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MODERN PHYSICS

(LEVEL III)

Radioactivity and its applications

Concepts to be learnt:

Introduction:

Antoine Henry Becquerel, was familiar with the work of Wilhelm Conrad Roentgen.Roentgen

discovered “mysterious" rays, called as X-rays. They werecapable of producing an image of

bones on a photographic plate. This excited scientists of his day, including Becquerel. Becquerel

chose to study the related phenomena of fluorescence and phosphorescence. In March of 1896,

quite by accident, he made a remarkable discovery.

Becquerel put his wrapped photographic plates away in a darkened drawer, along with some

crystals containing uranium. Much to his surprise, the plates were exposed during storage by

invisible rays or radiations coming from the uranium. This was the discovery of Radioactivity.

Becquerel did not pursue his discovery of radioactivitymuch but others did. Working in the

Becquerel laboratory, Marie Curie and her husband, Pierre, began what became a lifelong study

of radioactivity.

Becquerel had already noted that uranium radiations could turn air near the sample into

conductor of electricity. Pierre and Marie Curie measured the conductivity inducedby various

elements. They tested an ore of uranium, pitchblende, for its ability to turn air into a conductor of

electricity. The Curies found that the pitchblende (UO2) produced a current 300 times stronger

than that produced by pure uranium. After much grueling work, Curies were able to extract

enough polonium and another radioactive element, radium.

Radioactivity:

Becquerel, Pierre and Marie Curie worked with uranium salts. He kept the concentration of

uranium salt constant and measured the intensity of radiations by changing physical and

chemical conditions. He changed temperature, strength of electric and magnetic field, pressure

etc. The intensity of radiations remained the same. This clearly showed that the phenomenon

could not be due to orbital electrons. Therefore he concluded that the phenomenon was

connected to nucleus.

α, β & γ radiations, Properties of α, β & γ radiations, Mass Defect, Energy mass

relationship, Binding Energy. Fission, Radioactive transformation, Chain reaction,

Law of radioactive decay, half life time, Nuclear Fusion, Radioisotopes and their

uses.

Natural radioactivity is a spontaneous phenomenon. Certain heavy unstable elements emit

radiations from their nuclei and become stable. This property of the some substances is

known as radioactivity and the substances or elements which possess this property are called

as radioactive substances.

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Types of radiations: (α, β& γ radiations):

Rutherford (1897) first found that the radiations emitted by radioactive substances are more than

one type. He found some are more penetrating and some rays are less penetrating. He called the

less penetrating rays as α-rays and more penetrating rays as β rays. In 1900, Sir Villard found

third type of rays which were more penetrating than others. He gave the name γ rays.

Rutherford’s Experiment:

The experiment connected to radioactivity was conducted in vacuum. The radioactive sample

was kept in a cavity drilled in a lead block. Lead is good absorber of radiations. Sample radiates

the radiations in all directions. The radiations emitted in the direction of bore are emitted through

this source and rests of the radiations are absorbed by the block. Two parallel plates are arranged

near this radiated beam from the bore. Electric field sets up when potential is applied to it. In a

similar way, coils can be arranged so that magnetic field perpendicular to the direction of beam

can be applied. A well wrapped photographic plate was kept at some distance from the lead

block.The experimental arrangement is as shown below.

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Observations:

There was a single dark spot on the photographic plate when electric or magnetic field was not

applied. This indicated that the radiations travel in straight line in absence of electric or magnetic

field (force).

When electric field was applied then there observed three spots on the photographic plate. It

clearly indicated that the radiated beam splits in to three in presence of electrostatic force. The

inferences drawn are as follows:

i) The part of the beam that deflected towards the negative plate must be consisted of

the positively charged particles. These are known as αrays.

ii) The part of the beam that deflected towards the positive plate must be consisted the

negatively charged particles. These are known as β rays.

iii) The rays that went in straight lineeven in presence of electrostatic field do not contain

any charged particles. Further it was proved that these are electromagnetic waves.

These are known as the γ rays.

iv) β particles deflect more than α particles. It indicated that β particles are lighter than

αparticles.

These radiations were studied systematically further. It was found that α particles are

helium nuclei.β particles are electrons and gammas are electromagnetic radiations.

