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15 Cards in this Set
- Front
- Back
- 3rd side (hint)
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What is the difference between a gas filled ionization chamber and a semi-conductor?
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semi-conductor are solid state devices where the charge carriers are electron hole pairs, not electrons and ions as in a gas filled chamber. they operate the same way but use different charge carriers
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What is the primary advantage of a semi-conductor detector?
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improved energy resolution, that is the improved ability to tell the difference between radiations of different energies
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The energy resolution of a NaI(Tl) detector system (gas filled) system is _____% and the resolution of Ge(Li) detector (semi-conductor) system is about ____%
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The energy resolution of a NaI(Tl) detector system (gas filled) system is 8-15% and the resolution of Ge(Li) detector (semi-conductor) system is about 1-3%
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List 5 advantages of using semi-conductors
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1. linear response over wide energy range
2. high efficiency for a given size 3. can be constructed in a variety of shapes and sizes 4. Insensitivity to magnetic fields 5. ability to count in narrow windows, thus lowering background |
linear efficiency constructs magnetic windows
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What is the primary property of a semi-conductor?
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contains a material where electric current will flow and an insulator where it will not. will not conduct electricity at very low temperatures but at higher temperatures
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how do you describe the energy states of electrons in crystals?
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They exist in bands, the highest ground state being the valence layer, when excited beyond that they go to conduction band. as the name implies current cannot flow unless electrons raised to that band
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Explain the difference between conductors and insulators in reference to energy bands in crystals?
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an insulator has such a wide gap between the valence and conduction bands that no amount of applied current will cause electrons to be raised to conduction band. in a conductor electrons already partially fill conduction band and any applied current causes electrons to flow
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Explain what happens when an electron moves to the conduction band?
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It creates a positively charged "hole" in the valence band. these move in opposite directions in an electric field
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Why are impurities added to a crystal in a semi-conductor detector?
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to increase the number of charge carriers, making the material more conductive
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What effect is created when adding arsenic to silicon?
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silicon is a semi-conductor that has 4 valence electrons, making covalent bonds with neighboring silicon atoms, when you introduce arsenic with 5 valence electrons it creates a weakly bound 5th electron that can easily go to conduction layer. What type semi-conductor does this create?
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An N-type semi-conductor, or one that has an excess of negative charge
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What happens when the impurity of Gallium in added to the silicon semi-conductor?
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It has 3 valence electrons creating an excess positive charge creating a P-type semi-conductor.
What are the impurities gallium and arsenic known as in reference to electrons? |
Arsenic is a donor impurity while gallium is an acceptor impurity
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What is a depletion layer, how is it created and what does it do?
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It is the sensitive area of a semi-conductor detector (which is much more sensitive than ionizaiton chamber). when placing a p-type semi-conductor next to a n-type semiconductor you create a p-n junction, when you reverse the electric field no current flows and a depletion layer is created. this is where electron hole pairs are created when radiation enters
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How many electron pairs are created in relation to radiation energy that strikes the semi-conductor?
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3 eV/pair
example: a 5MeV alpha particle deposits all its energy in a semi-conductor detector. how many electron pairs will be created? |
5 x 10E6/3 eV/pair = 1.7 x10E6
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Why has it only been recently that it has been commercially feasible to create the electronics to make a S-C camera?
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a 128 x 128 pixel image contains 16,384 pixels and each pixel requires its own electronic channel
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What are the 4 advantages of a S-C camera?
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1. made of dense material, improving attenuation
2. since PM tubes not required camera can be much smaller, even portable 3. Good resolution is possible 4. Fast electronics What are the 3 disadvantages of a S-C camera? |
1. Cost
2. Uniformity 3. small camera FOV |
