TOPIC 4: THERMIONIC EMISSION

Thermionic Emission

Thermionic emission is the discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes (e.g., television picture tubes) in electronics and communications. The phenomenon was first observed in 1883 by Thomas A. Edison as a passage of electricity from a filament to a metal plate inside an incandescent lamp. The classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals becomes significant only at temperatures over 1000 K. The science dealing with this phenomenon has been known as “thermionics,” but this term is gradually falling into disuse.

Cathode Rays

Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes. Electrons were first discovered as the constituents of cathode rays. In 1897, British physicist J. J. Thomson showed the rays were composed of a previously unknown negatively charged particle, later named the electron. Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to create images in classic television sets.

The Production of Cathode Rays

Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they must first be detached from the atoms of the cathode.

Modern vacuum tubes use thermionic emission, in which the cathode is a thin wire filament heated by a separate electric current passing through it. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the filament’s surface into the evacuated space of the tube.

Since electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low-mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall, exciting the atoms and causing them to emit light, a glow called fluorescence.

Researchers noticed that objects placed in the tube in front of the cathode could cast shadows on the glowing wall, realizing that something must be traveling in straight lines from the cathode.

After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube. The current in a beam of cathode rays can be controlled by passing it through a metal screen of wires (a grid) to which a small voltage is applied.

The electric field of the wires deflects some electrons, preventing them from reaching the anode. Thus, a small voltage on the grid can control a much larger voltage on the anode. This principle is used in vacuum tubes to amplify electrical signals.

High-speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets). These are used in cathode ray tubes found in televisions, computer monitors, and electron microscopes.

The Properties of Cathode Rays

Properties of cathode rays include:

1. Cathode rays travel in straight lines, which is why they cast shadows of any solid object placed in their path. Their path is not affected by the position of the anode.
2. Cathode rays consist of matter particles and possess energy by virtue of their mass and velocity. They can set a paddle wheel into motion when placed in their path.
3. Cathode rays consist of negatively charged particles. When subjected to an electric field, they deflect toward the positively charged plate (anode). They also deflect in strong magnetic fields.
4. Cathode rays heat only the objects on which they fall. The particles possess kinetic energy, which transfers to the object upon impact, causing a temperature rise.
5. Cathode rays cause green fluorescence on glass surfaces where they strike.
6. Cathode rays can penetrate thin metallic sheets.
7. Cathode rays ionize gases through which they travel.
8. When falling on certain metals such as copper, cathode rays produce X-rays. X-rays are not deflected by electric or magnetic fields and can pass through opaque materials like black paper but are stopped by solid objects such as bones.
9. Cathode rays travel at speeds nearly equal to that of light.

The Application of Cathode Ray Tube

Applications of cathode ray tubes include:

Televisions

Before LCD or Plasma televisions, CRTs were used to create moving images. For black and white televisions, this worked well, as controlling the brightness of the beam was sufficient.

A CRT TV works by having the electron beam “scan” the screen faster than our eyes can perceive. It shoots across the screen like a machine gun, and the images we see are made from many fluorescent dots.

The fluorescence caused by the beam striking the screen lasts slightly longer so the next scan can occur without the previous image disappearing. It scans twice each time, first filling in the odd “holes” then the even ones. Each scan takes about 1/50 of a second.

Colour CRT TVs have three electron guns rather than one, a shadow mask, and a modified fluorescent screen. The three electron guns correspond to the three primary colours (Red, Green, and Blue) that can be adjusted in different amounts to create any colour.

The colours form due to the shadow mask, a layer with holes that controls the angle of incoming electron beams. The fluorescent screen is separated into multi-coloured phosphors placed adjacent to each other at small intervals.

Thus, it is not a single coloured pixel but rather three very small pixels that join to form a larger dot.

Cathode Ray Oscilloscopes

A Cathode Ray Oscilloscope (CRO) is a diagnostic device that allows one to “see” voltage. It is essentially a Cathode Ray Tube with two perpendicular sets of deflecting electric plates. The vertical set receives the input voltage for display.

The horizontal set is connected to a “sweep generator,” providing a constant, adjustable time base for sweeping. It creates a “sawtooth voltage,” causing the image to be animated and measured with a linear scale.

X-Rays

The Structure and Mode of Action of the X-ray Tube

X-radiation (composed of X-rays) is a form of electromagnetic radiation. Electromagnetic radiation (EM radiation or EMR) is radiant energy released by certain electromagnetic processes. Visible light is one type of electromagnetic radiation; other familiar forms include invisible radiations such as X-rays and radio waves.

Most X-rays have wavelengths ranging from 0.01 to 10 nanometers, corresponding to frequencies from 30 petahertz to 30 exahertz (3×10¹⁶ Hz to 3×10¹⁹ Hz) and energies from 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays.

In many languages, X-radiation is referred to as Röntgen radiation, after Wilhelm Röntgen, who is credited as its discoverer and named it X-radiation to signify an unknown type of radiation.

X-ray Tube

The X-ray tube consists of an emitter (either a filament or a cathode), which emits electrons into a vacuum, and an anode to accelerate the electrons, establishing a flow of electrons through the tube.

The cathode is a filament that emits electrons when heated. The anode is made of copper and carries the target.

A high potential difference between the anode and cathode is maintained by an external high-voltage source. A battery supplying high current heats the cathode filament, often made of tungsten. The cathode is coiled to provide high resistance to the current.

Production of X-rays

Electrons from the filament experience the potential difference and accelerate toward the anode. When they hit the anode, they are stopped and transfer their energy to the electrons of the anode material, producing X-rays.

Only a small percentage of their energy converts to X-rays; the rest transforms into heat.

Difference between Soft and Hard X-rays and their Production

X-rays may be classified as hard or soft depending on their wavelengths, which give rise to different properties.

Differences between Hard and Soft X-rays

The Properties of X-rays

Properties of X-rays include:

1. They travel in straight lines.
2. They readily penetrate matter.
3. They are not affected by electric or magnetic fields (they have no charge).
4. They cause fluorescence in certain substances.
5. They can be detected by photographic emulsion.
6. They ionize gases, causing the gases to conduct electricity.

The Application of X-Rays in Daily Life

The following are some uses of X-rays:

1. In the medical field
2. Crystallography
3. Astronomy
4. X-ray microscopic analysis
5. X-ray fluorescence
6. Security installations
7. Industries