All matter in the universe has its origin in an event called the ‘big bang’, a cosmic explosion releasing an enormous amount of energy about 14 billion years ago. Scientists believe that particles such as protons and neutrons, which form the building blocks of nuclei, were condensed as free particles during the first seconds.
As the universe expanded, the temperature decreased, leading to the formation of particle combinations such as deuterium (heavy hydrogen) and helium. For several hundred million years, the universe was a plasma, composed of hydrogen, deuterium, helium ions, and free electrons. The equations for these early conditions can be approximated as:
Equation 1: The density of plasma in the early universe is related to the temperature \( T \) and the number of particles per unit volume \( n \) as:
\( n(T) \propto T^3 \) for high temperatures, with the plasma being dominated by ionized hydrogen and helium.
As the temperature continued to decrease, electrons combined with protons and other ions, forming neutral atoms. This marked the conversion of the plasma into a large cloud of hydrogen and helium gas, which slowly condensed under gravity, leading to the formation of the first stars.
These stars initiated nuclear fusion. Nuclear reactions in stars can be represented by the following fusion equations:
Equation 2: The fusion of two hydrogen nuclei (protons) into helium-4:
\( 2\text{H} \rightarrow \text{He} + \text{energy} \)
As temperature and pressure inside stars increased, heavier elements were produced through nucleosynthesis. The fusion reactions led to the creation of elements such as carbon, nitrogen, and oxygen. This process can be described as:
Equation 3: The production of carbon-12 through the CNO cycle:
\( 4 \text{H} \rightarrow \text{C} + 2 \text{He} + \text{energy} \)
As stars aged, consuming their hydrogen fuel, they eventually exploded in supernovae, spreading heavy elements throughout the universe. This allowed the formation of new stars and planetary systems. These supernovae produced elements like iron, nickel, and uranium, which are crucial for the formation of planets like Earth.
Four and a half billion years have passed since the planet Earth was formed. In that time, most atomic nuclei consisting of unstable proton-neutron combinations have undergone radioactive decay to more stable combinations. However, some isotopes with very long half-lives remain, such as:
The discovery of radioactive elements dates back to the late 19th century. As Henri Becquerel discovered in 1896, some elements emitted radiation spontaneously. This led to the field of nuclear physics and chemistry. One of the most notable achievements was the work done by Marie Skłodowska-Curie and Pierre Curie, who further investigated the properties of radioactive substances, isolating radium and polonium from uranium ores.
"Radioactivity is a spontaneous process that occurs in unstable atomic nuclei, releasing energy in the form of particles or electromagnetic waves." — Marie Skłodowska-Curie
These discoveries contributed to the establishment of the study of nuclear reactions and decay, which would later revolutionize medicine, especially in the field of nuclear medicine.
George de Hevesy was a pioneer in using radioactive elements for scientific applications. He used the isotope \( ^{210}Pb \) as a tracer in studies of the solubility of lead salts. This was crucial in the development of the radioactive tracer technique in medicine.
In 1923, de Hevesy investigated the uptake of lead in plants using \( ^{212}Pb \), marking the first use of radioactive tracers in biology. This method was extended to clinical studies a year later, as Blumengarten and Weiss injected \( ^{212}Bi \) into a patient to study blood flow dynamics. Their findings showed that patients with heart disease had prolonged arrival times of the tracer, indicating delayed circulation.
"The use of radioactive tracers allows us to study biological processes in a way that was once thought impossible." — George de Hevesy