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Microchannel Plates

Microchannel plates (MCPs) are compact electron multipliers of high gain. They have been used in a wider range of particle and photon detection systems perhaps more than any other kind of detector.

A typical MCP consists of about 10,000,000 closely packed channels of common diameter which are formed by drawing, etching, or firing in hydrogen, a lead glass matrix. Typically, the diameter of each channel is ~ 10 microns. Each channel acts as an independent, continuous dynode photomultiplier. In astronomy, and in the many other fields that use MCPs, the detectors are generally used for distortionless imaging with very high spatial resolution.

Physical principles of MCP operation

The fundamental physical principals of MCP detectors are gain, efficiency, energy resolution, spatial resolution, time resolution, and dark noise.

X-rays interact with the channel plate glass and electrodes (and with the associated photocathode material) via the photoelectric effect. For X-ray energies below about 5 keV, detection proceeds in a 'single channel mode'. That is, no significant fraction of the X-ray beam entering a given channel penetrates the channel wall to illuminate the neighboring channels. At higher energies, this 'channel crossing' phenomena becomes important.

Another important property of MCPs is their relative immunity to magnetic fields. A single plate with typical operating parameters is completely unaffected by being immersed in a 0.5 Tesla magnetic field. Stacks of plates in certain orientations are immune to much higher fields. This property has only just begun to be exploited in the space astronomy world.

Gain

For single photon or charged particle detection, MCPs are typically used in one of several "high-gain" configurations which produce a saturated (or peaked) output pulse height distribution. Gains of 106 - 108 are achievable. The governing physical parameter which determines gain is the L/D ratio (length to diameter of the individual channels). The higher the ratio, the higher the gain. Typical values are in the range 75:1 - 175:1. Also, the most commonly used configurations-- the chevron (or 'V' shaped arrangement) and the Z-plate configuration -- will not only produce 107 gain factors, but reduce the ion feedback.

Diagram of a microchannel plate detector

The front plate of a chevron pair for use in X-ray astronomy usually has channels perpendicular to its front surface, since the X-rays emerging from a grazing incidence telescope do so along the surface of a cone. The channels of the rear plate are then at an angle of about 15 degrees. This arrangement, as far as we know, has never been proven to be optimal. It is just what is typically used. The gap between the plates is usually about 0. 1 mm. Often an intergap voltage field is also used. These two factors stop the electron cascade from spreading out, and thus reducing the spatial resolution of the detector system, as it crosses between the plates. Recently, other technologies (such as transparent metal meshes) have started to be used rather than the intergap field to produce the same result.

Theoretical and experimental evidence agree that the pulse height FWHM should decrease with decreasing channel diameter. Irrespective of geometry, however, minimal FWHM is achieved when (1) individual stages of the multiplier are independently operated in 'hard' saturation (the plate bias voltages independently exceed a level Vo where Vo = (8.94(L/D) + 450)V and (2) the interplate potential difference is well chosen. The best FWHM from the curved single plate, the V configuration, and the Z configuration is around 30.

Quantum Detection Efficiency

The quantum detection efficiency in X-rays for a single "bare" MCP is a low 1-10%. It is strongly correlated with photon energy (the higher the E, the less the eff) and with the angle of incidence. The efficiency curve has strong peaks associated with angles of incidence which correspond with the critical angle of X-ray reflection from the glass substrate and tends to be zero at both normal (0 degrees) and grazing incidence.

To enhance the efficiency, a material of high photoelectric yield is deposited on the front surface and the channel walls of the MCP. This can increase the overall efficiency to over 30%. Other, more complicated techniques, have been found to push the efficiency to over 60% for the optimum angle of incidence.

Energy Resolution

Until recently, this section would have stated simply "has none". However, CsI-coated chevron MCP detectors have now been shown to possess a limited degree of energy resolution (at least in the soft X-ray region). This occurs, however, only if the bias voltage is well below the saturation voltage. Ultimately, the resolution achieved will be determined by the properties of the coating used on the channel walls.

Spatial Resolution

For any multistage MCP detector at the focus of a grazing incidence telescope, the FWHM spatial resolution is the sum of terms related to the geometry of the X-ray interactions and terms related to the readout element/signal processing chain. Since the functions of the detection/amplification and of position encoding are separable in MCP detectors, a wide range of detector geometries has evolved, each with its resolution dominated by a different term in the sum. However, in all cases, the fundamental resolution is the channel diameter.

Temporal Resolution

In general, the time resolution of a satellite-borne MCP detector is determined by the telemetry rate. MCPs are intrinsically very fast detectors. The pulse transit time through the intense electric field is of order 10-10 seconds. The transit time for a single plate with a length to diameter ratio of 40:1 operating under typical voltages is about 50 picoseconds.

Dark Noise

Usually, the internal background count, or dark noise, in the current generation of MCPs is uniformly distributed across the plate with a value of 0.2 cts/sec/sq-cm. This is rather high compared to rates seen in the most commonly used proportional counters. However, it is more indicative of the sophistication of scintillator rejection techniques and the ignorance of MCP noise than any intrinsic behavior. Also, contamination by potassium and rubidium cause the background to be higher in MCPs. Better manufacturing will therefore lead to reductions in the dark noise.

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This page last updated: Monday, 05-Feb-2007 10:04:07 EST