Lecture 8 Flashcards
Why is gamma so good for detection
You can identify what is there via spectroscopy, it also travels the furtest distance
Two types of gamma radiation detectors
Hand-held monitors
Gamma spectrometers
Examples, positives and negatives of hand held gamma monitors
GM tube, Scintillations and micr-gamma spectrometers
ADV
Portable and real time measurements
DISADV
Cannot distinguish specific radionuclides
Positives and negatives of gamma spectrometers
ADV
can identify specific radionuclides
DISADV
measurements typically slow and expensive
What is secular decay
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
How do you detect Uranium and Thorium via gamma radiation and why
Secular decay and measurement of the daughter, because Ur and Th they emit alpha not gamma
For secular equilibrium how long does it take to get back to this state after one of the isotope groups has been removed
The time of the longest half life in the chain (think cascading waterfall feature)
In spectroscopy what is the Isotope we look beneath for all the answers
Radon-226
Considering Survey Methodology: what should be considered in terms of Flight Line Direction
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
Considering Survey Methodology: what should be considered in terms of Flight Line Spacing
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
Considering Survey Methodology: what should be considered in terms of Flight Altitude
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
Considering Survey Methodology: what should be considered in terms of Detector Volume
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
Considering Survey Methodology: what should be considered in terms of Sampling Rate
Normally data is collected once per second
As new data is collected the old data is being processed, this stretches the data collection area
Considering Calibrations and Processing: what should be considered in terms of Equipment and Dead Time
While one pulse is being analysed all other recieved pulses will be rejected - this time is called dead time and must be corrected for
Considering Calibrations and Processing: what should be considered in terms of Cosmic and Aircraft Backgrounds
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
Considering Calibrations and Processing: what should be considered in terms of Radon Background
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
How to work out background radon
Fly over the sea, remove aircraft and cosmic components and you are left with the radon background levels
What is ‘Stripping’
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.
What are ‘stripping ratios’
the ratios of the counts detected in one window to those in another window for pure sources of K, U, Th
Considering Calibrations and Processing: what should be considered in terms of Survey Altitude
The air that gamma rays pass through between source and detector will attenuate some of the rays
What is the equation associated to attenuation of gamma rays passing through air
Nh = N0e^(-uh)
Nh = background corrected and stripped count rate
N0 = count rate at ground level
u = attenuation coefficient
h = height above ground level
Considering Calibrations and Processing: what should be considered in terms of Radioelement concentrations
The corrected count rates can be used to estimate the concentrations of the elements K, U, Th.
What is the equation for working out the radioelement concentrations
C = N/S
C = concentration of element
S = broad source sensitivity for the window
N = count rate for each window after corrections applied
