Short answers, pulled from the story.
Radioactivity was discovered in 1896 by Henri Becquerel and independently by Marie Curie while working with phosphorescent materials. Becquerel found that uranium salts blackened a photographic plate wrapped in black paper, and Curie named the emissions rayons de Becquerel, the Becquerel Rays.
The three most common types of radioactive decay are alpha, beta, and gamma decay. Ernest Rutherford named them in increasing order of their ability to penetrate matter, with alpha carrying a positive charge, beta a negative charge, and gamma rays neutral.
The half-life is the time taken for half of a radioactive sample's atoms to decay, while the decay constant, lambda, is the reciprocal of the mean lifetime. The mean lifetime, tau, is the 1/e life, the point at which about 36.8 percent of the sample remains.
Radioactive decay is used for dating because it is truly random and its rate does not vary significantly over time. Carbon-14, with a half-life of 5700 years, becomes trapped when organic matter grows, so the decline in its 14 disintegrations per minute per gram lets scientists estimate an object's age, cross-checked against tree rings.
Researchers passed alpha particles through a very thin glass window and trapped them in a discharge tube, then studied the emission spectrum of the captured particles. This analysis proved that alpha particles are helium nuclei.
Most decay modes are unaffected by temperature, pressure, chemical environment, or external fields, but electron capture and internal conversion can shift slightly with electronic structure. Fully ionized rhenium-187 drops from a 41.6 billion year half-life to 32.9 years through bound-state beta decay, and beryllium-7 shows a 0.9 percent difference between metallic and insulating environments.
Early exposure to X-rays caused burns, hair loss, and worse, with cases reported as early as 1896, including Dr. Dudley's hair loss at Vanderbilt University. Marie Curie warned that radium is dangerous in untrained hands and later died from aplastic anaemia likely caused by ionizing radiation, and by the 1930s radium medicinal products had largely been removed from the market.