Calibrating the energy scale of the spectrum is essential to accurately determine the energy of the gamma lines by their channel numbers. Devices undergo a calibration process at the factory, but detector parameters may change over time, necessitating a calibration update. This requires sources with known isotopic composition and energies in advance. The calibration points are the vertices of the peaks on the gamma spectrum. The result of the calibration is the calibration factor values, which the device uses to convert the spectrum channel number into the radiation energy value in keV using a second-degree polynomial:

E = a₀ + a₁·x + a₂·x²

where

x is the channel number.

a₀, a₁, a₂ are calibration coefficients.

Calibration is conducted at three points to ensure sufficient linearity across the entire range of energies under study. To perform the calibration, the user will need a spectrum containing monoenergetic lines, preferably located at the beginning, middle, and end of the energy scale. For example, the following isotopes produce such lines:

Th-232, a naturally occurring element, is the most convenient and simple option for calibration. In most countries worldwide, small quantities of Th-232 are legal and available for purchase in local stores, for example, in the form of welding electrodes WT-20.

You can also use alternative sources or their combinations for calibration, such as Cs-137 and K-40. It is not necessary to collect the spectrum with both sources simultaneously; you can collect two separate spectra and memorize the channels where the lines appear during calibration. For instance, after collecting the spectrum of Cs-137, note that on channel 3 there was a peak with an energy of 32 keV, and on channel 66 there was a peak with an energy of 662 keV. After collecting the K-40 spectrum, by selecting these channels, enter the appropriate energy values (32, 662, 1461 keV).

For three-point calibration with a single source, it is best to use Th-232 based sources. However, a specific peculiarity must be considered: the first line is better chosen at the low-energy level, specifically the X-ray fluorescence line. If the source is a thoriated electrode, the X-ray fluorescence energy will be about 65 keV. If the source does not contain heavy elements, the line will have an energy of about 88 keV.

After calibrating the device, it is advisable to test it on sources with a different isotopic composition. Additionally, due to the nonlinearity introduced by the discrete nature of the channel step, the lines may not always align precisely with the intended channel. This is particularly true at the beginning of the spectrum.

If a rough calibration without a source is needed, the K-40 isotope is recommended. You should dial the background spectrum, where the peak at 1461 keV or potassium fertilizer will be clearly visible, and the peak should be entirely colored in yellow. Then, by entering the settings, adjust the calibration factor A1 in increments of 0.1 until the top of the peak reaches 1461 keV.

Invalid calibration factor values are displayed in red. The application prevents writing these values to the device.

You can manually correct the table data, and changes to energies and/or channel numbers will automatically recalculate the factor values.

If you need to restore the device's factory calibration or manually adjust the calibration factors, you can use the "Calibration factors" button in the “Device settings” dialog box.