Methods of detecting Ionizing Radiation

Question 1

State the basic principle behind ionization in a gas filled chamber

Answer 1

Detector has positive end (anode) and a negative end (Cathode)

A current is produced. Measured as an output mR/hr or counts per minute

Electron travel to Positive anode. Positive ions to cathode

voltage is increased between anode and cathode,

Question 2

Name the three counting regions associated with ionization in a gas filled chamber and give example of each

Answer 2

Ion Chamber Region- Most accurate but least sensitive.
Im-231/PD (X-rays) AN/PDR-73 (Nuclear Weapons)

Proportional Region- Boron Trifluoride (BF3)
An/PDR 70

Geiger-Muller Region- Most sensitive least accurate.
IM-271/PDQ with DT-304/PDR probe

Question 3

Alpha Contamination

Answer 3

MFR (IM-265/PDQ) with DT-681/PDQ
Mode of Operation- Scintillation (Solid state Detector) measure contamination
Prode- 100cm^2 of ZnS(Ag) scintillation photomultiplier connected tube ion crystal

Minimal detectable activity 50uuCi

probe 1/8 in from surface frisk 1-2 inches per sec

Question 4

BETA and Gamma contamination

Answer 4

IM-271/PDQ (or) IM-265/PDQ with DT-304/PDR probe
Mode of operation- Geiger-Muller
Used for frisking in NNPP operation
Survey is done holding probe 1/2 in from surface frisking 1-2 inches per sec

background radiation <100. if 100-300 rate of movement must be reduced. 300+ requires relocation of probe.

Question 5

Gamma only Contamination

Answer 5

PRM-5 with SPA-3 probe
mode of operation- Scintillation

Question 6

Beta and Gamma exposure

Answer 6

IM-265/PDQ with DT-680/PDQ
Mode of Operation- Geiger Muller
1. Rule of thumbs- 0.1 mR/hr near surface- 105 uuCi/100cm^2
2. 0.04 mR/hr at chest or abdomen equals 1 mCi of equivalent 60Co in lungs
3. 13.2mR/hr at 1cm over wound = 1 mCi of 60Co in wound

IM-231/PD
Ion chamber

Question 7

Neutron Exposure

Answer 7

AN/PDR-70
Proportional gas chamber

Question 8

Gamma only Exposure

Answer 8

IM-265/PDQ
Geiger Muller gas filled chamber

Question 9

Thumb rules for DT-304/PDR frisker in relation to Co-60

Answer 9

100 cmp above background = 450uuCi per 100cm^2 on swipe

100cmp above background= 450 uuCi per direct frisk

100 cmp above background on chest or abdomen of an uncontaminated person is >1 mCi of Co-60 retained internally

100 cmp above background on a 0.3m^3 Portable Air Sampler is equal to 1 x 10^-9 uCi/ml (Co-60 equivalent airborne particulate)

Question 10

Covert counts per minute into disintegration per minute

Answer 10

E= CCPM/DPM

CCPM- corrected counts per min, read minus background

DPM- decays per minute (from activity calculations)

E= CCPM/DPM

Question 11

Define Minimal Detectable activity and Type I and II errors

Answer 11

Minimum amount of radioactivity that can be detected with a detection system at a stated level of confidence 95%.

5% chance of false positive (Type I)

5% chance of false negative (Type II)

Question 12

State all required checks on RADIAC prior to use

Answer 12

1. Physical Integrity
2. Calibration Check- calibration sticker
3. Battery Check-
4. Source Check- ensure the device is detecting radiation. Check daily for contamination and weekly for radiation instruments