There properties are tabulated as follows.

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Sr.

No

properties α particles β particles γ rays

1 Nature Positively charged

helium nuclei, 2He4.

Negatively charged

particles called as

electrons.

Electromagnetic

waves of short

wavelengths

2 Charge Twice that of charge

on electron = 3.2 X

10-19

C.

= 1.6 X 10-19

C. No charge.

3 Mass = 6.62 X 10-27

Kg. = 9.1 X 10-31

Kg. No mass.

4 Velocity Ranges from 106 m/s

to 107 m/s

Ranges from 1.08

X 108 m/s to 2.94 X

108 m/s

Velocity of light

i.e. 3 X 108 m/s

5 Effect on

photographic

plate

Affects the

photographic plate.

Affects the

photographic plate.

Affects the

photographic

plate.

6 Ionization

power

Very strong, roughly

10000 times that of γ

particles and 100

times that of β

particles.

Moderately strong,

roughly 100 times

that of γ particles.

Very small

compared to α

and β particles.

7 Penetrating

power

Penetrating power is

very small, 1/10000

times that of γ rays.

Penetrating power

is moderate, 1/100

times that of γ rays.

Very high.

8 Effect of

Electric and

Magnetic field.

Deflected by electric

and magnetic field.

Deflected by

electric and

magnetic field.

No deflection

by electric or

magnetic field.

9 Fluorescence Produces

fluorescence in

fluorescent material

(ZnS).

Produces

fluorescence in

fluorescent material

(ZnS).

Produces

fluorescence in

fluorescent

material (ZnS).

10 Effect on Body Destroy living cells,

tissues and cause

biological damage.

As their penetration

power is large, they

cause greater

biological damage.

Penetration

power being

highest, cause

immense

biological

damage.

11 Applications They have large

momentum,

therefore large K. E.

They are used to

bombard atomic

nuclei to obtain new

nuclei.

They produce

x-rays when

stopped in tungsten.

They are very

useful in

treatment of

cancer.

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Mass Defect:

Scientists have accurately found the masses of the proton and the neutron. They calculated the

masses of various nuclei. When sum of the masses of individual protons and individual neutrons

is made for a particular nucleus, it was found that this ‘sum’ mass was always greater than actual

mass of the nucleus. This difference was termed as the mass defect. It was denoted by ∆m.

In simple words, the definition of mass defect can be stated as follows:

Definition: The difference between the calculated mass of unbound system and experimentally

measured mass of nucleus is called mass defect. It is denoted by Δm. It can be calculated as

follows:

Mass defect = (calculated mass of unbound system) - (measured mass of nucleus)

i.e, Δm= (sum of masses of protons and neutrons) - (measured mass of nucleus)

Classically a bound system is at a lower energy level than its unbound constituents. The energy

must have liberated in the form of γ rays and heat.

Energy mass relationship:

Mass–energy equivalence is the concept that the mass of a body is a measure of its energy

content. In this concept the total internal energy E of a body at rest is equal to the product of its

rest mass m and a suitable conversion factor to transform from units of mass to units of energy.

Albert Einstein proposed mass–energy equivalence in 1905. The equivalence is described by the

famous equation

whereE is energy, m is mass, and c is the speed of light in a vacuum. The formula is

dimensionally consistent and does not depend on any specific system of measurement units.

E = mc2 has sometimes been used as an explanation for the origin of energy in nuclear processes,

but mass–energy equivalence does not explain the origin of such energies. The origin of the

nuclear energy must be in the work done against ‘the strong nuclear force’. Instead, this

relationship merely indicates that the large amounts of energy released in nuclear reactions can

be understood through mass loss. Mass-loss may give released energy from the system.

Binding Energy:

Binding Energy is the energy required to disassemble a whole system into separate constituents.

A bound system typically has a lower potential energy than its constituent parts. This keeps the

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system together- often this means that energy is released upon the creation of a bound state. The

usual convention is that this corresponds to a positive binding energy.

In general, binding energy represents the mechanical work which must be done against the forces

which hold an object together. At the nuclear level, binding energy is also equivalent to the

energy liberated when a nucleus is created from other nucleons. This nuclear binding energy is

derived from the strong nuclear force and is the energy required to disassemble a nucleus into the

same number of free unbound neutrons and protons from which the nucleus is composed.

