The Physics
Opus in profectus

Radioactive Decay

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unstable isotopes

Quote that must be paraphrased

The turning point in the battle between theoretical physicists and empirical geologists and biologists occurred in 1896. In the course of an experiment designed to study x-rays discovered the previous year by Wilhelm Röntgen, Henri Becquerel stored some uranium-covered plates in a desk drawer next to photographic plates wrapped in dark paper. Because it was cloudy in Paris for a couple of days, Becquerel was not able to "energize" his photographic plates by exposing them to sunlight as he had intended. On developing the photographic plates, he found to his surprise strong images of his uranium crystals. He had discovered natural radioactivity, due to nuclear transformations of uranium. The significance of Becquerel's discovery became apparent in 1903, when Pierre Curie and his young assistant, Albert Laborde, announced that radium salts constantly release heat. The most extraordinary aspect of this new discovery was that radium radiated heat without cooling down to the temperature of its surroundings. The radiation from radium revealed a previously unknown source of energy. William Wilson and George Darwin almost immediately proposed that radioactivity might be the source of the sun's radiated energy.

Historical quote

These experiments show that the uranium radiation is complex, and that there are present at least two distinct types of radiation — one that is very readily absorbed, which will be termed for convenience the α radiation, and the other of a more penetrative character, which will be termed the β radiation.

Ernest Rutherford, 1899

The paths of alpha, beta, and gamma radiation through a magnetic field running into the page

Paths of α, β, and γ radiation in a magnetic field. Alpha particles deflect upward in this field obeying the right hand rule of a positively charged particle. Beta particles deflect the opposite way indicating negative charge. Gamma particles are unaffected by the field and so must carry no charge. In addition, the radius of curvature of the α particles is larger than that of the β particles. This shows that the alphas are more massive than the betas.

alpha decay


AZX → A−4Z−2Y + 42He

Alpha particles cannot penetrate a piece of paper or even the thin layer of dead skin that coats us all. They will quickly find and join with two electrons to become an atom of helium before they can do much harm. Alpha particles are most dangerous, however, when inhaled. The inside of our lungs are moist and sticky and not as well coated with expendable cells as our exteriors are. Were a bit of alpha emitting debris to find its way into our lungs, chances are pretty good that it would stick there long enough to emit an energetic and massive nuclear projectile into our tissues, ionizing and dissociating molecules along the way. Such activities are one source of lung cancer. Workers who handle plutonium (a significant alpha emitter) are well aware of this hazard and take great care to keep it outside of their bodies at all times.

beta decay

Also called beta minus decay.


n0 → p+ + e + ν0

Not isolated

AZX → AZ+1Y + 0−1e + 00ν

After photons, neutrinos are the most common particles in the universe.

positron emission

Also called beta plus decay.


p+ → n0 + e+ + ν0

Not isolated

AZX → AZ−1Y + 0+1e + 00ν

gamma decay

1899: Ernest Rutherford discovers that uranium radiation is composed of positively charged alpha particles and negatively charged beta particles

1900: Paul Villard discovers gamma-rays while studying uranium decay

AZX* → AZX + 00γ

neutron emission


AZX → A−1ZY + 10n

proton emission


AZX → A−1Z−1Y + 11p

decay chains


Five modes of nuclear decay on a graph of mass number vs. atomic number

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Uranium decay series
Uranium decay series on a graph of mass number vs. atomic number