Notes on the Practical Application of Segmented Gamma Scanning

Current segmented gamma scan systems allow for a largely automated operation: the measurement object is positioned on the scanner, some parameters are selected, and the measurement is subsequently started. After the measurement is completed, the data (measurement parameters, spectra, etc.) are stored, a gamma spectrometric analysis of the summed spectrum is initiated, and based on these results, a report of the radionuclides contained in the measurement object and their activities is created, ideally also including (traceable) details of the uncertainties of the determined activities.

This "automation" is well-suited for routine tasks where the activity inventory of the same measurement objects is to be examined. The prerequisites are correctly chosen parameters for the measurement process as well as for the subsequent data analysis and evaluation.

However, as soon as deviations from these routine tasks occur, the results obtained in this manner can lead to partially drastic misinterpretations and mislabelings, whether due to

• too low or too high activity values for the identified radionuclides, due to incorrect or insufficiently assumed assumptions (homogeneity, density, inner containers, etc.) for the evaluation model used,
• incorrect or missing radionuclide assignments during the spectral analysis due to incomplete nuclide libraries, which often contain "only" the routinely occurring radionuclides or fail to recognize more complex relationships in the analysis algorithm (same characteristic lines from different radionuclides).

This leads to the following:

The following points outline the type of information and their procurement (without claiming completeness, as the (facility-specific) conditions have a significant influence).

Origin

The first type of information is the origin of the measurement object. There are distinctions made between

• waste from ongoing operations
• waste from deconstruction
• older waste with partially unknown history
• other containers with radioactive contents

In the first case, containers with waste from ongoing operations, the content and composition are usually well known. This is where automated operation can be used for segmented gamma scanning.

Before conducting a measurement/campaign

• the set parameters of the measurement system (scan mode, collimator, number of sectors, layers or steps, measurement time, distance detector-measurement object, etc.) must be checked and adjusted if necessary;
• the nuclide library used in the spectrum analysis must be checked for the presence of all expected radionuclides and supplemented if necessary;
• the applied evaluation model must be checked for compatibility with the specified waste composition (i.e., are the assumptions underlying the evaluation model fulfilled?);
• the appropriate calibrations must be performed;
• if possible, test measurements with calibration containers of the same or similar composition should be carried out to verify and validate the chosen settings;
• all measurement-relevant information must be documented.

With waste from deconstruction, its origin is usually known (biological shielding, ancillary buildings, radionuclide laboratories, etc.). This information often allows conclusions about the material composition in the container as well as the expected activity inventory. Often, the corresponding nuclide vectors are known from radiochemical analyses of samples from the area to be deconstructed.

In these cases, prior to conducting a measurement/campaign, the same procedures as for containers with waste from ongoing operations should be followed. For example, the nuclide library should be checked to see if all (gamma-emitting) radionuclides of the nuclide vector, including their daughters, are included. The measurement method (scan mode) and the evaluation model must be adjusted to the respective waste composition.

If this information is partially or completely missing, the steps for older waste should be followed.

Older waste with partially unknown history poses the most challenging task for the characterization of radioactive waste. Here, it must first be generally assumed that everything(!) could be contained within the container. This concerns radionuclides, activities, materials, shielding, inner containers, etc.

Without further information, the results of a "normal" segmented gamma scan measurement on older waste are hardly reliable, if at all. They may provide information about the radionuclide inventory (qualitative assessment), but not about their activity values (at least with a credible uncertainty range). Here, one relies on additional a priori information to make the appropriate settings (measurement parameters, scan mode, nuclide library, evaluation model, etc.). Moreover, if supported by the measurement system, additional scan modes can be used to identify indicators of inner containers or internal shielding, as well as errors in the filling processes of inner containers, etc.

The last case, other containers with radioactive contents, relates, for example, to radionuclides contained in medical generators, shipping items with radioactive contents, such as calibration sources, or even volume sources, such as those used for calibration purposes.

These containers can also fundamentally be characterized using segmented gamma scanning. Usually, the dimensions, material compositions, and activity inventory are known. In these cases, the nuclide libraries used in the analysis and the selection of a suitable evaluation model must be adjusted accordingly.

A priori information

In addition to information about the origin of a measurement object, a priori information is an important part of quality control. On the one hand, it serves to verify the assumptions made in the measurements and evaluations, while on the other hand, it can provide complementary or missing information for selecting the correct measurement type (scan mode) and analysis methods.

A priori information can be categorized as follows:

• Information about the measurement object
• Information about the measurement system
• other information

Information about the measurement object:
The measurement object is typically a certified container. The associated data sheets contain information about its dimensions, mass, maximum load, etc. For other containers, dimensions can be obtained, for instance, through measurement and mass can be estimated from the dimensions, assumed wall thicknesses, and materials.

This information can be utilized in the evaluation models to account for the attenuation of the emitted gamma radiation in the container wall. Together with the result of a weighing, the net mass of the matrix can be determined under the assumption of the absence of further inner containers, as well as, along with the filling volume, which can either be taken from the datasheets or estimated from the outer dimensions and the (assumed) wall thicknesses, the average density of the (active) matrix can be determined.

In addition to the aforementioned points, many other information about the measurement object may be available. These are often qualitative rather than quantitative. One example is the information (e.g., in the transport documents) that the content of the container consists of cemented ashes. With the knowledge of the typical density of cemented ashes (e.g., from databases or previous (radiochemical) analyses) and the net mass available, the filling volume can be calculated. This can then give indications about the filling height, which can in turn be integrated into the parameters of the evaluation models. If the determined filling volume (significantly) exceeds the actual possible filling volume (container data), then this may indicate a mislabeling of the container contents.

Information about the measurement system
The mechanical components of the measurement system (i.e., the segmented gamma scanner in this case) determine the possible scan modes. Their availability may limit the deployment for specific measurement tasks and must be taken into account. For example, a measurement system consisting of a turntable for the measurement object and a lifting unit for the detector cannot provide indications about possible inner containers. An additional translation or tilt axis would be required for that purpose.

The used detector system, its settings, and any collimation also impact the suitability of a segmented gamma scan system for a specific measurement task. Without collimation, for example, measurements in open geometry are only meaningfully possible. The energy range set for the detector system must be able to capture all relevant characteristic lines of the radionuclides contained in the container. The energy resolution of the detector system must be sufficient to also separate relevant characteristic lines at similar energies.

Other information
Under other information, results from other measurement methods, such as dose rate measurements, transmission measurements (radiography or tomography), passive and active neutron measurements, as well as results from radiochemical analyses from representative sampling can be summarized. Other qualitative information also falls under this category. This information can again be used in determining the measurement and evaluation parameters and usually reduces uncertainties.

Often, the presence of additional measurement results from alternative measurement methods allows for the application of combined evaluations.