Figure 1: MCP Detector
An MCP (microchannel plate) detector (Figure 1) is an instrument for spatially resolved detection of events such as impacts from particles or photons. When an event is detected, the MCP detector outputs a pulse of electrons that can be recorded electronically or visualized on a phosphor screen. MCP detectors can detect particles such as electrons or ions. They can also detect photons of electromagnetic radiation with at least as much energy as ultraviolet light, such as X-rays. A pulse of electrons is more user-friendly for an experimenter than a single particle or photon. A phosphor screen is just a means of converting particle energy into light. The visual output of a phosphor screen can be viewed by eye or recorded electronically. An MCP detector requires a vacuum to operate so that the output electrons or the input particles are not impeded by the molecules of the air.
At the heart of the MCP detector is the MCP itself. An MCP looks like a thin, glassy wafer, typically dark in colour and circular. There is an iridescent effect across its faces due to its microstructure. The plate is a bundle of tiny, parallel, hollow glass tubes, side-by-side and fused together. These are the microchannels. Their internal diameter is around 10 µm and they are all at a slight angle relative to the flat faces of the plate. The manufacturing process involves repeated heating and drawing (stretching) of bundles of glass tubes that are originally macroscopic in size. The slight angle means that a particle meeting the MCP at or near normal incidence is likely to hit the wall of one of the microchannels. In doing so, several electrons are released from the wall. These are acclelerated through the microchannel by an applied electric field and each can collide again with the wall, liberating more electrons. This cascade effect continues until the pulse of electrons is released from the downstream end of the microchannel. A single input particle or photon might be amplified to 10 000 electrons at the output. Two or three MCPs can be stacked together for greater gain, but this leads to diminishing returns as the final output MCP cannot supply any more electrons and operates in a state of saturation. Operating at saturation can usefully provide a relatively consistent output pulse regardless of the conditions of the input event.
The detector consists of one or more MCPs and a means of detecting the output pulses. The pulse can be collected by a simple metal anode, but this loses spatial information that might be an important aspect of the detection. An array of anodes preserves some of the spatial information, but more often a phosphor screen is used to visualize the information. There must be an electric field gradient from the input face of the MCP to the output screen, going from negative to positive. Around 1 kV is applied between the faces of the MCP and the MCP output is held several kV more negative than the screen. The high voltages must be applied slowly to avoid damaging the MCPs through outgassing and arcing.
Figure 2: A rubber sheet depiction in SIMION 8.1 package of the electrostatic potential field between electrodes, with electron trajectories superimposed. The outermost electrodes near each end are at high negative potential relative to the other electrode.
To the cascading electrons, each carrying a negative charge, this field is like a hill to roll down from input to output. This can be a more useful analogy than being ‘attracted’ to things. Indeed, so-called rubber sheet visualizations are a useful tool for understanding charged particle trajectories (Figure 2); in the past actual rubber sheets and balls have been used to model field phenomena.
Generally, some part of this potential chain will be held at zero, that is, grounded, relative to the rest. To an electron, it does not matter which part of the detector is at ground. An electron will roll down a potential gradient from -5 kV to zero as happily as it will roll from zero to +5 kV. Grounding the MCP input face might be useful if your experiment involves detection of positive ions which might otherwise be repelled. Grounding at the screen might be convenient for safely mounting the detector to the vacuum chamber chassis.
A phosphor screen converts the energy of the output electrons to light. It is typically a clear glass disc with a thin coating of phosphor grains on the vacuum side. The phosphor might be a chemical such as a metal sulfide doped with small quantities of a different metal. Phosphors are typically identified by a P number designation. Characteristics of different phosphors include the colour of the emitted light, the brightness and the time taken to relax back to an off state (decay time). P20 glows brightly and is relatively easy to apply, but it contains cadmium and must be handled appropriately. P46 is a phosphor with a very short decay time. Construction of phosphor screens at TSL is accomplished in multiple stages. The phosphor itself is applied by a delicate technique known as brushing. The phosphor is typically applied so that the coating is a few grains thick. This thickness, along with the grain size of the phosphor, affects the resolution of the image.
Figure 3: A schematic depicting a cross-section through a phosphor layer with an aluminium coating applied (hν is shorthand for a photon).
To prevent the screen from accumulating electric charge, a conductive layer is applied. This might be a transparent indium tin oxide (ITO) layer beneath the phosphor, applied by TSL using a coating process known as reactive sputtering. Alternatively, we can coat a thin film of aluminium over the phosphor. The aluminium is applied so that it drapes across the phosphor grains rather than penetrating between them (Figures 3 and 4). This preserves its reflectivity so that stray light emitted in a direction back into the detector is reflected back towards the user.
Figure 4: SEM image of an aluminium coating overlying a layer of phosphor. The phosphor is visible through the damage to the aluminium. The row of dots beneath the image is the 30 µm scale bar.
The image produced on the screen by the electrons might be viewed by eye. That grainy green night-vision footage that you see on TV looks that way because it is on a phosphor screen. The screen is typically very close to the MCP to preserve the fidelity of the image (proximity focus). The image might be collected digitally, for example by a CMOS or CCD detector (as in a digital camera). Applying the phosphor to a fibre-optic plate rather than a glass disc is a convenient (but more expensive) way to transfer the image directly to the air-side surface of the screen assembly. The detector can be attached direct to the screen output with no further focusing required.
To sum up, an MCP detector turns a particle or photon strike into a pulse of electrons. An electron pulse is easier to process than a single particle or photon. The pulse can be recorded electronically or visualized on a phosphor screen. The design of the MCP means that spatial information about the event is preserved, so a detector with a phosphor screen can be thought of as an imaging detector.
