Strontium-90 (\({}^{90}\text{Sr}\)) is a well-known radioactive isotope, a product of nuclear fission. Its instability is supported by multiple lines of scientific evidence, ranging from the composition of its atomic nucleus to measurable physical phenomena. Examining the structural imbalance of the nucleus, the emission of detectable radiation, the quantifiable rate of transformation, and the final stable products provides a complete picture of why the \({}^{90}\text{Sr}\) nucleus is inherently unstable.
The Theoretical Basis for Instability
The instability of the strontium-90 nucleus is fundamentally linked to the ratio of neutrons to protons (N/P) within its structure. A nucleus of \({}^{90}\text{Sr}\) contains 38 protons and 52 neutrons, giving it an N/P ratio of approximately 1.368. For lighter elements, a stable nucleus typically maintains an N/P ratio close to 1:1. While the stable ratio increases for heavier elements, the configuration of \({}^{90}\text{Sr}\) places it outside the narrow range known as the “band of stability.” This isotope has an excessive number of neutrons relative to the number of protons needed for stability. This neutron-rich condition creates an intrinsic energetic imbalance that necessitates a change to achieve a more favorable state.
Direct Evidence: The Emission of Beta Radiation
The most direct, observable proof of nuclear instability is the emission of radioactive particles. The \({}^{90}\text{Sr}\) nucleus resolves its unstable neutron-to-proton imbalance by undergoing beta-minus (\(\beta^-\)) decay. In this decay, one excess neutron spontaneously transforms into a proton, which remains in the nucleus, increasing the atomic number by one. This transformation is accompanied by the expulsion of a high-energy electron, known as a beta particle, and a corresponding antineutrino. The energy released during this initial decay is 0.546 MeV. This emission is directly detectable using specialized equipment, such as Geiger counters or scintillation detectors. The instruments register the passage of the energetic beta particles, providing empirical evidence that mass and energy are actively being ejected from the strontium nucleus.
Measuring the Rate of Decay
The instability of \({}^{90}\text{Sr}\) is also quantitatively demonstrated by measuring the rate at which its atoms transform. This rate is expressed as the half-life (\(t_{1/2}\)), which is the time required for exactly half of the initial quantity of the radioactive material to decay. Strontium-90 has a finite, measurable half-life of 28.91 years. This relatively short time frame is a precise measure of the nucleus’s high degree of instability. If \({}^{90}\text{Sr}\) were a stable isotope, its half-life would be infinite. The consistent and predictable nature of this decay rate allows scientists to quantify the instability with high precision. Observing the exponential decrease in the number of \({}^{90}\text{Sr}\) atoms or the detected radiation over time serves as a robust, quantitative affirmation that the nucleus is rapidly transforming.
Confirmation Through Isotope Transformation
The final confirmation of \({}^{90}\text{Sr}\) instability is the analysis of the elements that remain after the decay process is complete. Nuclear instability is proven because the original strontium atom is entirely replaced by different chemical elements in a decay chain. The initial beta decay transforms the \({}^{90}\text{Sr}\) nucleus into Yttrium-90 (\({}^{90}\text{Y}\)), which has an atomic number of 39 compared to strontium’s 38. The daughter isotope, \({}^{90}\text{Y}\), is itself unstable, with a much shorter half-life of approximately 64 hours. It quickly undergoes a second beta decay, transforming into stable Zirconium-90 (\({}^{90}\text{Zr}\)). The eventual accumulation of this stable end-product serves as the ultimate proof that the original \({}^{90}\text{Sr}\) nucleus was unstable and underwent a complete transformation.