Question

What is alpha decay? What type of energy spectrum do alpha particles have?

Answer 1

Heavy nuclei are generally unstable because the nuclear force will not be able to counteract the electrostatic repulsion of the large number of protons. In such cases the nucleus can attain stability by emitting particles so that the size gets reduced. Alpha particle emission is one of the ways of increasing stability. An alpha particle is a helium nucleus.

Complete step by step solution:
Alpha decay is a type of radioactive decay in which alpha particles are emitted from the nucleus. An alpha particle is a helium nucleus having mass number 4 and atomic number 2. Hence when an alpha particle is emitted from the nucleus the atomic number of the parent nucleus reduces by 2 and the mass number of the parent nucleus reduces by 4. The alpha decay occurs in heavier nuclei.
The alpha decay from a parent nucleus X with mass number A and atomic number Z can be represented as
$_Z{X^A}{ \to _{Z - 2}}{Y^{A - 4}}{ + _2}H{e^4}$
Y denotes the daughter nucleus and $_2H{e^4}$ is the alpha particle.

We know that the protons having positive charge are held together in the nucleus with the help of strong nuclear force. The coulomb repulsion between the like charges is overcome by the strong nuclear force. But the nuclear force is a short- range force whereas electromagnetic force has unlimited range. The strength of the nuclear force thus depends upon the number of nucleons whereas the electromagnetic force is proportional to the square of atomic number. So, a nucleus having nucleons more than 210 is so large that the nuclear force holding it cannot counterbalance the large electromagnetic repulsion between the protons.

In such a case the nucleus emits alpha particles so that the size is reduced which leads to increased stability.
This emission of alpha particles from an unstable nucleus is known as alpha decay.

The alpha particles come out of the nucleus through quantum tunneling. Because the alpha particles do not have enough energy to pass over the potential wall confining it. Hence the Alpha particles escape from the nucleus by tunneling through the wall.
The energy spectrum of alpha particles is discrete. That is, in the emission of helium ions the energy spectrum that we get is a sharp line spectrum. In the spectrum there will be a number of discrete lines with varying intensity.

Note: Alpha decay is the emission of alpha particles which is primarily a helium nucleus, in order to increase stability of heavy nucleus. But alpha decay is not the only way with which an unstable nucleus attains stability. There are two other types of decay which are beta decay and gamma decay.