In the early stages of scientific exploration, nature was the primary supplier of radioactive nuclides used in experiments. Isotopes of uranium and thorium generated a variety of radioactive heavy elements such as lead. However, radioactive isotopes of light elements were not well known until the groundbreaking work by Marie Curie's daughter Irène Curie and her husband Frédéric Joliot.
Alpha-emitting sources had been used for years, such as those studied by Ernest Rutherford, who investigated the deflection of α particles in gold foils. This led to the understanding that atoms consisted of a tiny nucleus surrounded by orbiting electrons. However, the Joliot-Curies discovered that when α particles bombarded a foil (e.g., aluminum), they induced radioactivity. They observed that the half-life of this induced radioactivity was approximately 3 minutes and identified the emitted radiation as originating from ^{30}P created in the reaction:
\( \text{Al}^{27}(\alpha, n) \text{P}^{30} \)
This discovery opened the door for further exploration, where the Joliot-Curies hypothesized that other bombarding particles, such as protons and neutrons, could also induce radioactivity in various elements.
In the same year, Ernest Lawrence in Berkeley and Enrico Fermi in Rome validated these theories, with Lawrence's cyclotron producing the isotope ^{13}N and Fermi's experiments highlighting the power of neutrons in inducing radioactivity. Fermi's team used a neutron source created by sealing radium gas with beryllium powder, generating neutrons via the reaction:
\( \text{Be}^{9}(\alpha, n) \text{C}^{12} \)
These findings led to a series of exciting discoveries. Scientists like Hevesy used radioisotopes such as ^{32}P to study the uptake and elimination of phosphate in various tissues of rats, demonstrating the kinetic behaviors of essential elements in living organisms. This marked the birth of radiotracer technology, which is fundamental in modern biology and medicine.
Later, the cyclotron was used to produce biologically important radionuclides, such as ^{11}C, which was essential for studying photosynthesis and the fixation of carbon monoxide in humans. However, the short half-life of ^{11}C (20 minutes) posed challenges in longer studies.
The discovery of ^{14}C, produced through the reaction:
\( \text{C}^{13}(d, p) \text{C}^{14} \)
The creation of ^{14}C was serendipitous. During experiments, ammonium nitrate solution unintentionally became irradiated by neutrons. This led to the production of highly radioactive carbon-14 that could be easily separated from the target:
\( \text{N}^{14}(n, p) \text{C}^{14} \)
This discovery was pivotal, as it allowed the production of carbon-14 in large quantities, which was subsequently used in studying biochemical processes in living organisms. Similar discoveries followed, including the production of tritium (^{3}H) and the realization that cyclotrons were crucial in producing isotopes before nuclear reactors became the primary source of radionuclides.
Post-World War II, reactors became the main source of radionuclides, capable of producing a wide variety of radioactive elements. This development transformed the field of nuclear medicine, allowing for the use of radionuclides in clinical settings, such as ^{60}Co for radiotherapy and ^{131}I for thyroid treatment.
Accelerator-produced nuclides, such as ^{111}In, ^{123}I, and ^{201}Tl, remain critical in nuclear medicine, particularly for positron emission tomography (PET), which utilizes isotopes like ^{11}C, ^{13}N, ^{15}O, and ^{18}F.