An atom consists of a nucleus containing protons and neutrons surrounded by electrons. The nucleus is very small compared to the atom. Atoms are electrically neutral overall but have a positive nucleus and negative electrons that balance out. There are three main types of radiation emitted by radioactive atoms: alpha particles which are helium nuclei, beta particles which are electrons, and gamma rays which are electromagnetic radiation like x-rays. These three types of radiation can be differentiated using a magnetic field which deflects charged particles but not neutral gamma rays. Alpha particles have the highest energy but lowest penetration ability while gamma rays have the lowest energy but highest penetration. Prolonged exposure to nuclear radiation can cause health issues like cancer and radiation sickness.

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Module # 47
Nuclear Physics & Radiations
Atom
An atom, the fundamental constituent of matter, is composed of
three elementary particles called electron, proton and neutron.
Since an atom in normal state is a neutral particle, the number of
protons in a nucleus is equal to the number of electrons revolving
around it.
An atom consists of the following:
(I) It has a hard central core known as nucleus. It contains two
types of particles one is known as proton and carries positive
charge, the other is neutron which is electrically neutral i.e. it
carries no charge though it is as heavy as proton. The protons
and neutrons are very closely held together with tremendous
nuclear forces, in the nucleus.
(II) Revolving round the relatively massive nucleus, in more or
less elliptical orbits (or shells) are infinitesimally small particles
known as electrons. These electrons carry the smallest negative
charge and have a negligible mass. The mass of an electron is

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approximately 1/1840 that of a proton.
It has been found that the positive charge on a proton is
numerically equal to the negative charge of an electron. Normally,
an atom is electrically neutral because it consists of as many
protons as electrons. The number of protons in the nucleus of an
atom gives the atomic number whose atom it is. The total weight
of a nucleus (i.e. protons plus neutrons) is called the atomic
weight. The total number of protons and neutrons in the nucleus
of an atom gives its atomic mass number. If the number of
protons in a nucleus is changed, then transmutation of one
element into another can be achieved.
Electron
Electron is a very light and negatively charged particle. Electrons
move round the nucleus in closed orbits. The amount of charge
on an electron is exactly equal to that on a proton.
An atom may contain one or more than one electrons. When one
or more than one electrons are removed from an atom, it
becomes positively charged particle because of protons in the
nucleus; on the other hand, if electrons are added in a substance,
it becomes negatively charged due to excess of electrons. This is
the basic reason, why positive and negative charges appear over
an object.

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The electron is the fundamental negative charge of electricity. The
charge on an electron is 1.67 x 10-19
C. The rest mass of an
electron is 9.11 x 10-31
kg. It is 1/1836 of the mass of proton. Since
an atom in normal state is a neutral particle, therefore, the
number of protons in a nucleus is equal to the number of
electrons revolving around the nucleus.
Electron Theory
It has been established that all matter whether solid, liquid or
gaseous, consists of minute particles called molecules which are
themselves made up of still minute particles known as atoms.
Those substances whose molecules consist of similar atoms are
known as elements and those whose molecules consist of
dissimilar atoms are called compounds. The number of stable
elements so far discovered is 106 whereas the number of
compounds is unlimited.
Proton
Proton is a positively charged particle and is found in the nucleus
of an atom.
Proton is considered to be a heavy particle because it is about
1836 times more massive than electron. Each proton carries
positive charge equal in magnitude to the charge on an electron,
i.e. 1.66 x 10-19
coulomb.

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It has been determined that the mass of a proton is nearly equal
to the mass of a neutron which is equal to 1.67 x 10-27
Kg.
The proton is the fundamental positive charge of electricity. Mass
of proton is approximately 1836 times the mass of electron.
Number of protons in the nucleus is equal to the number of
electrons revolving around the nucleus. Each element has a
definite number of protons.
Radius of Proton = 1.2x10-15m
Neutron
Neutron is a neutral particle i.e. it has no charge on it. Neutrons
are present in the nucleus of an atom along with protons. The
mass of a neutron is slightly greater than the mass of a proton.
Yet, its mass is considered to be equal to the mass of a proton,
i.e., 1.67 x 10-27
kilogram. The number of neutrons in the nucleus
is denoted by the letter N. For a neutron the symbol 0n1 is used.
Nucleus
The most important source of energy other than sun is the atomic
nucleus which releases energy through fission and fusion.
OR
Nucleus consists of two types of particles, i.e., protons and

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neutrons. These particles are thus known as nucleons.
An atomic nucleus with a given atomic mass number is written as
ZXA
where X is any element.
Examples
Hydrogen gas has symbol H. There is only one proton in its
nucleus. So, in this case, Z = 1 and A = 1. The nucleus of
hydrogen is represented by symbol 1H1
.
For helium, symbol is He. Its nucleus carries two protons and two
neutrons.
Z = 2 and A = 4. So, the nucleus of He is represented by 2He4
.
In the same way, to represent the nucleus of uranium element, we
use symbol U.
For uranium, A = 238, Z = 92. So, the nucleus of uranium is
represented by 92U238
.
Size of Nucleus
Experimental measurements show that the nucleus is very small
in size and occupies a spherical region in space. The diameter of
a nucleus is of the order of 10-14
m, which is ten thousand times
smaller than the diameter of an atom, and depends on the atomic
mass number A.

