Plutonium (Pu) is a synthetic, highly radioactive element first discovered in 1940. This metallic element, with atomic number 94, is notable for its exceptional mass and complex physical behavior. Unlike most elements, its density is not a single, fixed value, presenting a unique challenge to materials scientists and engineers. Its density varies significantly due to its unusual internal structure.
Defining the Baseline Density
Density is fundamentally defined as mass per unit volume, typically measured in grams per cubic centimeter (g/cm\(^3\)). The most stable and common form of pure, unalloyed plutonium at room temperature is known as the alpha (\(\alpha\)) phase. This phase serves as the baseline measurement for the element’s density.
The density of alpha-phase plutonium is approximately 19.86 g/cm\(^3\) at standard conditions. This value is significantly higher than common heavy metals; for instance, it is almost twice the density of lead (about 11.34 g/cm\(^3\)). It is also slightly denser than gold, which has a density of around 19.3 g/cm\(^3\). This makes plutonium one of the densest elements known.
The Unique Phenomenon of Plutonium Allotropes
The reason plutonium’s density is not constant is due to its extraordinary property of allotropy, meaning it can exist in multiple distinct solid crystalline forms. Plutonium exhibits at least six different solid phases (\(\alpha\) through \(\epsilon\)) at ambient pressure, a complex polymorphism unmatched by any other element. Each of these phases possesses a different internal crystal structure and, consequently, a unique density.
The density of these various allotropes can range from a high of 19.86 g/cm\(^3\) for the alpha phase down to about 15.92 g/cm\(^3\) for the delta (\(\delta\)) phase. This difference represents a volume change of more than 25% between the densest and least dense phases. The change occurs with only minor shifts in temperature or pressure, demonstrating the material’s extreme sensitivity.
One of plutonium’s most unusual characteristics is the density change that occurs upon heating. In most metals, increasing the temperature causes thermal expansion and a corresponding decrease in density. However, when pure alpha-plutonium is heated, it transitions to the delta phase, which is significantly less dense. This substantial expansion upon heating is highly anomalous for a metal and complicates its use and fabrication.
Why Density Matters for Critical Mass
The density of plutonium is a factor that has profound practical significance in nuclear physics, particularly concerning the concept of critical mass. Critical mass is the minimum quantity of fissile material required to sustain a nuclear chain reaction. The probability of a neutron striking an atomic nucleus and causing fission is directly related to how closely the fissile atoms are packed together.
A higher density means the atoms are physically closer, which increases the likelihood that a neutron released during fission will hit another nucleus before escaping the material. This enhanced probability translates directly into a smaller critical mass requirement. In simple terms, a denser sphere of plutonium needs less total mass to achieve a sustained chain reaction than a less dense sphere of the same material.
In the design of nuclear devices, this relationship is exploited through density manipulation. Plutonium is often stabilized in the lower-density delta phase by alloying it with a small amount of gallium for easier machining and handling. To initiate a nuclear reaction, high-explosive lenses create an inward shockwave, which rapidly compresses the core. This compression forces a phase transition to the much denser alpha phase, dramatically reducing the critical mass and initiating the nuclear explosion.
Practical Challenges in Density Measurement
Measuring the exact density of plutonium involves significant experimental hurdles that complicate obtaining precise values. Plutonium is highly toxic and intensely radioactive, emitting alpha particles that necessitate remote handling within specialized enclosures called hot cells. This requirement alone introduces difficulties in performing routine laboratory measurements.
Furthermore, plutonium is subject to an intrinsic process known as self-irradiation damage. The continuous alpha decay of its isotopes causes the material to damage its own crystal structure over time, which can alter its phase stability and, consequently, its density. The decay process also generates heat, leading to self-heating of the material.
The self-heating effect makes it difficult to maintain the precise and uniform temperature control necessary for accurate density measurement, especially when studying phase transitions. Traditional methods, such as using the Archimedes principle, historically required immersing the metal in hazardous liquids like monobromobenzene. Scientists now use safer, more precise techniques, such as gas pycnometry, to determine the density of this uniquely challenging element.