Survey Instruments (2)

Question 1

What are the two types of survey meters used in radiography?

Answer 1

1. Ionization Chamber

2. Geiger Muller Detector

Question 2

Describe an Ionization chamber survey meter

Answer 2

• gas filled chamber (air) containing two electrodes
• amplifier
• readout device
• battery to supply power

Question 3

How does an Ionization chamber survey meter work?

Answer 3

relies on the ionization of the gas molecules to detect radiation.

Electric field in the chamber from the battery, when photons enter they interact with gas creating ion pairs, producing + and - ions that migrate to the opposing charge, creating a momentary drop in voltage allowing current to flow - flow is measured with an ammeter.

Question 4

Facts about Ionization chambers

Answer 4

• low sensitivity

- measure high radiation fields

Question 5

What is an Analog survey meter?

Answer 5

Two scales, four settings

top scale = high intensity radiation (1mR to 10R/h or 20uSv to 100mSv/h)

Settings:

• off position
• check position
• high scale
• low scale

Question 6

What do survey meters measure?

Answer 6

Dose Rate

Question 7

Describe the Geiger Muller Detector survey meter

Answer 7

• uses higher voltage
• gas = argon
• gas interacts with incoming ions

Question 8

Facts about the Geiger Muller Detector

Answer 8

• measure low radiation doses

- can become saturated if there is to many ion pairs

Question 9

How does a Geiger Muller Detector work?

Answer 9

Because of the high voltage Gas Multiplication occurs. # of electrons collected at anode depends on # of ion pairs produced

Question 10

What is gas multiplication or amplification?

Answer 10

The production of electrons (negative ions) is sped up due to high voltage

Question 11

Settings on a Geiger Muller Detector

Answer 11

off
x100 = 0 to 1000mR/h
x10 = 0 to 100mR/h
x1 = 0 to 10mR/h
x0.1 = 0 to 1mR/h

Question 12

External vs Internal Detectors

Answer 12

external = detect non-waveform radiation (read in counts-per-minute) referred to as Pancake Detectors or Contamination Detectors

Question 13

Survey Meter requirements

Answer 13

enforced by CNSC + Health Canada

• worn at all times in radiography
• dose dates 20uSv/h to 100mSv/h
• way to check battery
• calibrated every year
• readings accurate to +/- 20%