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Diameter of a Nucleus
The diameter of a nucleus is of the order of 10-14
m, which is ten
thousand times smaller than the diameter of an atom and
depends on the atomic mass number A.
Nucleon
The protons and the neutrons in a nucleus are collectively
referred to as nucleons. The mass of each nucleon is about 1836
times greater than that of an electron. Thus the mass of an atom
is mainly due to the mass of the nucleons, i.e. the mass of its
protons and neutrons. The nucleons are clustered together as
incompressible pieces of matter in the spherical region of the
nucleus.
Atomic Number
Number of electrons (revolving round the nucleus) or the number
of protons in the nucleus is called atomic number. It is denoted by
z. It is equal to the difference of mass number and number of
neutrons in the nucleus, i.e. Z = A-N.
The nucleus of an element X with mass number A and atomic
number Z can be represented by a symbol ZXA
.

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Atomic Mass Number
The total number of protons and neutrons in a nucleus of an atom
is called its atomic mass number and it is denoted by the letter A
such that A = Z + N
OR
Mass number of the atom is equal to the total number of neutrons
and protons in the nucleus. It is denoted by letter A such that
A = Z + N
Cesium
Cesium contains 55 protons. The atom also has 55 electrons
arranged in groups of 2, 8, 18, 18, 8 and 1. The last electron
cannot go into shell 4 (N) since it would violate the limit of 18
electrons in the next to the last shell. This electron cannot go into
shell 5 (O) since it would violate the limit of 8 electrons in the last
shell.
Argon Atom
Argon contains 18 protons. The atom also has 18 electrons
arranged in groups of 2, 8 and 8. The last shell is filled even
though its maximum quota is 18.

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Nuclear Physics
The branch of physics which deals with the nuclear particles is
called Nuclear Physics. It deals with the nuclear part of atoms and
the related phenomena, for example, nuclear fission, nuclear
fusion and nuclear forces, etc.
Radiation
Soon after the discovery of radioactivity, Rutherford and other
scientists found that radiations emitted from radio-active elements
are of three types, named as Alpha, Beta and Gamma rays.
These can be separated by a simple experiment.
A small quantity of a radioactive substance such as radium is so
placed in a cavity in a block of lead that the radiation from radium
can come out of the mouth of this cavity. A photographic plate is
placed at some distance above the lead block so that radiations
from radium fall upon it. This apparatus is placed in a vacuum
tight chamber which is evacuated by a powerful pump. This
chamber is then placed between the poles of a strong magnet.
Under the action of magnetic field, two of the three types of
radiations are deflected, so three separate images are formed on
the photographic plate. This shows that radiations coming out of
radium are of three types. One of these are those radiations
which bend towards left and give rise to a black spot on the plate

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close to the central one. The direction and the extent of deflection
shows that these radiations consist of positively charged particles
which are quite massive. Further experiments show that these
particles are nuclei of helium. These radiations are known as α-
rays.
The second type of radiations are those which bend towards right
and form a black spot on the plate at a comparatively larger
distance from the central spot. This indicates that these radiations
consist of negatively charged particles. Later experiments have
shown that these particles are actually electrons. These radiations
are known as β- rays.
The third type of radiations are those which give rise to the black
spot at the centre. The fact that these radiations move in the
magnetic field without any deflection shows that magnetic field
has no effect upon them. This shows that these rays are neither
positively charged particles nor negatively charged particles.
Further experiments showed that their nature is similar to light or
X-rays. These radiations are known as γ-rays.

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Alpha, Beta & Gamma Radiations, Differentiated
(Experiment)
Fig: The radiation from a radioactive source can be separated into
three components by using a magnetic field to deflect the charged
particles
Take a small quantity of a radioactive substance like Radium and
place it in a cavity in a block of lead in such a way that only a
collimated beam emerges from a small opening in the top of the
lead block as shown in figure.
A photographic plate is placed horizontally at a small distance
above the opening. The whole system is enclosed in an
evacuated chamber from which air has been pumped out. A
strong magnetic field is applied perpendicular to the plane of the
chamber. When the photographic plate is developed, it shows
three dark spots indicating where the radiation emitted by radium
has fallen. This shows that three types of radiations are emitted
by radium.

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One type of radiation consists of positively charged particles
indicated by the dark spot to the left. Here, the magnetic poles
determine the +ve/-ve nature of the radiations and the extent of
their deflection shows their massive character. The other consists
of negatively charged particles indicated by the dark spot to the
right. The central third spot is by the radiation which remained un-
affected by the magnetic field. These rays or radiations are
named as alpha, beta and gamma rays respectively.
Thus, due to the presence of strong magnetic field, oppositely
charged particles ( and - rays) are deflected in opposite
directions and neutral rays remain un-deflected.
Conclusion
From this experiment, it can be verified that -rays are positively
charged and -rays carry negative charge while the γ-rays are
electrically neutral.
Alpha (α) Particles
Alpha-particles are nuclei of helium.
Alpha Rays
Alpha rays are helium nuclei.

