1. Alpha Decay

Some nuclei are too large to be stable, and undergo alpha decay. A nuclide that undergoes alpha decay transforms into a different nuclide by emitting alpha particle. An alpha particle is a helium nucleus, 4He, two protons and two neutrons bound together, with total spin zero. When an alpha particle is emitted by a nucleus, each of its number of neutrons, N and number of protons or atomic number, Z values decreases by 2 and its mass number, A decreases by 4.

Alpha decay is possible when the mass of the original neutral atom is greater than the sum of the masses of the final neutral atom and a neutral \(^4_2 He\).

\(^A_ZX \rightarrow\; ^{A-4}_{Z-2}Y +\; ^4_2 He\)

\(\text{parent nucleus} \rightarrow \text{daughter nucleus} + \text{alpha particle/helium nucleus}\)


Example 1.1:

\(^{238}_{92}U \rightarrow\; ^{234}_{90}Th +\; ^4_2He\)

The example above shows the transformation of Uranium-238 after undergoing alpha decay. Uranium-238 becomes Thorium-234. Uranium is referred as the parent nucleus while Thorium is called as the daughter nucleus.


Example 1.2:

Another example is the decay of Radium-226. When Radium-226 undergo alpha decay, it becomes Radon-222.

\(^{226}_{88} Ra \rightarrow\; ^{222}_{86} Rn +\; ^4_2He\)

Radium is the parent nucleus and Radon is the daughter nucleus.


Example 1.3:

\(^{222}_{86}Rn \rightarrow\; ^{218}_{84}Po +\; ^4_2 He\)

Example 3 is the alpha decay process of radon to polonium.


2. Beta Decay

Another particle emitted by nuclide when transformed into a different nuclide is the beta particle. This type of decay is called beta decay. In general, beta decay process is of the form:

\(^A_ZX \rightarrow\; ^A_{Z+1}Y +\; ^0_{-1}e\)

\(\text{parent nucleus} \rightarrow \text{daughter nucleus} + \text{beta particle/electron}\)

Beta decay is of three different simple types: beta-plus, beta-minus, and electron capture.


2.1 Beta-plus Decay

Beta-plus decay occurs when the mass of the original neutral atom is at least two electron masses larger than that of the final atom. When the ratio between N and Z or the neutron-proton ratio, N/Z is too small for stability, the nuclide can emit a positron, which is the electron’s antiparticle. Positron is identical to the electron but with positive charge. The beta-plus decay process \(\beta^+\), is

\(p \rightarrow n + \beta^+ + v_e\)

where \(\beta^+\)is the positron and \(v_e\) is the electron neutrino.

In beta-plus decay, the mass number remains the same while the atomic number of the daughter nucleus is one less than that of the parent nucleus, \((A, Z-1)\).


2.2 Beta-minus Decay

A beta-minus decay can occur when the mass of the original neutral atom is larger than that of the final atom. A beta-minus particle, \(\beta^-\) is an electron. In this type of decay, there is no emission of electron from the nucleus, instead, there is transformation of a neutron into a proton, and electron, and an antineutrino. The beta-decay process is given as

\(n \rightarrow p + \beta^- + v_e\)

This occurs when the neutron-to-proton ratio is too large to be stable. In this process also, the change in the values of the following is: N decreases by 1, Z increases by 1, and A remains the same. The daughter nucleus of this type of decay has a mass number equal to that of the parent nucleus, and an atomic number which is one greater than the atomic number of the parent nucleus, \((A, Z+1)\).


2.3 Electron Capture

The third beta decay process is the electron capture. This occurs when the mass of the original neutral atom is larger than that of the final atom.

\(p + \beta^- \rightarrow n + v_e\)

In this process, N increases by 1, Z decreases by 1 and A remains constant.


Example 2.1:

\(^{24}_{11} Na \rightarrow\; ^{24}_{12} Mg +\; ^0_{-1}e\)

The example above is the beta decay process of sodium to magnesium. In this case, the atomic number of the daughter nucleus is greater than that of the parent nucleus, while the mass remains the same, thus we can conclude that sodium undergoes beta-minus decay.


Example 2.2:

\(^{212}_{82}Pb \rightarrow\; ^{212}_{83}Bi + ^0_{-1}e\)


Example 2.3:

\(^{227}_{89} Ac \rightarrow\; ^{227}_{90} Th +\; ^0_{-1}e \)


Gamma Decay

When a nucleus is in excited state, in any of the following process: bombardment with high-energy particles or by a radioactive transformation, it can undergo gamma decay. In gamma decay, the nucleus can decay to the ground state by emitting one or more photons. These photons are called gamma rays or gamma-ray photons. In general, gamma decay is of the form:

\(^A_ZX \rightarrow\; ^A_ZX +\; ^0_0 \gamma\)