The term space charge in an x-ray tube refers to which of the following?

The term space charge in an x-ray tube refers to which of the following?

• A. Free electrons
• B. Positive ions
• C. X-ray photons
• D. Neutral atoms

Answer: (A)

Space charge is the cloud of free electrons that forms near the cathode of an x-ray tube when the filament is heated. Thermionic emission sends electrons into the tube, creating a negatively charged region that repels additional electrons and can limit the tube current until the anode voltage is strong enough to pull them across. This is why current in a tube can be space-charge–limited if the voltage isn’t high enough to sweep the electrons away. Positive ions would come from ionized residual gas and are not the primary space-charge effect here, X-ray photons are produced when electrons strike the anode after acceleration, and neutral atoms don’t carry a net charge and thus don’t form the space-charge cloud.

Space charge is the cloud of free electrons that forms near the cathode of an x-ray tube when the filament is heated. Thermionic emission sends electrons into the tube, creating a negatively charged region that repels additional electrons and can limit the tube current until the anode voltage is strong enough to pull them across. This is why current in a tube can be space-charge–limited if the voltage isn’t high enough to sweep the electrons away. Positive ions would come from ionized residual gas and are not the primary space-charge effect here, X-ray photons are produced when electrons strike the anode after acceleration, and neutral atoms don’t carry a net charge and thus don’t form the space-charge cloud.