When the nucleons are grouped together to form a nucleus, they lose a small amount of mass,

i.e., there is mass defect. This mass defect is released as (often radiant) energy according to the

relation E = mc2; thus

binding energy = mass defect × c2.

Nuclear Fission:

We are aware that protons and neutrons are nuclear particles. Each proton has positive charge

and its magnitude is exactly equal to the magnitude of negative charge on electron. We know

that like charges repel each other. Neutrons are electrically neutral. Protons remain in small

region of nucleus because of strong nuclear force. This force is short range force; it is attractive

and approximately thousand times larger than electrostatic force. The attraction due to nuclear

strong force supersedes the electrostatic repulsion. This is one of the explanations of protons in

nucleus forming a stable system along with neutrons.

The mass of each nucleus was expected to be sum of the mass of all protons plus the mass of all

neutrons. This was relevant to law of conservation of mass. Actually the mass of each nucleus

was detected less than that calculated by sum formula. We have already seen ‘the mass energy

equivalence’. The protons and neutrons are bound by strong forces and some energy is removed

during the nucleus forming process. The mass corresponding to this energy (m = E/c2) appears as

less mass than expected. This is known as the ‘mass defect’.

As the atomic number of the nucleus goes on increasing the number of protons and the number

of neutrons goes on increasing. However the rate of growth of neutrons is more than rate of

growth of protons. Therefore the number of neutrons is more than protons in nucleus. As the

nucleus becomes very large in size like ,the nuclear strong force starts becoming

lesseffective. The nucleus becomes unstable and becomes susceptible to fission. It spontaneously

disintegrates and emits one or more of α, β& γ radiations. It also emits energy along with

radiations and comes to stable state.

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Figure 1 :Uranium Sample

If such nuclei are bombarded by slow neutrons then first the neutron is absorbed. The nucleus

becomes more unstable and breaks in to fragments. This is called as the induced fission. Otto

Hahn and Strassman first discovered that breaks it to [

] and [ ] if it bombarded

by slow neutrons. Barium and Krypton atoms produced are called as the daughter nuclei. They

travel in different directions with large speed. Large amount of heat energy is also evolved in this

reaction. The reaction can be stated as:

+

= [ ] + [

] + 3 + Energy. [1]

Nuclear Fission: The splitting of the heavy nucleus into two lighter nuclei together with the

large amount of energy is called as the nuclear fission.

The amount of energy released in each fission event is about 200 MeV.

There are three neutrons released in the reaction.These three neutrons can trigger another three

atoms and produce nine more neutrons. This multiplication can continue and self

generating fission may occur. This process of self generating fission is called as the ‘chain

reaction’.

The chain reaction of is represented by following figure.

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Nuclear fission as a source of energy:

The energy released can be calculated from the masses of fissionable atomss and the masses of

fragments. We have the following data

1 amu= 1.66 x 10-27

kg.

Mass of = 235.043915amu.

Mass of = 1.008665 amu.

Mass of = 140.913900 amu.

Mass of = 91.897300 amu.

From this data mass of constituents of left hand side of equation [1] = 236.052580 amu and that

of all product atoms and particles on right hand side = 235.837195 amu. Therefore it is clear that

the mass of the products is less than the mass of the constituents.

Mass difference = 236.052580 – 235.837195

= 0.215385 amu.

Any mass can be converted in to equivalent energy using Einstein’s equation E = mc2. The

energy equivalent of 1a.m.u. is 931MeV.

Therefore the energy released in above fission reaction E is

E = mass difference x 931 MeV

= 0.215384 x 931 MeV

= 200.5 MeV.

This is the amount of energy released when one atom of undergoes fission.

This energy is released in the form of heat.

The average life time of slow neutrons is 10-4

seconds and that of faster ones is 10-7

sec. It means

in one second, 104 to 10

7 times uranium atoms will undergo fission. The number of breaking

atoms (fragmentations) will increase in geometric progression. If we assume minimum number

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offragmentations 104 per sec, then [10

4]3

atoms will undergo fragmentation in one sec. It means

1012

atoms willfragment to produce approximately 1012

x 200 MeV = 2.00 x 1018

eV. = 0.32 J of

heat energy in one second. In the next second another 1036

fragmentations will produce 0.32 x

1036

J of heat energy. In third second 10108

fragmentations will take place producing 0.32 x 10108

J of energy. This is essentially the uncontrolled chain reaction which leads to the nuclear

explosion.

