By Timothy E. Raneyâ€¦Bald Engineer Guy with Glasses
This is a summary of the design and fabrication of two uraninite (UO2) check sources. These sources were designed to â€œconfirm the continued satisfactory operation of an instrumentâ€ (per ANSI N323-1978) or otherwise verify the proper functioning of radiation detection and measurement systems, e.g., Geiger-Mueller counters. Since their purpose is to verify functioning, the calibration period was only three minutes for each source vs. 20 or more minutes. These check sources used small (~0.3 to 0.4 gram) natural uraninite fragments that were not otherwise processed, other than being incidental chips from a larger specimen.
Design & Fabrication
The sources are comprised of 1â€ diameter aluminum (6061) blanks that are 0.625â€ thick. Each piece was cut from bar stock to within 0.060â€ of the final linear dimension and faced to length. A central hole for the uraninite sample was then drilled with a 0.375â€ diameter carbide 4-flute end mill with lathe spindle speed at 1000 rpm to leave a square-bottomed hole. The finished blank is deburred & the hole cleaned with a swab soaked in denatured alcohol. The sample is then placed in the hole with tweezers and two-part epoxy is poured over it. This is generally the 5-minute cure type epoxy, but other epoxies or potting compounds work fine too. In this case, the uraninite samples were completely immersed in epoxy. The emitted radiation is then predominantly beta and gamma since most (if not all) of the alpha particles are absorbed by the epoxy encapsulation. The image at right shows a completed check source with the embedded uraninite sample in the center. Note that the activity & type of radioactive material is noted on the label.
Testing & Calibration
In testing these two check sources, a Ludlum Model-177 ratemeter verified their activity. Check source #1 yielded a total 3-minute count of 6982 or 6982/3 = 2.3 X 103 counts per minute (CPM). Check source #2 yielded a total 3-minute count of 2767 or 2767/3 = 992 CPM. Ambient temperature was 20oC. These two results do not include background corrections since the normal background with this instrument at this location is a nominal 40 CPM. Since these are only check sources, there was no great effort in calibrating them further. They are perfectly adequate to verify the functioning of various particle & radiation counters, except for those instruments designed solely to detect alpha particles.
 B. Hall & R. Cantu, The Ludlum Report: More on Check Sources, Ludlum Measurements, Inc., Sweetwater, TX, 2004.