Outcome 3

Question 1

examples of survey meters

Answer 1

• ionization chamber
• GM gas detector

Question 2

timeline for calibration of both equipment.

Answer 2

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

Question 3

radiation that interactions with atoms/molecules results in?

Answer 3

ionization - creates ion pairs

Question 4

what are ion pairs?

Answer 4

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

Question 5

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

Question 6

simplest radiation detector =

Answer 6

gas-filled detector

Question 7

examples of gas filled chambers

Answer 7

• dose calibrator
• ionization survey meter
• Geiger counter

Question 8

what are the two essential parts to a gas detector?

Answer 8

1. chamber filled with gas
2. basic electric circuit

Question 9

neutralization (electrons that reach the anode) measured by?

Answer 9

• current - current mode
• voltage - pulse mode

Question 10

number of electrons depends on:

Answer 10

• 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

Question 11

list the voltage vs. amplitude regions.

Answer 11

• recombination region
• saturation/ionization region
• proportional counter region
• GM region

Question 12

what is current mode?

Answer 12

• 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

Question 13

what functions under current mode?

Answer 13

ionization chamber and dose calibrator

Question 14

what is pulse mode?

Answer 14

• 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)

Question 15

what functions under pulse mode?

Answer 15

GM counter

Question 16

ionization survey meters are read in?

Answer 16

rate mode (mR/hr or uSv/hr)

Question 17

integrate mode

Answer 17

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

Question 18

uses of ionization survey meters

Answer 18

• 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

Question 19

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

Answer 19

false.

Question 20

what type of gas does the GM use?

Answer 20

helium or argon gas

Question 21

GM features:

Answer 21

• 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

Question 22

what are the variation of configurations for the GM?

Answer 22

• pancake detector
• cylindrical detector; GM tube

Question 23

what is the metal backing on the pancake probe for?

Answer 23

it scatters the gamma rays back into the active volume

Question 24

what are the daily QC needed for the survey meter?

Answer 24

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

Question 25

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

Answer 25

ensures that there is no contamination

Question 26

what constancy qc is done for the survey meter?

Answer 26

- measures radioactive sources - reproducible geometry - readings should be within +/- 10%

Question 27

what is the annual qc done on the survey meter ?

Answer 27

accuracy

Question 28

is the accuracy qc done by techs?

Answer 28

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

Question 29

how does testing accuracy of the a survey meter happen?

Answer 29

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

Question 30

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

Answer 30

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

Question 31

gamma constant

Answer 31

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

Question 32

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

Answer 32

- contamination - incorrect measuring distance or reference standard

Question 33

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

Answer 33

- incorrect measuring distance or reference standard - shielding material between standard and detector

Question 34

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

Answer 34

dead time

Question 35

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

Answer 35

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