A liquid scintillation counter is a device used to measure ionizing radiation by detecting light produced when radiation interacts with a liquid scintillation solution. The solution contains organic compounds that emit photons when excited by radiation, allowing for the quantification of radioactive isotopes in a sample. These counters are commonly used in fields like nuclear medicine, environmental monitoring, and radiochemistry. They can measure various types of radiation, including alpha, beta, and gamma particles, making them versatile tools for researchers and scientists studying radioactive materials.