A Multi-Channel Scaler is fundamentally concerned with measuring pulses: When they arrive, how wide they are, how tall they are, how they are shaped, their total energy, and other parameters. They are typically called "photon counters" because the pulses being analyzed are most commonly from sensors that detect photons, such as photomultiplier tubes or photodiodes.
The photons themselves often come from radiation sensors (scintillators mated with photomultiplier tubes), though when non-photonic radiation sensors are used (such as ion chambers), these instruments are then magically called "gamma counters". Other popular photon sources include lidar and many types of fiber optic based sensors.
The general field of pulse analysis has many components with overlapping terms and many synonyms and redefinitions: Multi-Channel Analyzer, Pulse-Height Discriminator, Pulse Analyzer, Pulse Dwell Analyzer, and the list goes on and on and on.
Fundamentally, we care about several key pulse attributes:
- Polarity (direction) of leading edge
- Time of leading edge detection
- Rate-of-Rise of leading edge
- Pulse height (peak amplitude)
- Time of peak height
- Time of trailing edge detection
- Rate-of-Rise (well, fall) of trailing edge
- Pulse width
- Pulse area (energy)
The first and most critical feature of an MCS is clearly to obtain and amplify pulses with extreme linearity and precise timing fidelity: If you look at a narrow tall pulse, its frequency content can be staggering: MCS analog inputs must contain extremely good pulse amplifiers.
In any pulse sensor, it is common for more than one sources of pulses to be present. For example, a radiation detector used in medicine (such as in a PET scanner) will also pick up pulses from cosmic rays and natural background radiation, not to mention the X-Ray machine in the next room. So an important feature of an MCS is to "gate" the pulses based on any or all of the above pulse characteristics.
Once pulses of interest are identified, what is to be done with them? We count them. But since the pulses of interest will have characteristics that distinguish one from the next, we can choose characteristics of interest to create counting bins, or a histogram. For example, most radiation sensors convert gamma strength to pulse height, so we create bins according to pulse height. A lidar sensor cares more about pulse timing, so we would bin the pulses according to their delay following the initial laser illumination pulse.
An important characteristic of many pulse sources, especially radiation detectors, is that the pulse source is inherently random. Pulses may overlap, and there will be arbitrarily long or short delays between pulses. And every pulse detector has a characteristic of its own: It has a "recovery" or "dead" time after detecting a pulse before it is able to detect a subsequent pulse. This is where the "multi-channel" aspect comes in: Multiple pulse detectors are used in parallel, timed to hopefully ensure that at least one is always out of its dead-time, with anti-coincidence logic present to ensure that pulse detectors triggered by the same pulse don't cause that pulse to be counted twice.
Even with such precautions, every use of an MCS must be paired with a dead-time analysis, and an ensuing dead-time correction process (a statistical correction). The best MCS units will monitor themselves and provide dead-time alarms (no free pulse detector) and an estimate of the pulses missed due to dead-time effects. Many sensors also exhibit dead times, so the sensor itself must be carefully characterized, and its behavior statistically combined with that of the MCS to determine the overall dead-time penalty.
Some SR430 links:
http://www.physics.rutgers.edu/ugrad/389/muon/SR430m.pdfhttp://www.thinksrs.com/products/SR430.htmEnjoy!
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