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Unit of activity|Alpha Decay






4) Unit of activity

  • The most commonly used unit is the curie
  • Curie was originally based on the rate of decay of a gram of radium
  • There are $3.7 \times 10^{10}$ disintegrations per sec per gram of radium .This no is taken as a standard
    => One curie=$3.7 \times 10^{10}$ disintegrations per sec
  • One curie of activity is very strong source of radiation
    => 1 milli curie=1mCi=$10^{-3}$Ci
    1 microcurie=1μCi=$10^{-6}$ Ci
  • Another unit of activity is Rutherford
    $1rd=10^6$ dis/sec
  • Activity |dN/dt|=λN=.693N/T
    => A very short lived substance gives rise to large activity ,even it is present in minute quantities
  • The SI unit of radioactivity recently proposed is Becquerel (Bq) which is defined as activity done to one disintegration per sec hence
    1ci=$3.7 \times 10^{10}$ bq
    =37G bq


5) Alpha decay:

  • Nucleus before the decay is called parent nucleus and after the decay is called daughter nucleus
  • In Alpha decay, the parent nucleus AXZ emits an α particle (=4He2) leaving behind a daughter nucleus of four mass unit less and two charge units less i.e. A-4XZ-2
    Alpha decay
  • α decay shift the element two places to the left in the periodic tables of elements ex

  • All nuclides of A >= 210 and Z > 83 tends to decay by α emission
  • 209Bi is the heaviest stable nuclide in nature
  • α decay in heavy nucleus occur because a too heavy nucleus becomes unstable due to coulomb repulsion and by emitting an α particle the nucleus decrease its A and Z to moves towards stability
  • Now the rest mass energy of parent nucleus AXZ is greater then the sum of rest mass energies of A-4XZ-2 and 4He2
  • The difference between the rest mass energies of initial constituents and final products is called Q-value of the process
  • For α decay process ,Q value is
    Q=[mp -(md+mα)]c2
    where mp -> Mass of parent nucleus ZAX
    md -> Mass of parent nucleus Z-2A-4X
    mα -> Mass of parent nucleus 24He






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