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Show radiation, a 45 degree prism to separate the illumination beam from the scattered light, and another lens to refocus the scattered light onto the photomultiplier. The illumination beam itself is focused onto a photovoltaic detector to permit monitoring the alignment and measuring the attenuation of the beam as it passes through the window system and aerosol stream. Given this optical arrangement, the alignment system ensures that only when the image of the beam waist passes through the detector slit will scattering signals generated by particle trajectories passing through the sample volume (also on the beam waist centerline) be detected. Centering the slit on the beam waist image is a straightforward task accomplished by monitoring the photovoltaic detector output as one traverses the entire detector mount along the laser beam axis and transverse to the beam axis (Fig. 1). Precise centering in the vertical axis is unnecessary because the analog peak detector picks off the peak amplitude at the beam center. This alignment procedure is convenient for two reasons. First, variable-thickness windows which displace the foci can be inserted between the transmitter and receiver and precise realignment achieved by the above procedure without placing a test scatterer in the sample volume, a tedious process even under laboratory conditions. In a process stream at high temperature and pressure, placing a test scatterer would be totally infeasible. Second, under conditions where beam wander is severe, this method gives a continuous indication of the extent of beam steering. For severe conditions, this technique alerts the experimentalist when it is necessary to increase the slit width so that misalignment tolerances are adequate. There is still another benefit to be obtained in conjunction with this alignment system. The beam attenuation due to particle scattering, absorption, and window transmission losses is monitored by each of the photovoltaic detectors. Such a measurement is essential for properly renormalizing the absolute intensity response function to obtain the correct particle size calibration. If one separately measures the window transmission losses from the particle scattering and absorption losses, then one can also obtain a Sauter mean diameter. This additional information can be useful for wery high number density aerosols with mean diameters below the detection limit of the single particle counter. Figure 3 is a schematic diagram of the analog and data processing system. With this system the scattering signature from each particle is processed electronically. The fast analog system consists of three channels of information including pulse height spectra from small particles and large particles, as well as particle transit times, which give the velocity for either size range. For small particles the signature from each scattering pulse is amplified and then logarithmically scaled before being pulse-height-analyzed and digitized by the analog to digital converter (ADC). For large particles each signature is amplified and then split into two channels for transit time analysis and pulse height analysis, 7 |