Outcome 5

Question 1

scintillation detectors are more efficient than gas detectors. t/f

Answer 1

true

Question 2

can scintillation detectors differentiate energies?

Answer 2

it has energy discrimination

Question 3

what are the 4 examples of scintillation detectors?

Answer 3

1. well counter
2. uptake probe
3. gamma cameras
4. PET cameras

Question 4

what are the four performance characteristics of scintillation detectors?

Answer 4

1. high voltage setting
2. energy resolution
3. statistical variability of the detector
4. ability to determine absolute radioactivity

Question 5

what causes the high voltage of a detector to fluctuate?

Answer 5

unstable HV supply

Question 6

what causes gradual changes to HV in a detector?

Answer 6

loss of optical coupling or hermetic seal

Question 7

what is energy peaking?

Answer 7

it defines the relationship between the pulse size and energy of the gamma ray
- use of photopeak window

Question 8

calibration

Answer 8

measuring the stability of the system and checking the HV settings of the detector

Question 9

what is a reasonable fluctuation frame for constancy?

Answer 9

+/- 10%

Question 10

what QC are done DAILY?

Answer 10

• calibration
• constancy

Question 11

frequency of energy resolution qc?

Answer 11

quarterly or annually

Question 12

components of efficiency

Answer 12

• detector composition
• detector geometry
• energy window
• absorption and scatter

Question 13

sensitivity in terms of detector efficiency

Answer 13

detector’s ability to convert radiation emission into useful signals

Question 14

MDA

Answer 14

demonstrates capability of a detection system

Question 15

MDA must be _____ (higher/lower) than detection limit

Answer 15

lower

Question 16

reference standards to test detector efficiency needs to have these characteristics.

Answer 16

1. long-lived source
2. known absolute activity in dpm
3. emit gamma rays in similar energy to isotope of interest

Question 17

what are the possible causes of “lower than expected constancy reading”?

Answer 17

• incorrect high-voltage setting
• incorrect window setting
• incorrect source-detector geometry

Question 18

what are the possible causes of “higher than expected constancy reading”?

Answer 18

• contamination/source present
• incorrect source-detector geometry

Question 19

what are the possible causes of “widening FWHM”?

Answer 19

• decoupling of PMT from scintillation crystal
• damage to the crystal

Question 20

detector efficiency is dependent on what 3 factors?

Answer 20

1. detector type, size and shape
2. distance between detector and source
3. type of radiation

Question 21

frequency of efficiency qc?

Answer 21

annually

Question 22

frequency of chi-square qc?

Answer 22

monthly or quarterly

Question 23

efficiency is important for what reasons?

Answer 23

• performance of wipe tests (checking packages for contamination)
• thyroid bioassays

Question 24

for chi-square results, what is in the acceptable range?

Answer 24

p = 0.9-0.1

Question 25

chi-square result of >0.9 = ?

Answer 25

“too good to be true”

Question 26

chi-square result of < 0.1 = ?

Answer 26

variation too great

Question 27

what is the purpose of the efficiency qc?

Answer 27

ability of the system to correlate a measured count rate to an absolute activity

Question 28

what is the acceptable range for energy resolution for Cs-137? Tc-99m?

Answer 28

Cs-137 = 6-8%
Tc-99m = 10-12%

Question 29

formula for efficiency

Answer 29

efficiency = (cps/dps) *100%

Question 30

formula for energy resolution

Answer 30

% e reso = (FWHM/centre peak) * 100%

Question 31

higher efficiency = ____ (lower/higher) MDA

Answer 31

higher efficiency = lower MDA