This reaction can be controlled by controlling the number of fissionable atoms in the sample. It is

also controlled by using the moderators like heavy water and cadmium rods. The system in

which the nuclear reaction is controlled and executed is called as the ‘the nuclear reactor’. The

evolved energy is used to produce electricity. The energy released in atomic reactor is used to

heat the water and make steam. Further the steam is superheated to run the steam turbine and

generate electricity. In India we have such atomic power plants at Trombay, Tarapur,

Kalpakkam, Kota etc. Still India produces only about 1.5% of its power generation by atomic

energy.

Radioactive transformation:

A radioactive nucleus emits α and β radiations and becomes the nucleus of entirely new element.

This is also called as the ‘nuclear disintegration’. The laws of radioactive decay are stated as

follows

I) When α particle is emitted, the mass number of radioactive atom decreases by 4 and

atomic number decreases by 2.

→

+

II) When β particle is emitted, the mass number of radioactive atom remains the same and

atomic number increases by 1.

→

+

III) When γ rays are emitted there is no change in the mass number and atomic number of

the element. The nucleus of the exited atom comes to lower state of energy by emitting the γ

radiations and becomes more stable.

Test your understanding:

We have seen that the fission of one atom of U-235 generates 202.5MeV. You are aware that

1eV = 1.6 × 10−19

J. This energy is liberated in the form of heat.Suppose you have 1 kg of

material. Each atom undergoes alpha decay once only. All energy liberated in the form of

heat is used to heat the water for taking bath. Calculate how many people can take bath by hot

water using this liberated nuclear energy. Assume that, a normal person consumes 10 liters of hot

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water which is 300 C above the room temperature for his bath. (One mole contains Avegadro

number of atoms and 1 cal = 4.18 J.)

[Answer:

One atom emits 202.5 MeV = 202.5 X 106 X 1.6 X

-19 = 3.24 X 10

-11 J

Therefore in one mole will emit = 6.023 X 1023

X 3.24 X 10-11

J = 19.5 X10-12

J

Energy liberated in Alpha decay by 1 kg of U235

= Energy liberated per mole X number of moles

Energy liberated in Alpha decay by 1 kg of U235

= 19.54 (1000 /235) X 1012

= 83.15X 1012

J.

= (83.15 / 4.18) X 1012

= 19.90 X1012

cal.

A normal person consumes 10 lt.of hot water for taking bath, which is normally at 600C. If

water is heated from 300C to 60

0C, then

Amount of heat required for heating =10000 X 30 = 300000 = 3 X 105 cal.

Therefore with the above heat (19.90 X 1012

/ 3 X 105) = 6.63 X 10

7persons.

This means with this much amount of energy emitted only in Alpha decay, a huge population of

6.63 Cr can take the bath by hot water.

This gives better quantitative understanding about hugeness of amount of energy. ]

Laws of radioactive decay, half life time:

Rutherford and Soddy discovered that the rate at which radioactive substance disintegrates and

emits the radiations is independent of physical conditions like temperature, pressure etc. They

also proved that rate of radiation is independent of chemical condition. The rate of emission

remains same for that radioactive element when it is present in any compound. They also

detected that the substance reduces exponentially in its quantity. The law is stated as follows.

Law of radioactive decay: The rate at which number of atoms radiates at any instant is directly

proportional to the number of atoms present at that instant.

It clearly means the number of radioactive substance goes on reducing with time. The amount of

radioactive substance goes on reducing with time. The substance becomes half of its quantity

after some time.

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Half life time (T1/2): The half life time of any radioactive substance is defined as the time

required reducing that substance to half of its original quantity. It is characteristics of that

substance and it is denoted by λ.

Nuclear Fusion:

We have seen that heavy atoms are unstable. They disintegrate and emit radiations. They break

to from stable product nuclei. In another natural phenomenon very light atoms like hydrogen and

its isotopes combine to form more stable elements. This natural phenomenon is called as the

fusion.

