Calibration Sources
Suitable calibration sources are required for conducting calibration measurements. They should meet the aforementioned requirements for completeness, uniqueness, and external conditions. They should possess a corresponding calibration certificate. This allows for a traceability to national standards. Calibration certificates contain all necessary data for measurements, such as nuclides, activities, half-lives, transition probabilities, reference dates, etc., including information on their respective uncertainties.
Below are some typical examples of calibration sources or calibration source sets.
Point Sources
In practice, point-like enclosed calibration sources are often used. They have the advantages that
- measurements can be conducted with individual sources, meaning no overlap of characteristic lines with lines from other nuclides needs to be considered;
- in the evaluation of measurement results, self-shielding, which occurs with calibration sources of greater volume, does not need to be taken into account;
- with a suitable selection of nuclides, nearly any energy range can be covered with a sufficient number of lines;
- individual calibration nuclides can be replaced at any time (e.g., due to the decrease in activity because of a short half-life) without needing to repurchase the entire set;
- calculating the various calibration factors for point-like calibration sources is usually simpler.
In contrast, the disadvantage is that a separate calibration measurement must be performed for each nuclide to ensure identical measurement conditions (e.g., same distance and position). This is associated with a significantly increased time requirement. However, adjusted measurement times can account for the activity differences of the individual calibration sources that arise after several weeks or months due to their half-lives, within certain limits.
Photo of the 241Am point source from the set shown below. The source (dark point in the center of the red circle) is embedded in a rectangular plastic matrix.
An example of a set of calibration nuclides is illustrated below. It consists of the nuclides 22Na, 54Mn, 57Co, 60Co, 88Y, 133Ba, 137Cs, 203Hg, and 241Am. The energy range covered by these calibration sources extends from about 50 keV to about 1850 keV.
Photo of a set with point-like calibration sources embedded in rectangular plastic matrices and flat, round calibration sources.
The decrease of activities over a period of 20 years is illustrated below. The starting point is the data of the above calibration set, whose nuclides had an activity of about 0.4 MBq at the reference date, except for 203Hg (0.9 MBq). It is clearly recognizable that 203Hg and 88Y can no longer be used for calibration purposes after just under a year.
Temporal progression of the activities of the above set with point-like calibration sources (Note: activity axis is in logarithmic units).
152Eu Calibration Source
152Eu is usually produced through neutron activation of natural Europium, which consists of 47.8% 151Eu and 52.2% 153Eu. Therefore, a 152Eu calibration source typically always contains 154Eu, whose characteristic lines are also included in the calibration spectrum. Due to the shorter half-life of 154Eu (T1/2 = 8.593 years) compared to 152Eu (T1/2 = 13.516 years), its relative proportion decreases over time.
Calibrations with 152Eu cover the energy range between about 40 keV and 1500 keV very well in one measurement, thus reducing both the time required for measurements and evaluations, as activities and half-lives of different nuclides do not need to be considered.
Graph of the transition probabilities of the characteristic lines of 152Eu in the energy range from 39.5 keV to 1528.1 keV (a total of 51 lines). Note that for better visualization, the transition probabilities are shown logarithmically.
An example of a 152/154Eu calibration source is illustrated below. Activated europium was placed in a cylinder made of stainless steel.
Left: Photo of an enclosed calibration source with 152/154Eu in the lower section of the stainless steel container. Right: X-ray radiography of the calibration source.
Mixed Nuclide Calibration Sources
Commercially available mixed nuclide standards are also often used. These are offered as certified solutions and contain a range of different nuclides that can cover a wide energy range.
Excerpt from a calibration certificate of a mixed nuclide standard with an overview of the included radioactive nuclides, their half-lives, their strongest characteristic lines and their transition probabilities (Branching ratio) as well as their activities (at a given reference date).
Photo of a QCY calibration solution in a PE bottle.
These solutions can either be used directly for calibration measurements or to create derived standards in specific geometries.
However, these nuclide mixtures have the drawback that the composition changes over time due to the different half-lives of the individual nuclides. After about 1.5 years, 203Hg can no longer be evaluated in the measured calibration spectra or only with very large uncertainties due to its relatively short half-life of only about 46 days. After a few years, typically only the characteristic lines of 60Co, 137Cs, and 241Am can be evaluated, and the mixed nuclide standard must be replaced.
One advantage of these solutions is that almost any geometric shape and structure can be produced with them (such as the sand-filled Marinelli beaker mentioned above or the volume source described below).
Volume Source
Some institutions have the ability to manufacture calibration sources in different geometries themselves or have them made by commercial manufacturers. How such a volume source can be produced (assuming the necessary handling permits are obtained) is described below as an example for a sand-filled volume source. This is used, among other things, for the calibration of segmented gamma scanners.
"Recipe" for producing a Sand Volume Source
Goal: Creation of a homogeneous volume source with the nuclides 152Eu, 154Eu, and 241Am in a sand matrix.
Container: Aluminum cylinder with a wall thickness of 0.2 cm, 34.5 cm internal height, and 30.2 cm internal diameter
Starting activities in the form of stock solutions:
- Europium (Volume V = 1 ml)
A(152Eu) = 2.70 MBq
A(154Eu) = 0.34 MBq
- Americium (Volume V = 1 ml):
A(241Am) = 0.19 MBq
Production of the "active sand":
Into a polyvial, the following are filled in succession:
- 14 spatulas of dried sand,
- 100 µl of 152Eu/154Eu stock solution,
- 14 spatulas of dried sand.
The polyvial is dried in an oven at 90°C for 4 days and subsequently mixed for about 10 days in a shaker for homogenization.
Creation of the source:
In the aluminum container, the following are successively filled:
- 5 cm of dried sand (indicates the height of the layer in the aluminum container)
- 7 spatulas of "active sand"
This process is repeated three times and subsequently filled to a height of 5 cm with dried sand. The total height of the sand layer is approximately 25 cm.
In the next step, pre-cleaned 241Am solution (a total of 260 ml) is added to the sand.
The contents of the prepared aluminum cylinder are dried in an oven at 90°C. After closing the cylinder, the contents are homogenized over several days using a shaker.
This gave the volume source the following activities:
- A(152Eu) = 2.504 MBq
- A(154Eu) = 0.303 MBq
- A(241Am) = 49.6 MBq
The homogeneity is subsequently verified by measuring the spatial distributions with a segmented gamma scanner in the multi-rotation mode.
Photo of the sand volume source.
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Note:
241Am was introduced with a significantly higher activity than 152/154Eu because its most commonly used characteristic line at 59 keV has a very low energy and is therefore strongly attenuated in the sand matrix.
Spatial distributions for 241Am measured with a segmented gamma scanner in multi-rotation mode.
This large-volume calibration source is very suitable for calibrating segmented gamma scanner systems and can be created for various homogeneous matrix compositions in nearly any geometries. The "recipe" listed above for creating the calibration source with a sand matrix can be easily adjusted accordingly. For instance, if a matrix with lower density is needed, layered bubble wrap can be used instead of sand. The required activities can be injected statistically distributed using injection needles into the bubbles.
The chosen calibration nuclides 241Am and 152/154Eu allow for long-term use, as the diagram shows.
The temporal progression of the activities of the volume source shows its long-term usability.
