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Topics 1. Elements of Nuclear Physics 2. History of Nuclear Physics; beginning of nuclear energetics 3. Detecting nuclear radiations, radiation detectors 4. A nuclear power plant 5. Types of nuclear reactors based on usage of the reactors 6. Generations of nuclear power plants 7. Nuclear reactors in Hungary 8. Mini or small nuclear power plants 9. Reactor safety, radiation safety 10. Nuclear accidents, abnormal operations 11. Nuclear power plants and environmental protection 12. Fusion, fusion devices, fusion power plants 13. Natural nuclear reactors or nuclear reactors in nature Lecture #3. Detecting nuclear radiations, radiation detectors Chapter 3. Lecture 3. About detectors o Definition (detectors): o o o o Detectors help to observe and detect or sometimes help to specify some specialties of particles taking part in introductions in atomic physics. These particles are generating during nuclear processes. Simple detectors: • Counters • Its task is to indicate presence of a particle in a given place in a well defined time moment. Little complicated detectors: • It is able to specify some parameters of the particles (for example: identification, charge, mass, kinetic energy, pulse or moment measurement) Physical background of detection: Particle (or its radiation) interacts with the material of the detector (In most cases the type of the interaction is electromagnetic). Neutral particles cannot be detected directly. Neutral particles can be detected by charged particles generated by neutral particles. (Secondary particles.) Lecture #3. Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 1. Gas-filled counters 1. Ionization chamber: o Definition (Ionization chamber): This is the most simple particle detector, which is a gas insulated plane capacitor. o Schematics: o Operation: If particles propagate through the plane electrodes of a gasinsulated capacitor, then the working gas can be ionized. If the gas is ionized, then ions (particles with charges) will be generated. Due to the voltage fells on the electrodes the ions will propagate to the electrodes. In other words: directional flow of charged particles will be presented. It means: electric current will flow. The current can be measured by meters. This detector is well used for high intensity particle detection. Lecture #3. 1. Gas-filled counters 2. Proportional counter: o What does it do?: Number of particles generated by ionizing radiation and energies of these particles can be counted by proportional counters. o Operation: The detector is filled by natural gas. Free electrons moving by high electric field can excite the gas molecules. o If the electric field is big enough, then the electrons can absorb energy during two elastic collisions. The energy is equal to the ionization potential of the gas. During the coming collision every electron can ionize an atom. After this process the original electron and a new (just generated from ionization) electron will propagate. These two electrons can ionize one-one atom (in other words two atoms), and after the process another plus one-one electron will be generated. Etc… o Avalanche of electrons can be generated this way. The strength of the avalanche and the measured electric signal are proportional to the number of the primer electrons. After this effect the name is: proportional counter. (There is a gas which brakes the avalanche to grow up to infinity strength.) o Comments: • Size: several centimeters – meter; distance of the planes is several centimeters. • Gas fill: argon; Braking gas: methane (P-10) • U 2000 V • Application field: measuring neutron flux Lecture #3. 1. Gas-filled counters 2. Proportional counter: o Schematics: E= 𝑈 𝑟1 𝑟 ∙ 𝑙𝑛 𝑟2 r1 : radius of the cathode r2 : radius of the anode r : distance from axis, where electric field is measured U : voltage E : electric field Source: Wikipedia Lecture #3. 1. Gas-filled counters 3. Proportional chamber: o Operation: Numerous proportional counters are installed in a flat container. Numerous wires are outspreaded parallel to each other in the flat container. These are the anode wires. Above and below the theoretical planes of the wires solid metal planes are installed (see the figure below). These metal planes are the cathode planes. The gas fill is joint, but every single wires can work as independent proportional counters. o Schematics: Lecture #3. 1. Gas-filled counters 3. Proportional chamber: o Structure of the electric field inside the proportional chamber: o Comments: • Application fields: mainly used for detecting energetic radiations or particles • Size: from dm2 - to m2 • Density of wires: 1-2 wire(s) / 3mm • 102 – 105 wires are installed in every meters • Expensive detectors! Lecture #3. 