Discuss Transmutation by Photons and Alpha Particles?
Transmutation by Photons
The disintegration of atomic nuclei by the bombardment with high-energy photons is called transmutation by photons or photodisintegration.
The photon can only supply its kinetic energy to target nucleus in nuclear reaction because photon is massless. This kinetic energy must be at least as great as the binding energy of target nucleus. The photodisintegration reactions are endothermic having threshold energies of the order of 10MeV.
There are two exceptions for which photodisintegration does not occur with γ-rays from natural radioactive substance. These exceptions are called deuteron which has binding energy of only 2.2 MeV and nuclide 4Be9 in which one neutron is loosely bond.
The photodisintegration reactions are used to calculate binding energy of nuclear particles. For example, the threshold energy is just equal to binding energy of deuteron.
γ + d ⟶ p + n
The γ-rays of 17MeV energy are produced when Li is bombarded with protons. These γ-rays are successfully used to produce photodisintegration of other nuclides such as (γ, n) reaction.
15P31 + γ → [15P31]* → 15P30 + 0n1
The (γ, p) nuclear reaction requires still higher energies and can be observed with high-energy photons from betatron.
The γ-rays involved in nuclear reactions given below have energies about 50MeV.
13Al27 + γ → [13Al27]* → 11Na25 + 1H1 + 1H1
13Al27 + γ → [13Al27]* → 11Na25 + 1H1 + 1H1 + 0n1
Some examples of photodisintegration reactions are
(γ, n) Photo Reaction
(γ, p) Photo Reaction
(γ, 2p, n) Photo Reaction
Now we discuss transmutation by alpha particles.
Transmutation by Alpha Particles
The ɑ-particle is a helium nucleus (2He4). It was first time used by Rutherford in 1919 for the transmutation of nitrogen nucleus. The ɑ-particle must have sufficient energy for transmutation to over the Coulomb barrier.
The apparatus used by Rutherford is shown in diagram. There is an opening in one end of a box B which is covered by a silver foil F. A zinc sulfide screen is placed at S. The scintillations on the screen were observed by microscope M. The source of alpha particles is radium C placed on a small disc D. The nitrogen gas is introduced through side tubes T.
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The scintillation are observed on screen caused by particles ejected from nitrogen nucleus by the impact pf alpha particles. The measurement of the magnetic deflection of the particles suggested that they are protons.
The disintegration of nitrogen by alpha particle may be expressed as
2He4 + 7N14 → [9F18] → 8O17 + 1H1
The symbols on left side stands for reacting nuclides. The symbol in brackets stands for unstable nucleus formed as a result of the capture of alpha particle called compound nucleus.
The emitted proton and residual nucleus on right side are called products of reaction. This transmutation may also be represented as N14 (ɑ, p)O17
(ɑ, P) Nuclear Reaction
The general form of (ɑ, P) nuclear reactions is written as
2He4 + ZXA → [ Z+2CnA+4 ] → Z+1YA+3 + 1H1
The charge of residual nucleus increases by one unit and mass increases by three units.
Examples
2He4 + 5B10 → [ 7N14 ] → 6C13 + 1H1
2He4 + 11Na23 → [ 13Al27 ] → 12Mg26 + 1H1
(ɑ, n) Nuclear Reaction
The general form of (ɑ, n) nuclear reaction is
2He4 + ZXA → [ Z+2CnA+4 ] → Z+2YA+3 + 0n1
Examples
2He4 + 7N14 → [9F18] → 9F17 + 0n1
It is called Rutherford reaction.
2He4 + 4Be9 → [6C13] → 6C12 + 0n1
It is called Chadwick reaction.
(ɑ,𝜸) Nuclear Reaction
The capture of alpha particle by neutron does not always result in the emission of a proton by the compound nucleus when Be is bombarded by alpha particles, one of the products of the reaction is 𝛄-rays.
2He4 + 4Be9 → [6C13] → 6C13 + 𝛄
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