Effect of changing target metal to heavier metal in X-ray tube

[desmond iking]

Homework Statement

when target metal is changed to heavier metal in X-ray tube, the min wavelength will not change. but the wavelength of the characteristics line will become smaller. my textbook gives me the above statement explaining much. can someone please explain it? thanks !

Homework Equations

The Attempt at a Solution

 

[Fightfish]

Firstly, do you understand the process by which the X-rays are formed, and the difference between the Bremsstrahlung (braking or continuous radiation) and the characteristic X-ray peaks?

 

[desmond iking]

Firstly, do you understand the process by which the X-rays are formed, and the difference between the Bremsstrahlung (braking or continuous radiation) and the characteristic X-ray peaks?

i think i can understand it.. but can you explain when target metal is changed to heavier metal in X-ray tube, the min wavelength will not change. but the wavelength of the characteristics line will become smaller. why ?

 

[Fightfish]

Understanding the origin of the characteristic X-ray peaks will help you answer that question. What do the wavelengths of the characteristic X-ray peak(s) depend upon?

 

[desmond iking]

can i explain in this way?
we know that for diffraction to happen , the formula is d sin θ = m λ , as d(distance between atoms in metal) decreases (more closely packed) , the wavelength also decreases...?

 

[Fightfish]

Unfortunately the characteristic x-ray peaks have nothing to do with diffraction! They are due to the de-excitation of electrons from higher to lower energy levels when the inner shell electrons are ejected from the metal. That's why its important for you to understand the theory first before you can try to answer your qn.

 

[desmond iking]

Unfortunately the characteristic x-ray peaks have nothing to do with diffraction! They are due to the de-excitation of electrons from higher to lower energy levels when the inner shell electrons are ejected from the metal. That's why its important for you to understand the theory first before you can try to answer your qn.

i mixed up diffraction and production of x-ray! by the way, can you please tell me when target metal is changed to heavier metal in X-ray tube, the min wavelength will not change. but the wavelength of the characteristics line will become smaller? i really can't think of the reason.

 

[Fightfish]

Desmond, I don't mean to sound rude, but you've been here sufficiently long to know the standard PF rules - we are supposed to guide you to the answer, and not provide it directly, especially if there is insufficient evidence of effort on your part to understand and tackle the problem.
The short answer to your question is that the characteristic lines are dependent on the species of the target material, in particular the difference between the energy levels. On the other hand, the minimum wavelength is dependent only on the voltage supplied to the X-ray tube.
I'll leave it to you to read up on the basic theory behind the operation of the X-ray tube, which can be found in most standard introductory level physics texts. If you run into any other queries while reading up about the theory, I'll be glad to help out.

 

[desmond iking]

c

Unfortunately the characteristic x-ray peaks have nothing to do with diffraction! They are due to the de-excitation of electrons from higher to lower energy levels when the inner shell electrons are ejected from the metal. That's why its important for you to understand the theory first before you can try to answer your qn.

Can I explain in this way? for the heavier metal , there are more shells in the metal , because the atoms are ore closely packed , therefore it's more difficult for the photon to knock off the electron in the inner shell , higher energy is needed for this to occur. thus from E= hc/ λ , when E increases , λ decreases.

 

[Fightfish]

for the heavier metal , there are more shells in the metal , because the atoms are ore closely packed, therefore it's more difficult for the photon to knock off the electron in the inner shell , higher energy is needed for this to occur. thus from E= hc/ λ , when E increases , λ decreases.

For the heavier metals, there are indeed more electron shells, but this has nothing to do with the atomic packing. Atomic packing depends on a lot of other factors, including the geometry of the atom, shielding effects and bonding. There are more electron shells simply because the atoms have more electrons. While it is true that it will certainly be more difficult for the photon to eject the inner shell electron, this simply reduces the rate at which these events occur.

Furthermore, the characteristic X-ray lines observed have nothing to do with the energy required to eject the inner shell electrons. Just think about it: this is an energy absorption process, but the characteristic lines are due to emission. Also, the energy spectrum absorbed by the electrons when they are ionized is continuous and not discrete.

The lines occur when an electron from a higher orbital de-excites into the empty orbital left behind by the ejected electron. So, really, please do go read up on the basic theory of X-ray production. If you don't have access to a relevant physics textbook, even Wikipedia does a relatively decent job of explaining it.

 

[desmond iking]

but why the λ min still remained unchanged? is it because of energy to excite n=1 to n= infinity shell remained unchanged? why is it so?

 

[Fightfish]

but why the λ min still remained unchanged? is it because of energy to excite n=1 to n= infinity shell remained unchanged? why is it so?

The minimum wavelength is related to the maximum energy of the photons that can be produced. Evidently, this depends on the energy that you supply to the system i.e. the voltage that you apply to the tube.