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Properties of  – Rays
(1) Alpha particles are nuclei of helium. The mass of each alpha
particle is four times the mass of a hydrogen nucleus.
(2) The charge carried by an alpha particle is positive and twice
the charge on a proton.
(3) The -particles can cause ionization. The ionizing power of
-particles is greater than that of  and γ-particles.
(4) The penetrating power of α-particles is very small due to
their large mass.
(5) They can cause fluorescence in certain substances.
(6) They can produce artificial radioactivity in certain
substances.
(7) These rays produce burns and itches on human body.
(8) These rays can affect photographic plates.
(9) These rays are stopped after travelling few centimeters in
air.
(10) When these rays are allowed to fall on a thin foil of gold,
some of the α-particles are scattered at very large angles.

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Properties of β-Rays
(1) These rays consist of negatively charged particles. These
particles are fast moving electrons.
(2) These rays can cause fogging on a photographic plate.
(3) They possess less kinetic energy than that of alpha particles.
(4) These rays can produce fluorescence in certain substances
like barium platinocyanide.
(5) The ionization power of β-particles is less than that of α-
particles.
(6) The penetration power of β-rays is greater than that of α-
rays.
(7) The velocity of β-rays is nearly equal to the velocity of light,
i.e., β-rays can have velocity of the order of 9 x 107
m/s to 27x l07
m/s.
(8) Due to small mass as compared to α - particles, they are
easily scattered by the nuclei of atoms.
Gamma Rays
Gamma rays consist of electromagnetic radiations similar to X-
rays. Gamma rays have very small wavelength (less than 10-11
m)
but their energy is very high. They are emitted by the nucleus of

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certain radioactive substances. They are also released during
certain nuclear reactions. Some of their other properties are as
under.
Properties of γ-Rays
(1) These rays are electrically neutral i.e. γ- rays carry no
charge and are not affected by electric or magnetic field.
(2) The velocity of γ-rays is equal to velocity of light.
(3) The penetrating power of these rays is greater as compared
with α-rays or β-rays. Their penetrating power is about hundred
times that of the beta rays.
(4) When γ-rays are allowed to fall on a metal, they can emit
electrons from it. This phenomenon is called photoelectric effect.
(5) They produce feeble fluorescence when incident on a screen
coated with barium platinocyanide.
(6) The γ-rays can be absorbed by certain materials.
Radiation Hazards
Excessive radiations can be very dangerous to human body and
can cause cancer or incurable sickness to a human being.
Radiation from the radioactive element show everlasting effect in
the human body. The radiation in the large amount may cause

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wounds on the skin. However, in the intensive condition, radiation
may cause death.
Nuclear radiation is released in great quantities during nuclear
explosions and accidents in nuclear installations. This radiation is
not only dangerous for human beings but it also affects
agriculture, making vegetables, fruit and grains radioactive. Thus,
these vegetables and fruit cannot be used as food. One nuclear
plant in Chernobyl in U.S.S.R. caused great human and material
loss. It led to an extensive radiation fall out in areas hundreds of
kilometers away.
The destruction of the cell is caused by the ionizing properties of
the energy of radiation. The amount of ionization produced
depends upon the intensity and energy of radiation. Energetic
radiations will penetrate deep into the body and can destroy the
vital body cells. High intensity radiations can destroy large
number of vital body cells. The destruction of the body cells also
depends upon the exposure time to the radiation. A large number
of body cells will be destroyed if it is irradiated for a longer time.
The person, if strongly irradiated, may suffer the following
diseases:
(1) Anemia i.e. disease of red blood corpuscles.

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(2) Leukemia i.e. an increase in the white blood corpuscles also
known as blood cancer.
(3) Malignant tumors.
(4) Cataracts i.e. opacities of the eye lens.
(5) Several other diseases.
Safeguards against Radiations
(1) Protective shields and radiation detector should be used all
the times.
(2) Loose clothes made of light colored material including plastic
help in protecting the effect of radiation.
(3) Periodic medical tests should be taken to detect any ill-effect
of exposure to radiation.
Precautions to Minimize Radiation Danger
(1) One should keep a safe distance away from the radiation
emitting sources. Since, the source emits radiation in all
directions, so, its density falls according to the relation 1/r2
,
where, r is the distance of the source from the body.
(2) The doctor, while giving treatment to a patient by radiation,
should take the minimum possible time for radiation exposure.

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(3) The radiations for a reactor are shielded by thick concrete
walls.
(4) In a laboratory, where radioactive materials are handled, the
radio-active substance is covered in a lead box with a lid made of
lead. Lead is a high density material and therefore stops
radiations falling upon it. The experiments are performed in
separate rooms, and the students are instructed to handle the
apparatus carefully.