Theory of Operation

An opening in the top of the monitor admits particle fallout that settles onto a polished 7.62 cm. (3 in.) removable silicon wafer.  An infrared light-emitting diode (LED) with a limiting aperture is used to illuminate a portion of the silicon wafer.  This light is directly reflected by the silicon wafer in the absence of particle fallout.  As particles settle on the silicon wafer, the light is scattered.  The scattered light is measured by an optical assembly consisting of a lens, a long pass filter and chopper-stabilized electronics that remove the effect of ambient light, and a large area silicon detector.  The signal from the detector is digitized by a 12-bit analog to digital converter, logged in internal non-volatile memory for later download, and reported in real-time through the RS-232C serial port.

Sensitivity of the system is maximized by pulsing the LED during the sampling period.  The signal difference at the detector between the on and off states of the LED is then averaged to produce a relatively low noise signal.  This process improves the monitor’s performance beyond that achievable with only the 12-bit A/D converter.

The TAU-N100A battery power is sufficient to run the unit through more than 6,000 samples, independent of elapsed time, without recharging.  The liquid crystal display (LCD) and sampling electronics are inactive during quiescent periods to maximize battery life.  The LCD is activated with a momentary switch to display percent obscuration and raw counts, unit run time, memory utilization, and calibration parameters.