MDC is a revised type of the impact-ionization dust detectors on board HITEN and BREMSAT which successfully measured dust particles around the Earth [12], The dimensions of MDC are 136 x 127 x 181 mm3, its sensor aperture is 124 x 115 mm2, and its power consumption is 3.78 W [1]. The sensor is directed 45 degrees from the opposite direction of the spin axis of NOZOMI. Since the spin axis points to the Earth during cruise and circum-martian orbits, the sensor box can avoid direct solar light which would increase noise signals from photoelectrons.

The sensor has two charge collector plates which are biased by positive (+220V) and negative (-220V) voltages. All surfaces of the sensor interior are gold plated. When a dust particle impacts on the sensor interior, generated impact plasma is separated into positive ions on the negative collector (ion channel) and electrons on the positive collector (electron channel). In MDC-NOZOMI, charge signal on the grounded neutral target is also recorded to distinguish an impact signal from a noise signal. Charges are recorded by three channels for 200 /is through logarithmic amplifiers (Figure 1). From the total charge and risetime of the

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