MCP: Gain of Microchannel Plate

  • Definition and Function: MCPs are described as two-dimensional sensors that detect various particles and radiation (electrons, ions, VUV rays, X-rays, and gamma rays) and amplify the detected signals. This amplification is referred to as Gain.
  • Operating Principle for Gain: The process of achieving Gain involves establishing a potential gradient along the channels of the MCP by applying a voltage (VD) between the input and output sides. When an incident particle (like an electron) enters a channel and hits its inner wall, it causes the emission of multiple secondary electrons. These secondary electrons are accelerated by the potential gradient and travel in parabolic trajectories, repeatedly striking the channel wall and causing further secondary electron emission. This process continues, leading to an exponential increase in electrons as they move towards the output end of the channel, resulting in a large number of electrons being extracted from the output side.
  • Factors Determining Gain: The Gain of an MCP is determined by two main factors:
    • The ratio of the channel length (L) to the channel diameter (d), which is referred to as α (α=L/d).
    • The secondary emission factor inherent to the channel wall material. Standard MCPs are fabricated with an α value typically ranging from 40 to 60. The MCP thickness is determined by the required channel diameter and the design value of α.
  • Gain and Number of Stages: The number of MCP stages in an assembly can be selected to obtain the necessary Gain. The sources provide examples of typical Gain characteristics for 1-stage, 2-stage, and 3-stage MCPs, showing that increasing the number of stages generally increases the achievable Gain.
    • 1-stage MCPs can achieve a Gain of around 104 or 105 depending on the supply voltage.
    • 2-stage MCPs can achieve a Gain of around 106 or 107 depending on the supply voltage.
    • 3-stage MCPs can achieve a Gain of around 107 or 108 depending on the supply voltage. Specific models also list Gain values, for instance, the F14844 hybrid type detector, which combines an MCP and an avalanche diode (AD), can achieve a total gain of 106 or higher. It specifies the MCP gain as 1 to 104 and the AD gain as 1 to 102, with electron collision gain of 100 to 800. Other models like F2221 to F2226 assemblies list minimum Gains for different stages: 1x104 for 1-stage, 1x106 for 2-stage, and 1x107 for 3-stage.
  • Gain Measurement and Characteristics: The sources mention MCP gain characteristics and pulse height distribution (PHD) which relates to the distribution of output pulse amplitudes resulting from individual incident particles, reflecting the gain variation. The resistance of the MCP, which has a negative temperature characteristic (resistance decreases as temperature increases), affects the strip current which is related to the voltage applied and resistance; Gain characteristics are also typically dependent on supply voltage.
  • Impact of Operating Conditions on Gain: To correctly measure the MCP resistance, the MCP must be in a vacuum. Gas adsorption can occur on the MCP surface, potentially affecting performance; degassing is recommended before use or after storage by evacuating in a high vacuum for more than 24 hours before supplying a voltage. Voltage should be increased slowly when supplying power to the MCP or assembly. Operating ambient temperature is a specified condition for measurements like Resistance and Dark current. Vacuum level is critical for operation, typically below 1.3 x 10-4 Pa. The F14844 series can operate at higher pressure (1 Pa or less) but its gain at 1 Pa is listed as 1 x 106.
  • Gain Adjustment: Using multiple high-voltage power supplies allows the MCP gain to be independently adjusted. Using a divider circuit with a single high-voltage power supply is lower cost, but the MCP gain varies as the signal output changes.

In summary, Gain is the fundamental signal amplification capability of an MCP, achieved through a cascade of secondary electron emissions within its channels under an applied voltage. It is influenced by channel geometry (L/d ratio), material properties, and the number of MCP stages used in an assembly. The applied voltage and vacuum conditions are critical for proper operation and achieving specified Gain levels.