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NMED 270 - QC1 > Outcome 3 > Flashcards

Outcome 3 Flashcards

QC on Survey Meters

1
Q

examples of survey meters

A
  • ionization chamber
  • GM gas detector
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2
Q

timeline for calibration of both equipment.

A
  • before 1st use
  • following a repair that effects calibration
  • annually
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3
Q

radiation that interactions with atoms/molecules results in?

A

ionization - creates ion pairs

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4
Q

what are ion pairs?

A
  1. free electron
  2. remaining atom/molecule that is positively charged
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5
Q

*number of electrons is related to the amount of radioactivity/radiation present

A
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6
Q

simplest radiation detector =

A

gas-filled detector

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7
Q

examples of gas filled chambers

A
  • dose calibrator
  • ionization survey meter
  • Geiger counter
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8
Q

what are the two essential parts to a gas detector?

A
  1. chamber filled with gas
  2. basic electric circuit
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9
Q

neutralization (electrons that reach the anode) measured by?

A
  • current - current mode
  • voltage - pulse mode
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10
Q

number of electrons depends on:

A
  • strength of radiation source/field
  • energy of radiation
  • geometric configuration of the detector
  • composition of the gas
  • volume, pressure and temp of gas
    ***major determinant = applied voltage
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11
Q

list the voltage vs. amplitude regions.

A
  • recombination region
  • saturation/ionization region
  • proportional counter region
  • GM region
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12
Q

what is current mode?

A
  • measures the number of electrons/sec required to keep the anode/cathode charged
  • when neutralization occurs, electrons from the power source will flow to restore the charge
  • reaches steady state over time as long as the source field strength and detector-source distance are the same
  • needs a large amount of radiation
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13
Q

what functions under current mode?

A

ionization chamber and dose calibrator

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14
Q

what is pulse mode?

A
  • electrons are created by each radiation interrelation and pulse is a single entity
  • size of pulse represents the total charge deposited by a single radiation interaction
  • measure changes in voltage
  • pulse shaping
  • number of pulses/sec
  • requires low rate of radiation (=dead time)
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15
Q

what functions under pulse mode?

A

GM counter

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16
Q

ionization survey meters are read in?

A

rate mode (mR/hr or uSv/hr)

17
Q

integrate mode

A

total accumulated radiation exposure in mR (uSv) over a period of time

18
Q

uses of ionization survey meters

A
  • can be used interchangeably with the GM *but requires a large radiation flux
  • monitoring radiation levels of patients receiving radionuclides for therapy
  • ambient radiation levels
19
Q

ionization survey meters are good to check for contamination. t/f

A

false.

20
Q

what type of gas does the GM use?

A

helium or argon gas

21
Q

GM features:

A
  • very sensitive so good for detecting small amounts of contamination
  • reads out in cpm or exposure (mR/hr or uSv/hr)
  • audible
  • portable and raggedly built
22
Q

what are the variation of configurations for the GM?

A
  • pancake detector
  • cylindrical detector; GM tube
23
Q

what is the metal backing on the pancake probe for?

A

it scatters the gamma rays back into the active volume

24
Q

what are the daily QC needed for the survey meter?

A
  • check the battery
  • reading bkg levels
  • check on the constancy of the detector’s response to sources
25
Q

why are background readings needed to be done as daily qc for the survey meter?

A

ensures that there is no contamination

26
Q

what constancy qc is done for the survey meter?

A
  • measures radioactive sources
  • reproducible geometry
  • readings should be within +/- 10%
27
Q

what is the annual qc done on the survey meter ?

A

accuracy

28
Q

is the accuracy qc done by techs?

A

no, it is done by a commercial calibration lab or a NM product vendor

29
Q

how does testing accuracy of the a survey meter happen?

A

they compare readings from the detector from a source of known activity; compare to a range/scale

30
Q

if the accuracy is not right (out of the +/- 10% range), what should we do?

A
  • need to calibrate to bring back to compliance
  • apply correction chart that is supplied if out by 10-20%
31
Q

gamma constant

A
  • gives the radiation field strength (mR/hr)
  • produced from 1 mCi of a radionuclide at a specified distance
    **measuring something with known radiation strength
32
Q

what is a possible cause if there is a higher-than-expected constancy reading?

A
  • contamination
  • incorrect measuring distance or reference standard
33
Q

what is a possible cause if there is a lower-than-expected constancy reading?

A
  • incorrect measuring distance or reference standard
  • shielding material between standard and detector
34
Q

what is a possible cause if the reading decreases as distance decreases?

A

dead time

35
Q

what is likely the cause if the GM is non-responsive?

A
  1. batt dead
  2. cable wires broken
  3. thin end window damaged

NMED 270 - QC1 (6 decks)

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