Fabrication of “Source Organs”
for the
“Helgeson Simulated Livermore Phantom”
2-September-2001
Objective
The Helgeson Scientific Services (HSS) in vivo counters are calibrated using “point” sources distributed uniformly in styrofoam packages. Typically, 200 of these point sources are used so that different calibration geometries may be determined:
- total body,
- head,
- thyroid,
- lung,
- stomach,
- lower GI tract, and
- legs.
This document will describe the various processes required to accomplish the desired tasks.
- 1. Order the sources.
The specifications for the sources are ready quite simple: deposit a known quantity of the radionuclide of interest at the center of a plastic “dot,” and after drying, seal it. The quantity of the radioactive material in all of the “dots” is a function of its half-life: if the half-life is short, such as eight days for iodine-131, then the typical quantity would be approximately five microCuries. Radionuclides with significant live longer half lives, such as Co-60 or Cs-137, would contain one to two microCuries of activity.
It is not necessary that the individual point sources be referenced to NIST since we have found wide variations in the actual strength of each point source. See the graph, below, in Step 4.
- 2. Receive and Measure the Gross Activity of the Sources.
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On receiving the sources from the manufacturer verify the approximate activity using a survey meter or a whole body counter. This is necessary to be sure that the manufacturer has not made a significant error. For example, an order was placed for a total of one microCurie of barium-133. When the sources were received they were delivered in a lead shield and upon inspection we found that there was approximately 200 microCuries of total activity. The manufacturer had mistakenly fabricated sources of one microcurie for each “dot” rather than one microcurie for the total activity of all 200 sources.
- 3. Determine the Activity of the New Source by Comparison with a NIST Source
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The next step is to compare the activity from a NIST source to all of the point sources so that the actual activity of the point sources may be determined. This is most easily accomplished by measuring the two sets of sources under identical conditions. If the individual point sources have not yet been mounted on cards, then “identical conditions” are quite easily achieved. For example, place the NIST source on the bed of a “Do-It-Yourself” whole body counter so that the sources are directly beneath the center of the detector. The typical distance from the front face of the detector to the surface of the bed is 12 inches. The source should be counted for sufficient among of time such that the two sigma counting error is less than one percent. Next, the entire group of dot sources should be placed in the same spot as the NIST source was placed and should be counted so the two sigma error is less than one percent. The certificate that came with the NIST source will show the date of fabrication, the activity at the time of fabrication, and the half-life. The activity of the entire group of “dot” sources may now be calculated by correcting the NIST activity for decay to the time of the measurement of the point sources and multiplying that activity by a the total number of counts from the “dot” sources and dividing by the total number of counts from the NIST source. The following photographs show how this process was done in the HSS offices:
- 3.1 Choose a Reference Source-to-Detector Geometry
- 3.2 Calculate the Activity of the Unknown Source
- 4.0 Mount the "Dots" on a Source Card.
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Assuming that the “dots” have not already been mounted, the next step involves mounting these sources on stiff paper. A standard business card cut in half the long way is ideal. Use clear “Scotch” tape to mount the “dot” quite close to the end of the card, as shown in Figure 4 at the right. At this point in time, each of the cards should be individually numbered only. Later, the card will be labeled with the name of the radionuclide, the batch number, and its activity. The reason for this will be explained later. 
