Answer to Question #11029 Submitted to "Ask the Experts"

Category: Radiation Basics — Alpha Radiation

The following question was answered by an expert in the appropriate field:


We recently discovered a cornerstone in one of our research buildings where a researcher/scientist, in order to study the long-term biological effects of radiation, placed two vials of 238U with bacterial spores in 1960. I am wondering why 238U was chosen, but assume it was because it is an alpha emitter and is very long lived. It seems like it is also in a lead-lined vial. The intent of the experiment was for it to last for 100 years. Any light on this interesting research will help.


Thank you for your most interesting question. Without additional information about the design and details of the experiment, it is not possible to shed very much light of a definitive nature on what was done and why. However, I can offer some words albeit somewhat speculative as to why 238U may have been chosen for the experiment. I suspect that, in fact, what was chosen was natural uranium, which in 1960 was inexpensive and readily available from chemical supply houses and, as a naturally occurring substance, was not regulated by the Atomic Energy Commission nor by the states and hence did not require a license. Indeed, I daresay that most college chemistry laboratories had a quantity on hand for teaching and research purposes. Natural uranium was also of considerable interest at that time to the general public.

Natural uranium by weight is 99.275 percent 238U, 0.72 percent 235U, and 0.0055 percent 234U. All three of these uranium isotopes are radioactive with very long but significantly different half lives, and all decay by the emission of alpha particles. In addition, beta, gamma, and x rays, largely from the radioactive progeny produced by their decay, are also associated with natural uranium. Coupled with the ready availability and low price, the long half-life of uranium would make it desirable to be used for an experiment meant to last 100 years.  

While it may be reasonable to assume that this experiment was done with an eye towards determining any long-term biological effects on the bacterial spores, without additional information this assumption is speculative. If you can provide the details of the experiment it might be possible to come up with a reasoned explanation bearing in mind the more information that can be obtained, the less speculative the conclusions are likely to be.

Ronald Kathren, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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