Discovery of Artificial Disintegration

The artificial transmutation of one element into another is first accomplished by Rutherford in 1919.

The chamber 'c' was filled with a gas such as Nitrogen and Alpha particles from a radioactive source at 'A' were absorbed in the gas. A sheet of silver foil 'F', itself thick enough to absorb the alpha particles was placed over an opening in the side of chamber. A zinc sulphide screen 'S' was placed outside this opening and a microscope 'M' was used for observing any scintillatons - occuring on the screen 'S'. Scintillations were observed when the chamber was filled with Nitrogen, but when the Nitrogen was replaced by Oxygen or Carbondioxide no scintillations were observed.

Rutherford concluded that the scintillations were produced by high energy particles which were ejected from Nitrogen nuclei as a result of bombardment of these nuclei by alpha particles.

Magnetic deflection experiments indicated that these particles were Hydrogen nuclei or Protons.

Later experiments by Rutherford and Chadwick showed that these ejected Protons had Ranges upto 40cm in air.

Other light elements in the Range from Boron to Potassium were also disintegrated by bombardment with alpha particles.

The disintegration of Nuclei has also been studied with  Wilson cloud chamber. One of the first of these investigations was that of Blackett, who photographed the tracks of alpha particles in a Wilson cloud chamber containing 90% Nitrogen and 10% Oxygen. The majority of tracks photographed were straight tracks typical of alpha particle tracks.

Many of the tracks were observed to be forked tracks, indicating that an inelastic collision had taken place between an alpha particle and a Nitrogen Nucleus.

Measurement of the tracks showed that momentum of system was conserved but that the sum of kinetic energies of particles after impact was less than kinetic energy of alpha particle before impact.

On the basis of theory of nucleus advanced by Bohr, the disintegration of Nitrogen by bombardment with alpha particles may be thought as consisting of two separate parts.

The first is the capture of the alpha particle by Nitrogen nucleus which resulted in the formation of a new compound nucleus.

The second is the breaking up of compound nucleus into two particles, one of which is a Proton.

These two processes can be represented by means of a nuclear reaction equation analogous  to one representing a chemical reaction.

The nuclear reaction equation for this process is

₂He⁴ + ₇N¹⁴ --------->  (₉F¹⁸✷) ------->  ₈O¹⁷ ⁺ ₁H¹ + Q

Q is energy evolved or absorbed during nuclear reaction

Q --->  -Ve --->  energy has been absorbed (endothermic)
Q --->  +Ve ---> energy has been evolved (exothermic)
Q ---> nuclear reaction energy or disintegration energy

If sum of masses of the final particles exceeds that of initial particles, 'Q' must be negative; the energy absorbed in such a nuclear reaction must have been obtained from kinetic energies of the particle.

If 'E1' is kinetic energy of alpha particle just before capture, 'E2' the kinetic energy of Proton, 'E3' the kinetic energy of product nucleus,

Q = E2+E3-E1

In those cases in which Q is positive the sum of kinetic energies of final particles will be greater than kinetic energy of incident alpha particle.

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