Thermionic emission is the

thermally 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 work function of the

material.

THERMIONIC EMMISION AND RADIOACTIVITY

THERMIONIC EMMISION:- 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.

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.

FACTORS AFFECTING RATE OF EMMISION:-

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.

CONDITIONS FOR GOOD ELECTRON EMITTER:-

 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.

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).

CATHODE RAY TUBE:-

The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns, and a phosphorescent screen used to view images. It has a means to accelerate and deflect the electron beam(s) onto the screen to create the images.

Radioactivity:-▪ Radioactivity also known as radioactive decay is a

process in which unstable atomic nucleus loses energy by emitting radiation.

▪ Radiation is released in form of particles or electromagnetic waves.

▪ The rate of radioactive decay is described in half-lives.▪ The main source of radiation in Earth is sun. Radiation is

going through everywhere and all the time.

Pioneers in RadioactivityRoentgen:Discoverer of X-rays

1895

Becquerel:Discoverer

of Radioactivit

y 1896

The Curies:Discoverers of Radium

and Polonium

1900-1908

Rutherford:Discoverer Alpha and Beta rays

1897

Type of Radioactivity:-By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1)Alpha particles (a)

2)Beta particles (b)

3)Gamma-rays (g)

Alpha Particles (a)

Radium

R226

88 protons138 neutrons

Radon

Rn222

This is theatomic weight, whichis the number ofprotons plus neutrons

86 protons136 neutrons

+ n npp

a (4He)2 protons2 neutrons

The alpha-particle (a) is a Helium nucleus. It’s the same as the element Helium, with the electrons

stripped off ! A sheet of paper or human skin can stop Alpha Particles. These are only hazardous to human health if they are

inhaled.

Beta Particles (b)

CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+ e-

electron(beta-particle)

We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred:

n p + e ( + n )

Beta Particles (b)

They can travel a few feet in air but can usually be stopped by clothing or a few centimeters of wood.

They are considered hazardous mainly if ingested or inhaled, but can cause radiation damage to the skin if the exposure is large enough.

Unstable Neutron decays into a proton.

Gamma particles (g)

In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

A gamma is a high energy light particle. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

NeonNe20

10 protons10 neutrons

(in excited state)

10 protons10 neutrons

(lowest energy state)

+

gamma

NeonNe20

Gamma particles (g) Occurs when an unstable nucleus emits electromagnetic

radiation. The radiation has no mass, and so its emission does not change the element.

They penetrate matter easily and are best stopped by

water or thick layers of lead or concrete. Gamma radiation is hazardous to people inside and

outside of the body. Gamma rays have the lowest ionizing power, but the

highest penetrating power.

Uses of radiation

• Radioactive tracers are used to locate tumors, to study the functioning of a particular organ, or to monitor the flow of blood. Such as iodine-131 is used for thyroid problems.

• used to treat cancer may involve the use of implanted radioactive isotopes such as gold-198 or iridium-192.

• Radioactive particles often used to measure the thickness of metal goods.

• Radio isotopes often used for smoke alarms.• Nuclear fission is used to generate Electricity.• The age of fossil or rock or anything can be determined by

radio isotopes suck as carbon-14. though it can measure only 50,000years.

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