Survey for testing airborne full spectrum corrections in Gamman

Survey summary

Survey description 

Whenever a detector is operating at a large height, the gamma radiation it measures will be less than the pure radiation emitted by the ground. When the distance between the ground (which is the source of our radiation) and the detector increases, less terrestrial radiation will be received, since an increasing portion will be absorbed by air. Simultaneously, the detector will also collect gamma rays coming from cosmic sources and radioactive radon (²²²Rn) nuclides in the air. With this data set we take a look at the Airborne correction feature in Gamman. This feature takes the raw data and corrects it for height, cosmics and radon radiation.

Gamman offers two types of airborne corrections. The first is the “classic” method which uses height-dependent absorption models and an effective height based on pressure and temperature.
Another method, which will be used for this analysis, is the Full-spectrum elevation correction. This approach is based on results from Monte Carlo simulations. From these simulations, a collection of standard spectra at heights between 0.5 and 160 meters is created and contained in Gamman. In combination with the air pressure and temperature, a standard spectrum takes into account the absorption of terrestrial radiation in air for each data point with a certain height.

The survey chosen is a test (airplane-) flight with a Medusa 4 liter CsI spectrometer near Breckenridge, Canada. It is separated into three parts. First, some test flights were done between 60 and 240 meters above the ground. Then, a grid at 280 meter above the ground with line spacings of 200 meter is flown above the Gitineau hills. And lastly, cosmic calibration test flights were conducted at heights of 2400 to 3000 meters above sea level.

Data processing

We start off with the raw data loaded into Gamman. If we go to data view, we can display some information about the survey. With the available gps data it can be seen how the survey has been conducted. When plotting the longitude against the latitude, we spot a grid (see figure 1.) Combining this with information from the plot of spectrum number against altitude (see figure 2,) we see that this grid has been flown at a, more or less, constant height and that outside the grid, there are some variations in height. These measurements are done to test some features in Gamman; finding cosmic and carrier backgrounds, filtering out any radon presence in air and performing airborne corrections.

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Cosmic Wizard
The cosmic and carrier background can be found with the Gamman feature “Cosmic Wizard.” The cosmic background consists of radiation with energy of 511 keV, coming from electron-positron annihilation, or energy larger than 3 MeV which will be out of the energy range of interest (0-3 MeV.) Its spectrum has a constant shape and increases exponentially with height. Given that the number of counts from terrestrial radiation decrease with increasing height, this means that the cosmic background becomes more dominant at larger altitudes. The carrier background stays the same throughout the measurement.

The cosmic wizard needs data points obtained at a height that is larger than 600 meters. At smaller altitudes, the terrestrial radiation will be large enough and the cosmic background small enough, such that the cosmic background can be neglected. We select the data points in the survey measured towards the end of the measurement. In figure 2 can be seen that these points are obtained above 600 meter. When selected, the Cosmic Wizard can be opened under the tab Tools | Airborne corrections | Cosmics. The Cosmic Wizard tries to fit the cosmic and carrier background to the selected data points (see figure 3.) In the "summed spectrum" tab (see figure 4) can be seen that the selected spectrum does not fit very well to the standard (terrestrial radiation) spectra. This cosmics term comes from a so-called cosmics channel in the data. This can be a separate channel where all counts above 3 MeV are stored. It can also be the last spectrum channel, choose which is applicable. Since all radiation with energies above 3 MeV come from cosmic radiation and the cosmic background has a constant shape, the cosmic background scales linearly with the cosmics channel. In the background spectrum window, the red line represents the cosmic background found and the green line the carrier background.

If needed, several options can be adjusted to make a best fit for the backgrounds.

• The A1 adjuster fine-tunes the stabilization of the spectrum.
• The USE NNLS box lets you select non-negative least squares.
• The power of the 1/E model is normally between 1 and 1.5.
• The 1/E scaler adjusts the intensity of the 1/E continuum w.r.t. the model peaks in the cosmic spectrum. This should be around 5.

When pressing the “close” button, Gamman will save the created background spectra and they can then be seen in the “Background editor” (see figures 5 and 6) which can also be found under the “Tool” tab. In the cosmics spectrum, we can clearly see the peaks at 511 keV (coming from electron-positron annihilation due to cosmic radiation) and at 3 MeV (cosmics channel).

Radon Finder
The cosmic and carrier background are not relevant for determining radionuclide concentrations with measurements flown at small heights, such as that of the grid in this survey. These backgrounds can be neglected. What might be relevant is to determine the background coming from possibly present radioactive radon nuclides in the air. The Radon Finder in Gamman (found under Tools | Airborne corrections | Radon Finder) can be used for this.
It can be hard to distinguish the radon spectrum from the Uranium spectrum since 222Rn is part of the decay chain of 238U. The main reason why the spectra are different and therefore distinguishable is because of the geometry. The radon in the air is much closer to the detector than the uranium in the soil. That means that radiation from radon will have a lower chance to be absorbed by air, especially in the lower energy regions. The radon spectrum has thus more low energy counts than the uranium spectrum. The Radon Finder tries to calculate the radon correction using a simulated standard radon spectrum and the standard spectra of 40K, 238U, 232Th at the correct heights.
When opened (see figure) the Radon Finder asks for the detector-specific elevation correction database, the heights, pressure and temperature of the data points and gives a potential option to subtract the cosmic and carrier backgrounds that were created earlier (see figure). When choosing to substract the cosmic background, the number of records to integrate can also be adjusted to improve the stability of the solution. The elevation correction database we choose is the one for the "CsI4x4x16"  detector, since that is the 4 liter CsI detector that was used for the survey. For elevation, we choose the "alt radar" column, which represents the height above the ground (while the columns "alt" represents het height above sea level.) The pressure and temperature are stored in the columns "Pressure" and "Temperature"  respectively.  We neglect the contribution of cosmic and carrier background by ticking the box "No cosmic".
On the next page, the radon correction can be performed. Here is an option to choose the number of records to integrate as well as an NNLS and smoothening option. When clicking “Run Radon Processor” the radon concentrations will be calculated. In the radon viewer, the created radon concentrations can be visualized in a diagram (see figure). When opting “close” the radon concentrations will be stored as separate data in the column called "Radon".

Results
Now the corrected concentrations are calculated, we can analyze the elevation corrected results of the survey.

Full spectrum airborne corrections
With the radon data, we can perform the full spectrum height corrections. The Full-spectrum elevation correction can be found under Tools|Airborne corrections and gives a window similar to that of the Radon Finder (see figure). Again, the type of detector, height, pressure and temperature need to be selected. Additionally, the radon background, together with the cosmic and carrier background, can be subtracted from the raw data. The second window can run the elevation corrector and in the data viewer can be seen what the results are. The elevation corrector calculates the corrected total counts and concentrations of the different radionuclides as separate data sets.
*In order to perform height corrections, data of the nuclide concentrations need to be present. Then the corrector can calculate the new, corrected concentrations. If this data is not initially present in your data, the raw data need to be processed in Gamman first. Here, we already did this when determining the radon concentrations.