Methods of Detecting Ionizing Radiation

Question 1

State the basic principal behind ionization in a gas filled chamber.

Answer 1

Ionization of gas occurs when incoming radiation enters a detector. The ion migrates to the opposing charged electrode due to current production from the ion, and is detected as voltage. Voltage changes to the anode/cathode will increase forces on ions in the chamber causing greater acceleration of ions, each voltage range is associated with differing proportionality of ions in to output.

Question 2

Name the three counting regions (voltage ranges) associated with ionization in a gas filed chamber and give an example of each.

Answer 2

1. Ionization chamber Region: Ex. IM-231/PD.
2. Proportional Chamber Region (only useful for neutron): Ex. AN/PDR-70
3. Geiger Muller Chamber Region: Ex. IM-271/PDQ, IM-265/PDQ

Question 3

The DT-681 is used for alpha radiation. What is the minimum detectable activity (in Ci) and how many counts is that? How high off of the surface should you hold this probe?

Answer 3

MDA is 50uuCi
<8 counts per minute is below detectable activity.
Thumb Rule: 1 cpm = 6.2uuCi/100cm2, only accurate above MDA.
Hold 1/8” above surface.

Question 4

What is the RADIAC used for frisking in NNPP operations?

Answer 4

IM-271 with DT-304 Probe

Question 5

The DT-304 is used for gamma/beta contamination. What are the thumb rules? How high above the surface should it be held?

Answer 5

• 100cpm above background = 450uuCi/100cm swipe OR direct frisk.
• 100cpm above background on chest/abd is approx. 1uCi of Co-60 internally.
  1/2” above surface.

Question 6

The DT-680 is used for gamma/beta survey; what are the thumb rules for use?

Answer 6

• 30 clicks/min = 0.01mR/hr
• 0.04mR/hr on chest/abd equals 1 uCi equivalent Co-60 in abd/lungs.
• 13.2mR/hr at 1cm over wound = 1uCi equivalent Co-60 in wound.
• 0.1mR/hr at surface = 105uuCi/100cm2

Question 7

What is the counting efficiency ratio?

Answer 7

The ratio of detected to incident counts. Efficiency = CPM (counts/min) / DPM (decays/min).

Question 8

Define minimal detectable activity.

Answer 8

The minimum amount of radioactivity that can be detected with a radiation detection system at a stated level of confidence; usually 95% CI.

Question 9

Describe a type 1 error.

Answer 9

Detecting activity where none is present.

Question 10

Describe a type 2 error.

Answer 10

Failing to detect activity despite it being present.

Question 11

What are the 4 checks to a RADIAC prior to use?

Answer 11

Physical integrity check: cables, switches, etc.
Calibration Check: Annually done, check sticker.
Battery Check: Adequate power supply.
Source Check: Ensure it is detecting radiation from a known source; daily for contamination instruments and weekly for radiation instruments.