The diagram shows a Coolidge tube used for the production of X-rays. Which of the following physical processes causes X-rays to be emitted from the component labeled II? (A) Electron diffraction, (B) X-ray diffraction, (C) the thermionic effect, (D) the photoelectric effect, or (E) decrease in electron energy.
This component labeled II is known as an anode target. Let’s review the general process of what happens in the Coolidge tube in order for X-rays to be generated. A potential difference here powers and heats up component I of the Coolidge tube, which is called a cathode coil. This heating causes electrons to be released from within the coil in a process called thermionic emission, or the thermionic effect. The thermionic effect is responsible for the release of electrons in component I, not X-rays in component II. So answer (C) cannot be correct.
Then, another potential difference here causes a potential difference across the components within the Coolidge tube, creating a powerful negative charge on the cathode coil and a positive charge on the anode target. This causes the released electrons from the cathode coil to begin to accelerate towards the anode target. When an electron strikes the anode target, it releases an X-ray photon. So let’s look at which of these processes could cause this to happen.
Answer (A) and answer (B) both have to do with diffraction. Diffraction occurs when a straight wave traveling forwards encounters an obstacle or opening. This causes the waves to bend as they continue to move forwards. Diffraction describes a process that happens to light, not the production of light, which is what we want. So (A) and (B) cannot be it.
Answer (D), the photoelectric effect, is a process that occurs within atoms, in which an incident photon is absorbed by an electron. The electron now has too much energy to remain in its current energy level. And if the absorbed energy is high enough, it escapes the atom entirely. This is the opposite process that we want. We want X-rays to be emitted because of electrons, not electrons to be emitted because of photons. So the photoelectric effect cannot be it.
This leaves a decrease in electron energy, which is exactly what happens when an electron strikes the anode target. This decrease in electron energy usually occurs in two different ways. The first one in which the electron slows down from striking the target directly produces Bremsstrahlung or breaking radiation. The second type of decrease in electron energy occurs within the electrons of the atoms. An electron from the electron beam accelerates towards one of the electrons in the anode target and bounces against it, causing the electron to be knocked out of the atom. This leaves behind a space which causes one of the higher-energy-level electrons to transition downwards, releasing an X-ray photon in the process. This is known as an energy level transition.
This process, in combination with Bremsstrahlung, cause X-rays to be emitted from the anode target. And since both these processes rely on a decrease in electron energy, out of the following physical processes that cause X-rays to be emitted from the component labeled II, the correct answer is (E) decrease in electron energy.