Gas proportional counting in carbon dating
Gas proportional counting in carbon dating - dating a supervisor
Townsend Avalanche In a proportional counter, many electrons (10 - 10,000) reach the anode for each primary ion pair produced in the gas.
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This means that the proportional counter is more conveniently operated in the pulse mode (ion chambers usually operate in the current mode). The higher the operating voltage, the larger each avalanche becomes and the larger the pulse. The following diagram shows a charged particle traversing the detector gas.
Unlike the situation in a GM detector, the pulse size reflects the energy deposited by the incident radiation in the detector gas. Four primary ion pairs (and four resulting avalanches) are produced.
P-10 gas, developed by John Simpson in the 1940s, is the most widely employed gas for gas flow proportional counters.
Also, if the detector is designed so that the electrons dont have far to travel to the anode, there is less chance that they will interact with the oxygen.
Using air as the proportional gas allows the use of a thin window without the need for a gas flow system. In high humidity conditions, air proportional counters are prone to generating spurious pulses.The solution is to add a small amount of a polyatomic quench gas such as methane.The quench gas preferentially absorbs the photons, but unlike the fill gas (e.g., argon), it does so without becoming ionized.Otherwise, electrons heading towards the anode will combine with the electronegative gas.If this happens, a negative ion goes to the anode rather than an electron, and unlike the electron, the negative ion will fail to produce an avalanche.The fill gas in a proportional counter (and a GM detector) is usually a noble gas because noble gases are not electronegative and don't react chemically with the detector components.