Lecture 8

Question 1

Why is gamma so good for detection

Answer 1

You can identify what is there via spectroscopy, it also travels the furtest distance

Question 2

Two types of gamma radiation detectors

Answer 2

Hand-held monitors
Gamma spectrometers

Question 3

Examples, positives and negatives of hand held gamma monitors

Answer 3

GM tube, Scintillations and micr-gamma spectrometers

ADV
Portable and real time measurements

DISADV
Cannot distinguish specific radionuclides

Question 4

Positives and negatives of gamma spectrometers

Answer 4

ADV
can identify specific radionuclides

DISADV
measurements typically slow and expensive

Question 5

What is secular decay

Answer 5

State that occurs in certain radioactive decay chains when the rate of production of a radioactive daughter is equal to its rate of decay. Relies on the decay of production rate of the parent and the decay rate of the daughter remaining constant and unchanged

Question 6

How do you detect Uranium and Thorium via gamma radiation and why

Answer 6

Secular decay and measurement of the daughter, because Ur and Th they emit alpha not gamma

Question 7

For secular equilibrium how long does it take to get back to this state after one of the isotope groups has been removed

Answer 7

The time of the longest half life in the chain (think cascading waterfall feature)

Question 8

In spectroscopy what is the Isotope we look beneath for all the answers

Answer 8

Radon-226

Question 9

Considering Survey Methodology: what should be considered in terms of Flight Line Direction

Answer 9

right angles to geological structures of interest (strike)
mountains - follow contours
searching for radioactive objects - parallel to long axis of search zone
Fallout monitoring - right angle to wind direction

Question 10

Considering Survey Methodology: what should be considered in terms of Flight Line Spacing

Answer 10

Determined by budget available
reconnaissance scale geological survey - 1km typical
Detailed (ei uranium exploration) - as little as 100m spacing and flight height
Fallout - large spacing used initially for first look followed by more specific line spacing

Question 11

Considering Survey Methodology: what should be considered in terms of Flight Altitude

Answer 11

Maintaining the same height for the survey everywhere
Every 100m extra elevation increases the attenuation to a degree that half of the ground aplitude is lost

Question 12

Considering Survey Methodology: what should be considered in terms of Detector Volume

Answer 12

The detector volume is based on the weight that the aircraft can carry
Lower to the ground smaller detector volumes can be used because the signals are stronger
For fallout detection the signals can be strong enough to overwhelm the circuits on a bigger volume and so smaller detector volumes are favoured

Question 13

Considering Survey Methodology: what should be considered in terms of Sampling Rate

Answer 13

Normally data is collected once per second
As new data is collected the old data is being processed, this stretches the data collection area

Question 14

Considering Calibrations and Processing: what should be considered in terms of Equipment and Dead Time

Answer 14

While one pulse is being analysed all other recieved pulses will be rejected - this time is called dead time and must be corrected for

Question 15

Considering Calibrations and Processing: what should be considered in terms of Cosmic and Aircraft Backgrounds

Answer 15

Counts increase exponentially with height due to cosmic radiation. Can remove these by recording anything over 3 MeV as no terrestrial gamma rays have energies above this
Flights when there is an on-shore breeze ensure that the radon contribution is negligible

Question 16

Considering Calibrations and Processing: what should be considered in terms of Radon Background

Answer 16

The rate of diffusion depends on such factors as air pressure, soil moisture, ground cover, wind and temperature.

Radon concentrations near the ground are higher in the still conditions in the early morning than in the afternoon, after mixing has occurred

Question 17

How to work out background radon

Answer 17

Fly over the sea, remove aircraft and cosmic components and you are left with the radon background levels

Question 18

What is ‘Stripping’

Answer 18

The spectra of K, the U series and the Th series overlap meaning that the detection of one of them will contain traces of the other radioelements. Stripping is the removal of these traces.

Question 19

What are ‘stripping ratios’

Answer 19

the ratios of the counts detected in one window to those in another window for pure sources of K, U, Th

Question 20

Considering Calibrations and Processing: what should be considered in terms of Survey Altitude

Answer 20

The air that gamma rays pass through between source and detector will attenuate some of the rays

Question 21

What is the equation associated to attenuation of gamma rays passing through air

Answer 21

Nh = N0e^(-uh)

Nh = background corrected and stripped count rate
N0 = count rate at ground level
u = attenuation coefficient
h = height above ground level

Question 22

Considering Calibrations and Processing: what should be considered in terms of Radioelement concentrations

Answer 22

The corrected count rates can be used to estimate the concentrations of the elements K, U, Th.

Question 23

What is the equation for working out the radioelement concentrations

Answer 23

C = N/S

C = concentration of element
S = broad source sensitivity for the window
N = count rate for each window after corrections applied