THERMIONIC EMISSION AND RADIOACTIVITY

1. THERMIONIC EMISSION AND
RADIOACTIVITY
BY:-
VIPUL SHARMA 36
SHIV GHOSH 13
ABHINAV SAGAR 37

2. THERMIONIC
EMISSION

3. THERMIONIC EMISSION
Thermionic emission is the heat-induced flow of charge
carriers from a surface or over a potential-energy barrier. This
occurs because the thermal energy given to the carrier
overcomes the binding potential, also known as work function
of the metal. The charge carriers can be electrons or ions, and
in older literature are sometimes referred to as "thermions".
After emission, a charge will initially be left behind in the
emitting region that is equal in magnitude and opposite in
sign to the total charge emitted. But if the emitter is
connected to a battery, then this charge left behind will be
neutralized by charge supplied by the battery, as the emitted
charge carriers move away from the emitter, and finally the
emitter will be in the same state as it was before emission.
The thermionic emission of electrons is also known as thermal
electron emission.

4. EDISON’S EXPERIMENT
Thomas Edison on February 13, 1880, while trying to discover the
reason for breakage of lamp filaments and uneven blackening of the
bulbs in his incandescent lamp built several experiment bulbs, some
with an extra wire, a metal plate, or foil inside the bulb which was
electrically separate from the filament, and thus could serve as an
electrode. He connected a galvanometer to the output of the extra
metal electrode. When the foil was charged negatively relative to the
filament, no charge flowed between the filament and the foil. In
addition, charge did not flow from the foil to the filament because the
foil was not heated enough to emit charge. However, when the foil was
given a more positive charge than the filament, negative charge could
flow from the filament through the vacuum to the foil. This one-way
current was called the Edison effect. He found that the current emitted
by the hot filament increased rapidly with increasing voltage, and filed
a patent application for a voltage-regulating device using the effect on
November 15.

5. EDISON’S EXPERIMENT

6. FACTORS
AFFECTING
THE RATE OF
THERMIONIC
EMISSION

7. i. Nature of the metal surface
Lower the work function of the metal, greater is
the rate of emission of electrons from the surface.
ii. Temperature of the surface
Higher is the temperature, more will be the rate
of emission as the electrons will have more
kinetic energy to leave the surface.
iii.Surface area of the metal
Larger the surface area of the metal, more is the
rate of emission as thermionic emission to some
extent is like evaporation.

8. CONDITIONS
FOR GOOD
ELECTRON
EMITTER

9. i. Low work function
The work function of the body
should be low so that the
electrons could be emitted even
when the substance is not heated
to a high temperature.
ii. High melting point
The melting point of the substance
should be low so that the metal
does not get melted when heated.

10. CATHODE RAY TUBE
The cathode ray tube is a vacuum tube containing an
electron gun (a source of electrons or electron emitter) and a
fluorescent screen used to view images. It has a means to
accelerate and deflect the electron beam onto the fluorescent
screen to create the images. The images may represent
electrical waveforms (oscilloscope), pictures (television,
computer monitor), radar targets and others. CRTs have also
been used as memory devices, in which case the visible light
emitted from the fluorescent material is not intended to have
significant meaning to a visual observer (though the visible
pattern on the tube face may cryptically represent the stored
data).

11. CATHODE RAY TUBE

12. PARTS OF A CATHODE
RAY TUBE

13. ELECTRON GUN
• An electron gun (also called electron emitter) is an electrical
component that produces an electron beam that has a precise
kinetic energy and is most often used in television sets and
computer displays that use cathode ray tube (CRT)
technology, as well as in other instruments, such as electron
microscopes and particle accelerator. A direct current,
electrostatic thermionic electron gun is formed from several
parts: a hot cathode, which is heated to create a stream of
electrons via thermionic emission, electrodes generating an
electric field which focus the beam (such as a Wehnelt
cylinder), and one or more anode electrodes which accelerate
and further focus the electrons accelerators.

14. ELECTRON GUN

15. THE DEFLECTING
SYSTEM
•
It is the part of the cathode ray tube that deflects the electron
beam. It consists of two pairs of plates, one is kept horizontal
called the Y- plates and the other vertical called the X-plates.
These are placed with the plane parallel to the beam.
When a potential difference is applied across the plate the
electric field is set up which deflects the electron beam. The
Y-plates deflect the beam in vertical direction while the X-
plates deflect in horizontal direction.
It should be noted that the deflection caused by the magnetic
field is much larger than compared to the same length of
electric field.

