Types of Radioactive Emissions

In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether.

*NOTE! The number of protons in an atom defines the element, so a change in protons results in a change in the atom.

When a nuclide decays, the reactant decaying nuclide is called the parent and the product nuclide is called the daughter. Nuclide can decay in several ways

1.    Alpha Decay

 ²⁶³₁₀₆Sg → ²⁵⁹₁₀₄Rf + ⁴₂He

           The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged and is written as ⁴₂He or ⁴_{2 a} . An atom that emits an alpha particle is called an alpha emitter. The element that undergoes "alpha decay" reduces the atomic number by 2 units and it’s mass by 4 units. Alpha decay occurs when a nucleus has so many protons that the strong nuclear force is unable to counterbalance the strong repulsion of the electrical force between the protons. Because of its mass, the alpha particle travels relatively slowly (less than 10% the speed of light), and it can be stopped by a thin sheet of aluminum foil.

         Ernest Rutherford began experimenting to determine the nature of this radiation in 1898. One experiment demonstrated that the radiation actually consisted of 3 different types: a positive particle called "alpha," a negative particle, and a form of electromagnetic radiation that carried high energy.

2.      Beta Decay

 

_¹⁸8O → _¹⁸9F + ⁰-1e

_¹⁴7N → _¹⁴6F + ⁰-1e

Beta decay is a form of radioactive decay in which the nucleus of an atom undergoes a change which causes it to emit a beta particle.

Atoms undergo beta decay when they are unstable because they have too many neutrons or too many protons. To stabilize themselves, the excess neutrons or protons are converted, conserving mass and making the nucleus more stable. In the process, the atom also changes into another element, because while the overall number of particles in the nucleus remains the same, the balance of protons and neutrons changes.

In beta minus decay, an excess neutron becomes a proton, and the nucleus emits an electron. The electron is the beta particle symbolized as 0-1e. When a nucleus undergoes beta plus decay, a proton is converted into a neutron, with the nucleus emitting a positron. Beta particles can be electrons or positrons, as illustrated, depending on whether a nucleus goes through beta minus or beta plus decay. Before researchers realized that beta particles were just electrons or positrons, they referred to these particles as “beta rays,” which is why some antiquated texts contain references to beta rays.

          A beta particle has more penetrating power than an alpha particle, but less than a gamma particle. Beta particles can be stopped with a thick sheet of metal, a large pocket of air, or several sheets of paper. This makes them relatively safe to work around, as long as safety precautions are observed when people are around elements which undergo beta decay.

3.      Gamma Radiation

Gamma rays from radioactive gamma decay are produced alongside other forms of radiation such as alpha or beta, and are produced after the other types of decay occur. The mechanism is that when a nucleus emits an α or β particle, the daughter nucleus is usually left in an excited state. It can then move to a lower energy state by emitting a gamma ray, in much the same way that an atomic electron can jump to a lower energy state by emitting infrared, visible, or ultraviolet light.

²³⁸₉₂U → ²³⁴₉₀Th  +  ⁴₂He + 2⁰₀⅟

In gamma decay, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay. The characteristic energy is divided between only two particles.

4.      Positron Decay  

Positron decay involves the emission of a positron. A positron, symbolized by ⁰₁β, is the “antiparticle” of an electron. Positron decay occurs when a proton in the nucleus is converted into a neutron and a proton is released.

¹₁p → ⁰₁n + ⁰₁β

Positron emission is a byproduct of a type of radioactive decay known as beta-plus decay. In the process of beta plus decay, an unstable balance of neutrons and protons in the nucleus of an atom triggers the conversion of an excess proton into a neutron. During the conversion process, several additional particles, including a positron, are emitted.

¹¹₆C → ¹¹₅B + ⁰₁β

5.      Electron Capture

Electron capture occurs when the nucleus of an atom captures an electron from an orbital of the lowest energy level.

¹₁p + ⁰_-1e → ⁰₁n

¹⁹⁵₇₉Au + ⁰_-1e → ¹⁹⁵₇₈Pt

The theory of electron capture was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed by Luis Alvarez, in vanadium-48. He reported it in a 1937 paper in the Physical Review. Alvarez went on to study electron capture in gallium-67 and other nuclides

Electron capture is the primary decay mode for isotopes that have a relative superabundance of protons in the nucleus, but there is insufficient energy difference between the isotope and its prospective daughter with one less positive charge, for the nuclide to decay by simply emitting a positron. Electron capture also exists as a viable decay mode for radioactive isotopes that do have enough energy to decay by positron emission, and in that case, it competes with positron emission. It is sometimes called inverse beta decay, though this term can also refer to the capture of a neutrino through a similar process.