Gas-Filled Detectors Lecture Notes PDF

Gas-filled Detectors INSTRUMENTATION 1 John Michael B. Medina, RMT Overview 01 04 Principles of Gas Ionization Operation Curve 02 05 Components Geiger-Muller 03...

Gas-filled Detectors INSTRUMENTATION 1 John Michael B. Medina, RMT Overview 01 04 Principles of Gas Ionization Operation Curve 02 05 Components Geiger-Muller 03 06 Current vs Pulse Ion Chambers  Learning Outcomes Effectively use radiation detection Perform quality control on all types of equipment to monitor levels of nuclear medicine equipment and Recognize problems with the radioactivity or exposure analyze its results. equipment and know when to report and/or perform corrective measures. Operate variety of nuclear medicine Perform operational preventive equipment. maintenance in all equipment Radiation Detectors Radiation detectors are of paramount importance in nuclear medicine The detectors provide a wide range of information including: Radiation dose of a laboratory worker Positron emission tomography (PET) image of a patient Consequently, detectors with strongly differing specifications are used  Interaction Mechanisms Sensors that produce signals upon interaction with radiation Signals can be processed electronically to give requested information X-rays & Gamma-rays interaction mechanisms Photoelectric effect Compton scattering Pair production Relative importance depends on Radiation energy Interaction medium Result in production of energetic electrons These will eventually transfer their energy to interaction medium by ionization and excitation  Interaction Mechanisms Charged particles transfer their energy by ionization & excitation Ionization results in Charge carriers production: ⚬ Electrons and ions in a gaseous detection medium ⚬ Electrons and holes in a semiconductor material ⚬ Light quanta emission in scintillators Radiation detectors Charge or current forms signal Signal created by charge motion in applied electric field ⚬ Gas-filled detectors ⚬ Semiconductor detectors Light emission observed using light sensor that produces charge or current ⚬ Scintillation detectors Counting, current, integrating mode Radiology / radiotherapy radiation detectors Operated in current mode Intensities too high for individual counting of events Nuclear medicine Primarily use counting mode Energy information Arrival time information Personal dosimeters Detector used in integrating mode Dose is measured monthly Information extracted much later after actual interaction Detector requirements Radiation detector quality expressed in terms of Sensitivity Energy resolution Time and position resolution Counting rate performance Sensitivity Sensitivity depends on Subtended solid angle Detector efficiency for radiation interaction Relevant energy range is ~30–511 keV, where it’s governed by: Photoelectric effect Attenuation length (cm) ~ ρZeff 3–4 ρ = density, Zeff =effective atomic number of the compound Compton scattering Almost independent of Z Proportional to ρ ρ of gas-filled detector is 3 orders of magnitude smaller than for solid state detector Need highest possible ρ and Zeff at 511 keV Energy Resolution Strongly coupled to number of information carriers Number of information carriers Given by N =E/W E =Radiation energy W = Mean energy needed to produce information carrier Largest number produced in semiconductors Smallest number produced in inorganic scintillators + PMT’s Energy Resolution Mean energies W to produce information carriers Detector type W (eV) Gas filled (electron–ion) 30 Semiconductor (electron–hole) 3 Inorganic scintillator (light quantum) 25 Inorganic scintillator + PMT (electron) 100 Inorganic scintillator + Si diode (electron–hole pair) 35 Energy Resolution  Time Resolution Mainly important for PET in nuclear medicine Time resolution depends on 2 main factors Rise time of the signal pulses Height of the signal pulses Important because there is also noise Easier to determine pulse position when the pulse is higher relative to noise Time jitter due to pulse height (energy) variation is less important Inorganic Scintillators detectors preferred because they have Fast response Fast rise time Light sensors’ fast response Counting Rate Counting Rate Relation between R and T for non-paralysable and paralysable cases if τ = 0, than R = T  Gas-filled detectors The detector's basic operation involves the radiation entering the gas-filled chamber and ionizing the gas molecules. The ionized electrons move towards the positive electrode (anode), and the positive ions move towards the negative electrode (cathode). The resulting movement of charge generates a measurable current, which can be used to detect and quantify radiation.  