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Do X-rays' wavelength or frequency help them not excite electons...

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  1. Jan 14, 2016 #1
    ... to transmit through opaque objects.?

    So when it comes to bone the x-rays cannot pass through them, or the steel plate because, the energy is higher, and or those bones, and steel plate have more electrons in their shells, and so is it the more electrons that absorb the X-rays, or is it the energy of the electrons in bone and steel that make it absorb more than the tissue.
    Thank you for your help, anything helps, even a few words.
     
    Last edited: Jan 14, 2016
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  3. Jan 14, 2016 #2

    blue_leaf77

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    For crystalline materials, I would say it's the valence electron density which determines the amount of photons absorbed per unit volume of the material. The wavelength corresponding to this absorption is determined by the bandgap between valence and conduction bands. For amorphous stuffs like bones, I think the contributions from electrons (electronic excitations) and nuclear motion (vibrational and rotational excitations) separately will matter more. This is why, the absorption spectrum of amorphous materials are approximated well enough with the classical oscillator model for some frequency region.
     
  4. Jan 14, 2016 #3
    Great, thank you very much for your answer.
    So your saying more denser material, like bones, and the back metal plate, there are more electrons in shells absorbing the X-rays, than say in the human tissue which their is not so many electrons in the shells in the atom.
    Is this correct
     
  5. Jan 14, 2016 #4

    blue_leaf77

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    Roughly speaking, yes. But in greater detail, the plasma frequency (for metals) and absorption linewidth also have some influence to the degree of absorption.
     
  6. Jan 14, 2016 #5
    Nuclear motion does not matter at all for X-ray absorption. It is an atomic property. Band gaps only matter when one is doing high-resolution near-edge spectroscopy. Also melting only has an effect on minor details.
     
  7. Jan 14, 2016 #6
    Great, thank you for your help.
    When you have time, can you help with this other question, no answer to this on the internet.
    If you have a four inch cubic block of glass, and carbon, light passes through the glass no problem, but the carbon will absorb some red, yellow, green, and blue light, but if you look at the carbon absorption for light, not all blue, green, yellow, and red light get absorbed by carbon, like in the diagram below.
    https://mail.google.com/mail/u/0/?ui=2&ik=1a702d60a0&view=fimg&th=1523d25f444c13eb&attid=0.1&disp=emb&realattid=ii_1523cf2013009e68&attbid=ANGjdJ8MBdAFSGzV51YJ-soGg2CQn381_egRpCpWpfOMkB_UKZmCbpEj6taeUgk7Z-GWzjMK2_gT0xKe_17ThBRmOtxVpyjy2zPFCsEgvCMH-_Kxt5PLZQg4rX9xQmI&sz=w496-h406&ats=1452810140097&rm=1523d25f444c13eb&zw&atsh=1
    Some materials have larger band gaps than others, glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another, and back again.
    glass cannot absorb high wavelengths of light, but the glass will absorb ultraviolet waves, which have a smaller wavelength.
    So if the two four inch cubic block of glass, and carbon, are placed in a dark room with no light hitting the blocks at all, and you just shined the colors of light at the carbon, that did not excite the electrons to a higher shell energy level, what would happen.
    Question 1. Does the light from the blue, green, yellow, and red pass through the carbon block, but I think you would just see the block of carbon just be black right, even though certain colors of light are passing through it, is this correct.
    All light colors pass through the glass no problem, so for the carbon things are different, the amorphous material the glass is made from is not necessarily what is making the glass transparent, its the energy of the electrons in the glass that cannot get exited for the light, so the light gets transmitted through the block of glass.
    So for the carbon, does its electrons either absorb more energy, or because it has 2 electrons in shell 1, and 4 electrons in shell two, silicone which is mostly what glass is made from has two electrons in shell 1,and 8 in shell 2, and 4 in shell 3.
    So it cannot be the amount of electrons I think just the energy of electrons, but can you explain why the energy levels are different for some electrons.
    Here are a list of some ways to effect electron:
    1. Cold temperature, can this effect the way electron absorb photons of light.
    2. Certain wavelengths of light
    3. Amorphous material.
    4. energy of electrons.
    Do you know of any other ways the electron can not get excited.
    Thank you for your help, anything helps, even a few words.
     
