Question

The "quench gas" has what function in each of the following gas-filled detectors? a.) (3 pts) Proportional counter: b.) (3 pts) GM tube: 1.

Answer 1

A)In a proportional counter the fill gas of the chamber is an inert gas which is ionized by incident radiation, and a quench gas to ensure each pulse discharge teminates; a common mixture is 90% argon, 10% methane, known as P-10. creating a further voltage discharge giving inaccuracies in the measure from the device When the quenching gas migrates to the cathode it recombines at ground state and so does not present the potential to cause any further ionisation For cach electron collected in the chamber, there is a positively charged gas ion left over These gas ions are heavy compared to an electron and move much more slowly. Free electrons are much lighter than the positive ions, thus, they are drawn toward the positive central electrode much faster than the positive ions are drawn to the chamber wall. The resulting cloud of positive ions near the electrode leads to distortions in gas multiplication. Eventually the positive ions move away from the positively charged central wire to the negatively charged wall and are neutralized by gaining an electron. In the process, some energy is given off, which causes additional ionization of the gas atoms. The electrons produced by this ionization move toward the central wire and are multiplied en route. This pulse of charge is unrelated to the radiation to be detected and can set off a series of pulses. In practice the termination of the avalanche is improved by the use of quenching techniques. The quenching gas molecules have a weaker affinity for electrons than the chamber gas does; therefore, the ionized atoms of the chamber gas readily take electrons from the quenching gas molecul es. Thus, theionized molecules of quenching gas reach the chamber wall instead of the chamber gas. The ionized molecules of the quenching gas are neutralized by gaining an electron, and the energy liberated does not cause further ionization, but causes dissociation of the molecules B)A Geiger-Muller tube consists of a sealed metallic tube filled with argon or another noble gas mixed with a small amount of alcohol called the detecting gas whereas the bromine gas or alcohol yapours are referred to as the quenching gas. The gas mixture insi de the tube is at a pressure below atmospheric pressure. A thin metal wire runs through the gentre of the tube. An clectric potential of up to 1 kilovolt is maintained between the metal wire (the anode) and the cylinder (the cathode). In the absence of any radiation no current flows between the wire and the cylinder or bromine gas. I The argon gas is When a radioactive particle enters the tube it ionises an argon atom. The resulting electron is accelerated towards the metal wire or anode. As the electron approaches the metal wire it experiences an increasing electric field strength which in tum applies a greater accelerating force on the electron. The accelerating force becomes so strong that on collision with other argon atoms the electron can ionise them. The electrons from these ionisations can go onto to generate a cascade of further electrons, an cffect called the avalanche effect. The ignisation by one particle can result in millions of electrons striking the metal wire. The purpose of the quenching gas is to absorb the positive argon ions as they accelerate to the cathode. Without the quenching gas these positive ions will be etralised at the cathode in an exited state or could even also dislodge electrons from the cathode These dislodged electrons or excited atoms could trigger further ionisation