Radioactivity

Radioactivity

Natural Radioactivity

The process of spontaneous (i.e. without external means, by itself) disintegration of the nuclei of heavy elements with the emission of certain types of radiations is known as natural radioactivity.

• The disintegration of unstable nuclei is spontaneous and involves the emission of -particle [a helium nucleus, 2He₄] or -particle (electron or positron). The -emission or -emission may be accompanied by -rays. The-rays are extremely short-wavelength electromagnetic waves (photons).
• The emission of radiations transforms the radioactive element formed will further change into a still new element. This process goes on till a stable element (₈₂Pb²⁰⁸) is arrived at. After this, the radioactivity (i.e. radioactive decay) stops.
  
  
  Radioactive Disintegration
  
  (a) Law of radioactive disintegration
  At any instant the rate of decay of radioactive atoms is proportional to the number of atoms present at that instant ie.
  
  
  
  It can be proved that N =
  
  Where N = number of atoms remains undecayed after time t, N₀ = Number of atoms present initially (i.e., at t = 0)
  rate of decays, l = Decay constant or disintegration constant.
  
  Activity of atom is given by
  
  A =
  
  Where A₀ = Activity of t
  
  A = Activity after time t.
• Half life (T_1/2) is the time interval in which the mass of a radioactive substance or the number of its atom reduces to half of its initial value is called the half life of the substance i.e., if N = then t = T_1/2
  
  Hence from N =
  
  
  
  = .
• Nuclear Reaction
  
  A reaction in which there is a successful attempt by an external bombarding particle to change the content of a nucleus is known as a nuclear reaction.
  
  
  
  
  ₁₃Al²⁷ + ₂He⁴₁₅P³¹ ₁₄Si³⁰ + ₊₁e⁰ + ₀n¹
  
  ₇N¹⁴ + ₂He⁴ [₉F¹⁸] ₈O¹⁷ + ₁H¹- Q¹
  
  
  Nomenclature of Nuclear Reaction
  
  ₈O¹⁶ + ₁H² ₇N¹⁴ + ₂He⁴      (, ) reaction
  
  ₁H² + ₁H¹ + ₂He⁴      photo nuclear reaction
• Nuclear Fission
  
  ₉₂U²³⁵ + ₀n¹ [₉₁U²³⁶] ₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3₀n¹ + 200 MeV
  
  
  (1) The ₉₂U²³⁵ nucleus after absorbing a neutron had split into two medium-mass nuclei. This was startling, for until then the known nuclear reactions involved knocking out only a tiny fragment (for example, , or ) from the target nucleus.
  
  (2) A huge amount of energy (about 200 MeV per ₉₂U²³⁵ nucleus) was released in the reaction. It was quite surprising because the energy released in the known nuclear reactions was quite small.
  
  (3) The incident particle (i.e. slow neutrons) which initiated this strange nuclear reaction were present in the product particles. It was found that on the average 2 to 3 neutrons were released in the process. The released neutrons could further cause splitting of ₉₂U²³⁵ nuclei and lead to self-sustaining process.
  
  The splitting of a heavy nucleus (A > 230) into two medium-mass nuclei in a nuclear reaction with the release of huge amount of energy is known as nuclear fission.
• Atomic Nucleus
  
  The central core of the atom which contains all of the atom's positive charge and most of its mass is known as atomic nucleus.
  
  

  Protons: A proton is a positively charged particle and has a charge equal to that of an electron. However, the mass of a proton is about 1850 times that of the electron.
  Charge on proton, e = + 1.6 × 10^-19 C
  Mass of proton, mp = 1.6726 × 10^-27 kg = 1.007825 a.m.u.
  

  • Neutrons: A neutron is a neutral particle i.e. it has no charge. Its mass is almost identical to that of the proton.
    Charge on neutron = Zero
    Mass of neutron, mn= 1.6750 × 10^-27kg = 1.007825 a.m.u.
    
    These two constituents of a nucleus (protons and neutrons) are called nucleons. Although the hydrogen nucleus consists of a single proton alone, the nuclei of other elements consist of both neutrons and protons. The different types of nuclei are often called nuclides.
    
    Note. Since neutron is a neutral particle, it has high penetrating power and very low ionising power. Further, electric and magnetic fields have no affect on it.
  • Nuclear charge: It is the total charge carried by the nucleus and is numerically equal to its atomic number times the electronic charge, i.e.
    
    Nuclear charge = Ze
    where Z = atomic number
    e = electronic charge
  • Nuclear density. The ratio of mass of the nucleus to its volume is called nuclear density i.e.
    
  • Mass Defect
    
    The difference between the actual mass of the nucleus and the sum of masses of the constituent nucleons is called mass defect i.e.
    
    Mass defect, D m = M - A
    where M = actual mass of the nucleus
    and A = mass number
  • Nuclear Binding Energy
    
    The total energy required to liberate all the nucleons from the nucleus (i.e. to disintegrate the nucleus completely into its constituent particles) is called nuclear binding energy of the nucleus.
    
    Mathematically given as
    B.E = mass defect × C
    = (2M + (A - 2) M - M] × C Joules
    Hence, M = measured mass of the nucleus in kg.
    A = mass number
    Z = atomic number
  • Nuclear Fusion is the process of combining lighter nucleus into heavy nucleus at a very high temperature and pressure releasing extremely high amount of energy. Ex- Two deuteron atoms combining to give a Helium nucleus.