| RAU | radioactive uptake |
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| RI | radiation intensity; radioactive isotope; radioimmunology; recession index; recombinant inbred [stra... |
| RIHSA | radioactive iodinated human serum albumin |
| RISA | radioactive iodinated serum albumin; radioimmunosorbent assay |
| RIU | radioactive iodine uptake |
| elements, radioactive | Chemical elements which spontaneously transmute into another element with corpuscular or electromagnetic radiation. The natural radioactive elements are all those with an atomic number above 83, and some other elements, such as potassium (atomic number 19) and rubidium (atomic number 7), which are very weakly radioactive. (12 Dec 1998) |
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