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17 Cards in this Set
- Front
- Back
- 3rd side (hint)
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In general what effects the absorption of radiation in tissue?
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Higher Z material, like bone, attenuates more photons.
what are the units of absorbed dose? |
Rads = 100 ergs/g
Grays = 100 Rads |
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What does a unit of exposure attempt to measure and what is the primary unit for this?
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attempts to measure the complex relationship between radiation and its effect on matter and the amount of ionization that takes place.
The Roentgen (R) is the primary unit of exposure, specifically what does it measure? |
It is a measure of charge generated by the ionization of air by radiation or the electronic charge in air
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How does an free air chamber detector work?
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The beam of radiation interacts with the air in the chamber. A transducer converts ionization into electric current.(cathode and anode plates)
What is the limitation of a free air chamber? |
1. can measure only when photon flux is constant.
2. only practical to measure to 3 MeV 3. thickness of air required to achieve electronic equilibrium, which increases with energy, making free air chambers impractical outside of lab due to large size requirement |
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In general what effects the absorption of radiation in tissue?
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Higher Z material, like bone, attenuates more photons.
what are the units of absorbed dose? |
Rads = 100 ergs/g
Grays = 100 Rads |
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What does a unit of exposure attempt to measure and what is the primary unit for this?
|
attempts to measure the complex relationship between radiation and its effect on matter and the amount of ionization that takes place.
The Roentgen (R) is the primary unit of exposure, specifically what does it measure? |
It is a measure of charge generated by the ionization of air by radiation or the electronic charge in air
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How does an free air chamber detector work?
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The beam of radiation interacts with the air in the chamber. A transducer converts ionization into electric current.(cathode and anode plates)
What is the limitation of a free air chamber? |
1. can measure only when photon flux is constant.
2. only practical to measure to 3 MeV 3. thickness of air required to achieve electronic equilibrium, which increases with energy, making free air chambers impractical outside of lab due to large size requirement |
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What is the difference between a free air chamber and a air wall chamber?
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Air wall chambers have denser material than air so electronic equilibrium is reached faster allowing for a smaller more portable chamber
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Which way do the flow of electrons (current) travel, to the negative or positive?
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positive
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In a gas filled detector what is it called when the voltage is high enough to cause complete collection of all the ions produced?
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Saturation
What is the saturation voltage? |
The point where saturation begins, usually between 50 - 300 volts depending on the design of the chamber
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In the ionization region that is flat of the graph of signal vs. voltage on a gas filled detector, what does this region signify?
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This is the range of voltage that we can operate our ionization chamber
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What are ionization chambers used for?
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1. check the integrity of shielding
2. check the radiation rate from radiation sources 3. Perform radiation protection survey |
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What are dose calibrators and survey meters used in nuclear medicine?
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they are a combination of ionization chambers with very sensitive current measuring devices called electrometers
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What happens in a gas filled chamber when the current is increased beyond the plateau saturation voltage level region (range where ionization chambers operate)
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secondary electrons are produced by collisions and the ionizations accelerate so rapidly that the process is called gas amplification factor, called the proportional region
What counters operate in this range? |
proportional counters, suitable for counting individual events, mainly used to count alpha and beta samples in research, poor x and gamma ray detectors
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When the voltage is increased further in a gas detection chamber above the proportional region what happens?
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It plataues again as an "avalanche" occurs, when the voltage is so high causing so many secondary ionizations that no change in pulse is produced by different energies.
what detector operates in this range? |
Geiger-Muller counter, which this region is named. it produces a large signal, making it easy for simple electronics to pick up, making it suitable for a portable detector. although they are not able to distinguish between different types of energies. Once used primarily in NM in 50's now replace by other detectors and may be used for survey meters for radiation protection purposes
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What is the problem that happens when there is an "avalanche of ionizations" in a GM meter?
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It can produce UV light and cause more ionizations, causing a pulsating series of discharges. How is this problem solved?
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by using a quenching gas, argon or other noble gas
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How do you convert from counts per minute CPM to disintegrations per minute DPM?
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you must know the efficiency of the detector you are using and the background rate and apply the following formula:
CPM - BKG rate/Detector efficiency = DPM |
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How do you determine the efficiency of a detector?
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You compare it with a reference source making sure you use consistent units: see following problem as an example:
A Co-57 ref source is 0.102 microcuries on the day of testing. The source is counted in a well and a net count of 197,100 cpm is obtained. Step 1: convert: 197,100 cpm/37,000 dps x 60 =0.0888 micro curies Step 2: divide measured activity by ref source: 0.0888 microcuries/0.102 microcuries = 0.87 x 100 = 87% |
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