Source of Electron Emission in a Cathode Ray Tube (CRT)

A Cathode Ray Tube (CRT) is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images.

The fundamental part of a CRT that creates the image is the electron beam. For an electron beam to be formed, there must be a source that emits electrons.

Identifying the Electron Source in a CRT

Let's look at the options provided to understand which one is the source of electron emission:

• PN junction diode: A PN junction diode is a semiconductor device used for rectification or switching in electronic circuits. It is not the source of electron emission in a CRT.
• A barium and strontium oxide coated cathode: This describes a typical cathode used in vacuum tubes, including CRTs. This cathode is heated (thermionic emission), and the coating of barium and strontium oxides significantly lowers the work function of the material, making it easier for electrons to be emitted when heated. This is the primary source of electrons in a CRT electron gun.
• Post-accelerating anodes: These anodes are located after the deflection system in a CRT and are used to give the electron beam a final boost in energy before it hits the screen. They accelerate electrons but do not emit them.
• Accelerating anodes: These anodes are part of the electron gun assembly and are used to accelerate the electrons emitted from the cathode towards the screen. Like post-accelerating anodes, they accelerate but do not emit electrons.

How the Electron Source Works in a CRT

In a CRT, the electron gun assembly includes a heater, a cathode, and several anodes. The heater heats the cathode, causing it to emit electrons. This process is called thermionic emission. The cathode material is specifically chosen and often coated with materials like barium and strontium oxides to enhance this emission at reasonable temperatures. The emitted electrons are then shaped into a beam and accelerated by the various anodes before being directed towards the screen by deflection systems.

Therefore, the component responsible for initially emitting the electrons in a Cathode Ray Tube is the specially coated cathode.

Based on the function of each part of the electron gun and the CRT, the source of emission of electrons is the specifically prepared cathode.