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Proportional Counters & Geiger Mueller counters by Hiba Al-Sa’eed Physics 641 Prof. Nidal Ershaidat [email protected] 1 Proportional Counters • Introduction • The proportional counter is a type of gas-filled detector that was introduced in the late 1940s. • They rely on the phenomenon of gas multiplication to amplify the charge represented by the original ion pairs created within the gas. [email protected] 2 • Proportional counters are used in the detection and spectroscopy of low-energy Xradiation • They are also applied in the detection of neutrons [email protected] 3 Gas Multiplication • A. Avalanche Formation • At low values of the field, the electrons and ions created by incident radiation drift to there respective collecting electrodes • Many collisions occur with neutral gas molecules • Positive or negative ions achieve very little average energy between collisions because of their low mobility [email protected] 4 • Free electrons are easily accelerated by the applied field and may have significant kinetic energy when undergoing such a collision • Now, if this energy is greater than the ionization energy of the neutral gas molecule, it is possible for an additional ion pair to be created in the collision [email protected] 5 • Because the average energy of the electron between collisions increases with increasing electric field , there is a threshold value of the field above which this secondary ionization will occur . [email protected] 6 • The electron liberated by this secondary ionization process will also accelerated by the electric field . This electron undergoes collisions with other neutral gas molecules and thus can create additional ionizations . • So we can say that Gas Multiplication is a consequence of increasing the electric field within the gas to sufficiently high value . [email protected] 7 Regions of Detector Operation [email protected] 8 B. Regions of detector operation • At very low values of the voltage , the field is insufficient to prevent recombination of the original ion pairs , and the collected charge is less than that represented by the original ion pairs . • As the voltage is raised recombination is suppressed and the region of ion saturation is achieved . This is the normal mode of operation for ionization chambers . [email protected] 9 • as the voltage increases , the threshold field at which gas multiplication begins is reached . The collected charge then to multiply , and the observed pulse amplitude will increase • Over some region of the electric field , the gas multiplication will be linear , and the collected charge will be proportional to the number of original ion pairs created by incident radiation [email protected] 10 • That is the region of true proportionality and represents the mode of operation of conventional proportional counters • Increasing the applied voltage still further can introduce nonlinear effects Explanation is in the next slide [email protected] 11 • Although the free electrons are quickly collected , the positive ions move much more slowly and , during the time it takes to collect the electrons , they barely move at all. Therefore , each pulse within the counter creates a cloud of positive ions , they represent a space charge that can significantly alter the shape of the electric field within the detector [email protected] 12 • Because gas multiplication is dependent on the magnitude of the electric field , some nonlinearities will begin to be observed • These effects mark the onset of the region of limited proportionality [email protected] 13 • If the applied voltage is made sufficiently high , the space charge created by the positive ions can become completely dominant in determining the subsequent history of the pulse. Under these conditions the avalanche proceeds until a sufficient number of positive ions have been created to reduce the electric field below the point at which additional gas multiplication can take place . [email protected] 14 • The process ,here, is self-limiting and will terminate when the same total number of positive ions have been formed regardless of the number of initial ion pairs created by the incident radiation This is the Geiger-Mueller region of operation [email protected] 15 Applications • Proportional counters are used in resonance–electron Mossbauer spectroscopy. • The type used is the High‐temperature proportional counter. • “In order to avoid insulation difficulties due to electric leakage on the surfaces of heated insulators, the counter is carefully constructed so that the sensitive volume of the counter can be warmed up while keeping the insulators at room temperature; all insulating materials between the anode and cathodes are located outside the electric furnace in the counter system. The counter operation is influenced by thermal electrons emitted from the cathode material”. * • “The high‐temperature counter provides us with a new method to observe directly surface phenomena at high temperatures”.* • * http://rsi.aip.org/resource/1/rsinak/v52/i3/p413_s1?isAuthorized=no [email protected] 16 Geiger-Mueller Counters • Introduction • The Geiger-Mueller counter is one of the oldest radiation detector types in existence, having been introduced by Geiger and Mueller in 1928. • Commonly referred to as the G-M counter, or simply Geiger tube. • The simplicity, low cost, and ease of operation of these detectors have led to their continued use to the present time. [email protected] 17 • G-M counters comprise the third general category of gas-filled detectors based on ionization (first two types are the ion chambers and the proportional counters). • They are used to detect alpha and beta particles, gamma and X-ray. • In common with proportional counters, G-M counters employ gas multiplication to greatly increase the charge represented by the original ion pairs, but in a fundamentally different manner. [email protected] 18 • In proportional counter, each original electron leads to an avalanche that is basically independent of all other avalanches. Because all avalanches are nearly identical, the collected charge remains proportional to the number of original electrons. • In the G-M counters, substantially higher electric fields are created that enhance the intensity of each avalanche • Under proper conditions, a situation is created in which one avalanche can itself trigger a second avalanche