Safety · ARRT 2025
Radiation Physics & Radiobiology
X-ray production, photoelectric vs Compton interactions, SI radiation units, the law of Bergonie and Tribondeau, and acute radiation syndrome dose bands for the ARRT exam.
Overview
Safety is the biggest domain on the ARRT exam, and this chapter is its first half: how x-rays are made, what they do when they hit matter, how radiation is measured, and what absorbed energy does to tissue. Photoelectric interactions give you the image and give the patient dose; Compton interactions give you fog and give everyone in the room dose.
Production happens at the tube target, where about 99 percent of the projectile electrons' energy becomes heat and only 1 percent becomes x-rays, by exactly two mechanisms. Bremsstrahlung emerges across every energy up to the peak kVp; characteristic x-rays come out at discrete energies fixed by the target, and tungsten's K-shell binding energy of 69 keV means no K-characteristic x-rays below 69 kVp. In the patient, photoelectric absorption rises with the cube of atomic number and falls with the cube of photon energy, while Compton scattering is independent of atomic number, which is why raising kVp trades contrast away for scatter.
Measurement follows NCRP Report 184. Air kerma and absorbed dose take the gray; equivalent and effective dose take the sievert; 1 gray equals 100 rad, and the radiation weighting factor for x-rays is 1, so the numbers carry straight across.
Radiobiology closes the loop. The body is roughly 80 percent water, so indirect action through free radicals dominates, and a double-strand DNA break is the lesion that kills or mutates. Deterministic effects have thresholds and scale in severity; stochastic effects have neither. Acute radiation syndrome runs in dose bands: 2 to 10 Gy hematologic, 10 to 50 gastrointestinal, above 50 central nervous system, with an unsupported LD 50/60 of about 3.5 Gy.
What you’ll learn in this chapter
The 35 lessons in this chapter break down as follows. The full lesson content is unlocked when you start a free account.
Radiation Safety
- Radiation Physics & Radiobiology
Production
- Bremsstrahlung and Characteristic
- The X-ray Emission Spectrum
- Quiz
The beam
- Beam Quantity and Beam Quality
- Quiz
Interactions
- The Five Interactions
- Quiz
- The Photoelectric Effect
- Quiz
- Compton Scattering
- Attenuation
- Quiz
Measurement
- Radiation Units
- Quiz
Radiobiology
- The Target
- Quiz
- Direct and Indirect Action
- Quiz
Radiosensitivity
- LET, RBE and OER
- Quiz
- What Makes a Cell Sensitive
Effects
- Three Ways to Sort an Effect
- Quiz
- Deterministic Effects
- Acute Radiation Syndrome
- Quiz
- Stochastic Effects
- Irradiation in Utero
Recap
- What to Carry Out of This Chapter
- Quiz
- Quiz
- Quiz
- Quiz
- Quiz
Key terms in this chapter
These are the 8 terms most likely to appear on the ARRT registry from this chapter. Use them as a flashcard pre-quiz.
- Bremsstrahlung
- Braking radiation, emitted when a projectile electron is slowed and deflected near the target nucleus. It emerges at every energy from near zero up to the peak kVp.
- Characteristic X-ray
- A discrete-energy photon emitted when an outer-shell electron fills an inner-shell vacancy. Tungsten's K-shell binding energy of 69 keV sets the production threshold.
- Photoelectric Effect
- Total absorption of the photon by an inner-shell electron. Probability rises with the cube of atomic number, which is why bone and contrast media appear white, and it is the source of patient dose.
- Compton Scattering
- A photon ejects an outer-shell electron and continues in a new direction with less energy. It is independent of atomic number and is the source of image fog and occupational exposure.
- Sievert (Sv)
- The SI unit of equivalent and effective dose. 1 Sv equals 100 rem, and because the radiation weighting factor for x-rays is 1, absorbed dose in gray converts across numerically.
- Law of Bergonie and Tribondeau
- Radiosensitivity is highest in stem cells, young tissue, and cells with high metabolic and proliferation rates. Lymphocytes and spermatogonia top the list; muscle and nerve resist.
- Stochastic Effect
- An effect with no threshold, where probability rises with dose but severity does not. Cancer and leukemia are the examples, and the linear nonthreshold model assumes risk at every dose.
- Acute Radiation Syndrome
- The early deterministic response to a high whole-body dose: hematologic at 2 to 10 Gy, gastrointestinal at 10 to 50 Gy, central nervous system above 50 Gy.
Sample practice question: Radiation Physics
One free sample from the 79-question Radiation Physics bank. See the format, the rationale style, and the difficulty before you sign up.
Which of the following photon interactions is the primary source of radiographic contrast between bone and soft tissue?
Show answer and rationale
A, Incorrect: Compton scatter is the source of patient and operator dose, not contrast. It produces uniform fog that reduces contrast.
B, Incorrect: Pair production requires photon energies above 1.022 MeV, far above diagnostic range.
C, Correct: Correct. The photoelectric effect is highly dependent on atomic number (Z³) and photon energy (1/E³). Bone (high Z) absorbs photoelectrically far more than soft tissue, producing the contrast difference between them.
D, Incorrect: Coherent scatter is a minor interaction at diagnostic energies and contributes negligibly to contrast.
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Frequently asked questions
What does the ARRT Radiography Safety category cover?
Safety is the biggest domain on the ARRT exam, and this chapter is its first half: how x-rays are made, what they do when they hit matter, how radiation is measured, and what absorbed energy does to tissue. Photoelectric interactions give you the image and give the patient dose; Compton interactions give you fog and give everyone in the room dose.
How many lessons are in the Radiation Physics & Radiobiology chapter?
This chapter contains 35 lessons across 9 sections, plus a knowledge-check quiz at the end. The full lesson content is unlocked with a Premium subscription. The free tier includes 3 complete chapters.
Is this chapter aligned with the ARRT 2025 Content Specifications?
Yes. Every chapter on this site maps directly to the ARRT Radiography Content Specifications effective 2025. This chapter falls under the Safety domain of the official ARRT exam blueprint.