A brief introduction to radioactive decay

academics physics

The discovery of radioactivity revolutionized physics. What started as a study of phosphorescent materials in a Paris laboratory quickly grew to define a new area of physics, one that is vital to medicine, environmental studies, and national defense.

Here, we will briefly discuss the forms of radioactive decay and how we can measure it. 

Alpha, Beta, and Gamma Decay 

In radioactive decay, an unstable (or radioactive) nucleus loses energy via ionizing radiation. As it decays, the nuclide will change from its original form—a parent nuclide—to different atom—a daughter nuclide. The principle modes of decay for radioactive nuclei are called the alpha, beta, and gamma decay. The decay names cleverly take the first three letters of the Greek alphabet, as the ability for each particle to penetrate matter increases with each.  

Alpha decays occur when a nucleus emits an alpha particle, which is a particle comprised of 2 neutrons and 2 protons. Therefore, the daughter nuclide will have 2 less neutrons and 2 less protons than the parent nuclide. This type of decay is only seen with elements that have a large atomic number (Z>52). Alpha particles are the largest of the 3 types of decays, and therefore need the least amount of shielding to be stopped. An alpha particle can be stopped by no more than a sheet of paper. 

Beta decays occur when a neutron becomes a proton, or vice versa, by the emission of a very small, high energy beta particle and a quasi-massless neutrino. There is a bit more to this explanation but, in short, beta decays mean that the A of the parent nuclide and the daughter nuclide are the same, but the N and Z change by one. Beta decays require more shielding than Alpha particles, but can still easily be stopped by a sheet of aluminum foil. 

The final type of decay is Gamma decay, which will not be discussed in depth here. The parent nuclide has the same A, N, and Z as the daughter nuclide, but has a higher energy state than the daughter nuclide. A gamma ray is incredibly small and massless, with high energy that can penetrate thin shielding easily. A very thick layer of lead, or a much more substantial mass, is needed to stop a gamma ray.  


Radioactive decay is typically measured via a half-life, t1/2. Each radioactive atom decays in a different amount of time; some nuclides will take a few seconds, while others take millions of years. To be able to measure the decay of a radionuclide, it is best to imagine a group of similar radionuclides grouped together. By determining how long it takes for half of the group to decay, we can figure out the average lifespan of that nuclide.  

The half-life of a nuclide is important in measuring the activity, or the number of decays per unit time, of a radioactive sample, thereby allowing them to be used in all sorts of applications.


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