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.
