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 production of an X-ray spectrum with this shape due to an
electron beam striking a target? (A) Excitement of electrons in
high-energy states in target atoms. (B) Ejection of electrons in
low-energy states in target atoms. (C) Ejection of electrons in
high-energy states in target atoms. (D) Deceleration of free
electrons. (E) Acceleration of free
electrons.
When an electron, represented by
one of these blue dots here, strikes a target which is usually made of metal, it can
produce X-rays of various energies. These energies depend on two
things. The first is the initial energy of
the electron beam, given by the equation 𝑞𝑉 t, where 𝑞 is the charge of an
electron and 𝑉 t is the potential difference across the Coolidge tube that is
producing the electron beam.
Okay, now before we look at the
second thing that controls the X-ray photon energies, let’s clear some space. Okay, now, when an electron strikes
a target, there are two different processes that could potentially occur that could
produce X-rays. The first is when an electron slows
down when it strikes the target, the energy the electron had outside of the target,
which we’ll call 𝐸 one, is greater than the energy that it has after it slows down
inside the target, which we’ll call 𝐸 two. The energy that the electron loses
as it slows down is released as an X-ray or several X-rays depending on how many
steps it took to slow down.
When produced this way, these X-ray
photons are called bremsstrahlung or braking radiation. This is braking like the brakes on
a car not breaking like smashing something with a hammer. And the reason this is called
braking radiation is because it is produced when the electrons slow down or
decelerate.
The opposite case, where an
electron accelerates, does not produce any photons nor would it make sense for an
electron to accelerate when it strikes a target. So answer (E) is not correct
here.
But while an electron doesn’t
accelerate inside of a target, it doesn’t always slow down either. As the electron speeds towards this
target, there’s a chance that it directly strikes an already present electron in one
of the target’s atoms, which causes this already present electron to be ejected from
the atom entirely. This leaves behind an empty space
in that electron shell that is easy to fill with another electron, which will happen
if the atom happens to have an electron in a higher-energy shell.
The electron in the higher-energy
shell will drop down to the empty space in the lower-energy shell, releasing an
X-ray photon as it does so, the energy of which is equal to the difference in energy
levels that the electron transitioned through. This whole process is called an
energy level transition.
So then looking at our answers,
with both processes of bremsstrahlung and energy level transition in our minds, we
know that neither process involves the excitement of electrons in high-energy
states, only the ejection in energy level transition, which means answer (A) is not
correct.
And when we look at answer (C), the
ejection of electrons in high-energy states in target atoms, if an electron in a
high-energy state is ejected, leaving behind an empty space in this higher-energy
level, then it’s possible that it might never get filled by an electron from within
the atom since there may not be any yet higher-energy level electrons that could
transition down. Nor can we say that a lower-energy
level electron will transition up to fill this space since in order to do so, it
must absorb energy. And this does not produce a
photon. An X-ray photon is only produced
when there is an ejection of an electron in a low-energy state, meaning answer (C)
is not correct.
This leaves behind two possible
answers. (B) The ejection of electrons in
low-energy states in target atoms describes the process of an energy level
transition, while (D) describes bremsstrahlung. Now let’s consider this graph which
has been here the whole time and determine which of these processes will produce a
curve of X-rays like this.
The X-ray photons produced by
energy level transitions are of extremely specific energies since they’re always the
difference in energy of the energy levels that the electron transitions through. On an X-ray spectrum of X-ray
intensity versus X-ray photon energy, the photons produced by energy level
transitions will appear as peaks of very specific energy with little anywhere
else.
In contrast, bremsstrahlung is a
braking or slowing down of an electron. And this slowing down can occur all
in one big step or in several smaller steps, which means the photons produced will
create a very wide X-ray spectrum with a smooth curve where the average energies are
more likely to occur. Because the curve of this graph is
smooth and we do not see the sharp peaks of the energy level transitions, it must be
that this spectrum is caused by bremsstrahlung, which is the deceleration of free
electrons.
So the correct answer to “Which
mechanism produces this X-ray spectrum shape when an electron strikes a target?” is
bremsstrahlung, which is answer (D) the deceleration of free electrons.