| Figure 1, at the right, shows the spot on the bed of the HSS “DIYS” counter at the right end of the bed. |  |
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| Figure 2, at the right, shows a NIST Calibration Source, in this case, cobalt-60 |  |
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| Figure 3, at the right, shows the "unknown" source located on the "spot." At the time of writing this document, the sources for the barium-133 calibrations had already mounted in the styrofoam organ inserts, so they could not be used for this example and a cobalt-60 "thyroid source" was used for example purposes. If the sources are already mounted on cards, they should be placed in as tight a circle as possible with the center of the circle at the same point as the center of the NIST source is placed. |
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The following table shows the calculations of the activity of the new barium-133 sources:
| Calculation of the Activity of the Ba-133 Sources | |
| NIST Source Strength at Fabrication, Bq | 73,540 |
| Date of NIST Fabrication | January 1, 1999 |
| Date of DIYS Measurement | April 8, 2001 |
| Elapsed Time, days | 828 |
| Half-Life, years | 10.52 |
| Half-Life, days | 3,842.43 |
| Source Strength at Measurement, Bq | 63,337 |
| Total Counts from NIST Source in DIYS | 2,144,352 |
| Counting Time, seconds | 1,800 |
| Gross counting rate, c/s | 1,191.307 |
| Total Background Counts in DIYS | 13,398 |
| Background Counting Time, second | 1,200 |
| Background Counting Rate, c/s | 11.165 |
| Net Counting Rate, c/s | 1,182.142 |
| Two-Sigma, NIST Source, c/s | 1.627 |
| Two-Sigma, Background, c/s | 0.194 |
| Two-Sigma, Total, c/s | 1.639 |
| Precision, % | 0.139 |
| Total Counts from Unknown Ba-133 Sources in DIYS | 1,019,387 |
| Counting Time, seconds | 1,800 |
| Gross counting rate, c/s | 566.326 |
| Total Background Counts in DIYS | 13,205 |
| Background Counting Time, second | 1,200 |
| Background Counting Rate, c/s | 11.004 |
| Net Counting Rate, c/s | 555.322 |
| Two-Sigma, Unknown Ba-133 Source, c/s | 1.122 |
| Two-Sigma, Background, c/s | 0.406 |
| Two-Sigma, Total, c/s | 1.193 |
| Precision, % | 0.215 |
| HSS Ba-133 Activity, Bq | 29,803.38 |
| Two-Sigma, Bq | 25.34 |
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As was stated above, the actual activity on each dot can vary significantly. Therefore, it is necessary to measure the activity of each of the point sources individually. Helgeson Scientific Services uses a “4-pi” counting geometry by placing two 10-cm x 10-cm x 10-cm detectors facing each other with a gap between them of approximately 3-mm. See Figure 5 at the right. These detectors are shielded on all sides by 5-cm of lead in the form of 5-cm x 10-cm x 20-cm bricks. Directly above the gap between the detectors, there is a similar gap in the top lead bricks. The sources, mounted on their cards, are lowered on a simple holder into the gap so the “dot” is essentially at the center of the front faces of each of the detectors. 
- 5.1 Set Up the Energy vs. Channel Relationship for the Analytical Equipment.
Prior to the beginning of the measurement of the sources, the “energy versus channel” relationship should be established at approximately 5-KeV per channel. An exact calibration is not necessary since all measurements will be relative to each other. A background count must be made after any change in gain and zero settings for a length of time sufficient to ensure that the background counting error is a small fraction of the total counting error. Typical counting times are 20-minutes for the background and 2-minutes for the sources.
- 5.2 Counting Each Source
- We are now ready to start counting the individual “dot” sources. Standard HSS data acquisition software may be used. In this manner, each measurement will be recorded in its own file. All the operator has to do is enter the source card number in the “Comment No. 1" area of the program.
- A "Quick Basic" software program was written to read each of the data files and extract the necessary information: file name, gross and background counts under the photopeak of interest, sample and background counting times, and source identification number. The program then calculated the gross and background counting rates, the net counting rates, the two-sigma counting errors for gross and background, and the combined two-sigma counting error for the sample net counting rate.
After all of the data were collected, they were entered into a spreadsheet program and ranked from the lowest amount of radioactivity to the highest. The graph below shows the distribution of activities aith the mean and 2-sigma error. 
- The objective of this entire excersize has been to load the phantom styrofoam "organs" to equitably represent the distribution of radioactivity in a human. Helgeson Scientific services has long used the following distribution of activities in various parts of the human body (these distributions originated with ICRP-2):
Body Part Percent Head 7 Lung 31 Stomach 32 Lower G.I. Tract 21 Legs 9 Thyroid As desired -
The final results are shown in the next table:
Data for placing the fabricated Ba-133 sources in styrofoam: Becquerels 2-Sigma nanoCuries 2-Sigma Head 1,949.3 8.9 52.7 0.2 Lungs 8,3311.8 15.9 224.6 0.4 Stomach 8552.8 15.0 231.2 0.3 Lower G.I. Tract 5,690.6 112.5 154.8 0.3 Left Leg 1,238.6 5.8 33.5 0.2 Right Leg 1,1712.4 5.7 31.7 0.2 Thyroid 2,888.8 8.9 78.1 0.2 Total without Thyroid 26,914.6 27.9 727.4 0.8 Total with Thyroid 29,803.4 29.3 806.6 0.8