BS Physics (Foundation) Series

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Module # 50
Radioactivity & Half Life
Radioactivity
Some elements having atomic number greater than 82 emit
radiations from their nuclei. These elements are known as
radioactive elements and this phenomenon in which the nuclei of
radioactive elements emit powerful radiations is called
radioactivity.
OR
The spontaneous break down or disintegration of an unstable
atomic nucleus with the emission of particles and rays is known
as radioactivity. As this process is inherent and natural
characteristic of radioactivity, so it is called natural radioactivity.
Discovery of Radioactivity
The phenomenon of radioactivity was first of all discovered by a
French scientist named Henri Becquerel in 1896. He was studying
the properties of the materials which glow when exposed to
ultraviolet light.
He wrapped a photographic plate by a black paper such that it
was not influenced by sunlight even if placed in direct sunlight for
the whole day. Then, he placed some crystals of uranium salt on

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it. After developing the photographic plate, he found that uranium
salt had caused fogging on the photographic plate i.e., an image
of uranium salt crystals was formed on the photographic plate.
He repeated this experiment by placing a thin glass sheet
between the uranium salt and the photographic plate and found
the same results. By similar experiments, he concluded that
uranium emits such radiations which can pass through glass plate
and can affect photographic plate. Later on, it was found that
many other substances also emit radiations. The phenomenon of
emission of invisible radiation from uranium and other substances
was named as radioactivity. The element which emits radiation is
called radioactive element. A radioactive element emits three
types of radiation.
(i) -Rays (Alpha - rays)
(ii) -Rays (Beta-rays)
(iii) -Rays (Gamma-rays)
These rays are known as radioactive rays.
Artificial Radioactivity
Joliot and his wife Irene Curie discovered Artificial Radioactivity in
1934.

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Laws of Radioactive Decay
1 When an element disintegrates by the emission of an -
particle, it turns into an element with chemical properties similar to
those of an element two places earlier in the periodic table.
2 When an element disintegrates by the emission of a -
particle, it turns into an element with properties similar to those of
an element one place later in the periodic table.
Half Life of an Element
We know that radio-active elements continuously emit radiations;
as a result, they get transformed into new elements. The element
emitting radiations is known as parent element and the new
element formed as a result of the emission of radiations is called
daughter element. The emission of radiations from a radio-active
substance is a spontaneous (natural) process. The atom of a
radio-active substance can emit radiation any time. If we have an
atom of a radio-active substance, we cannot guess about when
this atom is going to decay. It is quite possible that it may decay
the next moment. It is also possible that thousand years may
pass and it may not decay. Hence, the age of an element cannot
be estimated by considering the decay of a single isolated atom.

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On the other hand, if we have a very large number of atoms, say
105
atoms, then, we can find out the time when half of them will
decay into daughter elements.
The time interval in which half of the atoms in any given sample
decay into daughter elements is known as the half-life of the
parent element.
Thus, if we start with a sample of 100000 atoms of a radioactive
element with a half-life of T, then, after a lapse of T time, 50000
atoms of the element will decay into daughter element and the
number of remaining parent atoms will also be 50000. After the
lapse of another period of time T, the number of atoms of the
parent element will reduce to 25000 and so on. Different radio-
active materials have different half-lives which may range from
1010 years to fraction of a second.
Methods for the Measurement of Half-Life
There are three methods for measuring different values of half-
life.
(1) Measurement of extremely short half-lives.
(2) Measurement of moderately short half-lives.
(3) Measurement of very long half-lives. In all these methods,
the decay constant "λ" is calculated, and then using relation

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T1/2 = 0.693/λ, the half-life T1/2 is calculated.
Radioactive Isotopes
Isotopes of an element have the same atomic number but
different mass number i.e., they differ only in the number of
neutrons. The chemical properties of the isotopes of an element
are the same due to the same number of electrons. The nucleus
of a carbon atom is usually composed of six protons and six
neutrons and this carbon atom is represented by 6C12
. However,
there are a few carbon atoms which have eight neutrons in their
nuclei and they are represented by 6C14. Carbon atoms with mass
number 14 are unstable and emit β-radiations. This isotope of
carbon is called a radioactive isotope. Thus, radioactive isotopes
are the unstable isotopes. They may emit α, β or γ-radiations.
The naturally occurring unstable isotopes which emit various
types of rays and get continuously transformed into different
elements are called radioisotopes or radioactive isotopes. Many
of the isotopes are naturally occurring radioactive isotopes.
Radioactive isotopes can also be produced artificially by
bombarding subatomic particles into some elements. For
example, a radioactive isotope of cobalt is produced by
bombarding it with neutrons.

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Atoms of an element having the same atomic number but different
mass number are called isotopes of that element. Thus, the
atoms of an element which have same number of protons but
different number of neutrons are called isotopes.
For example, 8O16
, 8O17
and 8O18
are isotopes of Oxygen.
1H1, 1H2 and 1H3 are isotopes of Hydrogen.
These isotopes of hydrogen are named as protium, deuterium
and tritium respectively.
17C34, 17C35 are isotopes of chlorine
92U234, 92U235, 92U238 are isotopes of Uranium.
Isotopes of Hydrogen
There are three isotopes of hydrogen.
(1) Protium 1H1
(2) Deuterium 1H2
(3) Tritium 1H3
Protium
An atom of protium consists of one proton and one electron. It
contains one proton in the nucleus, but it has no neutron. It is
most commonly found hydrogen. It can be represented as 1H1

