|Contents||In the laboratory, they have been developing an ultraviolet spectroscopic observation device to be loaded on Mercury's surface probe vehicle from BepiColombo, an international Mercury exploration project. The current critical development subject is to optimize the method of depositing photoelectric material (CsI) on a microchannel plate (MCP; see the upper part of Fig.1) defined as a 2D electron multiplier to improve the sensitivity of the extreme ultraviolet light detector (with an observable wavelength of 55 to 150nm) in this device.
The MCP is formed by slicing a bundle of several millions of lead-glass electron multipliers with a diameter of 10 to 20µm and a length of 0.5 to 1.0mm (see the upper part of Fig.1), in which each tube functions as an independent electron multiplier to generate a 2D image as a whole. Between the input and output sides of the MCP is applied a voltage of about 1kV to generate an electric field in the tubes. Light incident to each tube runs into its inner wall to emit photoelectrons, and the photoelectrons are accelerated in the tube to gain kinetic energy and run into the opposite wall to regenerate multiple secondary electrons. This process is repeated to multiply electrons in an exponential manner to achieve a gain of about 103 times with one MCP (see the lower part of Fig.1).
In order to improve the light detection efficiency of MCP, a material (CsI) easy to emit photoelectrons is deposited on its plane of incidence. Fig.2 shows a deposition apparatus. In this apparatus, a CsI evaporation mechanism and a vacuum sealing mechanism are provided in a resistive heater, the vacuum sealing mechanism being arranged to vacuum-seal the MCP detector before returning the resistive heater to the atmospheric pressure after depositing CsI on the MCP.
Their progress in development and future plans are as follows.
(1) MCPs are placed in the vacuum sealing mechanism and further in the resistive heater. After evacuating the apparatus, CsI on a boat is heated and evaporated to make a uniform film.
(2) The deposition is performed while rotating about the B axis to coat the MCP inner wall uniformly with Cs. The A axis is for determining the depth of deposition and is fixed during deposition.
(3) As for determination of deposition completion, two methods can be considered: (i) reading a film thickness meter and (ii) exposing MCP surface to ultraviolet light and measuring photoelectrons emitted from the surface as a current to depend on efficiency. Based on these methods, they will find an optimum one (to be development).
(4) The cover of the vacuum sealing mechanism is closed to return the resistive heater to the atmospheric pressure (development completed).
(5) The MCP is transferred into a EUV correction chamber and, after evacuating the chamber, the cover of the vacuum sealing mechanism is opened. The efficiency under extreme ultraviolet light is measured.
Their final goal is to find an optimum CsI deposition method. What is particularly important is to evaporate CsI crystal without bumping in a vacuum. We hope to work with your company on this point of view.
YOSHIKAWA, Ichiro, Professor
Graduate School of Frontier Sciences, Department of Complexity Science and Engineering
MCP Structure (upper): general-purpose MCPs have an effective diameter of about 15 to 50mm.
Schematic Diagram of Electron Multiplication (lower)