Lecture 1 Basic Electronics Unit II Gasfilled detectors

Lecture 1: Basic Electronics Unit II: Gas-filled 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. • 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.

Molecular Bonding • Electricity 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.

Molecular Bonding: Electrons and Protons • Electrons are negatively charged – Electrons are outside of the atomic nucleus • Protons are positively charged – relatively, positively charged things are stationary • Electrons can move – Electrons move toward a positive charge – When 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

Molecular Bonding: Atomic Orbitals • Covalent bonds – Form two types of orbitals from valence (outer) shell electrons • Bonding—lower energy state of electron • Anti-bonding—higher energy state – Bonding orbitals more pervasive in covalent bonds between two atoms since this requires less energy – Common in organic compounds and many inorganic molecules

Molecular Bonding: Atomic Orbitals • Delocalized Bonds – In some materials, many atoms are bound together by sharing all electrons in a “band” of electrons. – Happens often with metals • The whole piece of metal is the molecule with delocalized bonding – Valence band » Holds bonding orbitals – Conduction band » Holds anti-bonding orbitals

Molecular Bonding: Atomic Orbitals For electrons to move from the valence band to the conduction band requires energy. http: //www. vtaide. com/png/images/atom. jpg http: //oldsite. vislab. usyd. edu. au/photonics/devices/semic/images/valcond. gif

Conduction Properties of Materials • Conductors – Full valence band – Extra electrons in conduction band – Materials with small forbidden gaps can become conductors. • Insulators – Full valence band – 5 e. V forbidden gap or larger – Difficult if not impossible to get electrons to conduction band • Semiconductors – Full valence band – Small forbidden gap (about 1 e. V) – Heat will jump electrons to conduction band

Electrical Circuits • Closed Loop Circuit Electrons moving through a conductor and exciting gas in a light bulb.

Electrical Circuits: Voltage & Current • Voltage = 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. )

Electric Circuits: Resistance • If 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).

Ohm’s Law V=IR Georg Simon Ohm R=V I Or I=V R V—Potential (Volts) I—Current (Amperes) R—Resistance (Ω) For a given voltage… If you increase resistance, you decrease the current. http: //www. stegen. k 12. mo. us/tchrpges/sghs/aengelmann/Ohm. Georg. Simon 2. htm

Electric Circuits: Capacitance • Capacitor: • Two conducting plates separated by an insulator • Electrical potential builds up charge difference between plates • Charge on plates limited to number of electrons that can be crowded on • Electric field created between the plates • Mathematically expressed as: C is Capacitance in farads V is change in voltage Q is charge on one plate

Uniform electric field (Or area of potential difference [∆V]) Figure B-4: Capacitor + pole - pole

Some Electrical Symbols Bushong, Stuart, Radiologic Science for Technologist, 8 th Ed. , (St. Louis: Mosby Inc. 2004), p. 83.

Resistor-Capacitor Circuit Prekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. Fig B-5, p. 273

Electrical Units and Mathematical Relationships

Capacitance: Conversion of Charge to Voltage is easier to measure and manipulate than current Increasing resistance in an RC current results in a longer voltage pulse compared to the charge imposed. This is often desirable in NM Instrumentation Prekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. p. 272 q(t) is the charge on any plate C is the capacitance

Next: Finally… Gas-filled detectors! http: //www. aolcdn. com/uk_promo/homer_promo