Gamma Surveying

Question 1

What type of radiation is easiest to detect in the field?

Answer 1

Gamma

Alpha is hard to detect at distance from source

Question 2

What detector is best for measuring radiation in the field?

Answer 2

Gas-filled detector

• Produces the most intense output for lowest applied voltage.
• Hand-held can detect gamma or alpha but cannot identify specific radionuclides

Question 3

What has to be considered when choosing field detector?

Answer 3

• Size
• Weight
• Energy
• Intensity of radiation

Question 4

Why does radiation mapping only detect near surface sources?

Answer 4

Gamma rays can be stopped by water and other molecules so can only detect near surface.

Question 5

What is gamma ray emission usually from?

Answer 5

U-238 decay through to Pb-206 (daughter isotope)

Question 6

How can you use gamma ray spectrometry to work out alpha radiation?

Answer 6

Detect gamma and work out the equivalent activity.

Question 7

What is secular equilibrium?

Answer 7

Occurs when all of the members of the decay chain have short half-lives compared to the original parent and the system has been undisturbed for longer than the half-life of the longest lived daughter.

Question 8

What has to be taken into account when surveying?

Answer 8

1. Flight line directions
2. Flight line spacing - want high res, but wide area coverage.
3. Flight altitude - constant, generally 120m. Gamma rays are attenuated by air exponentially.
4. Detector volume - determined by aircraft capacity
5. Sampling rate - want continuous spectral data

Question 9

What has to be considered for calibration and processing?

Answer 9

1. Equipment dead time
2. Cosmic an aircraft background
   - cosmic radiation will increase exponentially with height
   - can correct by flying over water bodies and working out what is coming from above
   - usually ~3MeV
3. Radon background
   - can diffuse from ground, easily entrained
   - can remove by flying over water and working out aircraft and cosmic combined, then know whats radon
4. Stripping ratios
   - isotopes overlap in spectrum windows
5. Survey altitude
   - gamma rays attenuated with air
6. Radioelement concentration
   - corrected count rates used to estimate K, U and Th concentrations