The mystery of detector coupling

[Sesselmann]

I often see confusion over two wire and three wire detectors, and for many people new to radiation detection it seems like a complete mystery how some detectors have one connector and others need a separate cable for the signal.

The coupling scheme used in radiation detectors are almost universally the same, and very simple when you understand what's going on.

Typically there is a scheme where a wire is maintained at a high bias voltage and when an event such as a particle or wave interacts with the detector there is a rapid voltage change in the wire, usually a voltage drop but not always, some detectors are wired the other way.

A load resistor is placed inline with the HV wire to delay the time it takes for the wire to recover it's original potential after the pulse has passed, so the resistor together with the capacitance of the system will determine the pulse length or time constant.

A high voltage capacitor is used to listen for a pulse on the HV line, and when an event takes place, it can be heard through the capacitor and from there it can be amplified by the preamp. A capacitor coupling is necessary as it isolates the sensitive preamplifier from the 800 volt line.

The image below should demystify detector wiring, clear for all to see, the only difference is which end we place the coupling.

** Note, a small number of detectors are wired for negative bias and have a slightly different scheme.

coupling2.png