Introduction

So you want to learn more about segmented gamma scanning.

Let's first recap what you should already know – and if you're unsure about this, feel free to check out the beginner course again.

The aim is to identify the gamma-emitting nuclides contained within a confined object (e.g., a 200 L drum) and, if possible, quantify them as well. For this, a suitable detector system is used to measure the gamma radiation emitted from the object, more specifically the gamma spectrum. The detector system can be moved around the object with a mechanical device. This allows for the determination of the distribution of gamma radiation emitted from the surface of the object for each radionuclide and visualized as spatial distributions (OVTs).

From the peak areas of the radionuclides identified in the summed spectrum, which are converted into count rates, and a suitable evaluation model, the activities of the respective radionuclides can be calculated.

There are two measurement methods: open geometry and collimated geometry. In both cases, a suitable detector system is used in conjunction with a collimator that limits the area from which radiation can fall onto the detector.

In the measurement in open geometry, collimation is done based on the object dimensions. The collimator here serves only to shield from gamma radiation that is not coming from the object, i.e., from the environment. The detector therefore “sees” only the object. An improvement of the data reliability is achieved when the object is rotated during measurement, allowing the detector to “see” the object from all sides.

In collimated geometry, the collimator limits the field of view to a small section of the object. Through scanning of the entire surface of the object, it can be determined where gamma radiation emerges from the object's surface. This requires a mechanical device that allows for the corresponding positioning of the detector and the object relative to each other.

Segmented gamma scanning (often abbreviated as SGS) refers to a special method of obtaining measurement data using gamma spectrometry. The method is usually applied to large volume objects containing radioactive content, such as 200 L waste containers. With the help of the measurement data, the (gamma-emitting) radioactive nuclides contained in the object can be identified, and their activities and nuclide masses can be determined with suitable evaluation procedures. In the measurement, the surface of the object is scanned with a suitable collimated detector, the gamma spectrum is measured for each position, and stored. The detector, collimator, and mechanical device that controls the movement of the object and/or the collimated detector are referred to as a segmented gamma scanner.

The gamma spectra stored for each measurement position can be summed to create a summed spectrum, from which the radionuclides contained within the object can be determined based on their characteristic lines. Additionally, the net peak area of a characteristic line of a radionuclide can be determined for each measured position and visualized in a false color representation on a diagram whose axes correspond to the movement of the measurement.

The two images show the resulting spatial distributions from a multi-rotation scan on a 200 L waste container for the two nuclides ²⁴¹Am and ²²⁸Ac. The horizontal positions represent the angle positions, and vertically the height position of the container. From the two images, it is immediately obvious that the activities in the containers are not homogeneously distributed and are located at different positions within the container.

From the spatial distribution for ²⁴¹Am, it is also evident that the container is likely filled to the top since the radionuclide is detected just below the maximum possible fill height of the container at 760 mm. Additionally, the spatial distributions indicate that the two radionuclides are confined to very small volumes within the waste matrix. For ²⁴¹Am, it could potentially be two sources or a single source with a (narrow) absorbing part between the source and the detector at a rotation angle of approximately 60°.

Many other insights can similarly be derived from the spatial distributions of the various detected radionuclides and their characteristic lines.

With the following questions, you can determine whether you already possess sufficient knowledge for the section “Advanced” to make the most of this section. If you cannot answer the questions correctly or feel unsure, we recommend first reading the beginner course on the topic of segmented gamma scanning. However, you can also skip the questions and start directly with the first topic of the advanced course, which is the construction of a segmented gamma scanner. By answering the last question, you will also be redirected to this page.