Scientists have announced the production of Uranium-241 for the first time, filling a gap in the long list of the heavy metal’s isotopes. At an estimated 40 minutes, the half-life is far too short for any U-241 formed in supernovas or kilonovas to survive. On the other hand, unlike many isotopes with half-lives measured in micro or nanoseconds, this one is long enough to study its properties.
Elements are defined by the number of protons in their nucleus, but (hydrogen aside) also need neutrons there to hold things together. Elements can have varying numbers of neutrons, creating isotopes, distinguished by the number of protons and neutrons combined. Thus all uranium atoms have 92 protons and the new isotope has 149 neutrons, whereas Earth’s natural uranium atoms have 146 or 143.
Some isotopes are stable, never decaying to other elements unless forced by radiation bombardment. Uranium has no stable isotopes, but U-235 and U-238 take so long to decay that a significant portion of those atoms present when the Earth formed survive today, along with tiny amounts of U-233, U-234, and U-236. All the other isotopes of uranium have had to be produced in the lab, and scientists have been slowly discovering which are possible. Now, in a new paper, a team led by Dr Toshitaka Niwase of Japan’s High Energy Accelerator Research Organization has added U-241 to the list.
In 2021, the announcement of U-214’s production meant the arrival of the uranium isotope with the fewest neutrons, and therefore the lightest nucleus. Like most newly discovered isotopes, it survives for only tiny amounts of time. Its half-life (the time it takes for half the atoms in a sample to radioactively decay) is estimated at just 0.0005 seconds, although there is still some uncertainty about this – it could be as much as three times as long.
Uranium-220 remains undiscovered, but with the production of 241, every other isotope up to U-242 has now been made. Of these, the majority have half-lives of less than a second, usually much less, so while U-241’s half-life still needs to be measured, the estimate of 40 minutes puts it among the longer-lived of the 28 known isotopes, although nowhere near U-238’s 4.5 billion years. U-241’s discovery is the first for an isotope at the heavier end of the scale since 1979.
When U-241 decays it releases beta rays, converting one of its abundant supply of neutrons to a proton and forming neptunium-241, which in turn lasts for about 14 minutes before becoming plutonium-241 through the same decay process.
Niwase and co-authors fired uranium-238 and platinum-198 particles together and produced 19 different isotopes of protactinium, uranium, neptunium, and plutonium. Eighteen of these were familiar, but atoms of the U-241 were found in the mix by sorting the products by weight.
It’s unlikely that U-241 will prove to have any practical uses. We already have other ways of making its daughter isotopes, and no one is likely to want to replace the isotopes used in existing applications of uranium with such a radioactive version. Using platinum makes for an expensive production process even before you allow for most of the output being other isotopes. Nevertheless, investigating such a heavy isotope’s behavior could increase our understanding of how neutron-rich atoms behave.
The study is published in Physical Review Letters.