Catching High-Energy Photons: The X Ray Detectors Physics Quiz

If soft tissue is made of elements like Carbon, Oxygen, and Hydrogen, then these have low atomic numbers. If X-rays used in imaging are in the right energy range, then they will be absorbed by these atoms. Therefore, the photoelectric effect is the main interaction for creating contrast in medical images.

If classical physics could not explain why light energy depended on frequency rather than intensity, then a new theory was needed. If Einstein proposed in 1905 that light travels in discrete packets (quanta), then the photoelectric effect could be understood. Therefore, Einstein is credited with the explanation that won him the Nobel Prize.

If a detector is very thick, then an X-ray might be absorbed at various depths. If the resulting signal spreads out as it travels to the sensor surface, then the image loses sharpness. Therefore, there is a trade-off between "stopping power" and "image clarity."

If the nucleus pulls on the electron with an electromagnetic force, then energy must be added to break that "grip." If this energy is specific to each shell and element, then it represents how tightly the electron is held. Therefore, it is binding energy.

If an inner-shell vacancy is filled by an outer electron, energy is released. If instead of emitting an X-ray photon, that energy is transferred to another electron which then gets ejected, then this is a "radiationless" transition. Therefore, this specific process is the Auger Effect.

If X-rays pass through the body and hit the film, they turn it black. If bones contain Calcium (high Z) and absorb X-rays via the photoelectric effect, then fewer X-rays reach the film behind the bone. Therefore, the "shadow" or white area represents where absorption occurred.

If Silicon is a semiconductor, then it can turn absorbed energy into an electrical signal. If it is easy to manufacture at scale, then we can make high-resolution sensors. Therefore, its electronic properties and availability make it ideal.

If the total energy of the photon is 50 keV, then 10 keV must be used to "pay" the binding energy to free the electron. If energy is conserved, then the remaining 40 keV becomes the movement energy of the electron. Therefore, the kinetic energy is 40 keV (50−10=40).

If X-rays enter a tube and knock electrons off gas atoms, then those atoms become ions. If these ions and electrons create an electrical pulse that can be counted, then the device is a radiation counter. Therefore, this is the Geiger counter.

If an atom is at a high-energy state because an inner electron is missing, then it is unstable. If there are electrons in outer shells with higher potential energy, then one will naturally fall into the vacancy. Therefore, the "hole" is filled to reach a more stable, lower-energy state.

If an X-ray photon strikes an atom, then it can transfer all of its energy to a bound electron. If the energy is high enough, the photon disappears and the electron is ejected. Therefore, the photon is described as being "absorbed" because it no longer exists as a wave.

If an electron drops from the L-shell to the K-shell, then it loses a specific amount of potential energy. If energy is conserved, that lost energy must be released as a photon. Therefore, the energy of that photon is exactly equal to the gap between those two specific shells.

If a hole is left in an inner shell, then a higher-energy electron must drop down to fill it. If that electron drops, it must release energy, often as a "characteristic" X-ray. Therefore, these events are linked in a chain reaction.

If chemists and physicists label electron shells starting from the nucleus outward, then the first shell is K, followed by L, M, etc. If the K-shell is closest to the nucleus, then it is the innermost shell. Therefore, K-shell is the innermost.

If X-rays are difficult for standard digital sensors to see directly, then they must be converted. If a scintillator material absorbs an X-ray and re-emits that energy as multiple visible light photons, then a camera can record it. Therefore, it acts as a translator between X-rays and visible light.

If an atom starts with an equal number of protons and electrons, it is neutral. If the photoelectric effect removes one electron, then there are now more positive protons than negative electrons. Therefore, the atom now has a positive charge and is called an ion.

If the incoming X-ray energy is just slightly lower than what is needed to eject an electron, absorption is low. If the energy increases to exactly match or exceed that binding energy, the probability of absorption jumps up instantly. Therefore, this sharp increase on a graph is called an "edge."

If an atom has a high atomic number (Z), then it has more protons and more tightly bound inner-shell electrons. If there are more targets for the X-ray to hit, then the chance of photoelectric absorption increases significantly. Therefore, high-Z materials are much better at stopping or detecting X-rays.

If the ejection of the electron was caused by light (photo) energy, then the resulting particle is named to reflect that origin. If it is an electron emitted via this effect, then it is a photoelectron. Therefore, the term is photoelectron.

If the energy of an X-ray photon increases far beyond the binding energy of the electron, then the probability of interaction decreases. If the photon is moving "too fast" energetically, it is more likely to pass through. Therefore, lower-energy "soft" X-rays are actually absorbed more easily than high-energy "hard" X-rays.