We’ve all been there at least once or twice -- You’re walking through your house, and you have to ask yourself, “Where did I even get all this junk?”. It happens to everyone, including the Pacific Northwest National Laboratory. In their case, one such example is three small uranium cubes that they’ve had in their possession for a long time. The local rumors are that these cubes were confiscated from Nazi Germany at the end of World War 2, but it seems that nobody remembers for sure. Jesus Jiménez, in his article “Did Nazis Produce These Uranium Cubes? Researchers Look for an Answer.”, describes the lab’s current struggle, along with the University of Maryland, to uncover the secrets behind these mysterious radioactive cubes.
At the end of the war, around 500-600 cubes of this sort were confiscated from the Nazi stockpiles. However, most have been lost in the years since. Dr. Jon Schwantes, the researcher in charge of this effort, says that the lost cubes were “most likely… folded into our weapons stockpile”.
The key to this research is dating the cubes through knowledge of their radioactive decay. Uranium steadily decays into thorium over time, releasing mass through emitted alpha particles. By carefully measuring the ratio of Thorium to Uranium, the researchers can deduce approximately how long it’s been since the cubes were pure uranium (and, by extension, how long it’s been since their manufacture).
Shown here is the chemical equation for uranium-238’s decay into thorium-234. The alpha particle that is produced is emitted outwards and lost (and is the main source of the radiation that makes these materials so dangerous).
According to the Lawrence Livermore National Lab, there are a few conditions that need to be met for proper radiochronometric dating. One of the conditions is that the sample was totally pure at the time of creation. In this case, it means that when the cube was manufactured, it was entirely composed of uranium. Other conditions mentioned are that the decay must be simplifiable to a single consistent chemical equation (such as uranium’s alpha decay here), or that it has remained a closed system since manufacture, so the only thing affecting its composition is the radioactive decay. The final condition mentioned is that, of course, the analysis done must be accurate.
Radiochronometric dating has many uses beyond just cube identification, however. An extremely similar process is used to determine the age of rocks or other geological artifacts. The technique is slightly different because the time scales are in the hundreds or thousands of millennia rather than a few decades, but the overall principle is the same. When using this technique for dating rocks, fossils, or ancient artifacts, isotopes such as carbon-14 are crucial, as the half-life is in the thousands of years, which means that there is detectable carbon-14 in the sample for millennia. Once there is none of your parent isotope, you can only determine a minimum age.
Since 1907, the concept of radiochronometric dating has revolutionized our ability to determine the ages of mysterious objects. In a situation like with these uranium cubes, it’s the perfect technique to unravel the mystery of where exactly they came from, and if they really are a relic from Nazi Germany.
Boltwood, B. B. “Ultimate Disintegration Products of the Radioactive Elements; Part II, Disintegration
Products of Uranium.” American Journal of Science, vol. s4-23, no. 134, American Journal of
Science (AJS), Feb. 1907, pp. 78–88. Crossref, doi:10.2475/ajs.s4-23.134.78.
Carroll, Chris. "Tracing the Fallout of Hitler's Nuclear Reactor That Wasn't." Maryland Today, U of
Maryland, 1 May 2019, today.umd.edu/
tracing-fallout-hitlers-nuclear-reactor-wasnt-73221c6f-b7aa-4cfd-9402-fa0ccf5e0579. Accessed 14
Sept. 2021.
Jiménez, Jesus. "Did Nazis Produce These Uranium Cubes? Researchers Look for an Answer." The
New York Times, 10 Sept. 2021, www.nytimes.com/2021/09/10/science/
nazi-uranium-nuclear-program.html. Accessed 14 Sept. 2021.
Koeth, Timothy, and Miriam Hiebert. “Tracking the Journey of a Uranium Cube.” Physics Today, vol. 72,
no. 5, AIP Publishing, May 2019, pp. 36–43. Crossref, doi:10.1063/pt.3.4202.
Good title and graphic. "Radiometric" is perhaps a little technical. Your lede (as a journalist might say) paragraph sets up the problem very well and would help draw in a general reader. Your explanation of radiometric dating is very good. It's at about the right level and concise. The problem and the associated chemistry are very interesting. A very good post.
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