17. THE FLUORESCENT
SCREEN
• The end of the tube is made of a screen whose
inside part is coated with zinc sulphide or barium
platinocyanide etc. the electron beam on striking
the screen gives a bright spot.
If a varying potential is applied on the deflecting
plates, the beam deflects according to the
variation of potential on the plates and the spot
traces out a pattern.

18. FLUORESCENT SCREEN

19. USES OF CATHODE RAY
TUBE
• i)To investigate the wave form of the unknown alternating
potential by applying it on the Y-plates and a known
periodic time base potential on the X-plates.
• ii) In determining the frequency of an alternating potential
by applying it on one pair of deflecting plates.
iii) For checking the wave form of an electric signal.
iv) For measuring the short time interval.
v) In television as a picture tube.

20. USES OF CATHODE RAY TUBE

22. RADIOACTIVITY

23. RADIOACTIVITY AS
ALPHA,BETA AND
GAMMA RADIAIONS

25. ALPHA PARTICLES

26. ALPHA PARTICLES
• Alpha Particles-An alpha particle has two protons and two neutrons, so
it has a positive charge. (Since it has two protons it is a helium
nucleus.) It is produced from large nuclei.
• When an atom emits an alpha particle, the atom's mass
number decreases by four due to the loss of the four nucleons in the
alpha particle. The atomic number of the atom goes down by exactly
two, as a result of the loss of two protons – the atom becomes a new
element. Examples of this are when uranium becomes thorium,
or radium becomes radon gas due to alpha decay.
• Alpha particles are commonly emitted by all of the
larger radioactive nuclei such as uranium, thorium, actinium,
and radium, as well as the transuranic elements. Unlike other types of
decay, alpha decay as a process must have a minimum-size atomic
nucleus which can support it. The smallest nuclei which have to date
been found to be capable of alpha emission are the lightest nuclides
of tellurium (element 52), with mass numbers between 106 and 110.

27. ALPHA PARTICLE

28. BETA PARTICLES

29. BETA PARTICLES
• Beta emission is when a high speed electron (negative
charge) leaves the nucleus. Beta emission occurs in
elements with more neutrons than protons, so a neutron
splits into a proton and an electron. The proton stays in the
nucleus and the electron is emitted.
• Beta particles are high-energy, high-speed electrons or
positrons emitted by certain types of radioactive nuclei
such as potassium-40. The beta particles emitted are a
form of ionizing radiation also known as beta rays. The
production of beta particles is termed beta decay. They are
designated by the Greek letter beta (β). There are two
forms of beta decay, β− and β+, which respectively give rise
to the electron and the positron

30. BETA PATICLE

31. GAMMA PARTICLES

32. GAMMA PARTICLES
• Gamma radiation, also known as gamma rays, and denoted by the
Greek letter γ, refers to electromagnetic radiation of high frequency
and therefore high energy per photon. Gamma rays are ionizing
radiation, and are thus biologically hazardous. They are classically
produced by the decay from high energy states of atomic nuclei
(gamma decay), but are also created by other processes. Paul
Villard, a French chemist and physicist, discovered gamma radiation
in 1900, while studying radiation emitted from radium. Villard's
radiation was named "gamma rays" by Ernest Rutherford in 1903.
Natural sources of gamma rays on Earth include gamma decay from
naturally occurring radioisotopes, and secondary radiation from
atmospheric interactions with cosmic ray particles. Rare terrestrial
natural sources produce gamma rays that are not of a nuclear
origin, such as lightning strikes and terrestrial gamma-ray flashes.

33. GAMMA PARTICLE

34. SOME USES OF
RADIOACTIVITY

35. SOME USES OF RADIOACTIVITY

36. DISADVANTAGES OF
RADIOACTIVITY
• If radiation collides with molecules in the air or in your
body, it throws out of them electrons. By throwing out
electrons you produce charged particles called ions.
This means it is the radiation responsible for ionising
molecules.
• If this happens in our body, the cells may die or they
may undergo a change called a mutation. The result is
called radiation sickness. A large dose of radiation will
cause death!
• Small doses of radiation over a long period of time can
cause the cells to multiply. However, these cells are
mutated. Some time later cancer may occur.