Calibration Measurements

For a successful execution of a segmented gamma scan measurement, the measurement system must be calibrated before its initial commissioning. For commercial systems, the steps to be taken for this should be listed in a calibration and maintenance instruction that accompanies the system. This usually also specifies the intervals after which the respective calibrations should be checked or redone.

Regardless of these guidelines, any irregularities must also lead to the corresponding calibrations being repeated outside of the specified intervals, and if necessary, appropriate maintenance or repair actions should be initiated.

Typically, the following calibrations of a segmented gamma scanner should be performed or checked at regular intervals:

• Calibration of the mechanics (speed, positioning, and their reproducibility)
• Energy calibration of the gamma spectrometry system
• Efficiency calibration of the segmented gamma scan system
• Calibrations of other installed components (e.g., integrated weighing unit, dose rate probes, neutron counters, etc.)

Calibration of the mechanical components

The process of a segmented gamma scan measurement is computer-controlled, meaning that the various axis parameters such as start and end positions, positions in general, and the movement speeds between individual measurement positions are specified by the operator in an input mask in the corresponding units (e.g., cm, °, cm/s, °/s).

However, the implementation in the control requires the conversion of the entered values into the corresponding coding of the motor control, e.g., into steps or steps/s for stepper motor controls. The relationship between the different units is determined by parameters that need to be established in calibration measurements (for commercial systems, this calibration should already have been performed for the first time by the manufacturer).

Energy calibration of the gamma spectrometry system

The detector system of a segmented gamma scanner converts the radiation entering the detector (usually a semiconductor or a scintillation detector) into electronic impulses, which are registered in a multichannel analyzer. Each channel is assigned an energy interval. The determination of the parameters for this (linear or quadratic) assignment is carried out through the energy calibration (see also the corresponding video tutorial on LVis).

Efficiency calibration of the segmented gamma scan system

Depending on the chosen measurement and evaluation methods, the parameters of the various model descriptions, which do not depend on the measurement object but only on the respective measurement arrangement, can be summarized in independent efficiency calibration factors. Since these describe the efficiency of the respective measurement system, their determination is colloquially referred to as efficiency calibration (of a segmented gamma scan system).

If the values determined for an efficiency calibration for a specific measurement arrangement are tabulated, they can be used in future measurements with the same measurement arrangement in the respective evaluation. This means that, in principle, a single calibration measurement for a specific measurement arrangement upon its commissioning is sufficient.

 Depending on the type of measurement, different calibration procedures are applied:

• Efficiency calibration for measurements in open geometry
• Efficiency calibration for measurements in collimated geometry

Calibrations of other components

All other components of a segmented gamma scanner whose measurement results contribute in some way to the characterization of the measurement object must also be calibrated.

Here are some examples of possible components:

• Integrated weighing unit
• Electronic distance measurement (e.g., for the distance detector - measurement object)
• Dose rate probe(s)
• Neutron counter
• etc.

The execution and documentation of the calibration during initial use as well as in recurring tests (RKT) are usually described in the accompanying documentation for the measurement system.