  8. Jan 14, 2016 #7

    blue_leaf77

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    I should probably have replaced "amorphous" with "dielectric/isolator", the latter was actually the one I had in mind when writing that post. I forgot that metals are also probably amorphous, are they not? Anyway, in my second post I address the light absorption in general, I didn't exclusively devote my explanation to X-rays only.
     
    Last edited: Jan 15, 2016
  9. Jan 14, 2016 #8
    Metals are usually crystalline. It is difficult to make amorphous metals (metglass). They are not thermodynamically stable.

    None of this matters for the x-ray attenuation length, unless one looks at minute details.
     
  10. Jan 14, 2016 #9
    Great thanks, can you help with this tough question,
    When you have time, can you help with this other question, no answer to this on the internet.
    If you have a four inch cubic block of glass, and carbon, light passes through the glass no problem, but the carbon will absorb some red, yellow, green, and blue light, but if you look at the carbon absorption for light, not all blue, green, yellow, and red light get absorbed by carbon, like in the diagram below.
    https://mail.google.com/mail/u/0/?ui=2&ik=1a702d60a0&view=fimg&th=1523d25f444c13eb&attid=0.1&disp=emb&realattid=ii_1523cf2013009e68&attbid=ANGjdJ8MBdAFSGzV51YJ-soGg2CQn381_egRpCpWpfOMkB_UKZmCbpEj6taeUgk7Z-GWzjMK2_gT0xKe_17ThBRmOtxVpyjy2zPFCsEgvCMH-_Kxt5PLZQg4rX9xQmI&sz=w496-h406&ats=1452810140097&rm=1523d25f444c13eb&zw&atsh=1
    Some materials have larger band gaps than others, glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another, and back again.
    glass cannot absorb high wavelengths of light, but the glass will absorb ultraviolet waves, which have a smaller wavelength.
    So if the two four inch cubic block of glass, and carbon, are placed in a dark room with no light hitting the blocks at all, and you just shined the colors of light at the carbon, that did not excite the electrons to a higher shell energy level, what would happen.
    Question 1. Does the light from the blue, green, yellow, and red pass through the carbon block, but I think you would just see the block of carbon just be black right, even though certain colors of light are passing through it, is this correct.
    All light colors pass through the glass no problem, so for the carbon things are different, the amorphous material the glass is made from is not necessarily what is making the glass transparent, its the energy of the electrons in the glass that cannot get exited for the light, so the light gets transmitted through the block of glass.
    So for the carbon, does its electrons either absorb more energy, or because it has 2 electrons in shell 1, and 4 electrons in shell two, silicone which is mostly what glass is made from has two electrons in shell 1,and 8 in shell 2, and 4 in shell 3.
    So it cannot be the amount of electrons I think just the energy of electrons, but can you explain why the energy levels are different for some electrons.
    Here are a list of some ways to effect electron:
    1. Cold temperature, can this effect the way electron absorb photons of light.
    2. Certain wavelengths of light
    3. Amorphous material.
    4. energy of electrons.
    Do you know of any other ways the electron can not get excited.
     
  11. Jan 14, 2016 #10
    You are asking now not about x-rays, but about visible light. Then the band gap is important. Graphite does not have a band gap and is therefor opaque. Diamond has a large band gap and is transparent for visible light.
     
  12. Jan 15, 2016 #11
    To move from a lower to a higher energy level, an electron must gain energy. Oppositely, to move from a higher to a lower energy level, an electron must give up energy. In either case, the electron can only gain or release energy in discrete bundles.

    Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen:

    1. The substance absorbs the photon. This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears.
    2. The substance reflects the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted.
    3. The substance allows the photon to pass through unchanged. Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object.
    Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level. Physicists sometimes talk about this in terms of band theory, which says energy levels exist together in regions known as energy bands. In between these bands are regions, known as band gaps, where energy levels for electrons don't exist at all.

    Some materials have larger band gaps than others.

    Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. Photons of visible light -- light with wavelengths of 400 to 700 nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red -- simply don't have enough energy to cause this skipping.

    Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent.

