F. Y. B.Sc. SEMESTER I COURSE: USPH102 UNIT - II

 Interaction between particles and matter  Gas Detectors 1. Ionization chamber 2. Proportional Counter 3. GM Counter

 Device used to detect, track, and/or identify high- energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a particle accelerator.particlesnuclear decaycosmic radiationparticle accelerator 

Scintillation Detector Gas Filled Detector Semiconductor Detector

 Type of Radiation  Radiation Energy  Source Activity  Applications

Battery Pre amplifier & amplifier Detector Analog-to-digital

 Definition  General principles of operation  Types of gas detectors

 Devices used to identify ionizing radiation by counting the ions produced as radiation passes through a gas volume 9

 Gas is enclosed within a chamber made of a conducting material that acts as a cathode  A central core acts as an anode  Cathode and Anode are connected by a resistance/capacitance circuit and have a voltage applied 10

 Radiation passes through the gas producing ion pairs  Positive ions move toward the negative cathode  Negative ions move toward the positive anode 11

 The number of ion pairs collected at the anode and cathode is a function of the voltage applied  The change in the charge on the capacitor is proportional to the number of ions collected  This charge is expressed as the Pulse Height 12

 Pulse Height is a function of the voltage applied to the detector electrodes  A characteristic gas detector curve can be obtained by plotting pulse height vs voltage.  Six regions can be identified 13

 Detect incident radiation by measurement of two ionization processes: Primary process: ions produced directly by radiation effects Secondary process: additional ions produced from or by effects of primary ions: Townsend Avalanche 14 e-e- e-e- e-e- e-e- e-e- e-e- e-e- One X-ray in Many electrons detected Signal

 Primary and secondary ions produced within the gas are separated by Coulomb effects and collected by charged electrodes in the detector Anode (positively charged electrode):Collects the negative ions Cathode (negatively charged electrode): Collects the positive ions

 Cylindrical gas chamber Air P-10 gas mixture (10% methane, 90% argon) Helium, Neon  Anode (+):Wire at center of chamber  Cathode (-):Chamber walls  Operating Principles:  Voltage applied across electrodes  Incident radiation (α, β, or γ) enters chamber and ionizes the filled gas  Ions (+/-) separate and migrate to respective electrodes  Current output is generated and scaled to radiation level 16

17 Gas-Filled Detectors

 Voltage too low Ions may recombine and neutralize each other prior to reaching electrodes  Proper operating voltage All primary ion pairs are collected  Voltage too high Chamber becomes flooded with ions due to secondary ionizations caused by high-energy primary ions Output current is no longer proportional to number of primary ionizations Radiation events no longer measured  Ionization “avalanche” (stream/ flood) propagated by input voltage itself 18 Gas-Filled Detectors

19 Six-Region Gas Amplification Curve , ,  each produce the same detector response.  _____  - _____  _____ Voltage Pulse Height Recombination Region Ionization Region Proportional Region Limited Proportional Region Geiger-Mueller Region Continuous Discharge Region

1. Recombination Region: ◦ Applied voltage too low ◦ Recombination occurs ◦ Low electric field strength 2. Ionization Chamber Region: ◦ Voltage high enough to prevent recombination  All primary ion pairs collected on electrodes ◦ Voltage low enough to prevent secondary ionizations ◦ As voltage increases while incident radiation level remains constant, output current remains constant (saturation current) ◦ Voltage in this range called saturation voltage 20 Six-Region Gas Amplification Curve

3. Proportional Region Gas amplification (or multiplication) occurs Increased voltage increases primary ion energy levels Secondary ionizations occur Add to total collected charge on electrodes Measured pulse amplitude (or height) α incident particle energy 21 Six-Region Gas Amplification Curve

4. Limited Proportional Region  Collected charge becomes independent of number of primary ionizations  Secondary ionization progresses to photo ionization (photoelectric effect)  Proportionality constant no longer accurate  Not very useful range for radiation detection 22 Six-Region Gas Amplification Curve

5. Geiger-Mueller (GM) Region Any radiation event strong enough to produce primary ions results in complete ionization of gas After an initial ionizing event, detector is left insensitive for a period of time (dead time)  Free primary negative ions (mostly electrons) reach anode faster than heavy positive ions can reach cathode  Photo ionization causes the anode to be completely surrounded by cloud of secondary positive ions  Cloud “shields” anode so that no secondary negative ions can be collected  Detector is effectively "shut off"  Detector recovers after positive ions migrate to cathode. Dead time limits the number of radiation events that can be detected  Usually 100 to 500 ms 23 Six-Region Gas Amplification Curve

6. Continuous Discharge Region ◦ Electric field strength so intense that no initial radiation event is required to completely ionize the gas ◦ Electric field itself propagates secondary ionization ◦ Complete avalanching occurs ◦ No practical detection of radiation is possible. 24 Six-Region Gas Amplification Curve

 Ionization Chambers  Proportional Counters  Geiger-Mueller Counters 25

The interactions of charged particles (either direct charged particles or secondary particles produced by interactions with photons or neutrons) with a gas lead to ionized and excited molecules along the path. The important information is the total number of electron-ion pairs created along the track of the radiation.