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(A = 1, Z = 1).
Deuterium
An atom of deuterium consists of one proton and one neutron in
the nucleus whereas one electron revolves around the nucleus. It
is represented by 1D2
or 1H2
(A = 2, Z = 1).
Tritium
It is the hydrogen atom which consists of one proton and two
neutrons in the nucleus while one electron revolves around the
nucleus. It is represented by 1T3
or 1H3
(A = 3, Z = 1).
Achievement or Preparation of Radioisotopes
To make the radioisotopes (or radioactive isotopes) of an
element, place the element in an atomic reactor, where free
neutrons are found in abundance. These neutrons interact with
the nuclei and make them radioactive. In this way, a large number
of radioactive isotopes are made in the laboratories, e.g., 11Na23
and 11Na24
are the artificially made radioactive isotopes. 11Na24
is
produced when magnesium is bombarded with neutrons.

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Similarly, when nitrogen is bombarded by neutrons, radioactive
carbon, 6C14
is produced, which decays with the emission of a
beta particle and gets transformed back to nitrogen.
Properties of Radioisotopes (or Radioactive Isotopes)
(1) The radioisotopes emit characteristic rays by which they can
be identified.
(2) Some of isotopes have very short half-lives i.e., of the order
of 10-6s whereas some other radioisotopes have large half-lives of
the order of billion years, e.g., half-life of Polonium is 10-6
sec,
whereas half-life of thorium is 13.9 billion years.
Uses of Radioisotopes of Elements in Medicine
(1) Radioisotopes have helped in understanding the basic
working of many of the internal organs and vital metabolic
processes. They have also helped in the diagnosis and cure of
many complicated diseases. Radioisotopes have also played a
vital role in determining the effectivity and absorption of medicine
in various parts of the body.
(2) A radio-active isotope of iodine-131 is very useful for
detecting thyroid disorders. If the thyroid gland is overactive or
cancerous, then, huge amounts of iodine-131 injected into the
body is accumulated in the abnormal region of gland and the

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intense radioactivity emitted by this iodine isotope can destroy the
diseased tissue.
(3) Radio phosphorous-32 is used to locate precisely the
position of tumor in the brain.
(4) The intense radiation given off by cobalt-60 is used to
destroy malignant cells in cancerous tumors.
(5) Radio sodium has been extremely useful in tracing the blood
circulation in the body.
(6) Radio strontium has been found effective in the treatment of
internal hemorrhages and wounds.
(7) Radio phosphorous has been found effective for treating
Leukemia. Radiation emitted by it destroys the excess production
of white blood corpuscles.
(8) These elements help in locating the cancerous portions of
the body precisely. Cancerous tissues of some kinds of cancer
absorb more radioactive atoms than surrounding healthy tissues.
This makes it possible to locate cancer and also helps in treating
it.
(9) Medical instruments and bandages are sterilized after
packing by brief exposure to gamma rays.

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(10) Food and meat are made to stay fresh by treating them by
gamma rays for a brief period.
(11) Carbon-14 is present in almost every organic molecule. The
details of how food molecules are digested, and to what parts of
the body they are diverted, can be traced by following the
movement of radioactive molecules through body using a Geiger
counter.
In the twentieth century, radioisotopes have brought about a
revolution in the diagnosis and treatment of many diseases
thought incurable earlier. Their role will continue to grow in the
21st century.
Uses of Radio Isotopes in Agriculture
(1) These elements are used to increase agricultural production
in agriculture. The importance of varieties of seeds, which resist
attacks of pests and yield higher production, cannot be over
emphasized. Such varieties of seeds for various agricultural
commodities have been produced after mutation through
radiation.
(2) Such radio elements are used to kill bacteria and
preserve food stuff.

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(3) Radiations are used to determine the optimum amount of
fertilizers and other nutrients intake by plants.
Fig: Detecting Radiation in a Plant
A very minute amount of radioisotope of the chemical whose
absorption by the plant is to be determined is mixed with the
ordinary chemical. The whole is then dissolved in water. The plant
is then irrigated with this water. A detector (see figure) placed
near various parts of the plant can identify the location of
absorbed chemical. The count rate will determine how much of
the chemical has been absorbed in various parts of the plant. In
this way, the exact location of the absorbed chemical as well as
the quantity of chemical absorbed can be precisely determined.
(4) The compounds of 6C14
are used to determine the effect of
medicines on different plants.
Uses of Radioactive Elements in Industry
(1) Radioactive elements are helpful in every step of industrial
progress, e.g., in the manufacture and uses of tools.

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(2) Water mark in printing is made with the help of beta rays
released from sulphur-35.
(3) CO-60 emits gamma rays which are used to detect cracks in
welding and to determine the defects in molding.
(4) Radioisotopes are used to determine the defects in machine.
In research into wear in machinery, a small amount of radioactive
iron is introduced into the bearing and the rate of wear is found
from the resulting radioactivity of the lubricating oil.
(5) To maintain the thickness of paper, radioisotopes are used.
In the manufacture of paper, the thickness is checked by having a
-source below the paper and a G.M. tube and counter above it.
(6) Radioisotopes can be used to detect leakage in the pipes.
This is done by introducing small quantities of radio isotope into
the fluid in the pipe. A radiation detector can be used to check
whether the radio isotope is leaking anywhere in the pipe, thus
indicating a fault in the pipe.
(7) Level indicators also depend on the absorption of radiation
and are used to check the filling of toothpaste tubes and packets
of detergents.