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Modern Physics - Part I |
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If we look at all the nuclei that are present in nature, we find that a large number of heavy mass nuclei like uranium (U), thorium (Th), radium (Ra) spontaneously decay to small mass nuclei. The process of decay by which one species of nuclei transform into another is called radioactivity or radioactive decay. The decay occurs with certain rules or laws of radioactivity. The decaying or unstable nuclei are called radioactive elements. Radioactivity can be either spontaneous or induced. Spontaneous radioactive elements occur naturally like uranium, thorium, radium, etc. Induced radioactivity is achieved by nuclear reactions, either by fusing two small nuclei (fusion reaction) or fissioning or breaking up larger nuclei (fission reaction). For example, all transuranic elements, man made elements beyond uranium are radioactive. What
we will study in this chapter
: 1.
There is a very strong force acting within the nucleus, which is able to overcome the Coulomb’s repulsion. The strong forces do not depend on the charge of the particles. Strong forces between proton-proton, neutron-proton and neutron-neutron are found to be identical. Strong forces can be experienced only in the dimension of the nucleus (10-15m). Strong force does not differentiate whether the particle is a proton or a neutron, hence these particles together are referred to as nucleons. It is seen that in nature, for low mass atoms, N and Z are nearly equal. But as Z increases, N becomes more than Z, so as to counter balance the proton-proton Coulomb repulsion. But as mass of atoms becomes very large, they tend to be unstable and break up. The break up results in emission of energetic particles. Unstable nucleus is called a radioactive nucleus. The instability in the nucleus can be spontaneous or can be induced by a nuclear reaction. The energetic radiation given off by a radioactive nucleus is called radioactivity. Radioactivity is categorized as : Alpha particle : ( Very rarely a radioactive atom may also emit neutrons. Radioactivity was discovered by accident by Henry Bacquerel in 1896. He had left a salt of uranium in one drawer of his desk and a photographic plate in another drawer. He noticed the next day that the photographic film had become clouded. He inferred that the photographic film was being bombarded by invisible radiation from the uranium salt that was capable of penetrating the layers of the drawer. Other experiments with radium by Marie and Peirre Curie proved that some elements emit invisible radiation. This phenomena was named as radioactivity by Marie Curie. To
understand more about radioactivity, do the following experiment. Keep a
radioactive substance like uranium that emits Keep
two metal plates P1 and P2 in the chamber. Let these plates be connected to
a battery so that P1 acquires a negative charge and P2 acquires a positive
charge. Keep a photographic
plate parallel to the hole in the lead pot so that the radiation coming out
from the radioactive substance is incident on the photographic plate. Expose
the photographic plate to the radiation for a short time. Then develop the
plate. You will notice three traces or dot. These indicate that there are
three types of radiation. The central dot shows that this particular
radiation is not affected by the presence of an electric field. This is
called the More
investigations showed that the Some
radioactive substances also emit neutrons. Neutrons are highly penetrating
as they are neutral particles and do not have any electrical interactions
with other atoms. Also it has to be mentioned here that [1]
As mentioned before, there are two types of beta radiations : electrons
shown as [2]
eV is the unit of energy in electron volt. 1eV is the work done to take
a charge of 1 electron through a voltage difference of 1 volt. 1eV = 1.6
x 10-19 joules. One
million electron volt is written as 1 MeV.
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- radiation
.
Although
