Sensors

The principal components of the MENA sensors are illustrated in the figure below. A fan-shaped collimator assembly, consisting of nineteen parallel plates alternately set at +10 kilovolts (nominal) or ground, deflects incident charged particles with energies up to 130 keV/charge and collimates the beam of incoming neutral atoms to 4 degrees in the spin plane. The incident ENAs enter the sensor through a gold grating. The function of this grating is to block ultraviolet light, principally Lyman alpha photons scattered from the geocorona, which can create false counts in the detector. Gratings rather than foils are used in the MENA instrument for UV blocking because of the relatively low energies of the ENAs that it is designed to detect. Foils, such as those used in the HENA imager, would cause neutrals in the MENA energy range to experience significant scattering and energy loss, with a corresponding degradation in the instrument's performance.

Mounted on the back of the grating is an ultra-thin carbon foil that produces secondary electrons when penetrated by incoming ENAs. The secondary electrons thus ejected from the foil are accelerated by an extraction grid downward into the microchannel plate (MCP) detector, where they trigger the start pulse for the time-of-flight (TOF) analysis. The TOF stop pulse is generated by the impact of the ENAs on the MCP.

The detector assembly consists of a Hamamatsu MCP and an anode that employs a novel "capacitive charge division" technique to determine the position of the ENA impacts on the MCP. A transmissive mesh with a -12 volt bias positioned ~1.5 mm in front of the MCP enhances the efficiency of the detector and suppresses noise caused by secondary electrons ejected from the surface of the MCP and scattered in the sensor.

The three sensor heads are mounted side by side on top of the DPU. The middle sensor looks straight ahead, with a 107-degree field of view in the plane of the spin axis; the look directions of the two side sensors are offset from that of the middle sensor by 20 degrees. This 20-degree offset compensates for a 20-degree blind spot in the center of each detector. The resulting MENA field of view in the plane of the spin axis is 147 degrees.

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