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Nuclear Radiations

According to Rutherford’s experiment, a sample of radioactive substance is put in a lead box and is allowed to emit radiation through a small hole only. When the radiation enters into the external electric field, it splits into three parts: α-rays, β-rays, and γ-rays.


Nearly 90% of the 2500 known nuclides are radioactive; they are not stable but decay into other nuclides
  • When unstable nuclides decay into different nuclides, they usually emit alpha (α) or beta (β) particles.
  • Alpha emission occurs principally with nuclei that are too large to be stable. When a nucleus emits an alpha particle, its N and Z values each decrease by two and A decreases by four.
  • Alpha decay is possible whenever the mass of the original neutral atom is greater than the sum of the masses of the final neutral atom and the neutral helium-atom.


There are different simple type of β-decay: β , β+, and electron capture.
  • A beta minus particle (β+) is an electron. The emission of β involves transformation of a neutron into a proton, an electron, and a third particle called an antineutrino (v).
  • β decay usually occurs with nuclides for which the neutron to proton ratio (N/Z ratio) is too large for stability.
  • In β decay, N decreases by one, Z increases by one and A does not change.
  • β decay can occur whenever the neutral atomic mass of the original atom is larger than that of the final atom.
  • Nuclides for which N/Z is too small for stability can emit a positron, the electron’s antiparticle, which is identical to the electron but with positive charge. The basic process called beta plus β+ decay.
    p n + β+ + v (ν = neutrino)
  • β+ decay can occur whenever the neutral atomic mass of the original atom is at least two electron masses larger than that of the final atom
  • The mass of v and v is zero. The spin of both is 1/2 in units of h/2π. The charge on both is zero. The spin of neutrino is antiparallel to its momentum while that of antineutrino is parallel to its momentum.
  • There are a few nuclides for which β+ emission is not energetically possible but in which an orbital electron (usually in the k-shell) can combine with a proton in the nucleus to form a neutron and a neutrino. The neutron remains in the nucleus and the neutrino is emitted.
    p + β+ n + v


The energy of internal motion of a nucleus is quantized. A typical nucleus has a set of allowed energy levels, including a ground state (state of lowest energy) and several excited states. Because of the great strength of nuclear interactions, excitation energies of nuclei are typically of the order of 1 MeV, compared with a few eV for atomic energy levels. In ordinary physical and chemical transformations the nucleus always remains in its ground state. When a nucleus is placed in an excited state, either by bombardment with high-energy particles or by a radioactive transformation, it can decay to the ground state by emission of one or more photons called gamma rays or gamma-ray photons, with typical energies of 10 keV to 5 MeV. This process is called gamma (γ) decay.
All the known conservation laws are obeyed in γ-decay. The intensity of γ-decay after passing through x thickness of a material is given by I = I0eμx (μ = absorption co-efficient).

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