Fluorescent Tube Detector Electronics

The electronics component of the detector is an important part of a Cosmic Ray Telescope and comes in two main parts.

Detection Circuit

One of the problems with building a detector using fluorescent tube is that although it functions very much like a Geiger-Müller tube the voltage is higher; the signal has a very high output impedance and is capacitive coupled. So the following considerations needed to be included in the circuit design:

1. The amplifier section must tolerate high voltages and have a high input impedance.
2. Capacitive coupling introduces an AC signal characteristics and amplitude variations, so the output needs shaping through a Schmitt trigger for better stability and noise immunity.
3. The high voltage power supply must have good filtering and be regulated to decrease amplitude variations and introduction of noise.

Coincidence Circuit

The main problem with the physical muon detector is that in essence it is a radiation detector and along with muons showering down from the skies there also equal amounts of terrestrial radiation present in the environment including the detector itself. Although this is in very small quantities it is sufficient to make it difficult to discriminate between a terrestrial or cosmic events.

However, a muon does have sufficient energy to pass through the physical detector easily, whereas terrestrial radiation will not. Therefore anything detected in two or more detectors placed one above the other, simultaneously (coincidence) is almost certainly a cosmic event. Consequently, having electronics that can measure coincidence across two or more detectors is essential.

To do this task the output of the Schmitt Trigger is feed into a logic gate eg: AND Gate where the output could be captured by a data logger recording counts over time or other devices.

Pulse Shortening
Tests using other more reliable detector systems such as Geiger–Müller Tubes have shown that a simple coincidence detector using a AND Gate by itself is problematic due to the response and decay time (Pulse Width) of the detector when an ionising particle has been detected.

Consequently, the wider the Pulse Width the greater the number of false positives. The means a pulse shorting or quenching circuit is also needed to shorten the Pulse Width to a period closer to the expected flight time of the Muon between tubes, but not too narrow that the electronics cannot measure relative coincidence. Some improvement might also be achieved by spacing the tubes further apart, but this also has the negative effect of decreasing the aperture of the detector.