Radioactivity

 

Natural Radioactivity

The phenomenon of spontaneous emission of radiations by heavy elements is called radioactivity.The elements which show this phenomenon is called radioactive elements.
Radioactivity was discovered accidentally by Antoine Henri Becquerel in 1896.
α-particle:It is a helium nucleus (alpha_particle) having two protons and two neutrons.
β-particle: It is a fast moving electron (electron).
Note: β-particle and antineutrino  download (8) do not exist inside the nucleus but they are created at the time of emission.
γ-rays: γ-rays are the packets of electromagnetic radiation energy and are known as photons.They do not have any charge and their rest mass is zero.
Properties of α-particle
  1. They have positive charge equal to + 2e, where daum_equation_1424017617333.
  2. They are emitted with velocities ranging between daum_equation_1424017759746
  3. They are deflected by electric and magnetic fields.
  4. They cause fluorescence in certain materials like zinc sulph

    ide,etc.

  5. They have low penetrating power.
  6. The have rest mass equal to 4 times the mass of a proton.
  7. They have high ionising power.
  8. α-particles are capable of producing heating effect when fall on a substance and they can cause skin burns.
Properties of β-particle
  1. β-particle has a negative charge equal to the charge on an electron.
  2. The rest mass of a β-particle is equal to the mass of an electron..
  3. They are emitted with a velocity of the order of daum_equation_1424017834764
  4. They are deflected by electric and magnetic fields.
  5. They cause fluorescence in zinc sulphide and other fluorescent materials.
  6. The ionising power of β-particles is about 1/100 times the ionising power of α-particles.
  7. The penetration power of β-particle is more than that of α-particle.
 Properties of γ-rays
  1. They are the packets of energy of electromagnetic radiations.
  2. They have no charge.
  3. The rest mass of γ-rays is zero.
  4. They always travel with the speed of light in vacuum (daum_equation_1424017877610)
  5. They are not deflected by electric and magnetic fields.
  6. They cause ionisation but their ionising power is about 1/100 times the ionising power of β-particles.
  7. They have very high penetration power. Their penetration power is 100 times greater than that of β-particles.
  8. They cause less fluorescence (in substances like willimite).

Laws of Radioactive Decay (i.e. disintegration)

  1. Radioactive Decay is a spontaneous process and is not affected by the external conditions such as pressure,temperature,etc.
  2. When a radioactive element decays by emitting an α-particle,its position goes down by two places in the periodic table.alpha-decay-reaction
  3. When a radioactive element decays by emitting a β-particle,its position is raised by one place in the periodic table.beta-decay-reaction
  4. When a radioactive element decays by emitting a γ-rays ,its position remains the same in the periodic table.gamma-decay-reaction
  5. The rate of disintegration of a radioactive substance is directly proportional to the number of atoms remained undecayed in the substance.This law is called radioactive decay law or disintegration law.
Conside a radioactive substance having No number of atoms before decay (i.e. at t = 0).When radioactive disintegration begins,the number of atoms in the substance decreases.Let after time t, the number of atoms remained undecayed in the substance be N.If dN is the number of atoms decayed in small time dt, then the rate of disintegration = dN/dt.
According to decay law,
Laws-of-radioactive-disintegration2
radioactive-decay-2

This equation is known as exponent

ial decay equation.

halflife

Radioactive decay constant or disintegration constant(λ)

Radioactive decay constant (λ) is the reciprocal of the time during which the number of atoms in the radioactive substance reduces to 36.8% of the original number of atoms in it.
Significance of decay constant:
If the value of λ of a radioactive substance is small ,it will decay slowly.If the value of λ of a radioactive substance is large ,it will decay rapidly as shown in figure below.
Radioactive_Decay_Law_Decay_Constants

Half Life of a radioactive substance (daum_equation_1424014053653)

The time during which half of the atoms of the radioactive substance disintegrates is called half life of a radioactive substance.
Radioactive-Decay
Image705
half-life of radioactive substance is inversely proportional to the decay constant(λ).
Mean-life or Average life of a radioactive substance
The sum of lives of all atoms divided by the total number of atoms is called as mean-life or average life of a radioactive substance.
daum_equation_1424016470879
daum_equation_1424016507834
Activity of a Radioactive substance
The activity of a radioactive substance is defined as the rate of disintegration of the substance.
daum_equation_1424016954431
Activity is inversely proportional to half-life.
Units of activity
  1. Curie (Ci)
  2. Rutherford (rd)
  3. becquerel(Bq)
1 Curie (Ci) = 3.7 × 1010 disintegration per second
1 Rutherford (rd)= 106 disintegration per second
1 becquerel (Bq) = 1 disintegration per second

 

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