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Beta Decay

The second kind of radioactive decay called beta decay, involves the emission of an electron. The simplest kind of beta decay that one can observe is for a single neutron. If we put a collection of neutrons it would start to disintegrate with about half of them disappearing in the first.

When a neutron decays it produces several particles the most obvious of which are a proton and an electron both particles that carry an electrical charge and are therefore vary easy to detect.

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Beta Decay Definition

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Beta decay is the phenomenon of emission of an electron from a radioactive nucleus is called beta decay. Beta decay is a process in which either
  1. a neutron is converted into a proton or
  2. a proton is converted into a neutron.
Thus the ratio N/Z is altered in beta decay.

When a neutron is converted into proton an electron and a new particle named anti neutrino are created and emitted from the nucleus.

n $\rightarrow$ p + e + $\bar{\nu }$

The electron emitted from the nucleus is called a beta particle. The anti neutrino is supposed to have zero rest mass, is charge less and has spin quantum number ±1/2

This beta decay process is denoted by

ZXA $\rightarrow$ z+1YA + -1e0 + $\bar{\nu }$

Beta Decay Equation

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  1. Beta particle decay is the radioactive decay process in which a beta particle is emitted from an unstable nucleus.
  2. Beta particle decay like alpha decay always produces a nuclide of a different element.
  3. The mass number of the new nuclide is the same as that of the parent nuclide.
However the atomic number has increased by 1 unit. The general equation for beta decay is

ZXA $\rightarrow$ -1$\beta$0 + z-1YA

At this point we may be wondering how a nucleus which is composed only of neutrons and protons ejects when no such particle is present in the nucleus. Explained simply a neutron in the nucleus is transformed into a proton and a beta particle through a complex series of steps; that is

Neutron $\rightarrow$ proton + beta particle

0n1 $\rightarrow$ 1p1 + -1$\beta$0


Once it is formed within the nucleus the beta particle is ejected with a high velocity. Note the symbols used to denote a neutron and a proton.
  1. 0n1 - no charge and a mass of 1 amu
  2. 1p1 - +1 charge and a mass of 1 amu

Beta Plus Decay

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A proton in the nucleus is converted into a neutron a positron and a tiny weakly interacting particle called a neutrino, resulting in the atomic number decreasing by one. The mass number however does not change, even though the actual mass does change very slightly.

A proton becoming a neutron after all should have absolutely no effect on the mass number because both protons and neutrons are nucleons or particles contained in the nucleus and converting one to the other does not change the overall number of particles in the nucleus.

Beta plus decay follows the form of the equation below, with the atomic number being decreased by one and the mass number remaining the same.

AXB $\rightarrow$ B-1YA + e+ + $\nu$

Beta Minus Decay

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A neutron is converted into a proton, emitting in this case an electron and an anti neutrino. Again the mass number remains the same through out the decay because the number of nucleons remains the same; however the atomic number increases by one. In case this is the reverse of beta plus decay.

Beta minus decay follows of the equation below, with the atomic number being increased by one and the mass number remaining the same.

AXB $\rightarrow$ B+1YA + e- + $\bar{\nu }$

Beta Decay Example

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Elements with an atomic number less than 83 do not usually exhibit alpha decay. Instead various isotopes of these elements emit beta-particles.

For example, carbon-14 (6C14) decays by beta particle emission forming (7N14). The decay process can be represented by the equation.

6C14 $\rightarrow$ 7N14 + -1e0

Notice that both mass and charge are conserved in beta decay as in alpha decay. This means that a neutron in the 6C14 nucleus is converted to a proton + an electron.

0n1 $\rightarrow$ 1p1 + -1e0

The proton remains in the nucleus but the electron is ejected as a beta particle. Notice that the ejected electron comes from the breakdown of a neutron in the nucleus. It is not one of the electrons in the shells of the isotope. Thus overall effect of beta decay is an increase of one proton in an isotope and a corresponding decrease of one neutron.
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