Fusion: The process of combining lighter atoms to form stable nuclear atom by emitting energy

is called as the nuclear fusion.

We all know that the life on earth is due to energy that we receive from Sun. Sun is the huge ball

constituted of about 71% of hydrogen, 27.1% of helium. There are other elements found and they

are in rest of about 2%. These elements are oxygen, carbon, nitrogen, silicon, magnesium,

neon,ironand sulphur. We note here that the compositions given in reference books and on

internet are different and list of elements also is different. We will go with Wikipedia as the most

authentic source of information.

The spectroscopic measurements state that the surface temperature of the Sun is nearly 5800 K

while the estimated temperature of the core of the sun is about millions of Kelvin. The reaction

on the surface of the Sun is the fusion reaction of hydrogen to helium.

6C14

–Isotope and carbon dating:

Cosmic rays essentially contain showers of high energy charged particles as well as high energy

neutrons. Most of the charged particles are absorbed in the ionosphere and they increase the

thickness and density of the ionosphere. This is due to earth’s magnetic field. Few charged

particles reach the atmosphere penetrating the entire atmosphere.

There are high energy neutrons also in the cosmic rays. These neutrons are absorbed in the upper

layers of atmosphere. These neutrons combine with atmospheric nitrogen and form new isotope

of carbon. The reaction is given by

0n1 + 7N

14→ 1H

1(proton) + 6C

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6C14

isotope of carbon is radioactive. It is reacts with the atmospheric oxygen to form carbon

dioxide molecules. The percentage of radioactive carbon dioxide in atmosphere remains almost

constant. This is because radioactive carbon dioxide decays by emitting β particles to form again

nitrogen. The reaction is stated as

6C14

→ 7N14

+ 0e-1

(beta particle)

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Green plants consume carbon dioxide during photosynthesis. Hence plants absorb this

radioactive carbon along with normal non radioactive carbon dioxide. All living animals eat

plants and they grow. Hence each living thing possesses a very small percentage of radioactive

carbon in them. Radioactive carbon decays to convert itself in nitrogen in living animals also.

Hence the percentage of radioactive carbon is constant till the animal is living. The amount of

radioactive goes on reducing when the animal dies. Hence by measuring the rate of beta particle

emission, date at which animal died, can be calculated.This process is known as the carbon

dating.

Half life period of 6C14

carbon is extremely large, 5730 years. Hence carbon dating is extremely

useful in archeology, in deciding the age of the dugout samples. So also it is useful in deciding

the age of bones and teeth of the buried human and animalbodies.

Uses of Radioactivity:

1 )Uses in medicine:

The affected part of the patient’s body is exposed to radioactive source. Generally γ radiations

are used. The deceased tissue dies and helps to control the decease. This therapy is called as

‘radiation therapy’. Radiation therapy is used in cancer, tumor, leukemia etc.

Some radioactive salts like radioiodine, radioactive NaCl etc are used is detecting decease

affected areas.

2) Scientific uses:

To study age of rocks, planets and other ancient objects.

To study growth of plants.

To study nature of nuclear forces.

Concept:

Thermionic emission: We have seen that there are free electrons inside the metal when we

learnt the current electricity. These electrons are mobile within the metal as a result of ‘metallic

bond’ which is characteristic of metals. It is customarily said that they form an electron gas

inside the metal. Therefore each electron is said to possess certain average kinetic energy. This

kinetic energy is a function of absolute temperature (T) of the metal. It is assumed that the

Thermionic emission, Cathode ray tube, Cathode rays and their properties, Deflection

of cathode rays by electric and magnetic field, X-rays, their properties and uses,

Radiation hazards.

hazards.

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electrons do not come outside the metal on its own at room temperature. This is because metal as

a whole is neutral and it tries to remain neutral. It will remain with one ‘e’ positive charge if an

electron leaves the piece of metal. This provides binding on the electron to remain within the

metal. In terms of kinetic energy it is said that the average kinetic energy of an electron is

insufficient to cross the surface of the metal.

