Exam category: Safety → Radiation Physics and Radiobiology
Why it matters: The kVp and mAs you select control how many x-rays the tube produces and how penetrating they are, which affects both the patient's dose and the image. The registry tests how x-rays are produced at the target and what controls the quantity and quality of the beam.
Electrons are boiled off the filament, accelerated across the tube by the kVp, then slam into the target and stop almost instantly. That sudden stop is where x-rays come from — two different ways an electron can lose its energy at the target produce two different kinds of x-rays.
Counting the parts. The atomic number (Z) is the number of protons in the nucleus; it defines the element, and in a neutral atom it also equals the number of orbiting electrons. The mass number (A) is the number of protons plus neutrons. So an atom with 15 protons and 16 neutrons has Z = 15 and A = 31. Atoms with the same number of protons but different numbers of neutrons are isotopes — same element, same chemical behavior, different mass number.
Shells and binding energy. Electrons occupy shells, named K, L, M… outward from the nucleus. Each shell has a capacity given by the 2n² rule, where n is the shell number — so the innermost K shell holds a maximum of 2 electrons, L holds 8, M holds 18, N holds 32.
Each shell also represents a different binding energy — the energy holding an electron to the atom. The nearer a shell sits to the nucleus, the harder its electrons are to pull loose, so K-shell electrons have higher binding energies than L-shell electrons, and take the most energy to remove. For tungsten, K-shell electrons have a binding energy of 69.5 keV and L-shell electrons are bound by 12.1 keV — which is also why a K-to-L transition emits a characteristic x-ray of 69.5 − 12.1 = 57.4 keV.
Ionization. Removing an orbital electron from a neutral atom leaves a positively charged atom and a free electron; together those two are an ion pair, and the process is ionization. It is specifically the electron that gets removed — protons and neutrons stay in the nucleus at diagnostic energies. That is what "ionizing radiation" names, and it is the property that makes x-rays both useful and hazardous.
Why the electron is the particle that matters. The shell arrangement is what governs how an atom reacts chemically (and because a neutral atom has as many electrons as protons, the proton number ultimately sets it). Electrons are also the particle that produces characteristic x-rays — an outer-shell electron dropping into an inner-shell vacancy. Protons and neutrons do neither.
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An atom has 15 protons and 16 neutrons. What is its mass number (A)?
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