Question Video: Understanding the Formation of Characteristic Lines in an X-Ray Spectrum | Nagwa Question Video: Understanding the Formation of Characteristic Lines in an X-Ray Spectrum | Nagwa

Question Video: Understanding the Formation of Characteristic Lines in an X-Ray Spectrum Physics • Third Year of Secondary School

The graph shows the relative intensity of X-rays in an X-ray spectrum of different X-ray photon energies. Which of the following mechanisms could result in the appearance of thin lines in the X-ray spectrum produced due to an electron beam striking a target? [A] Acceleration of free electrons [B] Deceleration of free electrons [C] Electrons in high-energy states in target atoms being ejected [D] Electrons in low-energy states in target atoms being ejected [E] Electrons in high-energy states in target atoms being excited

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Video Transcript

The graph shows the relative intensity of X-rays in an X-ray spectrum of different X-ray photon energies. Which of the following mechanisms could result in the appearance of thin lines in the X-ray spectrum produced due to an electron beam striking a target? Is it (A) the acceleration of free electrons? (B) The deceleration of free electrons. (C) Electrons in high-energy states in target atoms being ejected. (D) Electrons in low-energy states in target atoms being ejected. Or (E) electrons in high-energy states in target atoms being excited.

When X-ray photons are produced due to an electron beam striking a target, such as what happens in a Coolidge tube, the mechanism by which these X-rays are produced relies on the electrons striking the target and slowing down, which is to say the appearance of these X-rays depends on the deceleration of these free electrons as they strike the target, not the acceleration. However, we’re not looking for the general production of X-rays. We’re looking for the production of X-rays that create these thin lines in an X-ray spectrum. These thin lines are called characteristic lines, and they are produced by a process called energy level transition.

The rest of the spectrum, which forms this smooth curve, is produced from Bremsstrahlung. Bremsstrahlung, or breaking radiation, is so called because any time an electron slows down or breaks, it can produce X-ray photons. So the deceleration of free electrons usually refers more to Bremsstrahlung, which will produce the smooth line rather than the characteristic lines that we’re looking for. So (B) is not it. To see which of these other answers is correct, we’re going to take a closer look at energy level transition. But before we do so, let’s clear some space. We’re going to keep only the last line of the question and shorten the remaining answers. We’ve also removed answers (A) and (B) since we already eliminated them.

Now then, characteristic lines appear in X-ray spectra due to energy level transition. An energy level transition occurs when an electron in an electron beam strikes not just the target but specifically one of the electrons inside one of the target atoms. This electron from the electron beam is coming in with a very high speed such that if it strikes one of the electrons present in the target atom, it can knock it entirely out of its shell, leaving behind an empty space that needs to be filled. To fill it, one of the electrons in a higher energy level, already in the target atom, transitions downwards, releasing an X-ray photon in the process.

So, in order for an energy level transition to occur, an electron in a lower energy level has to be ejected because it is necessary for a higher-energy-level electron to transition downwards, producing a photon and taking its place. If, instead of a low-energy-level electron, a high-energy-level electron was ejected, then there would still be a gap. But there wouldn’t be any higher-energy-level electron to transition downwards to fill it, meaning that no photon would be produced, and so no X-rays. So characteristic lines are not from high-energy-level electron ejections, nor are they from high-energy-level electron excitations, since the only way that you can excite a high-energy-level electron is with an incident photon, not an electron beam striking it.

So the mechanism that would produce these thin lines, or characteristic lines, of the X-ray spectrum is an energy level transition in which an incident electron knocks out one of the lower-energy-level electrons, ejecting it from the atom. So the answer is (D) low-energy-level electron ejection.

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