4th Week: Decays, Tunneling and Cross Sections C

Radioactive Decay

The nucleus of an atom is made up of protons (Z) and neutrons (N). The total number of protons and neutrons is $Z+N=A$ In nature, an atom undergoes Radioactive Decay when the number of protons and/or neutrons in the nucleus changes. There are three main processes by which these changes can occur. These mechanisms are Alpha, Beta, and Gamma Decay.

Alpha Decay

An alpha particle is just another name for a ${^{4}He}$ atom. an atom, $(Z,A)$, can be said to undergo alpha decay if the nucleus of the atom loses two neutrons and two protons, ejecting an alpha particle as shown here:

$\left(Z,A\right)->\left(Z-2,A-4\right)+{^{4}He}$

As a simple example, the equation for a Lithium atom undergoing alpha radiation to decay into $^{3}H$ would be as follows:

$\left(3,7\right)->\left(1,3\right)+{^{4}He}$

Beta Decay

Beta Decay is another way in which a nucleus can undergo change. Beta particles are electrons or positrons, which are the antiparticle to the electron and have the same properties with opposite charge. Beta decay occurs like this: A nucleus with too many protons or too many neutrons spontaneously transforms one of the protons into a neutron, or vice versa. This type of reaction can occur in different ways. Three type of beta decay are $\beta ^{-}$ decay, $\beta ^{+}$ decay, and electron capture. In $\beta ^{-}$ decay, a neutron decays into a proton, and an electron and antineutrino, which are ejected from the nucleus, showing here:

$\left(Z,A\right)\rightarrow \left(Z+1,A\right)+e^{-}+{\bar {\nu _{e}}}$

$\beta ^{+}$ decay occurs when a proton decays into a neutron, and a positron and neutrino, which again are ejected:

$\left(Z,A\right)\rightarrow \left(Z-1,A\right)+e^{+}+\nu _{e}$

In electron capture, a free electron is snatched up by the nucleus and combines with a proton, creating a neutron in its place and ejecting a neutrino:

$(Z,A)+e^{-}\rightarrow (Z-1,A)+\nu _{e}$