Detector

Microchannel Plate Operation

An MCP is made of a glass wafer that has a 50mm diameter (47mm active) and 1.5mm thickness. The “microchannels” are small pores in the glass 25μm in diameter with a 35μm center to center spacing. This arrangement gives the active diameter over a 50% open area ratio so that is a particle hits the MCP, it is more likely to hit a microchannel than not. The glass is coated so that up to 1200V can be applied across the glass. This potential difference creates an electric field within the microchannels.

A hit on a microchannel by an energetic charged particle causes electrons to be ejected from the side of the microchannel. These electrons are accelerated through the MCP by the electric field. The microchannels are a 7° angle to the surface of the detector. This incline makes it so that ejected electrons cannot escape the microchannel without hitting the channel wall. The Electric field is such that by the time an ejected electron has hit the channel wall, it has been accelerated to a sufficient energy to eject more electrons. The result of one electron causing the ejection of many electrons repeated over the channel length causes the signal amplification. To boost the gain, two MCPs are used in conjunction with each other. Their arrangement has the 7° angle of the microchannels pointed 180° apart. This configuration is referred to as a “chevron”.

Delay Line Detector Operation

A delay line detector is made of pairs of wires that form a grid. This research involved both two and three axis detectors. When an electric current hits a wire pair, the signal propagates toward each end of the wire. A timer is started once a signal arrives at one end of the wire - presuming the signal did not arrive in the middle of the detector in which case the signals would arrive at the same time. The time between the two signals arriving at the ends of the wires is translated to the position of signal origin.

There are two wires on each axis for the purpose of discerning background noise from the desired electric current from the MCP. This is accomplished by holding one wire to a voltage V, and the parallel wire to V+ΔV. This makes the electrons from the MCP prefer to hit the wire at a higher potential. The signal from the higher potential wire is subtracted from that of the other wire to give the desired signal.

For position imaging of single particle impacts, the two axis detectors sufficient since all that is required is an X and Y position. A problem arises when two particles hit the MCP – and two signals hit the DLD – simultaneously. The detector relies on time difference between signal receptions to position the particle impact on each axis. On a two axis detector, information is lost on one of the axes. The advantage of a three axis detector is that position information can be reconstructed for simultaneous hits due to redundant data. Even with information lost from one axis, the other two can be used to position X and Y.

Technical information from MCP Delay Line Detector Manual