Truly speaking, each atom gives one or more electrons to form a metallic bond and it becomes an

ion. The average positions of these ions remain fixed. They are said to vibrate about their mean

positions. They possess some average kinetic energy. The electrons which are free also possess

the average kinetic energy. As the temperature of the metal goes on increasing the average

kinetic energy of ions as well as electrons goes on increasing. When theaverage kinetic energy of

an electron increases beyond certain limit, the electron comes out of the metal surface. As the

electron comes out due to thermal energy, it is known as the ‘thermion’.

Thermionic emission: The phenomenon of emission of electrons from the surface of a metal due

to increase in temperature is called as the ‘thermionic emission’.

Work function: The minimum energy required for an electron to ooze out from the metal

surface is known as the work function of that metal. It is conventionally denoted by ‘Ф’.

The work function is the characteristics of each metal. Therefore the work function is different

for different metals.

The work function is normally measured in ‘eV’, which is the unit of energy.

One eV:The energy acquired by the electron when it accelerates through one volt is called as one

eV energy.

Rate of emission of thermions:

It is experimentally found that the rate of emission of thermions depends on following three

factors:

I) Nature of the metal surface: The rate of thermions is inversely proportional to work

function of the metal. i.e. If the work function of the metal is low then the rate of

emission is high and if the work function of the metal is high then the rate of

emission is low.

II) Temperature of the metal: The rate of emission is directly proportional to the

temperature of the metal. The rate of emission of thermions increases with increase

in temperature.

III) The surface area of the metal: The rate of emission of thermions is directly

proportional to surface area of the metal, i.e. larger the surface area greater is the

rate of emission.

Characteristics of good electron emitter:

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I) A metal is a good emitter if its melting point is high. The metal does not melt quickly

on heating.

II) A metal becomes good emitter when the work function is low.

The melting point of tungsten (W) is 3655 K. Tungsten is normally used for making filaments.

The work function of tungsten is 4.6 eV. Thermions start emitting from 2500 K.

Thoriated tungsten filament is the tungsten filament coated with Thorium dioxide. The work

function of such filament is 2.6 eV. The filament starts emitting electrons from 2000 K onwards.

If tungsten is coated with alkali metal oxides then electron emission is at fairly low temperatures.

The work functions of such compositions are in the range 1 eV to 1.5 eV and the electron

emission starts at about 1000 K.

Application: Thoriated tungsten filaments are used for obtaining electron beam in Cathode ray

tubes. CRTs are extensively used in Cathode ray oscilloscopes, TVs, computer monitors and

monitors in medical application.

Thotiated tungsten filaments were used in incandescent lamps. The use of such lamps is reduced

as the their efficiency is low. Still they are used in automobile lamps.

Cathode ray tube, Cathode rays and properties, Deflection of cathode rays electric and

magnetic field:

The modern atomic theory is also nearly 100 years old. J.J. Thompson, an English chemist

discovered a particle 1836 times lighter than the Hydrogen atom. It was named as the ‘Electron.’

He developed an instrument called as ‘Cathode Ray Tube’ (CRT). The schematic diagram of the

CR tube is as shown below:

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In construction, the CR tube consists of a thick glass tube. The shape of the tube is further

modified according to the application. It is evacuated to a high degree. It mainly consists of

electron gun.

The typical electron gun

Each electron gun consists of directly or indirectly heated cathode.The filament of the electron

gun is heated with dc or ac source as per rating specified in the design. Thermions are emitted.

The rate of emission of electrons is monitored by the temperature of the filament, i.e. it is

controlled by the filament voltage. These electrons are accelerated toward positive electrodes.

Generally there are two positive electrodes. Positive electrodes are cylindrical in shape. There

exists small hole on front side. The two cylinders are given different positive potentials. It forms

the electrostatic lens. The electron gun forms a sharp beam of electrons which focuses on the

screen after passing through the electrostatic or magnetic horizontal and vertical deflection

system. Such beam of electrons coming out of the electron gun is called as ‘cathode rays’.

