So far we have not made any specific statements about the position of the detector relative to the container, i.e., where it is in relation to a radiation source inside the container. We want to address this now:
Let us consider a point-like radionuclide located inside the container, for two different arrangements:
Arrangement 1: The radionuclide is located near the inside of the container wall, at the shortest distance to the detector.
Arrangement 2: The radionuclide is located near the inside of the container wall, at the greatest distance from the detector.
Did you notice?
Arrangement 1 corresponds to Case 2 that we discussed in the section Attenuation:
The gamma radiation must pass through the container wall in order to reach the detector and is therefore attenuated in the container wall.
Arrangement 2 is addressed in the task for the section Attenuation (Case 4):
The radionuclide is again at the inside of the container wall, but now on the side farther from the detector. This time, the gamma radiation has to pass through the entire cement matrix in addition to the container wall. The impulses measured in the detector will therefore be significantly lower than in Arrangement 1.
Information:
For this measurement, we used the radionuclide Europium-152 (Eu-152) because it has a variety of characteristic lines in the energy range of 121.8 keV to 1408.0 keV.
In the first measurement, the Europium source was placed in the tube closest to the detector; in the second case, it was placed in the farthest tube. Let's take a closer look at the results of the two measurements. The measurement time, i.e., the duration of the measurement, was the same for both measurements (7803 seconds). For a better comparison, we have plotted the two spectra together in a diagram.
First, let’s look at the measured gamma spectrum for the first arrangement, shown in black. We can recognize a variety of characteristic peaks of Eu-152 across a wide energy range.
If we now compare this spectrum with the gamma spectrum measured in the second arrangement (shown in red), the following points immediately stand out:
- The number of pulses is lower throughout the entire energy range (i.e., it is well below the black curve).
- Characteristic lines are only recognizable at high energies.
- The peak areas of these characteristic lines are significantly smaller.
The reason for these differences lies in the attenuation, as you might have realized (if not, we recommend reviewing the section on Attenuation). What we are learning here is that the attenuation by a material is stronger the lower the energy of the gamma radiation. Therefore, we do not see characteristic lines at low energies in the gamma spectrum for the second arrangement. The associated gamma radiation is attenuated in the cement to the extent that it practically does not reach the detector.
The following video demonstrates the spectroscopy programs during both measurements, highlighting the different growth of the two spectra for the first 150 seconds after the start of each measurement. The small image displayed in the upper right shows the measurement trend for the distant source position, while the larger image shows that for the detector-close source position. The time course of the two measurements is synchronized, meaning a direct comparison of the two spectra is possible at any point in time.
We can clearly see that significantly more gamma radiation is registered by the detector for the detector-close source position than for the detector-far position. This is the effect of attenuation!
What else can we learn from the previous considerations?
- The heights (or areas) of the peaks depend on the positions of the radionuclide in the container and the detector relative to each other!
This property can be exploited to approximately determine the distribution of radionuclides in the container, i.e., to find out where they are located in the container (approximately):
Using a collimator placed in front of the detector, we can limit the area from which gamma radiation from the container reaches the detector.
Schematic sketch of the arrangement of container, collimator, and detector relative to each other. Only gamma radiation from radionuclides that are within the field of view of the detector can reach the detector through the collimator.
This enables us to perform so-called segmented measurements: A larger number of measurements are conducted, with each measurement taken at a different position of the container.
For example, for a cylindrical container (such as one of these yellow drums), measurements can be conducted at 10 height positions. This type of segmented measurement is called vertical scanning: the container is scanned at a point throughout its height (from hstart to hstop).
Introduction
Schematic representation of the container to be measured.
End Position
Last and highest position that the detector moves to on the lifting axis for a measurement. Start position: Height position of the detector for the first measurement. After completing the measurement, the next measurement position is approached by moving along the lifting axis. This is repeated until the end position is reached.
Detector
Schematic representation of the detector in the first measurement position (start position), surrounded by a cylindrical collimator.
Start Position
Height position of the detector for the first measurement. After completing the measurement, the next measurement position is approached by moving along the lifting axis. This is repeated until the end position is reached.
The vertical scan can have two disadvantages: on one hand, if the collimator opening is too small, the container will not be fully captured in its width, meaning radioactive contents can be "overlooked". On the other hand, only a rough statement can be made about the distribution of the radioactive content because, while a statement can be made about the height at which it is located (roughly), it cannot be determined whether it is located "at the back", "in front", or "to the side" (refer to the descriptions of cases 2, 3 and 4 for reference). A quantitative determination is thus hardly possible or only with extremely large uncertainties.
Container
Schematic representation of the container to be measured.
Measurement Positions
The container is rotated 360° (i.e., makes a complete turn). Measurements are taken at various positions during the rotation.
Detector
Schematic representation of the detector in the first measurement position (start position), surrounded by a cylindrical collimator. The area from which radiation can enter the detector (i.e., the area that the detector "sees" in its respective position) is represented by the gray area.
Schematic simulation of the measurement workflow of a multi-rotation scan.
For a cylindrical container (for instance, one of these yellow barrels), measurements can be conducted at 10 height positions, with 12 measurements at each position, each at a 30° angle (i.e., one full rotation: 12 · 30° = 360°). For each of these 120 measurement positions (10 height positions · 12 angular positions = 120 positions total), the measured spectrum is stored, meaning in this case we have a total of 120 individual spectra available for evaluation (determining the distribution and amount of the radioactive content).
As we learned in the section Information of a Gamma Measurement, we can identify the radionuclides contained in the container based on the peaks in a measured spectrum.
