In this particular article, Alpha decay or radioactive emission of alpha particles we are going to discuss about the properties of α-particle. We will also discuss the phenomena of α-decay in detail along with in introduction of disintegration energy.
Alpha decay or radioactive emission of alpha particles
- Alpha-particle: α-particles are doubly ionized helium atoms. Their charge is positive and equal to twice the electron charge. Their mass is equal to the sum of masses of two protons and two neutrons.
- Alpha-emission: The process of emission of α-particle from the nucleus of an atom is called α-decay and the emission process of these particles can be shown by following nuclear reaction.
By the emission of α-particle from the nucleus, the atomic number of the parent nucleus decreases by two and mass number by four i.e. the new element is shifted to places backward in the periodic table. In the nucleus of the new element, the number of protons and neutrons are decreased by two.
Properties of α-particle
- These are strongly ionizing rays and weakly penitents radiation.
- These particles are positively charged and hence deflected by electric and magnetic field.
- α-particles move with the velocity 10⁹ cm per second.
- Example of α-particle is 2He⁴.
- α-particles Get scattered when they passed through the metal sheet.
- They produced the heating effect when stop.
- α-particles can produce fluorescence effect in substances like ZnS.
- They produced intense ionization in air.
- They produce the effect on the photographic plate.
- Deflection with the electric field and the magnetic field is very small. α-particles Are heavy particles and have the heavy moment of inertia.
- After traveling the certain distance in air α-particles losses their ionizing power. And this distance is called range of α-particles.
- The energy of α-particle decreases by the ionization of the gas(medium). Before ionizing the gas it travels a certain distance. This distance is called range. It represented by R. The range of alpha particle depends on its energy and the nature of the gas. The relation of the mean range R‾with the energy E of the alpha particles are given by; R‾=0.318E¹⁄²
- Geiger and Nuttall proved on the basis of their experiments that greater the decay constant of a radioactive element, the greater its velocity, energy, and range of the alpha particle emitted by it. The decay constant of the element and the range of alpha particle have the following relation; logλ=A+Blog R
- The energy of all alpha particles are emitted from the nucleus of the element, γ-rays are also emitted along with it. Because the nucleus of an atom gets excited as soon as it emits the alpha particle. When the excited nucleus returns to its ground state, it emits γ-rays. Whose energy is equal to the difference of energies of its excited state and ground state.
Nuclei having 210 or more nucleons are so large that short-range attractive nuclear force that holds the nucleus are barely able to counterbalance the mutually repulsive forces among the protons. Alpha-decay occurs in such nuclei as a means of increasing its stability by reducing the size of the nuclei. Because of the alpha particle having a very high binding energy more than 28MeV and its mass sufficiently smaller than that of the heavy nucleus, its formation within the nucleus makes available kinetic energy, the particle must have escaped from the nucleus. To illustrate this point let us consider the following alpha-decay.
Using the law of conservation of mass-energy in it;
Thus energy Q is released when various particles are emitted by the decay of the nucleus. This energy is called disintegration energy. This energy is sum of the kinetic energy of the product nucleus and kinetic energy of alpha-particle.
Where Vy and Vα are the velocities of the final nucleus and alpha particle respectively. from the law of conservation of linear momentum,
Substituting this in equation (2),
Thus the kinetic energy of the alpha particle is less than the disintegration energy. For the mass numbers of nearly all alpha-emitters greater than 210, the disintegration energy appears as the kinetic energy of the alpha particles.
For an example, let us consider the decay of Bi²¹² into Th²⁰⁸ with the emission of the alpha particle. The kinetic energy of alpha-particle is Eα=10.54MeV. The mass of the alpha particle is mα=4 amu and mass of the final particle is My=208 amu
Thus most of the disintegration energy is supplied to the kinetic energy of the alpha particle.
The alpha decay from ₉₂U²³⁸ is accompanied by the release of energy 5.4MeV. This value agrees with the values predicted by the nuclear masses involved in it. For the emission of proton and ₂He³ the required energy are 6.1 MeV and 9.6 MeV and respecting which has to be supplied from outside.