MEDRADSC 2F03 Lecture Notes - Lecture 12: Photocathode, Ionization Chamber, Phosphorescence
14 Dec 2016
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Energy is deposited in matter by ionization/scintillation
• Energy is converted into thermal energy
• Photons are tiny so the amount of energy deposited in matter is very small, so detectors need signal
amplification.
Methods of Detecting Radiation:
• Light emission
• Scintillation
o The intensity of light emitted is proportional to the amount of energy absorbed in the crystal
o Electron charge emission
o Gas-filled detectors (ionization chamber):
▪ Consists of a cylinder of gas between 2 electrodes, ionization occurs and creates an arena for
the electrical impulses to be collected/measured
• Dosimeter: indicate net amount of energy deposited in the detector by multiply interactions
Luminescence, Fluorescence, Phosphorescence
• Luminescence: the ability of a material to emit light in response to excitation
o Wavelength: characteristic of luminescent material
• Fluorescence: the ability to emit light instantly in response to excitation
o Prompt emission of light (within 10-8sec of stimulation)
o Fluorescent material:
▪ Cesium Iodide (CsI), little balls of light, used in Image Intensifier
(II)→fluoroscopy
• X-ray exits pt and is transmitted through the glass
envelope and interacts with the input phosphor, which
emits light to the photocathode
• Photocathode→ electrons move to anode, electrons go
through towards the output phosphor
• Phosphorescence: the ability of a material (TLD or imaging plate) to delay
emission of light in response to excitation
o Electrons are trapped in some localized energy state
o TLD/CR imaging plates- exposed to x-rays/stored
o Thermoluminescence Dosimetry Steps:
▪ Exposure to ionizing radiation
▪ Heated
▪ Will give off light in proportion to the dose
▪ Measurement of the intensity of emitted light
▪ Record with a Photomultiplier Tube (PMT)
o Lithium Fluoride (LiF)
• Characteristic colour
• Can be subdivided based on timing of response
find more resources at oneclass.com
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Document Summary
Energy is deposited in matter by ionization/scintillation: energy is converted into thermal energy, photons are tiny so the amount of energy deposited in matter is very small, so detectors need signal amplification. Idc: (-1 and +1, want it close to 0: agfa: (logarithmic mean, 1. 7-2 lgm, fuji: (s number, 150-250, as it increases, the dose goes down (backwards)) Phototimers: aec, coated with phosphor, pmt phototimer (aec) Increase in signal of the pmt depends on the number of dynodes, and the gain/amplification at each dynode: pmt gain = gn (g = dynode gain/amplification, n = # of dynodes) Digital radiography: radiographic imaging technology producing digital projection images such as those using: Indirect: psp (in cr, ir: direct, ampophous selenium, amorphous silicon, metal oxide semiconductor-field transistor (mosfet) charge-coupled device (ccd) In digital radiography, a digital computer is used to process attenuation data collected from pts using. Idc: f number: not enough exposure= grainy image.