The deflection system:

The beam further passes through the horizontal and vertical deflection systems. These systems

are either electrostatic or magnetic or a combination of both. The electrostatic deflecting system

consists of two sets ofparallel plates; one for horizontal deflection and other for vertical

deflection. When potential difference (p.d.) is applied to parallel plates, the electron beam

deflects towards the positive plate. The amount of deflection is proportional to applied electric

field i.e. the applied p.d. Generally one plate is connected to ground and signal potential [output

of horizontal (H) and vertical (V) amplifiers] is given to other plate in each system. The gains of

H and V amplifiers are adjusted such that the beam of electrons scans entire screen of the picture

tube. The output of H amplifier is saw tooth voltage. This takes the electron beam from left to

right and it flies back to left in short time. This is called as “the time base”. The signal is taken

from vertical amplifier and applied to V deflecting plate simultaneously. The two signals ‘write’

the picture on the screen.The electrostatic deflecting system is shown in the diagram of CRT.

(For derivation, see Resnik-Halliday 6th

edition page 664./ any other standard reference book.)

The magnetic deflection system consists of coils. The output of H and V amplifier send the

current in two different coils. The magnetic field created due to signal current in the coil deflects

the electron beam in proportion with the current. The gains of the amplifiers are adjusted to scan

the entire screen. The magnetic deflection system is not shown in above diagram.

The electron beam is further made incident on the screen. The screen is coated by fluorescent or

phosphorescent material according to the demand of the application. Normally they are greenish

or bluish in colour.

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Monitor of your computer, Picture tube of T.V. in your home, various monitors in speciality

hospitals, CRO- cathode ray oscilloscopes in modern laboratories essentially contain the CR

tubes. They are extensively used in today’s modern lifestyle.

Properties of the Cathode Rays:

1. Cathode rays are negatively charged particles. They are the stream of electrons.

2. The energy of the electron is decided by the positive potential on the anode.

3. Cathode rays produce heat when they are retarded in metallic anode.

4. They are charged particles, therefore they can be deflected by electric and magnetic field.

5. They are energetic particles therefore they produce fluorescence and phosphorescence

when they strike certain materials like ZnS, Al2S3, soda glass, quartz, etc.

6. Cathode rays produce X-rays when they are stopped in metals of high density, e.g.

Tungston, Copper etc.

7. Cathode rays emerge normally from the cathode-surface and are not affected by the

position of the anode.Cathode rays travel in straight line and form shadows.

8. Cathode rays are particles having mass; they carry momentum along with them.Cathode

rays exert mechanical pressure.

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9. Cathode rays can produce physical and chemical changes: The cathode rays affect the

photographic plate and turn the colour of lithium chloride into violet.

10. Cathode rays can ionize gases : Cathode rays, on colliding with the atoms of gases, eject

electrons from them.

11. Cathode rays can penetrate through metal-foils : Cathode rays penetrate through light

metals. If an aluminium foil is placed normally in the path of cathode rays, the rays are

found on the other side of the foil.

Background Radiations:

There are some sources of radiations which continuously emit radiations. Further it is not

possible to control them. These radiations are fairly small in amount therefore generally we do

not take precautions also. Whenever we count the radiations from a source or from certain

affected body / object, these radiations also exist. We subtract the ‘background count’ in

precision measurements. Otherwise these radiations are ignored. These radiations are called as

the background radiations.

We ignore the hazards of background radiations because they are too small in amount and it is

impracticable to take corrective measures all the time.

The background radiations are due to

i) Cosmic rays reaching the earth constantly.

ii) Radioactive substances present around like radium, carbon-14, potassium-40,

polonium etc.

iii) Radioactive rocks present in the Earth’s crust.

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X-Rays:

On December 22 1895, Roentgen "photographed" his wife's hand,

revealing the unmistakable image of her skeleton, complete with

wedding ring. Roentgen's wife had placed her hand in the path of

X-rays. Roentgen's discovery of these "mysterious" rays, capable of

producing an image on a photographic plate excited scientists of his

days.

Production of X-rays:

Roentgen discovered X-rays while working with crookes tube.

Later on Coolige tubes were discovered for the production of X-

rays. It was known to Roentgen that when high energetic electrons

are stooped in dense target like tungsten or copper, X-rays are produced. It also produces large

amount of heat, hence tubes with water cooling systems for target were developed. A typical

tube is as shown below.

