Calibrating the energy scale of the device

Calibrating the energy scale of the spectrum is necessary to accurately determine the energy of the gamma lines of the spectrum by the channel number. The devices undergo a calibration process at the factory, but detector parameters may change over time, necessitating a calibration update. It would require 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 values of the calibration factors, which are used by the device to convert the spectrum channel number into the radiation energy value in keV using a polynomial of degree 2 kind:

x is the channel number;
a₀, a₁, a₂ are calibration coefficients.

Spectrum energy scale calibration procedure

The calibration is performed at three points, which ensures sufficient linearity over the entire range of energies under study. To perform the calibration, the user will need a spectrum containing mono- energy lines, preferably located at the beginning, middle and end of the energy scale. For example, the following isotopes produce such lines:
K-40 (1461 keV);
Am-241 (59 keV);
Lu-176 (202, 307 keV);
Cs-137 (32, 662 keV);
Th-232 (238, 583, 2614 keV);
Ra-226 (78, 351, 609 keV).

For the most correct calibration, it is recommended to take several sources, for example Cs-137 and K-40. It is not necessary to collect the spectrum with both sources at once, you can collect two separate spectra and memorize the channels where the lines are located during calibration. For example, after collecting the spectrum of Cs-137, remember 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. And after collecting the K-40 spectrum, by selecting these channels, enter the appropriate energy (32, 662, 1461 keV).

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

After calibrating the device, it is desirable to test it on sources with a different isotopic composition. Also, because of the nonlinearity introduced by the discreteness of the channel step, the lines may not always fall exactly into the channel where they are supposed to be. This is especially true at the beginning of the spectrum.

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

Calibration procedure

1) Place the device and the reference source to be measured as close as possible to each other. The detector should be placed with a "+" sign on the source or, if the source is powerful, pointed in its direction with a "+" sign. It is desirable that the count rate should not exceed 500 cps during calibration.

2) In the "Spectrum" tab, reset the spectrum by clicking on
and select "Restart Accumulation" from the drop-down menu.

3) Having determined the choice of reference points, wait until all spectral lines (Gaussians), on which the calibration will be carried out, are colored yellow, which will indicate sufficient statistical reliability of the spectrum data.

4) Tap on
and select "Energy Scale Calibration" from the drop-down menu. At the top of the tab, you will be prompted to enter the energy value:

5) Touch the graph and, by moving the marker lines, select the first point on the graph. Enter the energy corresponding to the point next to the "Next" button. Tap the "Next" button.

6) Repeat this manipulation twice and select the "Done" button.

7) The application will check the channel number values for validity. If the values are invalid, a diagnostic message about it will be displayed. If the values are acceptable, a window will be displayed with a table with energy values and corresponding spectrum channel numbers, and under the table the calculated values of the calibration factors:

Invalid values of calibration factors are displayed in red. The application blocks the ability to write such values to the device.

You can manually correct the table data, and changing energies and/or channel numbers will cause automatic recalculation of the factor values.

8) Tap the "Program factors to device" button to complete the calibration. If the calibration was initiated from the View tab, the button will save the spectrum open for viewing to the library with new coefficients.

If you need to return the factory calibration of the device or correct the calibration factors manually, you can use the "Calibration factors" button in the dialog box.
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