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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Limited X-Ray / Radiologic Technology › Radiation Physics & Safety for Limited Radiologic Technology › Lesson 2
HLT-RADI64-02 · Lesson 2 · 28 clock hours · demo

X-Ray Production: The X-Ray Tube, Circuits, and Emission Spectrum

Objective. Students will trace the path of electron production, acceleration, and interaction at the anode to explain how both characteristic and bremsstrahlung X-rays are generated.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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How X-Rays Are Made

X-rays used in diagnostic radiography do not exist in nature waiting to be captured — they are produced on demand inside a device called an X-ray tube. Understanding how that tube works is essential for every limited radiologic technologist, because the settings you choose directly control the quantity and quality of radiation you send through a patient.

Anatomy of the X-Ray Tube

The X-ray tube is a vacuum-sealed glass or metal envelope. A vacuum is necessary because air molecules would stop the electrons before they could do useful work. Inside the envelope are two main electrodes.

The cathode is the negative electrode. It contains a coiled tungsten wire called the filament and a surrounding focusing cup made of molybdenum. When electric current flows through the filament it heats up — a process called thermionic emission — and a cloud of free electrons boils off the filament surface. The focusing cup shapes that electron cloud into a narrow stream aimed at the anode.

The anode is the positive electrode. In most diagnostic tubes it is a rotating disk that distributes heat. The face of the anode is coated with tungsten (or a tungsten-rhenium alloy) because tungsten has a very high atomic number (Z = 74) and an extremely high melting point, making it ideal for withstanding the enormous heat of X-ray production. The small area on the anode where the electron beam strikes is called the focal spot (or target). X-rays exit the tube through a port — a window in the tube housing that is thinner than the surrounding shielding.

The High-Voltage Circuit

The tube is connected to a high-voltage circuit that creates a large potential difference — measured in kilovolt peak (kVp) — between cathode and anode. This potential difference accelerates electrons from cathode to anode at very high speed. A separate, lower-voltage filament circuit controls the current through the filament. Filament current determines the temperature of the filament and therefore how many electrons are released. This quantity of electrons is described by milliampere-seconds (mAs) — the product of tube current (mA) and exposure time (s).

Two Types of X-Ray Production

When the high-speed electrons slam into the tungsten target, their kinetic energy is converted mostly into heat (about 99%) and a small fraction into X-rays through two separate mechanisms.

Bremsstrahlung Radiation

"Bremsstrahlung" is a German word meaning "braking radiation." When an incoming electron passes close to the nucleus of a tungsten atom, the strong positive charge of the nucleus deflects the electron. The electron slows down and changes direction. The kinetic energy lost in this braking is released as an X-ray photon. Because the electrons can lose varying amounts of energy depending on how close they pass to the nucleus, bremsstrahlung photons are produced across a continuous range of energies, from nearly zero up to the maximum energy set by the kVp. Bremsstrahlung accounts for the majority of X-rays in a diagnostic beam.

Characteristic Radiation

Characteristic radiation is produced when an incoming electron has enough energy to knock an inner-shell electron (usually a K-shell electron) completely out of a tungsten atom. This creates a vacancy in the inner shell. An outer-shell electron immediately drops down to fill the vacancy, and the difference in binding energy between the two shells is released as an X-ray photon. Because the energy levels of electron shells are fixed for a given element, the photons produced have specific, discrete energies — they are "characteristic" of the target material. For tungsten, characteristic X-rays appear at certain energies above approximately 69 kVp (the K-edge binding energy of tungsten). Below that kVp, characteristic radiation from the K shell is not produced.

The Emission Spectrum

When you plot photon number against photon energy for a diagnostic X-ray beam, you get the emission spectrum. The bremsstrahlung component appears as a broad, hill-shaped curve from low energies up to the kVp maximum. Characteristic radiation appears as sharp spikes rising above that curve at specific energies. The entire spectrum shifts when you change technique:

Safety Considerations

Most of the electron energy becomes heat, not X-rays. Overloading the tube by exceeding its rated capacity can destroy the anode and damage the tube — always consult technique charts and tube-rating charts. The lead-lined tube housing and collimator limit the useful beam to the area of clinical interest, reducing unnecessary patient dose. As a limited technologist, you control both kVp and mAs; choosing the lowest dose consistent with a diagnostic image is a core professional and legal responsibility. Never defeat interlocks or modify shielding. If equipment appears damaged, report it before making any exposure.