    At wavelengths smaller than visible light, photons begin to have enough energy to move glass electrons from one energy band to another. For example, ultraviolet light, which has a wavelength ranging from 10 to 400 nanometers, can't pass through most oxide glasses, such as the glass in a window pane. This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.


    If an electron is in the first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it must have -3.4 eV of energy.

    Let's say the electron wants to jump from the first energy level, n = 1, to the second energy level n = 2. The second energy level has higher energy than the first, so to move from n = 1 to n = 2, the electron needs to gain energy. It needs to gain (-3.4) - (-13.6) = 10.2 eV of energy to make it up to the second energy level.

    So here's the question, If an electron is in the first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it must have -3.4 eV of energy.
    Let's say the electron wants to jump from the first energy level, n = 1, to the second energy level n = 2. The second energy level has higher energy than the first, so to move from n = 1 to n = 2, the electron needs to gain energy. It needs to gain (-3.4) - (-13.6) = 10.2 eV of energy to make it up to the second energy level.
    If it takes 10 eV to move the electron in shell 2 , in the glass, and the iron block, then why does the glass electron not get excited when hit by a photon.
    Why is the iron electron absorbing.
    What makes a four inch cubed block of glass different to a four inch cubed block of opaque solid iron, what is the difference in the glass, and irons electrons, why is the glass block allowing transmission, and the iron block absorbing.
    Is it the amount of electrons in the shells in the iron, or is it the electrons in the glass need more eV compared to the irons electrons.
    What I am trying to do is find all the ways the electron in the atom can not be excited, by light, or any type of EM radiation, or another way to look at it is, how the electron can be kept in the ground state shell 1 the "1 shell" (also called "K shell"), and not get excited to shell 2.
    https://mail.google.com/mail/u/0/?ui=2&ik=1a702d60a0&view=fimg&th=15247bba1386a248&attid=0.2&disp=emb&realattid=ii_15247bad4c5acd4e&attbid=ANGjdJ_gB_S7X_IC6st087aolohQiGHeOtNC_3il-P0E4z3F-CUQOQR_q43rRRsTIDqe_YlK2AlLrjWiYTkFRSctQkU5Znv7P5E6sZNIDv5Yz6cBpDC4rbnb5uu7Avs&sz=w746-h320&ats=1452905705409&rm=15247bba1386a248&zw&atsh=1
    Thank you for your help.
     
  13. Jan 16, 2016 #12
    The material with the largest band gap is lithium fluoride, LiF, about 11 eV. Beyond that, everything absorbs any vacuum-UV or x-ray. If you need windows, people use thin foils of low-Z elements like beryllium or boron nitride or carbon.
     
  14. Jan 17, 2016 #13
    3 effects contribute to the absorption of x-rays in matter - condensed matter or gas: The Compton effect, the photo-electric effect, and pair production.

    Pair production is only significant for very high photon energies, above 2x 511 keV as an electron-positron pair has to be created.

    The Compton effect depends on the total electron density. Relative to the photo-electric effect it is most important at photon energies above ~100 keV.
    It depends essentially only on the total electron density in the absorber, as almost all materials do not have binding energies anywhere near this range (notable exceptions are Pb and actinides).

    Below 100 keV the most important effect is the photo-electric effect.

    The photo-electric effect depends strongly on the binding energy of electrons. The cross section (probability of absorption) is particularly high when the photon energy is just above the binding energy of an electron in the absorber. This is why "light" materials such as Beryllium or hydrocarbons absorb weakly and are nearly transparent to x-rays.
    The dependence of the absorption above a binding energy ("absorption edge") depends on the electronic configuration of the absorbing atom, e.g. the charge, chemical bonds, etc. This "footprint" is used in x-ray absorption spectroscopy.

    Note that typical bandgaps in semiconductors are in the 1-10 eV range, whereas x-ray energies are in the keV range. The most used characteristic energy is the Cu K alpha line at about 8 keV. The bandgap and the difference between conductors, semiconductors and insulators therefore has very little effect on the absorption of x-rays.

    http://www.x-ray-optics.com/index.php?option=com_content&view=article&id=51&Itemid=61&lang=en [Broken]

    https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
     
    Last edited by a moderator: May 7, 2017
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