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12 Cards in this Set
- Front
- Back
- 3rd side (hint)
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What is the purpose of a preamp?
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To act as an interface between the detector and the amplifier, ie. matches the electrical waveform of the detector to the amplifier
What is the magnification factor of a preamp? |
detectors put out signals as small as a few millivolts and preamps magnify the signal by 5 to 20 times
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What does an amplifier do?
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Increase the signal from the detector
What is its amplification factor and max output |
a factor of 1000 is common and will peak at 10 V
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What is energy discrimination
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the ability of a detector to identify between different radiations and energies.
Which detectors are able to do this? |
Proportional counters do this best but are only good with beta and alpha emissions, Ionization chambers can discriminate as well and can pick up x and gamma (though very poorly) and GM cannot differentiate at all
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Explain a differentiating or CR circuit
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It consists of a capacitor and a resistor. it causes an incoming signal to decay faster than it would normally, in effect cutting off the end of the signal. What effect does this have?
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It does not allow low frequency signals to pass through which is why it is also know as a high pass filter
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What is the opposite of a CR circuit?
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A RC circuit or Integrating circuit which is a low pass filter. also consisting of a capacitor and a resistor but the output is taken across the capacitor. It does not allow high frequency signals to pass.
What is the importance of having these discriminating and integratng circuits? |
The "noise" of high and low frequencies would be amplified as well
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What is windowing?
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The combination of a LLD (low level discriminator) which uses a high pass circuit with ULD (upper level discriminatior) which uses a low pass circuit
only pulses received in the "window" are recorded how does this apply to nm cameras? |
selective windowing allows a specific photopeak to be selected corresponding to the peak output of a particular isotope, scattered photons are not seen
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When dealing with efficiency what is "self attenuation"?
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absorption and scatter of radiation by the source itself, esp important in beta and alpha emitters, also in imaging the patient absorbs a portion of gamma energy
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What is geometric efficiency?
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Size, shape and distance and the detector all determine how efficient the radiation is received. efficiency decreases with increased source to detector distance
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What is intrinsic efficiency?
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a function of detector thickness, compostion (atomic #) and energy of radiation to be detected. also can be explained by the following ratio:
Intrinsic eff = # radiations interacting with detector/# radiations striking the detector |
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Explain how intrinsic efficiency is energy dependent?
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a high energy photon is more likely to penetrate a detector than to interact with it
What is more efficient, a gas detector or a sodium iodide detector for detection of gamma rays? |
sodium iodide is quite good, almost 100% while gas is very poor, around 1-3%
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What is deadtime?
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A counting system takes time to process individual detected events and when there is overlap in these events there is a distorted or non-existent pulse
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How does deadtime effect counts?
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Actual counts are actually higher than the recorded ones and at very high rates the recorded counts decrease or stop altogether in a paralyzable system.
explain non-parlyzable and parlyzable systems? |
non-paralyzing systems simply ignore an event that occurs during the deadtime of a preceding event while a paralyzing system introduces a deadtime whether it is counted or not, where additional events would not be recorded, (most detectors are paralyzable systems)
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