Gegier Mullier (GM)Counter

1. GegierMullier (GM)Counter Dr.Sayed abbas NMT 244 Lecture 4 .

2. Learning objectives • After the lecture student should be able to • What are gas filled detectors • What is there mechanism • GM region and how it works • Different uses of GM counter • What is Quenching

3. Gas-Filled Detectors • Gas-filled detectors function by measuring the ionization that radiation produces within the gas. • There are several types of detectors that operate on this general principle, but they differ greatly in the details of construction and in the manner in which the radiation-produced ionizationis. • each type has one or more applications for which it is best suited.

5. A dc voltage is placed between the two electrodes. • As a result when the radiation interacts with a gas atom the electron will move towards the positive electrode and the ion will move towards the negative electrode.

6. Principles of Measurement 2- Charge Flow A- Measuring Current • A related approach is to measure the flow of the charges that ionizing radiation produces in a gas-filled detector. • The ions produced by the radiation are charged particles. • The negative particle is either a free electron or an oxygen or nitrogen molecule that has absorbed a free electron.

8. As the voltage increases in a gas detector the ions collected increases. The proportional region ends. Streamer mode Geiger mode Continuous discharge Higher Voltage

9. The gas-filled detector has both a positive anda negative electrode (Ionization Chamber)

10. The ionization chamber consists of a gas-filled detector energized by a relatively low dc voltage. • We will first of all make an estimate of the voltage pulse height generated by this type of detector.

11. The positive and negative ions produced in the gas by the radiation move in opposite directions, positive ions toward the negative cathode and the negative ions toward the anode. • This movement of ions (charges) is an electric current, which can be detected by a sensitive meter. • The current between the electrodes is a measure of the amount of incoming radiation. • The ionization chamber, is a practical instrument that functions in this way.

13. Wilhelm "Gengar" Geiger •  Walther Muller • In 1926 • Kiel University.

14. B- Counting Pulses of Current • The Geiger counter is an example of this type of detector. • The rate at which counts occur in such detectors is a direct measure of the amount of incoming radiation.

19. The process of ionization, collection of the charges produced, and recording of the count takes place very quickly, but it is far from instantaneous. • Time is required for the ions to reach their respective electrodes and for the detector to return to its resting state.

20. Uses of GM counter

22. Quenching • Is to save the counter from spurious counts. • When first pulse is detected by hitting the electrons to the anode, it emits secondary electrons. These secondary electrons may generate false counts. • The quenching gas will slow these electrons and make the detector ready for next pulse.

23. How to minimize quenching • Internal quenching. • a small amount of gas(butane or ethanol) is added into the GM counter is called internal quenching. • External quenching. • a digital circuit is connected with the GM counter to reverse the polarity of the counter's electrodes, after the pulse is detected, to stop the secondary electrons.

24. Questions

25. Task • What is Dead time • How it can be minimized.

26. Thank you