# 5. Fitting schemes

Processing data in Gamman is the step where measured gamma spectra will be analysed, which result in a set of activity concentrations. When processing data in Gamman, you have to select one of the available fitting schemes. When performing a moving measurement, the fitting scheme you choose, and the options you select have an effect on the accuracy of the activity concentrations and on the spatial resolution of the data.

A spectrometer collects data not from a point, but from an area surrounding the spectrometer. For a carborne measurement, data can be collected easily from a circle with a radius of 3 meters or more. For an airborne measurement, the circle is much larger and can be over 100 meters, depending on the altitude of flight. While performing a moving measurement, the circle moves along with the spectrometer, resulting in a covered area as shown in the figures below.

1 second at 1 Hz

## B

5 seconds at 0.2 Hz

## C

5 seconds at 1 Hz

## D

5 seconds at 5 Hz

Figure 1: The area contributing to the recorded gamma spectra during a moving measurement. The circle shown has a radius of 5 meter, speed during the measurement is 4 m/s (14.4 km/h). A: The area contributing to a single measurement of 1 second; B: The area contributing to a single measurement of 5 seconds; C: The effect of combining 5 measurements of 1 second each; D: Similar to C, but with 0.2 seconds per measurement.

### 5.1 Fitting schemes

As you can see from figures 1A and 1B, read-out frequency has an effect on the area covered while recording a gamma spectrum. Increasing the measurement period results in a larger area covered, which is often undesired. However, shorter measurement time will result in larger uncertainties. Gamman allows for combining multiple spectra through the Process dialog.

##### Fit each

Each individual spectrum will be processed separately.

##### Smart fit

Combines spectra before analyzing the data. The smoothing factor determines how many spectra will be combined. A smoothing factor of 5 for spectra recorded at 1 Hz results in 5 seconds of data being combined for analysis. The same area recorded at 5 Hz would require a smoothing factor of 25 to get the same results. The Smart fit performs a running average, it first combines spectra 1 to 5, then 2 to 6 etc. Auxiliary data, such as GPS position is averaged.

##### Repeated fit

Much the same as the Smart fit, but does not overlap analysis intervals. When combining 5 spectra, Gamman will first combine spectra 1 to 5, then 6 to 10 etc.

##### Conditional fit

This fitting scheme will combine spectra until all required uncertainties are being met.

Performs a traditional 3 windows analysis, where all other fitting schemes use the more advanced full spectrum analysis. It is mentioned on the page for reference only.

### 5.2 Summary

Fitting schemes allow for combining gamma spectra. Combining more gamma spectra results in more accurate nuclide concentrations, at the cost of spatial resolution.