypton, used in calorimetry for high radiation intensity applications, or for large time projection chambers (TPCs), where even low levels of radiation, together with long drift paths, can cause significant space-charge effects
Summary
The term space charge refers to the underlying distribution of charge, usually positive ions, reaching a level capable of significantly affecting the electric field established by the voltage on the electrodes of an electronic device. The change in an electric field affects the motion of the main charge carriers, usually electrons, and changes the signal being read out This subject was discussed first in the context of gas diodes [1,2,3], before becoming relevant for particle detectors, namely, in the context of drift chambers with long collection times [4,5]. Ionization detectors without gas multiplication can be affected by space charge as well, if the time necessary to dispose of the positive ions becomes long enough to cause significant accumulation of positive charge density This situation occurs in detectors based on dense media, such as liquid argon or krypton, used in calorimetry for high radiation intensity applications, or for large time projection chambers (TPCs), where even low levels of radiation, together with long drift paths, can cause significant space-charge effects.
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