1. Gas-filled counters 4. Drift chamber: o Definition (drift chamber): Drift chamber is a proportional chamber, where the distances between the wires fell to the range of 10 cm. Drift chamber is a special proportional chamber. o Operation: There is proportionality between the position of the generated ion column and the time interval needs for collecting ions by the anode wire. This proportionality can be used for precise describing of the position of the particle trace. o By time measurement position of the penetrating particle can be characterized coordinate detector Electric field must be kept homogeneous. Path – flying time connection (proportionality) is linear. Lecture #3. 1. Gas-filled counters 5. Geiger – Müller-counter: o Operation: If the power of a proportional chamber is increased above the normal operating power, a threshold can be reached, where the gas amplification will grow up to infinite value. In this case, the discharge generated by one electron is going to be selfsupported. If the discharge will be extinguish externally, a new type of counter can be designed: penetration of the propagating particle elicits the electron pulse (the effect), but the strength of the effect will be independent of the energy of the particle. Trigger counter. o It is possible to find a mixture of gas fill to get a trigger counter. It means that the counter goes back to starting position after one discharge cycle. Trigger counter. Gas mixtures possible: • Argon – argon vapor for lower intensity • Argon – halogen gas (bromine vapor) Lecture #3. 1. Gas-filled counters 5. A Geiger – Müller-counter: o Scheme: Lecture #3. Source: Wikipedia 1. Gas-filled counter 6. Spark chamber: o Operation: Spark chambers consist of a stack of metal plates placed in a sealed box filled with a gas such as helium, neon or a mixture of the two. When a charged particle ray travels through the box, it ionizes the gas between the plates. Ordinarily this ionization would remain invisible. However, if a high enough voltage can be applied between each adjacent pair of plates before that ionization disappears, then sparks can be made to form along the trajectory taken by the ray in effect becomes visible as a line of sparks. In order to control when this voltage is applied, a separate detector (often containing a pair of scintillators placed above and below the box) is needed. When this trigger senses that a ray has just passed, it fires a fast switch to connect the high voltage to the plates. The high voltage cannot be connected to the plates permanently, as this would lead to arc formation and continuous discharging. o Comments: • At the beginning there is a 100 V voltage to clear the field from the ions stay there for longer time. • Working voltage is several kV. • Pictures can be taken by photo machine coordinate detector operation. Lecture #3. Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 2. Scintillation counters o Definition (Scintillation counter): A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillator material, and detecting the resultant light pulses. o Structure of the device: a. Scintillator material, where the energy of the radiation is transformed to energy of light. b. Photomultiplier, the device which transforms light pulses to electric signals. c. Amplification of electric pulses, analyzing of electric pulses, register by electronics. Lecture #3. 2. Scintillation counters 1. Scintillator: o Scintillating materials: • Inorganic crystals (ZnS – zink-sulfid, NaI – sodium-iodid, CsI, LiI) • Organic single crystal (anthracene, naphthalene, stilbene) • Scintillating liquids(toluol) • scintillator (polimerized liquid scintillator) o Radiations (α-, β-, γ-radiation) transformed to light pulses. Lecture #3. 2. Scintillation counters 2. Photomultiplier: o Definition (photomultiplier): This is an electron tube which holds two parts: photocathode (it is sensitive for light) + amplifier part o Photocathode: It transforms the light pulses to electron current o Amplifier: It amplifies the electron current to stronger values. Lecture #3. 2. Scintillation counters 3. An important application: detection of gamma radiation o o Lecture #3. NaI - Sodium-iodide as scintillating screen Photoeffect, Compton-scattering and pair-generation – some of them is needed to generate the effect. (Light pulses in the crystal) Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 3. Semiconductor detectors 1. Parameters: o o o o o 2. From the beginning of 1960´s Small size Good energy distribution capability It is nicely used for radiating surfaces with small radiation Silicon, Germanium are mostly well known materials Operation + definition (semiconductor detectors): o 3. We can say that, semiconductor detectors are special ionization chambers, where the ionization occurs in solid, semiconductor material. (Instead of gas the ionization works is solid material.) Comparison with gas-filled ionization chamber: o o o o o