Components Electrode System Gas High-Voltage Supply A central anode and a cathode are The electric field created by applying placed inside the detector chamber. The ionizing radiation interacts with voltage across the electrodes guides the gas molecules, producing ion pairs the ions and electrons toward their The chamber is filled with a specific (positive ions and electrons). respective electrodes, creating an gas, such as air, argon, or other noble electric current. gases. Electrode System Energetic electrons Produce secondary electrons travelling through gas Secondary electrons drift to anode & ions to cathodes  Electrode System Relatively low V Recombination region Produces weak electric field E E too weak to efficiently separate the (-) and (+) charges Some will recombine Full signal not observed Increasing V decreases recombination Relatively high voltage V Full ionization Heavier charged particles & higher rates = higher V Signal becomes constant over wide V range Typical operating V of ionization chamber: 500 to 1000 V  Electrode System Relatively low V Recombination region Produces weak electric field E E too weak to efficiently separate the (-) and (+) charges Some will recombine Full signal not observed Increasing V decreases recombination Relatively high voltage V Full ionization Heavier charged particles & higher rates = higher V Signal becomes constant over wide V range Typical operating V of ionization chamber: 500 to 1000 V Electrode System Energetic electrons Produce secondary electrons travelling through gas Secondary electrons drift to anode & ions to cathodes Radiation Detectors Pulse height as a function of applied high V for gas filled detectors  REGIONS Recombinant Ionization Proportional In this region, all ion pairs are In this region, the electric field is At very low voltages, ion pairs strong enough to cause gas collected, and the current may recombine before reaching amplification. Electrons gain generated is directly proportional the electrodes, leading to enough kinetic energy to ionize to the radiation intensity. incomplete detection. other gas molecules, leading to This is the operational region for an increased number of ion ionization chambers. pairs.  REGIONS Continuous Discharge Geiger-Müller Region Non-Proportional As the voltage increases further, If the voltage is increased In this region, a single ionizing the proportionality between the further, continuous ionization event can cause a large avalanche initial ionization and the signal occurs within the detector, of ionizations, leading to a starts to break down, and non- leading to a constant current, saturated pulse output. linearities are introduced in the even in the absence of The size of the pulse is the same response. radiation. This region is regardless of the energy of the undesirable for radiation radiation. AKA Limited Proportionality detection.  Counting Rate Operation at stronger electric field E Examples: cylindrical detector geometry thin anode wire in centre metal cylinder as cathode At VT =threshold voltage E near anode Very strong Drifting electron gains enough energy to ionize gas atom Proportional region For gain M ≈104, M is independent of deposited energy proportional counter At normal temperature and pressure ET ≈106 V/m. For parallel plate geometry with depth ~1 cm, VT ≈10 kV à not practicable Due to the r–1 dependence manageable V can be applied for proportional operation (1–3 kV) Counting Rate Operation at stronger electric field E At further increased V Space charge effects start to reduce effective E Affect the gain Process will start at lower V for higher primary ionization density events Limited proportionality region is entered At further increased V Pulse height will become independent of the deposited energy Geiger–Müller region is entered V further increased Ionization zone expands Avalanche & significant amplification obtained  MODES Current Mode Pulse Mode In this mode, the detector operates Here, each ionizing event is treated as a continuously and measures the average discrete pulse of current. The pulses are current produced by the ionization events. counted individually, which allows for the The current is directly proportional to the measurement of each event’s characteristics intensity of the radiation. This mode is (e.g., energy). Detectors operating in this typically used in devices such as ion mode include proportional counters and chambers for measuring radiation dose rates. Geiger-Müller (GM) counters.  LIMITATIONS -Dead time: In pulse mode, the detector has a period called "dead time" after each event, during which it cannot register new events. This can limit the detector’s ability to handle high radiation rates. - Gas composition and pressure: The type and pressure of the gas inside the chamber significantly affect the detector's efficiency and sensitivity. Higher pressure can increase the number of ion pairs produced but may also require stronger electric fields. - Saturation: In current mode, extremely high levels of radiation can saturate the detector, leading to inaccurate readings. This occurs when all ion pairs are collected, and additional ionization events don’t produce any measurable increase in current.  Dead-time Paralyzable Non-Paralyzable This means that each new event resets the This means that while some events may be dead time countdown. missed, the dead time does not extend due to the missed events. As a result, if a detector experiences a high radiation flux, it can become "paralyzed," As a result, the detector can handle high leading to significant underreporting of radiation rates more effectively, although it events. will still undercount. Thank You!