Spontaneous emission of Alpha, beta and gamma radiation from
the unstable nuclei is called radioactivity. This generally happens with the
heavy nucleus like uranium and thorium. We cannot change the radioactivity by
playing high-temperature, high-pressure or electric field. Nuclei can
disintegrate immediately or it may take in finite time.
Alpha decay
The Alpha particle is nothing but a helium nucleus. It is a
positively charged particle whose charge is equal to two times a charge of the
photon. It is deflected both electric and magnetic fields.
With the emission of the Alpha particle, the atomic number of
the nucleus is reduced by two and its mass number is reduced by four.
Beta decay
The emission of the fast moving electrons from the nucleus is
called beta decay. The neutron inside the nucleus splits up into proton and
electron. The electron is rejected out like beta particle. The proton reminds
inside the nucleus itself. Because of this the atomic number is increased by
one when there is emission of beta particle will. Anyway the mass number of the
nuclei is going to remain same as the number of the neutrons decreases by one
and the number of the protons increases by one.
As the beta particles are having a charge they are also
deflected by electric and magnetic fields. They have ionization power less than
that of Alpha particles and penetration power better than that of Alpha
particles.
Gamma Decay
Gamma rays are nothing but electromagnetic radiation of short
wavelength. There is no effect on atomic number as well as the mass number
during this emission. During the emission of Alpha Ray and beta Ray electrons
goes to excited states. They cannot stay in the excited state for a long time
and they will come back to their earlier states.
During this process they emit
some energy and that energy is observed in the form of this electromagnetic
waves. Gamma rays alone cannot be emitted. As a consequence of Alpha Ray or
beta Ray, they can be emitted.
Law of radioactivity
Basing on experimental observations and analysis of
radioactive material this is formulated. According to this law the rate of
radioactive decay at any instant is directly proportional to number of nuclei
present at that instant and is independent of physical conditions like temperature,
pressure and chemical composition. Radioactive decay is nothing but the number
of the nuclei decaying per unit time.
We can derive the relation between number of the nucleons
present with respect to the initial number of the nucleons after a specified
time as shown below.
Half life period
The time interval during which the number of radioactive
nuclei of a sample disintegrates to half of its original number of nuclei is
called a half life period.
We can derive the relation between half life period and the
decay constant basing on their respective definitions as shown below.
Half life period of a material is its characteristic property
and it cannot be changed by any of the known methods. We can express the
relation between the initial number of nucleons and the final number of
nucleons with respect to number of the half lives as shown below.
It can be mathematically noticed that after half of the half
life period 70.7% of the initial number of the nucleons are going to be
present. It means approximately 30% of the nucleons were disintegrated in the
first half of the half life period. You might notice that it is not 25% but 30%
. More disintegration is happened because initially there are more number of
nucleons.
Average life
The phenomenon of radioactivity is random and we cannot
predict which one of the atoms will decay first and when. Each atom will decay
in its one-time and to determine the average of all the decays, we have defined
average life. It is defined as the ratio of total lifetime of all the nucleons
to the number of nuclei. It can be mathematically proved as the reciprocal of
the decay constant. We can derive the relation between half life period and the
average life as shown below.
Problem and solution
The half-life period of the Cobalt is 72 years. How much time
does it take for three by fourth of its initial mass to disintegrate?
We can solve this problem basing on the Basic derivation is
that we have made in the above pages. We can calculate the number of half life
period is as shown below.
Problem and solution
A radioactive sample can decay in two different processes
simultaneously with a different half life periods. Find the effective half
liquidate of the sample?
We can solve this problem basing on the law radioactive
decay. According to this law the number of the nucleons disintegrated per
second is directly proportional to initial number of nucleons. The same element
is going through two different processes and hence the initial number of the
nucleons are same. The total rate of disintegration is equal to the sum of rate
of disintegration of both the cases. We can solve this problem as shown below.
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