X-ray Tube Controls and Spectrum
In an X-ray tube, electron rate is controlled primarily by tube current and electron energy by accelerating voltage. Target material sets characteristic lines; voltage sets the continuous cutoff and must exceed shell thresholds for characteristic emission.
Why this shows up in the exam
Interpreting Coolidge-tube spectra · Separating tube-current and tube-voltage effects · Selecting medical-imaging exposure settings
Learn the idea
Tube current mainly changes X-ray intensity, while voltage changes photon energies and the cutoff. Sending more electrons to the target creates more X-ray photons, making the spectrum taller. Accelerating each electron more strongly gives access to higher photon energies and shifts the short-wavelength edge.
🧠 Memory hook: Current changes how many; voltage changes how energetic.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- X-ray intensity proportional to electron arrival rate — first-order effect of tube current at fixed voltage and target
- lambda_min = h c/(e V) — voltage control of the continuous-spectrum cutoff
How to approach it
- 1Classify the requested change as intensity, cutoff, or line position
- 2Assign current, voltage, or target material respectively
- 3Check threshold conditions when characteristic lines are involved
Common slip-ups that cost marks
- •Shifting characteristic-line positions by changing tube current
- •Changing cutoff wavelength when only electron count changes
- •Assuming all electron energy becomes useful X-rays
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Photons of energy 5 eV illuminate a metal of work function 2 eV. Find the stopping potential.
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