Ghost / Multichannel Analyser MCA-1

The light analysed by a spectrometer such as the Tandem Interferometer is measured by a suitable light detector. For maximum sensitivity the detector is normally a photomultiplier used in a single photon counting mode. The output from the photon counter is a random series of short TTL pulses, each pulse corresponding to a detected photon. In order to record a spectrum the spectrometer is scanned while at the same time counting the pulses in successive finite time intervals into an array of digital data channels. The spectrum thus appears as a histogram in which the number of counts in a channel corresponds to the intensity at that spectral interval. This array of data channels is known as a multichannel analyser.

When recording very weak signals a very long recording time may be needed in order to count a sufficient number of photons. For an instrument like the Tandem Interferometer long single scans are not possible because the instrument will drift out of alignment. In this case repetitive scanning is used in order to reach long recording times. During each scan the detected photons in a given spectral interval are counted into the same data channels, and so the recorded intensity slowly builds up with time.

The Ghost multichannel analyser performs the above recording functions. Three external signals are required: DATA, START and CLOCK. The Clock signal advances the channel number. Up to 1024 channels may be recorded. Ghost is available either as a stand-alone unit as in the picture, or as a PC board built into the control unit of the Interferometer.

The stored data is read into a standard IBM compatible computer via an RS-232 port. Software is provided for data analysis, allowing calibration, curve-fitting and deconvolution of the instrumental function. The software permits multi-tasking.