X-ray tube:

C is the filament, generally made up of tungsten. Uh is the filament supply. ‘A’ is thick copper or

tungsten target. It works as the anode. Ua is the high voltage dc source ranging up to 100 kV. Win

and Wout are the inlet and outlet for the water to cool the target. Water removes heat from the

target and increases the life of the target i.e. the life of the X-ray tube. The whole assembly is

fitted in a highly evacuated thick glass tube.

When filament heats, it emits electrons.Anode is positive and it is given high voltage. The

electrons emitted from the cathode are subjected to high electric field. They are accelerated to

high value. They hit the target and lose their K. E. to the target. As they are continuously slowed

down, they emit continuous X-rays. They knock out electrons from innermost K or M shell of

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target atoms. When electrons from outer shells take their position, they emit characteristic X-rays

of the target atoms. Rest of the energy of incident electron is converted in to heat energy. Heat is

taken away by water else the target melts or ‘burns’ out.

X-rays is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to

10 nanometers, corresponding to frequencies in the range from 3 × 1016

Hz to 3 × 1019

Hz and

energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and

longer than gamma rays.

Properties of X-rays:

i) X-rays are e. m. waves therefore they can be reflected, refracted, diffracted,

interferred and polarized.

ii) Their wavelengths are in the range of 0.01 nm to 10 nm and frequencies are in the

range of 3 × 1016

Hz to 3 × 1019

Hz.

iii) They are e. m. waves they travel with the speed of light.

iv) They are high energy waves, therefore they affect photographic plate, and they

produce fluorescence and phosphorescence.

v) They penetrate through materials, papers, tissues etc.

vi) They are stopped by the metals, bones and teeth. Hence shadows of bones and teeth

can be taken by X-rays.

Uses of X-rays:

Medical uses:i) to photographs the fractured bones ii) tooth photography iii) tomography

of tissues iv) radiotherapy and there are many other medical uses of X-rays. X-ray crystallography in which the pattern produced by the diffractionof X-rays through the

closely spaced lattice of atoms in a crystal is recorded and then analysed to reveal the nature of that lattice. A related technique, fiber diffraction, was used by Rosalind Franklin to discover the double helical structure of DNA.[46]

X-ray astronomy, which is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects.

X-ray microscopic analysis, which uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.

X-ray fluorescence, a technique in which X-rays are generated within a specimen and detected. The outgoing energy of the X-ray can be used to identify the composition of the sample.

Industrial radiography uses X-rays for inspection of industrial parts, particularly welds. Airport security luggage scanners use X-rays for inspecting the interior of luggage for security

threats before loading on aircraft. Border security truck scanners use X-rays for inspecting the interior of trucks for at country

borders. X-ray fine art photography X-ray photoelectron spectroscopy is a chemical analysis technique relying on the photoelectric

effect, usually employed in surface science.

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Radiation Hazards:

Nuclear radiations are energetic particles. They impart energy when they are incident on the

body. Especially ‘γ rays’ possess high energy. They may knock out electrons from organic

molecules. If a certain person remains exposed to energetic radiations for long time then

molecules may break up or their normal behavior will be hampered. Sometimes the death of that

organism is also possible.

There are two main types of damages.

1. Pathological damages:If certain organ is exposed to high dose of radiations then that

organ stops its normal functioning and it becomes dead. Such damage is called as the

pathological damage.

2. Genetic damage: The high energy radiations may affect the amino acids at the gene

sites. Such damage will not be seen immediately. It will show its effects in next

generations. Such damages are called as the genetic damages.

It clearly gives the message that we must take proper precautions while handling the

radioactive materials and working in system having radioactive materials.

Safety precautions:

1. Nuclear reactors should be embedded in thick concrete walls. This is to prevent from the

radioactive gases and neutrons.

2. Nuclear material must be kept in thick lead containers with proper lead covered windows.

3. Nuclear material should be handled with mechanical tongs.

4. Workers in nuclear establishments must wear lead lined aprons, gloves and they should

wear special lead glasses for protection.

5. Workers must wear special film badges. These badges are also exposed to the radiations

along with the persons. These badges indicate the amount of dose received so that proper

care can be taken.

6. Periodic and compulsory medical checkup is essential to the persons working in the

nuclear establishments.

References:

1) http://www.accessexcellence.org/AE/AEC/CC/radioactivity.php

2) Wikipedia