In this chapter, the authors discuss the unique ability of some micropattern detectors, in particular, GEM, MHSP, COBRA, and MICROMEGAS, to suppress the positive ion flow from the multiplication region of the detector back to the drift space. This effect is based on how electrons and ions move in these detectors. It will be shown that an optimized cascaded detector can so efficiently block the ion back flow that only 10-3% of the avalanche ions can penetrate back into the drift region. This feature makes these detectors attractive for applications such as photodetectors combined with highly efficient solid photocathodes or time projection chambers for tracking high fluxes of charged particles when the penetration of the avalanche ions back to the drift region may strongly disturb the detector operation. For example, in the case of photodetectors, the ions cause undesirable feedback preventing high gain operation necessary for single photoelectron detection. In the case of the time projection chambers, positive ions may disturb the uniformity of drift filed and thus affect the particle identification.
Top1. Introduction
Positive ions produced in the avalanche process will follow the electric field lines backwards, sometimes all the way back to the cathode of the drift region. When they interact and recombine at the cathode they may cause a number of problems. They can destroy the high efficient photo cathode by sputtering at low pressure or deposition of contaminants, emit new electrons that can trigger a second avalanche, produce photons that may trigger a new avalanche far away from the first one etc.
One of the interesting features of some micropattern detectors, in particular hole-type and MICROMEGAS, is their capability to suppress the number of positive ions penetrating from the avalanche region back into the drift space.
Recall that in classical gaseous detectors, such as single–wire proportional chambers, MWPC or a single-step PPAC, all positive ions created in the avalanches are collected on the cathode electrode. This could be undesirable in some applications. For example, the first gaseous detectors combined with photocathodes sensitive to visible light were based on a PPAC amplification structure and they suffered from feedback of photons from the ion recombination which prevented them from operating at high gas gains (Peskov, 1995).
The use of capillary plate multipliers for detection of visible photons, and later GEM and COBRA, enabled a significant reduction of the feedback process. The photon feedback is efficiently suppressed by the geometrical shielding of the visible light produced in the avalanche by walls of the holes. Also, ion feedback is suppressed due the partial collection of positive ions on the electrodes of the hole-type structure, so that only a small fraction of them can reach the photocathode and cause emission of secondary electrons (Peskov, 1999, Lyashenko, 2009).
Another relevant example is the time projection chamber (TPC) invented in 1974 by D. Nygren (1974).
The principle of the TPC is illustrated in Figure 1. The detector consists of a long drift region (often called a “field cage”) combined with an electron multiplication structure in each end serving as position-sensitive collection systems of electrons and ions. A highly uniform electric field is applied in the drift region, e.g. by letting the field cage consist of conducting rings surrounding the cylindrical shape volume whose potential is decreased in steps from the anode to the cathode. When a charged particle traverses this volume it ionizes the gas and liberate a track of electrons and ions along the trajectory. An external detector, e.g. a silicon detector, generates a start signal of when the incident particle enters the TPC and this signal is the start of the drift time measurements. The electrons from the track begin drifting towards the anode of the multiplication structure, whereas the ions drift more slowly towards the cathode electrode. By measuring the position of each arriving electron in the amplification structure one can determine the projection of the incident trajectory on this plane. By also measuring the arrival time of each individual electron one can calculate the axial (Z) coordinate of the origin of the electron. By combining the position and the time information of each electron, a 3-D image of the incident trajectory can be reconstructed. Similarly, the ions drifting in the opposite direction in some designs may give another 3D reconstruction of the incident particle trajectory.
Figure 1. The principle of a time projection chamber (TPC) (from O. Schäfer, http://www.lctpc.org/e8/e57671/). When a charged particle traverses the gas filled chamber it ionizes the gas a produce a track of electrons and ions. An electric field applied along the axis of the volume cause the electrons to drift towards the anode, where they are detected and the ions to drift towards the cathode. The arrival time, and the detected positions of the electrons, give a three dimensional localization of the incident particle trajectory.