Key terms8

Thermionic emissionThe release of electrons from a heated filament; the source of the electron cloud at the cathode of an X-ray tube.
Bremsstrahlung radiation"Braking radiation" produced when electrons are decelerated and deflected by atomic nuclei in the target; produces a continuous spectrum of X-ray energies.
Characteristic radiationX-rays produced when an incoming electron ejects an inner-shell electron from a target atom and an outer-shell electron fills the vacancy, releasing a photon of discrete, element-specific energy.
kVp (kilovolt peak)The peak voltage applied across the X-ray tube; controls the maximum energy and overall penetrating ability (quality) of the X-ray beam.
mAs (milliampere-seconds)The product of tube current and exposure time; controls the total number of X-rays produced (beam quantity) without changing their energy.
Focal spotThe small area on the anode target where the electron beam strikes and X-rays are produced; its size affects image sharpness.
Emission spectrumA graph of X-ray photon number versus photon energy for a given technique; shows both the continuous bremsstrahlung curve and any characteristic radiation spikes.
Focusing cupA negatively charged molybdenum cup surrounding the filament that shapes and directs the electron stream toward the focal spot on the anode.

Worked examples4

1. A technique uses 80 kVp and 10 mAs. The technologist doubles the mAs to 20 mAs while keeping kVp the same. How does the emission spectrum change?
  1. Recall that mAs controls the number of electrons reaching the anode, not their energy.
  2. Doubling mAs doubles the number of electrons, which doubles the number of X-ray photons produced at every energy level.
  3. kVp is unchanged, so the maximum photon energy and the shape of the spectrum do not change.
  4. Conclusion: the entire emission spectrum curve doubles in height (more photons) but retains its original shape and energy range.
Answer: The beam quantity doubles; beam quality (energy distribution) is unchanged. The spectrum curve is taller at every energy but otherwise identical in shape.
2. A technique is changed from 70 kVp to 90 kVp while mAs stays the same. What happens to (a) the maximum photon energy and (b) the presence of tungsten K-characteristic radiation?
  1. The maximum photon energy in a bremsstrahlung beam equals the peak voltage: at 70 kVp the maximum is 70 keV; at 90 kVp it becomes 90 keV.
  2. Tungsten K-shell electrons require approximately 69 keV to be ejected. At 70 kVp the beam just barely exceeds this threshold, so a small amount of K-characteristic radiation is possible.
  3. At 90 kVp, electrons arrive with up to 90 keV of energy, comfortably above the ~69 keV K-edge, so characteristic K-radiation spikes are clearly present in the spectrum.
  4. Because more electrons also carry enough energy to cause bremsstrahlung at higher energies, the overall curve shifts to the right and the average photon energy increases.
Answer: (a) Maximum photon energy increases from 70 keV to 90 keV. (b) K-characteristic radiation spikes are more prominent at 90 kVp because electrons exceed the tungsten K-edge binding energy by a greater margin.
3. A tube operates at 200 mA and an exposure time of 0.05 seconds. What is the mAs value?
  1. Use the formula: mAs = mA × time (in seconds).
  2. mAs = 200 mA × 0.05 s.
  3. mAs = 10 mAs.
Answer: The mAs value is 10 mAs.
4. A radiographer uses 100 mA and 0.2 s for a chest image, then wants to use 400 mA to stop motion. What new exposure time gives the same mAs?
  1. First calculate the original mAs: 100 mA × 0.2 s = 20 mAs.
  2. Set up the equation for the new technique: 400 mA × new time = 20 mAs.
  3. Solve for new time: new time = 20 mAs ÷ 400 mA = 0.05 s.
Answer: The new exposure time is 0.05 seconds (50 ms), maintaining 20 mAs while reducing motion blur.
In class: Observe instructor demonstration of a sectioned or transparent X-ray tube model; label a cutaway diagram identifying cathode, filament, focusing cup, anode, target material, glass envelope, and port. · Use simulation software (or projected tube-voltage/current graphs) to manipulate kVp and mAs settings and record how each change shifts the emission spectrum, documenting observations in a lab notebook. · In pairs, sketch and narrate the bremsstrahlung and characteristic radiation production processes on a whiteboard, then receive peer and instructor feedback against a provided rubric.
Materials: Sectioned X-ray tube model or equivalent simulation software; emission spectrum graph handouts; lab notebooks; whiteboard; Bushong textbook
Check: Scored diagram labeling exercise (10 structures, 100 points possible); pass mark 70%. Instructor verifies each student can verbally explain bremsstrahlung vs. characteristic production.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Radiation Physics
1. Which component of the X-ray tube is the source of electrons used to produce X-rays?
Radiation Physics
2. Bremsstrahlung X-rays are produced primarily because of which interaction?
Radiation Physics
3. Which of the following best explains why tungsten is used as the anode target material in diagnostic X-ray tubes?
Radiographic Technique & Image Production
4. A technologist increases kVp while keeping mAs constant. Which of the following correctly describes the resulting change to the X-ray beam?
Radiation Physics
5. Characteristic radiation from the K shell of tungsten will NOT be produced if the tube is operated below approximately which kVp value?
Radiation Physics
6. On an emission spectrum graph, what does the area under the bremsstrahlung curve represent?
Radiographic Technique & Image Production
7. A radiographer uses 50 mA and 0.4 s for an exposure. What is the mAs?
Radiation Protection & Safety
8. From a radiation safety perspective, why must a limited radiologic technologist use the lowest technique (kVp and mAs) consistent with a diagnostic image?
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