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Gamma Decay

Atoms consist of three subatomic particles, electrons, protons, neutrons.

Some elements are not stable and easily disintegrated to small nucleus with the emission of ionizing radiations like alpha, beta or gamma rays. These ionized radiations are termed as radioactive rays, elements called as radioactive element and the property of emission of radiations is known as radioactivity.

Radioactive elements have high neutron to proton ratio which makes them unstable. Since there is a repulsion force between positively charged protons located in a small region called as nucleus. Hence there must be some strong force greater than the repulsion force between protons which can make nucleus stable.
If there are a large number of protons in an atom, the repulsion force becomes very high and nucleus disintegrated in small nucleus with the emission of energy. The energy emitted in the form of radiation that is known as radioactive radiations.

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Gamma Emission

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Hence nuclear decay or radioactive decay is the process in which a nucleus of an unstable atom lose converts in a stable by the emission of ionizing particles or ionizing radiation. This unstable nucleus called as parent nuclide and the new element formed in nuclear decay is known as daughter nuclei and the nuclear decay also called as nuclear transmutation.

On the basis of emitted particles, radioactive decay can be different types. 

  1. Alpha decay: alpha particles (2He4) emitted during nuclear decay.
  2. Beta decay: beta particles (-1e0) emitted during nuclear decay.
  3. Gamma decay: gamma rays (Ï’) emitted during nuclear decay.
  4. Positron decay: positive beta particles or positron (1e0)  emitted during nuclear decay.

Gamma Emission       

Out of all these radiations; alpha, beta and positrons are charged particles while gamma rays are neutral in nature. Beta particles and positrons are similar in all properties except their charges. Let's compare alpha and beta rays in their different properties.

Alpha rays
Beta rays
Constituent particle Alpha particle
Beta particle
Particle structure Alpha particle  Beta particle  
Charge on rays Positively charged Negatively charged
Deflection towards electrodes
Move towards negative electrodes
Move towards positive electrodes
Velocity of rays
160000 km/sec 160000 to 240000 km/sec
Penetration power
100 times more than alpha
Ionization power
Less than alpha particle
Change in atomic number of daughter nuclei during decay
Decreases by two units Increases by one unit
Change in mass number of daughter nuclei during decay
Decreases by four units  Remain same as in parent nuclei
83AT211 $\rightarrow$ 81Bi207 + 2He4
1H3 $\rightarrow$ 2He3 +-1e0 + U

Apart from these charged particles, there is a possibility of emission of gamma rays also. Emission of gamma rays during any radioactive decay is termed as gamma emission. Gamma rays are high energy and high frequency electromagnetic radiations.

The frequency of gamma rays is more than 1019 Hz and energies above 100 keV with the wavelength less than 10 pico meters.

Electromagnetic Wave

Gamma Ray Emission

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Gamma radiations are similar to X-rays but have very low wave length and high energy. They are charge less and mass less; hence not show any deviation in both electric and magnetic field. Gamma rays have very high penetrating power compare to alpha and beta rays, hence can penetrate even a thick lead piece.

Gamma Ray Emission 
Generally gamma rays emission takes place with alpha and beta decay. Hence the gamma rays emissions are secondary effects of radioactive disintegration. Due to the emission of alpha and beta rays, nucleus gets excited and move to higher energy level which is not a stable state. Hence it drops to ground state by the emission of energy in form of electromagnetic radiations that are gamma rays.

The emission of gamma rays requires only 10−12 seconds, means it occurs instantaneously. Since gamma rays are neutral and mass less, there is no change in atomic number as well as mass number in daughter nuclei compare to parent nuclei in gamma decay. That is the reason, gamma emission is also known as isotopic transition which involves intermediate metastable excited states of the nuclei. The gamma emission in any nuclear process may cause photoelectric effect.
When the energy of gamma rays transfer to one of the most tightly bound electrons, that electron ejected from the atom and results photoelectric effect. The best example of gamma emission is beta decay of cobalt-60 forms Nickel-60 with the emission of gamma rays involve two nuclear reactions.

27Co60 $\rightarrow$ 28Ni60 + e- + Ï… +Ï’+1.17Mev
$\rightarrow$ 28Ni60+Ï’ + 1.33MeV

Nuclear Reactions

In this gamma emission which follows beta decay, first Cobalt-60 show beta decays and converts into excited Nickel-60 nuclei with the emission of an electron and anti neutrino particle. The energy of released electron is 0.31 MeV.

In second steps this excited Nickel -60 drops down to the ground state with the emission of two gamma rays of 1.17 MeV then 1.33 MeV.                                

Gamma Emission Equation

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Some examples of gamma emission are as follows.
  • 56Ba137 $\rightarrow$ 56Ba137 + $\gamma$ rays
                                                Gamma Emission Equation
  • 5B12 $\rightarrow$ beta particle + Anti neutrino+ 6C12$\rightarrow$ 6C12 + $\gamma$ rays
                                                   Examples of Gamma Emission
  • 66Dy152 $\rightarrow$ 66Dy152 + $\gamma$ rays
  • 95 Am241 $\rightarrow$ 93Np237 +2He4 + $\gamma$ rays
  • 77Ir192 $\rightarrow$ 78Pt192 + -1e0+ $\gamma$ rays

Induced Gamma Emission

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In 1934, Frederic Joliot and Irene curie found that certain light elements like aluminum, boron and magnesium when bombarded with alpha particles, change into radioactive isotopes of other elements. These radioactive isotopes further disintegrated in the same way as the naturally occurring radioactive elements like uranium etc. This process is termed as induced radioactivity or artificial radioactivity.

For example, bombardment of alpha particle on magnesium forms radioactive Silicon-27 which further disintegrated into Aluminium-27 and positron particle.

12Mg24 +2He4 $\rightarrow$ 14Si27 +0n1
$\rightarrow$ 13Al27 +1e0

If the induced radioactivity involves the emission of gamma rays, it termed as Induced Gamma Emission (IGE). It is a process of emission of gamma rays from excited nuclei of a specific nuclear isomer. Induced gamma emission is just like fluorescence process in which there is an emission of a photon instead of gamma rays by an excited electron in atom.

Some radioactive nuclear isomers can store significant amounts of energy for a long time and serve as nuclear fluorescent materials.
Induced Gamma Emission
When a photon is absorbed by target nucleus in its initial state, the nucleus will be moved to at high energy level and becomes excited. The excited nucleus emitted energy in the form of a cascade of transitions which is also called as "gateway state" or "trigger level" or "intermediate state". In this process fluorescent photons are emitted but after some delays of the initial absorption and this process is an example of induced gamma emission.                                                  
Induced Gamma Emissions
Another example of induced gamma emission is neutron absorption analysis (NAA). Neutron absorption analysis is a sequence of events starting from the neutron capture by a target nucleus through a non-elastic collision and forms a compound nucleus in an excited state. The energy of excited state is because of binding energy of the neutron with the target nucleus and due to high energy, it disintegrated instantaneously into a more stable nucleus by the emission of gamma rays.

Generally the new nucleus formed during the NAA process is radioactive in nature and further decays through the emission of delayed gamma rays at a much slower rate.
Gamma Emission

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