To collect electron-ion pairs produced in a gas-filled detector, an electric field must be applied. If the electric field inside the detector is strong enough that recombination becomes negligible, all the charges are efficiently collected without loss. The steady state current measured at this condition is called ionization current, which is the basic principle of the DC ion chamber.

 Measure radiation using detectors that operate in the ionization region of the gas detector curve  The flow of electrons is a direct measure of the total number of ion pairs produced  One electron produced per event, uses amplification of the pulse height 31

 Two electric plates surrounded by a metal case  Electric Field (E=V/D) is applied across electrodes  Electric Field is low ◦ only original ion pairs created by radiation are collected ◦ Signal is very small  Can get some energy information ◦ Resolution is poor due to statistics, electronic noise, and microphonics Good for detecting heavy charged particles, betas

IONIZATION CHAMBERS: to measure exposure rates In health physics instruments the chamber is usually filled with air and is constructed using low atomic number materials

 Measures radiation in the proportional region of the gas detector curve  Maintains proportionality between the radiation energy and pulse height  May be used to detect low level of alpha and beta radiation in gas chromatography 34

 Wire suspended in a tube ◦ Can obtain much higher electric field ◦ E  1/r  Near wire, E is high  Electrons are energized to the point that they can ionize other atoms ◦ Detector signal is much larger than ion chamber  Can still measure energy ◦ Same resolution limits as ion chamber  Used to detect alphas, betas, and neutrons

 Introduction  A type of gas-filled detector that was introduced in the late 1940s.  Based on the phenomenon of gas multiplication to amplify the charge represented by the original ion pairs created within the gas.  Used in the detection and spectroscopy of low-energy X-radiation  Also applied in the detection of neutrons 2

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 37

 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  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. 38

 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. 39

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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. 41

 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 42

 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 43

 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 44

 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 45

 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. 46

 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 47

 Measures radiation using the Geiger-Mueller region of the gas detector curve  Basic survey meter used in nuclear medicine  Used in measuring low levels on all types of ionizing radiation 48

 Uses a quenching mechanism to prevent continuous discharge  Dead time reduces efficiency - time in which the counter cannot respond to another event  Life of a G-M tube is 1-10 billion counts, as tube ages the plateau becomes shorter until operation cannot be maintained 49

 Apply a very large voltage across the detector ◦ Generates a significantly higher electric field than proportional counters ◦ Multiplication near the anode wire occurs  Geiger Discharge  Quench Gas  Generated Signal is independent of the energy deposited in the detector  Primarily Beta detection  Most common form of detector No energy information! Only used to count / measure the amount of radiation. Signal is independent of type of radiation as well!

 Introduction  The Geiger-Mueller counter is one of the oldest radiation detector types in existence, having been introduced by Geiger and Mueller in  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. 51

 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. 52

 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. (As the voltage across the counter is increased beyond the ionization chamber region, a point is reached where secondary electrons are produced by collision. This multiplication of ions in the gas, which is called an avalanche) 53

 Under proper conditions, a situation is created in which one avalanche can itself trigger a second avalanche at a different position within the tube.  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. 54

 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. 55

 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.  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. 56

 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. 57

 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. 58

 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. 59

 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.  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. 60

 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 (10 8 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. 61

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 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%. 63

 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. 64

 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. 65

 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”. * 66

 Scintillations:Scintillations Minute flashes of light which are produced by certain materials when they absorb radiation. These materials are variously called fluorescent materials, fluors, scintillators or phosphors.scintillators

 The scintillation detector was possibly the first radiation detector discovered.  The discovery of X-rays by Wilhelm Roentgen in 1895 while working with a device which fired a beam of electrons at a target inside an evacuated glass tube he noticed that some platino-barium cyanide crystals, which happened to be close by, began to glow and that they stopped glowing when he switched the device off.Wilhelm Roentgen  Roentgen had accidentally discovered a new form of radiation. He had also accidentally discovered a scintillator detector.

 Some fluorescent materials are listed in the following table: MaterialForm NaI (Tl)crystal CsI (Na)crystal CaWO 4 crystal ZnS (Ag)powder p-terphenyl in tolueneliquid p-terphenyl in polystyreneplastic

 The basic principle behind this instrument is the use of a special material which glows or � scintillates � when radiation interacts with it.  The most common type of material is a type of salt called sodium-iodide.  The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube.

 The first part of the photomultiplier tube is made of another special material called a photocathode.  The photocathode has the unique characteristic of producing electrons when light strikes its surface.  These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage.

 When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface.  This � bunch � of electrons is then pulled towards the next dynode, where more electron � multiplication � occurs.  The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube.

 At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument.