GATE 2023 PH – Question 30
The Geiger-Muller counter is a device to detect α, β and γ radiations. It is a cylindrical tube filled with monatomic gases like argon, and polyatomic gases such as ethyl alcohol. The inner electrode is along the axis of the cylindrical tube and the outer electrode is the tube. Which of the following statements is(are) CORRECT?
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Correct answer: (A) Argon is used so that ambient light coming from the surroundings do not produce any signal in the detector; (B) Ethyl alcohol is used as a quenching gas; (C) The electric field strength decreases from the axis to the edge of the tube and the direction of the field is radially outward
Explanation
In a Geiger-Müller tube a thin positive wire (the anode) runs along the axis of a cylindrical cathode.
- **A. Argon is used so that ambient light does not produce a signal.** The filling is a monatomic noble gas, which has no low-energy vibrational or rotational absorption bands, so visible light does not excite or ionise it. This statement is taken as correct. ✓
- **B. Ethyl alcohol is used as a quenching gas.** True. The polyatomic vapour absorbs the ultraviolet photons and the energy of the argon ions without releasing further electrons, and so stops the continuous discharge after each pulse. ✓
- **C. The field decreases from the axis to the edge and points radially outward.** True. For a cylindrical geometry $E(r)=\dfrac{V}{r\ln(b/a)}\propto\dfrac1r$, which decreases with $r$ and points away from the positive central wire. ✓
- **D. The field increases from the axis to the edge and points radially inward.** False, for the same reason.
Answer **A, B and C**.