Question

Properties of the tungsten that make it a good material for making the anode in an X-ray tube are:
(A). High vapor pressure at high temperature
(B). High atomic number
(C). High melting point
(D). Good thermal conductivity
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

Tungsten ($Z=74$) is the ideal anode material because it combines High Z (efficient X-rays), High Melting Point (heat resistance), Good Thermal Conductivity (heat dissipation), and Low Vapor Pressure (prevents tube damage).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
In an X-ray tube, fast-moving electrons collide with the target on the anode to produce X-rays.
However, more than 99% of the kinetic energy of these electrons is converted into heat, and less than 1% is converted into X-rays.
Therefore, the anode target must be made of a material that can withstand extreme heat while efficiently producing X-rays.

Step 2: Detailed Explanation:
Let us evaluate the physical properties of tungsten (W, atomic number 74):
- (B) High atomic number ($Z = 74$): X-ray production efficiency is directly proportional to the atomic number of the target material. Tungsten's high Z value ensures efficient production of Bremsstrahlung X-rays. (Correct)
- (C) High melting point ($3422^\circ\text{C}$): The massive heat generated at the focal spot can easily melt common metals. Tungsten's high melting point prevents it from melting under electron bombardment. (Correct)
- (D) Good thermal conductivity ($174\text{ W/m}\cdot\text{K}$): It dissipates the intense localized heat away from the focal spot quickly to prevent damage to the target. (Correct)
- (A) High vapor pressure: This is incorrect. The target must have a low vapor pressure at high temperatures. If it had a high vapor pressure, the tungsten would vaporize easily, depositing a metallic film on the glass envelope, which would cause electrical arcing and tube failure.
Therefore, the correct properties are (B), (C), and (D).

Step 3: Final Answer:
The correct properties are represented by option (D).