Types of radioactive decay - Alpha Decay, Beta Decay - eSaral

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Types of radioactive decay

(a) Alpha decay

$_{z} X^{A}$ (parent nucleus) $\longrightarrow_{Z-2} Y^{\mathrm{A}-4}$ (daughter nucleus)

$+{ }_{2} \mathrm{He}^{4}$ (alpha particle)

e. g. ${ }_{92} \mathrm{U}^{238} \longrightarrow{ }_{90} \mathrm{Th}^{234}+{ }_{2} \mathrm{He}^{4}$

1. Alpha particle consists of 2 neutrons, 2 protons and carries a positive charge in magnitude 2 electrons. It is doubly ionized helium nuclei.
   
2. $\alpha$ emission takes place when the size of the nucleus becomes too large. The decay reduces the size of the nucleus.
   
3. $\alpha$ emission is explained on basis of the quantum mechanical tunnel effect.
   
4. The energy released in $\alpha$ decay $\mathrm{Q}=\left(\mathrm{M}_{\mathrm{x}}-\mathrm{M}_{\mathrm{y}}-\mathrm{M}_{\alpha}\right) \mathrm{c}^{2}$
   
   
5. The kinetic energy of $\alpha$ particle $\mathrm{E}_{\alpha}=\left(\frac{\mathrm{A}-4}{\mathrm{~A}}\right) \mathrm{Q}$ where A is mass number and Q is disintegration energy

(b) Beta decay

• Electron emission
  
  $\left(\beta^{-}\right)$ ${ }_{z} X^{A} \longrightarrow_{z+1} Y^{A}$
  
  $+_{-1} \mathrm{e}^{0}\left(\beta^{-}\right.$ particle $)+\bar{v}$
  
  e.g. ${ }_{6} \mathrm{C}^{14} \longrightarrow{ }_{7} \mathrm{~N}^{14}$
  
  $+_{-1} \mathrm{e}^{0}+\bar{v}$ (antineutrino)
  

1. $\beta^{-}$ Particles are fast-moving electrons carrying a negative charge
   
2. $\beta^{-}$ Particles are emitted when the nucleus has too many neutrons relative to a number of protons i.e. N/Z ratio is larger than required.
   
3. The emission of electrons takes place when a neutron is converted to a proton inside the nucleus. This helps in the correction of the N/Z ratio. ${ }_{0} \mathrm{n}^{1} \longrightarrow{ }_{1} \mathrm{p}^{1}+{ }_{-1} \mathrm{e}^{0}+\overline{\mathrm{v}}$
   
   
4. The interaction responsible for $\beta$ decay is weak interaction.

• Positron emission
  
  $\mathrm{z} \mathrm{X}^{\mathrm{A}} \longrightarrow \mathrm{z}_{-1} \mathrm{X}^{\mathrm{A}}$
  
  $+_{+1} e^{0}\left(\beta^{+}\right.$ particle $)+v$
  
  $\mathrm{eg} \cdot{ }_{29} \mathrm{Cu}^{64} \longrightarrow{ }_{28} \mathrm{Ni}^{64}$
  
  $+_{+1} \mathrm{e}^{0}+v$ (neutrino)
  

1. $\beta^{+}$ Particles are positrons with a mass equal to an electron but carry a unit positive charge.
   
2. $\beta^{+}$ Particles are emitted when the nucleus has too many protons relative to a number of neutrons i.e. N/Z ratio is smaller than required.
   
3. The emission of positron takes place when a proton is converted to a neutron inside the nucleus. This increases the N/Z ratio.
   
   ${ }_{1} \mathrm{p}^{1}={ }_{0} \mathrm{n}^{1}+{ }_{+1} \mathrm{e}^{0}+\boldsymbol{v}$

(c) Gamma decay

${ }_{Z} \mathrm{X}^{\mathrm{A}^{*}} \longrightarrow{ }_{Z} \mathrm{X}^{\mathrm{A}}+\gamma$

e.g. $\quad{ }_{5} \mathrm{~B}^{12} \longrightarrow{ }_{6} \mathrm{C}^{12^{\star}}+{ }_{-1} \mathrm{e}^{0}+\overline{\mathrm{v}}$

${ }_{6} \mathrm{C}^{12^{*}} \longrightarrow{ }_{6} \mathrm{C}^{12}+\gamma$

1. $\gamma$ rays are electromagnetic radiations that are chargeless and massless
   
2. $\gamma$ rays are emitted when the nucleus has excess energy
   
3. $\gamma$ rays are emitted when the nucleus jumps from the excited state to a lower level or ground state. This reduces the energy of the nucleus.
   
4. $\gamma$ rays are electromagnetic radiations of short wavelength $\left(\sim 10^{-12} \mathrm{~m}\right)$ which travel with speed of light.

(d) Electron capture

${ }_{\mathrm{Z}} \mathrm{X}^{\mathrm{A}}+{ }_{+1} \mathrm{e}^{0} \longrightarrow+\underset{\mathrm{z}-1}{\mathrm{Y}}^{\mathrm{A}}+\mathrm{v}$

${ }_{4} \mathrm{Be}^{7}+{ }_{-1} \mathrm{e}^{0} \longrightarrow{ }_{3} \mathrm{Li}^{7}+\mathrm{v}$

1. This process takes place when the nucleus has too many protons relative to a number of neutrons. i.e. N/Z ratio is larger than required.
   
2. This process occurs when a parent nucleus captures one of its own orbital atomic electrons and emits a neutrino.

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