DDT7

• how is radiation detected

  by indirect means as it cannot be detected by our natural senses (sight, touch, taste and smell)
• what groups of people are susceptible to radiation exposure

  nuclear radiation works and hospital workers such as radiologists, nurses etc .....
• what is the simplest radiation detector

  the film badge
• describe the use of the film badge

  shows the cumulative exposure to radiation
• how does the film badge determine the cumulative exposure to radiation

  degree of blackening of the photographic film
• what is the composition of a single badge ?

  contains a series of filters of different thicknesses and materials
• how is the precise choice of filter and material determined

  by the environment to be monitored
• why are several different thickness filters used ? 


  an estimation of the approx. wavelength/freq of the incident radiation
• what examples of radiation detectors apart from a film badge

  scintillator counters and Geiger-Muller tubes
• what are radiation detectors based on ?

  detecting ionisation caused by radiation instead of radiation itself
• what are the key elements of a Geiger -Muller tube

  thin mica window to allow radiation to enter and cause an avalanche/cascade of ionisation , wire electrode, electrode is at high voltage in contrast to the metal casing, metal tube, tube filled with a gas, typically a noble gas such as argon, helium or neon, Geiger counter.
• • Output signal is measured and indicates that an event has taken place (a cascade reaction), usually by an audible click
• Operation of a Geiger Muller tube:

  • Alpha, beta/gamma radiation enters the tube via the mica window and causes ionisation of some gas molecules
  • Electrons are accelerated towards the high voltage wire electrode and ionise more molecules as they do
  • Positive ions accelerate towards the walls of the tube (negative cathode) and cause further ionisation
  • This results in an avalanche of charged particles, causing a short, intense pulse of current which is amplified and detected
• what will be heard on the Geiger counter ?

  slow clicking if there are low levels of radioactivity detected but rapid clicking if it is highly radioactive
• what must be done after an event has been detected ?

  quenching
• what is the period for quenching known as

  the dead time, no further radiation can be detected, typically between 100-500 milliseconds
• what is scintillation ?


  production of small flashes of visible light by certain materials (scintillators) resulting from absorption of ionising radiation.
• what are the missing elements to a GM tube

  unable to distinguish between different types of radiation and identify a radioactive spill in radio pharmacy
• what does a scintillator consist of

  transparent crystal, Na I, which when struck by ionising radiation fluoresces
• what happens when the flash is produced by the crystal

  it is detected and then amplified by the photomultiplier tube(PMT)
• how can we determine the original photon energy

  since the no. of photons per meV is fairly constant, we can use the intensity of the scintillation flash to find the incident photon energy
• what are key elements of a scintillation counter


  scintillator, photocathode, dynodes, PMT, anode, focusing electrode
• describe the operation of a scintillation counter

  photon emitted by the crystal is absorbed by the photocathode and produces a photoelectron (photoelectric effect)
• describe the pathway of the photoelectron in a scintillation counter

  the photoelectron is accelerated towards the first dynode (cathode) which is coated with a material that emits electrons, producing additional electrons and then these electrons are accelerated towards the second dynode which is at a voltage 100V greater than the first dynode, causing subsequent electrons to be produced. The 'geometry' of the dynode chain causes a cascade reaction resulting in the exponential production of electrons at each stage.
• what happens at the anode in the SC ?

  accumulation of charge occurs at the anode, a pulse of current is registered - an indication that a photon has been detected
• what are scintillation counters used in ?

  gamma cameras
• what are the elements of a gamma camera ? 


  large flat crystal of sodium iodide, a bank if PMT tubes arranged in a hexagon fashion behind the crystal and a collimator
• what does the hexagon arrangement of PMT tubes allow for

  optimal surface area
• what does the collimator consist of 


  thick lead sheets with thousands of adjacent holes drilled through them
• what do collimators allow for 


  spatial discrimination of the source of the detected photons and allows the specific location of the gamma rays to be detected ( map of activity as a function of position)
• how are PMT tubes arranged ?

  close-packed array to allow for minimal gaps between tubes
• what is the role of PMT tubes in gamma cameras

  conversion of light into an electronic signal and magnification of signal
• where immediately follows the PMT tubes

  position logic circuits
• what is the role of the logic circuits in gamma cameras

  receive impulses from the PMT and determine where each scintillation event has occurred within the detector crystal
• what is the operation of gamma cameras ?

  intensity of radioactive activity within the organ under study and and plots the intensity as a function of position
• what happens during gamma camera imaging

  a radiopharmaceutical is administered to the patient and is preferentially absorbed by the organ to be studied or introduced directly into it. the gamma camera detects the intensity of radioactive activity and plots it as a function of position
• what is the application of gamma camera imaging

  used to study organ function, kidney function and conduct full body scans to assess the possibility of bone disease (osteoporosis)
• what indicated abnormalities in gamma camera imaging

  abnormalities lead to an increased blood supply to the diseased region which causes a large uptake of tracer in the diseased region clearly visible as hotspots
• what are the properties of an ideal radiopharmaceutical

  short radioactive and biological half-life, non-toxic and emit gamma rays with little more energy than needed for detection (low patient dose)
• why would alpha or beta radiation not be used as radionuclides

  cause secondary ionisation