Bremsstrahlung, photoelectric effect & internal conversion?

[Reshma]

Bremsstrahlung is the process in which an accelerated electron radiates and loses energy in the form of a photon(production of continuous X-rays).

In photoelectric effect, an atom absorbs a photon and an electron from one of the shells is ejected.

So, is it reasonable to regard X-ray production and photoelectric effect as inverse processes?

On a similar note, in the process of internal conversion, an excited nuclear state comes down to ground state without the emission of a gamma ray(photon). The energy [itex]\Delta E[/itex] involved in this nuclear transition gets transferred directly to the bound electron of the atom and such an electron gets knocked out of the atom. Does this process(internal conversion) share an analogy with photo-electric effect?

 

[marlon]

In photoelectric effect, an atom absorbs a photon and an electron from one of the shells is ejected.

In the photoelectric effect the incident EM radiation is not absorbed by an atom but by an entire METAL. The emitted electron comes out of the electronic conduction band, which is a concept that arises due to the fact that you have many atoms making up a solid which we call metal (if the bandgap is almost 0 eV). This conductionband arises due to the overlap of the outer electronic valence shells making up a continuous energy band. The electrons in this band no longer belong to one atom but to the entire metal because they are less tightly bound to the constituent atoms.

So, is it reasonable to regard X-ray production and photoelectric effect as inverse processes?

No, since in X ray production, the EM radiation comes from an atom losing it's energy. Actually, you have both a continuous (ie the Bremsstrahlung)and caracteristic part in the X-ray spectrum coming from two processes. This is a manifestation of conservation of energy. The photo-electric effect requires a solid/metal to be executed.

On a similar note, in the process of internal conversion, an excited nuclear state comes down to ground state without the emission of a gamma ray(photon). The energy [itex]\Delta E[/itex] involved in this nuclear transition gets transferred directly to the bound electron of the atom and such an electron gets knocked out of the atom. Does this process(internal conversion) share an analogy with photo-electric effect?

Again, no because of the "individual atom <---> metal" contradiction.

marlon

 

[Meir Achuz]

1. Bremsstrahlung is the process in which an accelerated electron radiates and loses energy in the form of a photon(production of continuous X-rays).
2. In photoelectric effect, an atom absorbs a photon and an electron from one of the shells is ejected.

3. So, is it reasonable to regard X-ray production and photoelectric effect as inverse processes?

4. On a similar note, in the process of internal conversion, an excited nuclear state comes down to ground state without the emission of a gamma ray(photon). The energy [itex]\Delta E[/itex] involved in this nuclear transition gets transferred directly to the bound electron of the atom and such an electron gets knocked out of the atom. Does this process(internal conversion) share an analogy with photo-electric effect?

1. Correct, except that the whole EM spectrum can be emitted.
2. As M described, the e in the photo-effect cannot be considered as coming from a single atom. What you describe is "photo-ionization".
3. In the sense that the conduction electron absorbs a photon, it is somewhat like the inverse process to brem.
4. There are two differences. IC takes place within a single atom.
IC can be desribed with a virtual photon emitted from the nucleus, but the photo-effect has real photons.

 

[marlon]

3. In the sense that the conduction electron absorbs a photon,

The conduction electron does NOT absorb the photon. This is a misconception that occurs very often on this forum. Besides an electron can never absorb a photon because of the well known conservation laws that need to be respected.

IC can be desribed with a virtual photon emitted from the nucleus, but the photo-effect has real photons.

Sorry but this is a bit far fetched no ? One does not need the concept of virtual photons to fully explain the two phenomenae at hand here. There really is no need for bringing in the "virtual photon" since it is not necessary in the actual QM explanation of the processes the OP is asking about.

marlon

 

[Reshma]

Thank you very much Marlon and Meir Achuz.
Just a few more questions on the nature of the spectrum. In photoelectric effect and in internal conversion, emission of electrons occur. Are their spectra continuous or do they produce a line spectrum?

 

[marlon]

Thank you very much Marlon and Meir Achuz.
Just a few more questions on the nature of the spectrum. In photoelectric effect and in internal conversion, emission of electrons occur. Are their spectra continuous or do they produce a line spectrum?

The relation between the photo-electron kinetic energy and the incident EM-radiation frequency is nearly linear and starts at a non zero x-value for a 0 y-value (this is one of the important aspect). Look at the first link a provided.

The spectrum of internal conversion is a line spectrum (i am being a bit naive here) since he emitted electrons are orbital electrons. Internal conversion can be explained as the interaction between the nulcear multipole E fields and the orbital electrons. (i am assuming you know about multipole interactions). If this interaction is strong enough, one of these electrons can be emitted out of the atom.

But to be complete other interactions (like gamma decay) take place so the spectrum you will see is a superposition of all the contributions of each interaction.

Be sure that you understand the difference between gamma decay and internal conversion. Some professors or teachers like to ask this, trust me :)


marlon

 

[Astronuc]

Bremsstrahlung is the process in which an accelerated electron radiates and loses energy in the form of a photon(production of continuous X-rays).

Yes. http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html#c2

In photoelectric effect, an atom absorbs a photon and an electron from one of the shells is ejected.

Adding to what marlon and Meir Achuz mentioned, the classic photoelectric effect involves low energy photons in the eV range which are absorbed by conduction electrons, which are emitted from a metal (for which Albert Einstein received the Nobel Prize in 1921 - http://nobelprize.org/physics/laureates/1921/index.html) . In the context of radiation, the photoelectric effect means that the photon is entirely absorbed by an electron, which can be any electron in an atom, as long as the photon has some threshold energy, which is basically the binding energy of the particular electron. This is contrast to Compton scattering, in which an electron absorbs some energy and a lower energy photon is 'scattered'. At higher photon energies, pair production and photo-neutron emission may occur.

So, is it reasonable to regard X-ray production and photoelectric effect as inverse processes?

Yes, in a sense that X-ray production by brehmßtrahlung is emssion as opposed to absorption, but the X-ray from brehmßtrahlung is not from a bound electron. X-rays are also produced by electrons falling into the K or L shells of ionized atoms. Usually, electrons on the order of several keV are made to collide with atoms (e.g. W) knocking out K or L electrons (http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/moseley.html#c1). Another electron fills the empty shell releasing a characteristic X-ray in the process (the process discovered and elucidated by Henry Moseley(http://en.wikipedia.org/wiki/Henry_Moseley)) . Incidentally Henry Moseley was killed in World War I before he could be awarded the Nobel Prize for his work (http://nobelprize.org/physics/laureates/1924/press.html) .

On a similar note, in the process of internal conversion, an excited nuclear state comes down to ground state without the emission of a gamma ray(photon). The energy involved in this nuclear transition gets transferred directly to the bound electron of the atom and such an electron gets knocked out of the atom. Does this process(internal conversion) share an analogy with photo-electric effect?

Are you thinking of isomeric transition where the nucleus entering a lower energy state by gamma-emission.

Internal conversion is another electromagnetic process which can occur in the nucleus and which competes with gamma emission. Sometimes the multipole electric fields of the nucleus interact with orbital electrons with enough energy to eject them from the atom.

See - Internal conversion - http://hyperphysics.phy-astr.gsu.edu/Hbase/nuclear/radact2.html#c5

Isomeric transition and X-ray production produce photons of discrete energy (with some line broadening) because they involve transitions between quantum states. Bremstrahlung would produce more of a continuum is the electron is not necessarily bound, nor transition between quantum states.