By Ron B. Davis Jr., Georgetown University
The discovery of the neutron in 1932 is arguably the seminal event that ushered in the nuclear age. Neutrons also provided a versatile tool that could be used to build atoms up, as was the case with neptunium and plutonium. Moreover, just as they had aided in the discovery of neptunium and plutonium, neutrons would ultimately provide the key to filling in nearly all of the actinoids on the periodic table after plutonium—the so-called trans-plutonic elements.
The Discovery of Plutonium
Neptunium and plutonium were elements heavier than uranium that formed in early nuclear reactors when some nuclei captured neutrons to become even more massive. Instead of breaking apart, these more massive nuclei then emitted beta radiation, each time changing a neutron to a proton, increasing their atomic number and transmuting into an even larger element.
In fact, in certain nuclear reactors, so much plutonium formed, and persisted long enough, that plutonium could be collected and used in the nuclear bomb that destroyed Nagasaki in 1945.
And yet, with the discovery of plutonium, our understanding of the phenomenon of neutron capture and its impact on the periodic table was only really getting started.
The Manhattan Project
Of course, most of us know that the ultimate goal of the Manhattan project was to produce an atomic bomb, a super-weapon so powerful that it would bring the second great war to a conclusion. During the Manhattan project, in order to develop the first uranium and plutonium weapons, dozens of test explosions were carried out, many of them in America’s desert southwest.
The clouds of material created in these explosions contained high levels of radioactive material, and this turned out to be more than just smaller nuclei from the fission process.
Moreover, based on the explosive yield measured when the ‘Fat Man’ plutonium bomb was detonated over Nagasaki, it is estimated that only 16% of the plutonium atoms in the bomb’s core actually split. So, what happened to the remaining 84% of the plutonium?
The Test-bomb Explosions
Through the 1940s and 1950s nuclear testing was booming. Detonation of devices using uranium and plutonium were carried out routinely, and the products of the explosions continued to be of particular interest.
In the violent, hot, energetic and neutron-rich environment of a nuclear explosion, it appears that some of the remaining nuclear payload that did not undergo fission actually transmuted and became even larger atoms. Many of the more massive actinoid elements were first detected in the dust and debris from test-bomb explosions. This prompted the question—could these larger atoms be created in a more controlled environment and studied? The answer is yes.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Neutron Flux Can Produce Heavier Elements
Many of the larger actinoids were first studied in the controlled laboratory environments of what would become Argonne National Lab, near Chicago and the Berkeley Radiation Laboratory, in California. Three resulting elements we find in the periodic table read like an address for the Berkeley ‘Rad Lab’. Elements 97, 98, and 95 are Berkelium, Californium and Americium.
These elements are clustered together just to the right of element 94 plutonium in the f-block.
The important thing in all this was that the nuclear weapons test clearly showed that with sufficient neutron flux under the right conditions, elements more massive than even plutonium could be made. And if 93-neptunium and 94-plutonium can both be made by neutron bombardment of 92-uranium, in the more controlled confines of a nuclear reactor, could even heavier elements be produced using this technique? Analysis of nuclear fallout seemed to suggest that it could.
Americium
One of the first of the heavy actinoids discovered and synthesized using this technique, was element-95, americium. The man responsible for this achievement was Glen T. Seaborg, a Manhattan Project physicist with years of experience refining the synthesis of 94-plutonium from 92-uranium using neutron bombardment.
Seaborg was continuing his research into neutron capture in 1944, at the University of Chicago, when he and a team of physicists successfully created element 95. They did so by bombarding a sample of plutonium-239 with neutrons. Because its discovery took place in the United States, and given the element’s location on the table just below europium, Seaborg named his new element ‘americium’.
Consistent Emission of Ionizing Radiation
The half-life for the decay process from plutonium-241 to americium-241 is about 14 years. Thus, understandably, the collection of Seaborg’s new element took time. Over the decades since his discovery, however, many kilograms of it have been collected, and we all probably have a small sample of it in our home right now.
Americium’s challenging synthesis makes it an expensive element. At a cost of about forty-two thousand US dollars per ounce, americium is somewhere around thirty times as expensive as gold.
Yet, most of us own a small sample of this element, in our smoke detectors. Americium is valued for its clean, consistent emission of ionizing radiation. In these devices, a small amount of americium is used to generate a stream of alpha particles between two electrodes charged by a conventional battery.
The Workings of a Smoke Detector
Americium’s ionizing radiation can cause molecules in the air to become charged. As these charged particles form, they flow to the positive or negative end of the ionization chamber, creating a steady, measurable amount of electrical current. But smoke molecules tend to be large and more resistant to ionization. Hence, if smoke molecules infiltrate the ionization chamber of our smoke detector, they impair the unit’s ability to produce that steady current. As the current drops, the smoke detector sounds the alarm and possibly saves a life.
Interestingly, as alpha decay means losing two protons it changes element 95, americium to element 93, neptunium, which has a half-life of over two million years.
Thus, if we look up and see a smoke detector on a ceiling, chances are we are looking at two synthetic heavy elements, both of which got their start as uranium atoms in a nuclear reactor.
Common Questions about the Use of Neutron Flux in Extending the Periodic Table
The first example of how useful neutrons might be in extending the periodic table came from the discovery of neptunium and plutonium.
The nuclear weapons test clearly showed that with sufficient neutron flux under the right conditions, elements more massive than even plutonium could be made.
Americium is valued for its clean, consistent emission of ionizing radiation.
