By Ron B. Davis Jr., Georgetown University
While the 75,000-year half-life of ionium proves useful in radio dating samples that are around one hundred thousand years old, uranium offers a solution to dating geologic samples on a different timescale—the age of the Earth itself.
It is serendipitous, indeed, that most common isotope of uranium, uranium-238 is a relatively abundant, lithophilic atom whose half-life comes in at 4.5 billion years. On this time scale, all of the subsequent nuclear decays in the uranium-238 decay chain might as well be a split-second, by comparison.
On geologic timescales, this convenient fact allows us to think of the conversion of uranium-238 to lead-206 as a two-state system in which uranium-238 becomes lead-206 in a concerted transition, ignoring all intermediate nuclei.
Geologic samples that solidified billions of years ago trapped a given amount of uranium-238. As that uranium-238 decays into lead, the ratio of these two isotopes changes like a ticking clock. The greater the proportion of lead to uranium, the older the rock.
The relative abundance, lithophilic character and long half-lives of thorium and uranium have made both candidates for use in certain applications, like radiometric dating of geological features, and the generation of nuclear power.
Thorium’s use in geologic dating goes back to the discovery of its very special ‘ionium’ isotope, thorium-230. In fact, the technique of ionium-thorium geologic dating relies on the assumption that both thorium-230 and thorium-232 are chemically identical, but that one is primordial, while the other is a product of uranium decay.
When it comes to uranium, once locked in a mineral or sediment, each isotope decays but thorium-230 does so with a half-life of about 75,000 years. Thorium 232 on the other hand, with its whopping half-life of 14 billion years, decays hardly at all in that time. By comparing the ratio of these two isotopes in geologic samples, dates up to 400,000 years can be accurately determined.
Ushering in the Nuclear Age
It wasn’t until 1932 that the nuclei of atoms were shown to contain a second type of nucleon—the neutron. Only then, did it become possible to understand that Uranium-238 was a nucleus with 92 protons and 146 neutrons. Thus, neutrons not only put the last piece of the nuclear puzzle in place, they also ushered in the nuclear age during the next decade.
Uranium and thorium are primordial elements—elements as old as the supernova that seeded the elements in the birth of our solar system. Some atoms of those elements that existed at the time our planet formed are still here even today. And more and more are always decaying radioactively to create lighter, non-primordial elements such as actinium and protactinium.
It is those other decays that create enough of these elements to detect and even collect from natural pitchblende samples.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Nuclear Processes and Alpha Particle Emission
However, the nuclear binding energy curve points in another direction-in the direction of iron. This means that spontaneous nuclear processes involving such heavy elements most often begin with an alpha particle emission. It is responsible for reducing that nucleus’ atomic number and mass and shifting it toward the lighter and more stable nuclei on the curve.
And yet, beta radiation—which involves emission of a high-energy electron—can increase the atomic number of a nucleus by changing a neutron to a proton. Mass still balances because the mass of a neutron is slightly more than the mass of a proton—and the small mass of the emitted electron amounts to the difference.
The Nuclear Stability Curve
Thus, it is beta radiation which allows an element to step to the right on the periodic table, producing a nucleus of higher atomic number. One reason that elements don’t always do this is because of the nuclear stability curve. By increasing atomic number through beta radiation, the proton-to-neutron ratio becomes too high, and we move away from the curve of stability. That won’t last.
In the 1930s and 40s, nuclear physicists started to ask whether new elements, even more massive than uranium, might be created by bombarding uranium itself with neutrons.
Edwin McMillan and Philip Abelson
In theory, captured neutrons could throw the ratio of protons to neutrons outside the optimal ratio. Could nuclei with a big excess of neutrons respond by emitting a beta particle (a high-energy electron), converting one or more of the extra neutrons into protons? If so, that would re-establish more optimal proton-to-neutron ratio and produce new heavier nuclei with higher atomic numbers?
Proof of this concept was offered by Edwin McMillan and Philip Abelson from Berkeley, California in 1940. McMillan and Abelson achieved the synthesis of element 93 using this technique, irradiating uranium with low-energy neutrons. During this process several nuclear reactions take place, but the reaction of interest is the capture of two neutrons by uranium-235.
The resulting uranium-237 is more strongly radioactive and decays by the emission of a beta particle with a half-life of just 6.75 days. The resulting has 93 protons, and a half-life of just over two million years!
The reaction used to produce the first isotopes of element-94, plutonium was similar. Plutonium-239 can also be produced by neutron bombardment, this time of the most common isotope of element-92, uranium-238. The resulting U-239 undergoes two beta decays over time, converting two neutrons to two protons, raising the atomic number from 92 to 94, and forming plutonium-239.
And this next element was discovered just a few months after Neptunium, so that made it especially natural for its discoverers to name this next element after Pluto, which had been regarded at the time as the planet discovered after Neptune.
Common Questions about Uranium, Thorium, and Geologic Dating
Uranium-238 decays into lead and the ratio of these two isotopes changes like a ticking clock. The greater the proportion of lead to uranium, the older the rock.
The technique of ionium-thorium geologic dating relies on the assumption that both thorium-230 and thorium-232 are chemically identical, but that one is primordial, while the other is a product of uranium decay.
Edwin McMillan and Philip Abelson from Berkeley, California, in 1940, achieved the synthesis of element 93 by irradiating uranium with low-energy neutrons.