Microchannel plate (MCP)
A thin glass plate perforated by millions of parallel channels, each 2–25 µm wide, that act as continuous electron multipliers while preserving the position of the input event. One plate at about 1 kV gives a gain of 10³–10⁴; two in a chevron stack give 10⁶–10⁷, with timing spread of a few tens of picoseconds.
A microchannel plate (MCP) is an electron multiplier made from a disk of lead glass, typically 0.4–1 mm thick, containing millions of parallel channels with diameters from about 2 to 25 µm. The inner channel walls are treated to be slightly conducting and to emit secondary electrons, and both faces carry metal electrodes with roughly 1 kV between them. An electron, ion, or energetic photon striking a channel wall near the input releases secondary electrons, which the field accelerates down the channel into the wall again, and the cascade repeats along the full length. Each channel works as a tiny continuous-dynode photomultiplier; because the channels are independent, the electron cloud leaves the plate at the same position the event arrived. MCPs therefore amplify images as well as single events, with gains of 10³–10⁴ per plate.
Structure and gain
The gain of a channel depends on the applied voltage and on the ratio of channel length to diameter, , and not on the absolute size, so plates with very small channels work as well as large ones. Typical ratios are 40:1 to 60:1; a 10 µm channel at 60:1 is 600 µm long. Channels are tilted by a bias angle of about 8–13° to the plate normal so that incoming particles cannot pass straight through without striking a wall.
Plates are stacked to reach higher gain:
| Configuration | Plates | Typical gain |
|---|---|---|
| Single | 1 | 10³–10⁴ |
| Chevron (V-stack) | 2 | 10⁶–10⁷ |
| Z-stack | 3 | 10⁷–10⁸ |
In a chevron stack the two plates' bias angles oppose each other. This also blocks ion feedback: positive ions created in the high-field output end of a channel drift backward, and in a single straight channel they would reach the input and start spurious secondary pulses.
At high gain the end of each channel saturates through space charge. The pulse-height distribution of a single plate is broad and close to exponential, while a saturated chevron gives a peaked distribution, which makes discrimination between real events and noise in photon counting simple.
The fraction of the input face that is channel opening, the open-area ratio, sets the detection efficiency for particles hitting a bare plate. For circular channels on a hexagonal pitch ,
so 10 µm channels on a 12 µm pitch give 63%. Funneled input surfaces raise the ratio further.
Image intensifiers and gated cameras
An image intensifier places a photocathode on the input window, an MCP a fraction of a millimeter behind it, and a phosphor screen just behind the MCP. A photon releases a photoelectron through the photoelectric effect, the MCP multiplies it, and the electron cloud lights a spot on the phosphor, which is viewed by eye or by a camera. Night-vision devices use multialkali (second generation) or GaAs (third generation) photocathodes. Pulsing the cathode voltage turns the intensifier on for a few nanoseconds or less, the basis of gated intensified cameras for time-resolved imaging and spectroscopy. The streak camera uses an MCP in the same way to amplify the swept photoelectron image before its phosphor.
Bare MCPs, open to vacuum, detect ions, electrons, extreme-ultraviolet light and X-rays directly. They are standard in time-of-flight mass spectrometers and in the XUV spectrometers used for high-harmonic generation. A phosphor and camera or a position-sensitive anode records where each event landed.
MCP-PMT timing
In an MCP photomultiplier the dynode chain of a conventional tube is replaced by a chevron MCP close to the photocathode. The electrons travel a very short distance through uniform fields, so the transit-time spread is a few tens of picoseconds, compared with a few hundred picoseconds for a dynode tube. This low timing jitter makes MCP-PMTs a detector of choice for time-correlated single-photon counting, fluorescence-lifetime and time-of-flight measurements. They are also much less sensitive to magnetic fields than dynode tubes, particularly fields along the tube axis.
Pitfalls
- Vacuum and contamination. MCPs operate only in high vacuum, better than about 10⁻⁴ Pa, and must be conditioned after exposure to air; moisture and hydrocarbons degrade gain and cause discharges.
- Lifetime. Gain falls with the total charge extracted per unit area, so heavily illuminated regions age faster and show as burned-in patterns.
- Local count-rate limit. A channel recharges through the high resistance of its wall in milliseconds, so a bright spot saturates locally even when the total rate is modest.
- Dark counts and afterpulses. Field emission, residual radioactivity in the glass and ion feedback produce dark counts that matter in long exposures and weak-signal imaging.
Common questions
How does a microchannel plate differ from a photomultiplier tube?
A PMT multiplies electrons through 8–12 discrete dynodes and gives one output for the whole cathode. An MCP multiplies within millions of parallel channels, keeping the spatial position of each event and giving a shorter, more uniform electron path, hence better timing and imaging at the cost of lower maximum count rate and shorter life.
Does an MCP detect photons directly?
Visible photons do not release electrons from the channel glass, so for visible and near-infrared light an MCP is always paired with a photocathode. Ultraviolet, XUV and X-ray photons can release photoelectrons from the channel walls directly, and coatings such as CsI raise that efficiency.
References: J. L. Wiza, "Microchannel plate detectors," Nucl. Instrum. Methods 162, 587 (1979); W. Becker, Advanced Time-Correlated Single Photon Counting Techniques (Springer, 2005); G. F. Knoll, Radiation Detection and Measurement, 4th ed. (Wiley, 2010).