What is meant by the term attenuation in connection with radiation emitted by radioactive nuclides? To understand this, we first need to make a few general considerations.
It schematically shows for three different types of radiation (alpha (α), beta (β), and gamma (γ) radiation), how they are attenuated when they penetrate different materials.
In connection with segmented gamma scanning, we are only interested in the behavior of gamma radiation. From the schematic drawing, we can deduce that gamma radiation can penetrate “light” materials relatively unchanged, meaning the “amount” of gamma radiation before and after the material remains approximately the same. “Light” materials include paper or aluminum, i.e., materials with low density. In contrast, for “heavy” materials, such as iron or lead (materials with high density), this behavior changes. Here, the “amount” of gamma radiation after the material is lower.
This reduction in the “amount” of gamma radiation when passing through a material is referred to as attenuation of gamma radiation.
However, we have not yet considered the thickness of the material in these considerations! The thicker a material is, the stronger its attenuating effect.
Thus, we have two effects that determine the attenuation of gamma radiation:
the density of the material, and
the thickness of the material.
Both together determine how much gamma radiation comes out at the other end.
All clear? Then you can now answer the following task with ease. Also, take a look at the following photo.
Photo of a simple measuring setup for measuring gamma radiation. Left: detector; Middle: 2 cm thick plastic plate; Right: transparent container with residues from the combustion of radioactive waste.
Task:
Very good! Your answer is correct!
Let us now apply your previous knowledge:
Let’s imagine a container, for example, one of those yellow barrels. This is completely filled with cement. An example of this – even though as a container in gray color – is shown in the following photo. The cement-filled calibration barrel contains 13 vertical pipes for holding radioactive emitters (= radioactive sources) for the calibration of measurement systems.
200 L calibration barrel with a cement matrix. The cement matrix contains 13 vertical pipes for holding radioactive calibration sources.
Note: Radioactive substances are often mixed into cement to ensure they are bonded in a solid environment (= cement matrix).
Now we can ask ourselves whether and how the measured pulses for the characteristic lines of a point-like radionuclide (with equal measuring time) differ when the radionuclide is …
Case 1: … directly at the container wall in front of the detector,
Case 2: … in the vertical pipe in the cement matrix that is closest to the detector, or
Case 3: … in the vertical pipe located in the middle of the barrel?
Answering this question leads us directly to the concept of attenuation.
Information: Point-like radionuclides are those whose shape has very small dimensions. For example, they can be spheres with a few millimeters in diameter.
Let’s consider
Case 1: Between the radionuclide and the detector is only air. We will ignore its attenuating properties on gamma radiation here, as it is a very light material that can be almost completely penetrated by gamma radiation. All gamma rays emitted from the radionuclide towards the detector reach the detector. Note: In all animations on this page, gamma rays that are not emitted towards the detector by the radionuclide are not shown!
Case 2:
The gamma radiation must first penetrate a small piece of cement and then the container wall on its way from the radionuclide to the detector. We will once again neglect the subsequent path through the air. Both in the cement and in the container wall (usually steel), the gamma radiation is attenuated. This means that only part of the gamma radiation “flying” towards the detector actually arrives there. If you look closely, you may notice that not all gamma rays reach the detector, but some get “stuck” in the cement.
Case 3:
It should now be clear what happens in Case 3.
Think for yourself before continuing.
The amount of gamma radiation registered in the detector is even lower than in Case 2, as this radiation has to traverse an additional distance through the cement. Consequently, the gamma radiation is further attenuated before it reaches the detector. You can also recognize this in the animation by the lower number of gamma rays that reach the detector compared to Case 2.
With the same strength (activity) of the radionuclide and the same measuring times, the peaks in the three spectra will thus (significantly) differ, i.e., be of different heights.
Task: Now let’s consider a fourth case (Case 4), in which the radionuclide is located in the “farthest” vertical pipe in the cement matrix.
Correct!
Now that you understand how the effect of attenuation influences the heights of the peaks, we can move on to the next concept:
Since we use the peak areas for calculating activities, we need to know where the radionuclides are located;
near the inner edge of the container (Case 2 or Case 4),
Moreover, we need to know whether they are actually
point-like radionuclides or
extended areas
with radioactive substances.
Note: We have simply passed over the topic of peak areas without examining it in detail. As can be seen from the spectra presented, the peaks have a certain width, meaning they do not represent "lines." For further understanding, it is only necessary to know that the corresponding activities can be deduced from the peak areas.
You have now learned that it makes a difference where in the container the radioactive material is located for gamma measurement.
In practice, of course, you have no influence on its positions! But you can position the detector at different locations around the container. And then, depending on the position of the radioactive material, you will get peaks of different heights.
And this leads us to the second concept, segmentation.
