Q.(a) Explain the construction and working of a Geiger-Muller Counter. OR

(a) A GM counter detects individual ionizing particles by using the strong field around a fine anode wire to turn a single ionization event into an avalanche pulse large enough to count. (b) A photoemissive cell converts light falling on a photosensitive cathode into a measurable photoelectric current, with applications in light-operated switching and sound reproduction. Both alternatives are answered in full below.

(a) Geiger–Müller (GM) counter

Construction: A GM counter consists of a cylindrical metal tube (acting as the cathode) filled with an inert gas (commonly argon) at low pressure ($\approx 0.1\,\text{atm}$), with a small admixture of a halogen vapour (e.g. bromine) as a quenching agent. A fine tungsten wire, stretched along the axis of the tube, acts as the anode. One end of the tube has a thin mica window to allow ionizing radiation ($\alpha$, $\beta$, or $\gamma$) to enter with minimal absorption. The anode wire is connected through a high resistance $R$ to a high-voltage supply ($\approx 1000$–$1200\,\text{V}$), and the voltage developed across $R$ is fed (after amplification) to an electronic counter/scaler or a loudspeaker.

Working: When an ionizing particle enters the tube through the window, it collides with gas molecules along its path and ionizes them, producing a number of ion pairs (free electrons and positive ions) — this is the primary ionization. Because the anode wire is very thin, the electric field close to it is extremely strong; the free electrons produced are accelerated so strongly toward the anode that they themselves ionize further gas molecules by collision, producing more electrons, which are in turn accelerated and cause still more ionization. This chain reaction, called a Townsend avalanche, multiplies a single initial ion pair into a large pulse of charge collected at the anode within microseconds — enough to produce a sharp, measurable voltage pulse across $R$, regardless of the energy of the original particle. Each pulse is counted (or heard as a click), so the counter registers the number of ionizing particles entering the tube.

The halogen quenching gas absorbs the ultraviolet photons emitted during the avalanche (which would otherwise liberate more electrons from the cathode and cause a continuous, spurious discharge), and helps the tube recover quickly (dead time $\sim 200\,\mu\text{s}$) to detect the next particle.

(b) Photoemissive cell (photocell)

A photoemissive cell consists of an evacuated glass or quartz bulb containing a curved photosensitive cathode (coated with a low-work-function material such as caesium or potassium) and a thin wire or rod anode placed at the centre of curvature. The cathode and anode are connected, through a battery and a sensitive galvanometer/microammeter, in an external circuit.

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