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12 Cards in this Set
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- Back
- 3rd side (hint)
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When radiation reacts with matter producing light by the deexcitation of ions this material is known as _____________
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When radiation reacts with matter producing light by the deexcitation of ions this material is known as scintillators
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What is the most common form of organic scintillator used in radiation detection that is a liquid cocktail that absorbs energy and releases it as light?
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flor
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What is the most common inorgainic scintillator used in nuclear medicine?
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Sodium Iodide (NaI)
Why does this work poorly by itself? and what has to be added to it to make it functional? |
An impurity, known as an activator has to be added, like Thallium to improve the scintillating properties |
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List the 6 properties of an ideal scintillation material
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1. efficient in converting radiation energy into detectable visible light
2. proportional in visible light to radiation energy 3. transparent to its own light 4. quickly emits the light 5. machinable material 6. refraction index near that of glass |
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What are the 4 advantages of NaI crystals?
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1. relatively dense material and attenutates X and gamma rays well.
2. efficient detector, yielding 1 light photon per 30 eV of radiation absorbed 3. transparent to its own light photons 4. proportional in its output to the energy absorbed What are the 2 disadvantages of NaI? |
1. is fragile and can break with impact and temp change 2. hygroscopic, absorbs water well, so must be sealed from atmosphere otherwise will become opaque with water absorbtion |
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In a crystal when the atom is in a ground state the electrons are in the ________ band, when excited the electrons move to the __________ band.
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In a crystal when the atom is in a ground state the electrons are in the valence band, when excited the electrons move to the conduction band.
When an electron moves to the conduction band it leaves behind a hole in the valence band. What is this pair called? |
an exciton |
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What is the purpose behind adding an impurity like Thallium to the crystal?
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It creates additional energy states between the valence and conduction bands creating an activation center. when excited by photons, electrons or excitions it emits a photon when they return to a ground state
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What are the 6 sequence of events in the emission of a light photon from radiation that can be picked up by the scintillation camera?
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1. Ionization radiation interacts with crystal
2. electrons raised to conduction band 3. Holes are created in valence band 4. Excitons are fromed 5. Activiation centers raised to excited states by absorbing energy 6. de-excitation is followed by emission of light photon |
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Why can't the same detectors be used for PET detectors that are used in conventional nuclear medicine?
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511 keV photons are much more penetrating so greater stopping power is required.
What is the currently preferred material used in PET scanners? |
BGO (bismuth germanate), LSO and GSO are gaining in popularity |
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What is a photomultiplier tube and why is it necessary?
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The light emitted from the crystals is very small so the signal has to be amplified.
How does it do this and to what magnitude? |
Light strikes a diode that produces secondary electrons which in turn strikes additional diodes that continue to multiply the output. multiplied by a factor of 10 to the 6th or 7th power |
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What is dark current?
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electrons emitted from mostly heat and not from the photocathode. also known as noise
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What is done to optimize the efficiency of the transfer of light from the crystal to photocathode tubes?
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having a light pipe or optical coupling gel to match the refractive index so light is not lost or misdirected
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