Electrical characteristics of the fluorescent tube detector.

The resistance of the gas within a fluorescent lamp is virtually an open circuit at room temperature until a sufficient electric field is applied where the gas atoms absorb enough energy to emit a free electrons resulting in ionization.

Once the gas is ionised the electrical characteristics of the fluorescent lamp changes and begins to conduct electricity with a negative differential resistance and so more current flows. Consequently, the electrical resistance of the fluorescent lamp drops dramatically from many mega ohms to hundreds of ohms.

Ionisation of the gas within fluorescent lamp occurs at around 300 to 600V depending on the size of tube, environmental factors and the type of gas mixture used by the manufacturer this is usually a low pressure mercury vapour and a mixture of either argon, xenon, neon, or krypton.

In the cosmic ray (muon) detector the electrodes are not in direct contact with the gas inside the tube but capacitive coupled through the glass wall as identified in Diagram 1.

Where C1 represents the capacitive effect of these electrodes and R1 in series with VD represent the gas inside the tube. CP represents the parallel capacitance of the overall coupling effect from the two electrodes across the entire tube.

When a high voltage is applied E (~600 to 1200V) across the circuit it creates an electrostatic field within the gas and when this charge reaches sufficient energy the gas ionises (emitting a flash of light) causing the field to collapses and discharging C1 until the charge can build once again. If not quenched rapidly or the voltage applied is too high the circuit forms a basic relaxation oscillator and the process repeats itself rapidly proportional to the time constant of C1 and R1.

In the detector configuration in below the voltage E is set at a point just below ionisation.

!he 1M Resistor forms a series LC network with the tube capacitance C1 ensuring that only a small amount of energy is stored which can be easily quenched shortly after being triggered. The 4 10Meg Resistors also play an important role by reducing parallel capacitance, preventing the voltage across the tube C1 to run away due to the high impedance of the supply causing oscillation and also ensures a zero bias to the filaments of the fluorescent tube reducing the production of spurious pulses and discharges within the tube.

Diagram 2

Consequently, when a Muon passes through the tube, some of the gas molecules are ionised, creating positively charged ions, and electrons. The strong electric field created by the electrodes accelerate the ions towards the negative side of the tube wall and the electrons towards the positive side of the tube walls. The ion pairs should gain sufficient energy to ionise the gas further through collisions along the way, creating an avalanche of charged particles and discharging the energy in C1 resulting in a short pulse of current which can be measured across the 1 Meg resistor as a negative travelling pulse of voltage, not dissimilar to a Geiger-Müller tube.