Unit II Gasfilled detectors CLRS 321 Nuclear Medicine Physics and Instrumentation 1 Objectives Mostly from your text Distinguish between covalent and delocalized molecular bonding and briefly describe the electrical conductivity of insulators conductors and semiconductors ID: 1031342
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1. Lecture 1: Basic ElectronicsUnit II: Gas-filled detectorsCLRS 321Nuclear Medicine Physics and Instrumentation 1
2. Objectives(Mostly from your text)Distinguish between covalent and delocalized molecular bonding, and briefly describe the electrical conductivity of insulators, conductors, and semiconductors.Define and utilize basic terms and units of electricity including Coulomb, current, voltage, resistance, and capacitance.Diagram a RC circuit and discuss its uses in radiation detectors.
3. Molecular BondingElectricity is a flow of electrons within a circuit.The molecular bonding that comprises the materials making up the circuit impacts the ability of electrons to flow through the circuit.
4. Molecular Bonding:Electrons and ProtonsElectrons are negatively chargedElectrons are outside of the atomic nucleusProtons are positively chargedrelatively, positively charged things are stationaryElectrons can moveElectrons move toward a positive chargeWhen electrons are removed from a source that source becomes positively charged.The unit of charge is a Coulomb A Coulomb is 6.24 X 1018 moving electrons
5. Molecular Bonding:Atomic OrbitalsCovalent bondsForm two types of orbitals from valence (outer) shell electronsBonding—lower energy state of electronAnti-bonding—higher energy stateBonding orbitals more pervasive in covalent bonds between two atoms since this requires less energyCommon in organic compounds and many inorganic molecules
6. Molecular Bonding:Atomic OrbitalsDelocalized BondsIn some materials, many atoms are bound together by sharing all electrons in a “band” of electrons.Happens often with metalsThe whole piece of metal is the molecule with delocalized bondingValence bandHolds bonding orbitalsConduction bandHolds anti-bonding orbitals
7. Molecular Bonding:Atomic Orbitalshttp://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gifhttp://www.vtaide.com/png/images/atom.jpgFor electrons to move from the valence band to the conduction band requires energy.
8. Conduction Properties of MaterialsConductorsFull valence bandExtra electrons in conduction bandMaterials with small forbidden gaps can become conductors.InsulatorsFull valence band5eV forbidden gap or largerDifficult if not impossible to get electrons to conduction bandSemiconductorsFull valence bandSmall forbidden gap (about 1 eV)Heat will jump electrons to conduction band
9. Electrical CircuitsClosed Loop CircuitElectrons moving through a conductor and exciting gas in a light bulb.
10. Electrical Circuits:Voltage & CurrentVoltage = potential electrical energy (Joules/Coulomb)Current = movement of electrons over time (1 Coulomb/Second = Ampere)When voltage is applied to a copper wire, current moves through it.Voltage is like the suction on a straw, if electrons are present, they’ll get sucked up.The more suction, the more electrons, the more current.(Insulation keeps the current from moving outside the wire.)
11. Electric Circuits:ResistanceIf we reduce the diameter of our wire, it will reduce the flow of electrons and thus the current.If we use a coffee stirrer instead of a drinking straw to suck up electrons, we will suck up less electrons over a given period of time.This effect of reduction is called Resistance and is measured in Ohms (Ω).
12. Ohm’s Lawhttp://www.stegen.k12.mo.us/tchrpges/sghs/aengelmann/OhmGeorgSimon2.htm V=IR R=V I Or I=V RV—Potential (Volts)I—Current (Amperes)R—Resistance (Ω)Georg Simon OhmIf you increase resistance, you decrease the current.For a given voltage…
13. Electric Circuits:CapacitanceCapacitor:Two conducting plates separated by an insulatorElectrical potential builds up charge difference between platesCharge on plates limited to number of electrons that can be crowded onElectric field created between the platesMathematically expressed as:C is Capacitance in faradsV is change in voltageQ is charge on one plate
14. Figure B-4: Capacitor+ pole - pole Uniform electric field(Or area of potential difference [∆V])
15. Some Electrical SymbolsBushong, Stuart, Radiologic Science for Technologist, 8th Ed., (St. Louis: Mosby Inc. 2004), p. 83.
16. Resistor-Capacitor CircuitPrekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. Fig B-5, p. 273
17. Electrical Units and Mathematical Relationships
18. Capacitance: Conversion of Charge to VoltagePrekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. p. 272q(t) is the charge on any plateC is the capacitanceVoltage is easier to measure and manipulate than currentIncreasing resistance in an RC current results in a longer voltage pulse compared to the charge imposed. This is often desirable in NM Instrumentation
19. Next: Finally…http://www.aolcdn.com/uk_promo/homer_promoGas-filled detectors!