Lecture #3. Solid state is denser material than gas solid state needs weaker radiation to start the ionization Taking same irradiation, in solids more charged particles can be generated, than in gas ionization chamber. In case of solid semiconductor the measurement is more stable, errors are lower (lower error bars during the measurements) In case of semiconductors the energy resolution is better than in gases Specific conductivity is better for semiconductors 3. Semiconductor detectors 4. Main application fields: o o o o Lecture #3. Measurement on heavy charged particles Detecting electrons Detecting gamma-radiations Observing neutrons Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 4. Cherenkov-effect 1. Definition (Cherenkov radiation): If a particle is moving in a medium and its speed is higher than the speed of the light for the same medium, then the particle emits a radiation in a cone-like structure. This emitted radiation use to call as Cherenkov-effect or Cherenkov radiation after famous Russian Nobel Prize laureate physicist scientist Pavel Alekszejevics Cserenkov Cherenkov. Cherenkov (1904 - 1990) Lecture #3. Very typical blue light shows presence of Cherenkov radiation Geometry of Cherenkov radiation Direction of the radiation 𝑣 𝟏 𝒄, 𝐜𝐨𝐬 𝝋 = = 𝒏𝜷 𝒗 Direction of the radiation 𝑣 𝛽= 𝑐 Lecture #3. n: index of refraction of the medium 𝜑: angle of cone, cone angle 4. Cherenkov-counters 1. Cherenkov-counter: It is an application for detection of high speed charged particles. Cherenkov-effect can be used for detectors. o 2. Schematics: photomultiplier Radiator Cylindrical mirror slit Medium of the radiator is gas. Lecture #3. Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 5. Particle trace detectors Detectors Counters It shows information of the presence of a particle - in a given time minute - in a given place. Lecture #3. Particle trace detectors Particle leaves mark or trace along with their path during their movement. This trace can be visually projected or saved. Operation of particle trace detector is based on showing the trace of the moving particle. 5. Particle trace detectors 1. Cloud chamber – Wilson-type cloud chamber: o The oldest trace detector ever built! 1912. o When a charged particle (for example, an alpha or beta particle) interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight). When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge. Lecture #3. 5. Particle trace detectors • Three steps of the operation: 1. 2. 3. 4. Expansion of the gas fill Trace generation Taking photos on the particle traces Preparation of the chamber for the next experiment Source: Paks Nuclear Power Plant Lecture #3. 5. Particle trace detectors Lecture #3. 5. Particle trace detectors 2. Bubble chamber: o Cloud chambers work on the same principles as bubble chambers, but are based on supersaturated vapor rather than superheated liquid. o While bubble chambers were extensively used in the past, they have now mostly been supplanted by wire chambers and spark chambers. Historically, notable bubble chambers include the Big European Bubble Chamber (BEBC) and Gargamelle. Lecture #3. Source: Wikipedia 5. Particle trace detectors 3. Solid state trace detectors: o o When a heavy (charged) particle travels through solid insulators, single crystals, glass-like materials or organic polymers, permanent changes occur. These permanent changes are sub-microscopic, but they can be observed under microscopes. Main features of these type of detectors: • They are threshold detectors, it means that they can register energies above a given energy threshold or limit. (Energy threshold value is material specific.) o 𝑑𝐸 They gives possibility to measure energy, and/or determine , 𝑑𝑥 identify particles, diagnose direction and depth of penetration • They have good geometrical resolution (5 – 20 nm) • They are not sensitive for extreme environmental impacts • It is relatively easy and fast to prepare them, to develop them, and to see them Main application fields: • Investigation of nuclear fission • Investigation of reaction caused by alpha particles • Identification of heavy particles during high energy reaction • Investigation of primary components of cosmic radiation • Lecture #3. 5. Particle trace detectors 3. Figure of particle trace on solid state detector: Lecture #3. Types of Detectors 1. Gas-filled counters 2. Scintillation counters 3. Semiconductor detectors 4. Cherenkov-radiation and counters 5. Particle trace detectors (Visual detectors) 6. Neutrino detectors Lecture #3. 6. Neutrino detectors 1. Countering neutrino detectors: o o In these devices liquid scintillation detectors alternate with drift chambers which are used as coordinate detectors. Seconder particles are detected. Seconder particles can be generated from neutrino interactions. Lecture #3. Cherenkov circles in neutrino scattering