at a different position within the tube. [email protected] 19 • At a critical value of the electric field, each avalanche can create, on the average, at least one more avalanche, and a self-propagating chain reaction results. • At still greater values of the electric field, the process becomes rapidly divergent and, in principle, an exponentially growing number of avalanches could be created within a very short time. [email protected] 20 • Once this Geiger discharge reaches a certain size, however, collective effects of all the individual avalanches come into play and ultimately terminate the chain reaction. • Because this limiting point is always reached after about the same number of avalanches have been created, all pulses from a Geiger tube are of the same amplitude regardless of the number of original ion pairs that initiated the process. • A Geiger tube can therefore function only as a simple counter of radiation-induced events and cannot be applied in direct radiation spectroscopy because all information on the amount of energy deposited by the incident radiation is lost. [email protected] 21 • 1. The Geiger Discharge When excited molecules produced by an avalanche return to their ground state they emit photons whose wavelength may be in the visible or ultraviolet region. These photons are the key element in the propagation of the chain reaction that makes up the Geiger discharge. [email protected] 22 • These photons may be reabsorbed elsewhere in the gas by photoelectric absorption, creating a new free electron which will migrate toward the anode and trigger another avalanche. [email protected] 23 • The termination of Geiger discharge. Positive ions are created along with each electron in an avalanche. The mobility of these ions is much less than that of the free electrons, so they remain motionless during the time necessary to collect all the free electrons. When the concentration of these positive ions is sufficiently high, their presence begins to reduce the magnitude of the electric field in the vicinity of the anode wire. [email protected] 24 • Each Geiger discharge is terminated after developing about the same total charge, regardless of the number of original ion pairs created by the incident radiation. All output pulses are therefore about the same size, and their amplitude can provide no information about the properties of the incident radiation. [email protected] 25 2. Quenching • After the primary Geiger discharge is terminated, the positive ions slowly drift away from the anode wire and ultimately arrive at cathode of the counter. • Here they are neutralized by combining with an electron from the cathode surface. • In this process, an amount of energy equal to the ionization energy of the gas minus the energy required to extract the electron from the cathode surface (the work function) is liberated. [email protected] 26 • If this liberated energy also exceeds the cathode work function, it is energetically possible for another free electron to emerge from the cathode surface . This will be the case if the gas ionization energy exceeds twice the value of the work function. [email protected] 27 • The probability is always small that any given ion will liberate an electron in its neutralization, but if the total number of ions is large enough, there will likely be at least one such free electron generated. This electron will trigger another avalanche, leading to a second Geiger discharge. The entire cycle will now be repeated and this will produce a continuous output of multiple pulses. • Special precautions must be taken in Geiger counters to prevent the possibility of excessive multiple pulsing. [email protected] 28 • External quenching consists of some method for reducing the high voltage applied to the tube, for a fixed time after each pulse, to a value that is too low to support further gas multiplication. • One method of external quenching is simply to choose R (see the next slide) to be a large enough value (108 ohms) so that the time constant of the charge collection circuit is of the order of a millisecond. • This method has the disadvantage of requiring several milliseconds for the anode to return to near its normal voltage, and thus Geiger discharges for each event are produced only at very low counting rates. [email protected] 29 [email protected] 30 • Another method is the internal quenching, which is accomplished by adding a second component called the quench gas to the primary fill gas. • It is chosen to have a lower ionization potential and a more complex molecular structure than the primary gas component and is present with a concentration of 5-10%. [email protected] 31 • This gas prevents multiple pulsing through the mechanism of charge transfer collision. • When the positive ions make collisions, some of these collisions will be with molecules of the quench gas and, because of the difference in ionization energies, there will be a tendency to transfer the positive charge to the quench gas molecule. • The original positive ion is thus neutralized by transfer of an electron and a positive ion of the quench gas begins to drift in its place. [email protected] 32 • If the concentration of the of the quench gas is sufficiently high, these charge-transfer collisions ensure that all the ions that eventually arrive at the cathode will be those of the quench gas. • When they are neutralized, the excess energy may now go into dissociation of the more complex molecules in preference to liberating a free electron from the cathode surface. Thus no second avalanche will occur. [email protected] 33 Applications • Geiger – Mueller tubes are used in monitoring the environment near nuclear power sources. • “The environment near nuclear power sources is generally monitored by a number of peripheral instrument stations, each with two Geiger- Muller tubes. lf release of activity occurs, it is immediately detected and a signal is fed to a computer-controlled monitoring system. The computers trigger alarm warnings and provide an indication of the location of probable 'downwind' areas where precautions should be taken”. * * www.centronic.co.uk/tube_applications.htm [email protected] 34 THE END [email protected] 35 References • Radiation Detection and Measurement ,Third Edition, Glenn F . Knoll • http://rsi.aip.org/resource/1/rsinak/v52/i3/p4 13_s1?isAuthorized=no • www.centronic.co.uk/tube_applications.htm [email protected] 36