What should be the diameter of tungsten filament used as cathode:
**Core Concept**
The tungsten filament in an X-ray tube serves as the cathode, emitting electrons when heated. The diameter of the filament is crucial in determining the electron emission characteristics and, consequently, the X-ray production efficiency.
**Why the Correct Answer is Right**
The correct diameter of the tungsten filament used as a cathode is typically around 0.1-0.2 mm. This small diameter is necessary to achieve high electron emission velocities, which are required for effective X-ray production. A smaller diameter filament has a higher surface area-to-volume ratio, allowing for better heat dissipation and, consequently, more efficient electron emission. This is because the electron emission process is influenced by the work function of the material, and a smaller diameter filament enables a more uniform and efficient emission of electrons.
**Why Each Wrong Option is Incorrect**
* **Option A:** Using a filament diameter of 1 mm would result in poor electron emission velocities due to the lower surface area-to-volume ratio, leading to reduced X-ray production efficiency.
* **Option B:** A filament diameter of 0.5 mm is too large and would still lead to inadequate electron emission velocities, compromising the X-ray production process.
* **Option C:** A filament diameter of 0.05 mm is too small and may lead to overheating and filament burnout, causing the X-ray tube to malfunction.
**Clinical Pearl / High-Yield Fact**
A smaller diameter tungsten filament is essential for achieving high electron emission velocities, which are critical for efficient X-ray production in radiography.
**Correct Answer:** C. 0.15 mm.