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X-ray Ionisation Q's

  1. Mar 5, 2010 #1

    I am fascinated by the properties of x-rays and their many uses, but i need to clarify a few things:

    1) Can x-rays eject electrons from non-metals and possibly even gases, or is compton scattering constrained to metals only? (ie. photoelectric effect)

    2) How do you synthesize x-rays?

    3) How dangerous can x-rays be if used improperly?
  2. jcsd
  3. Mar 5, 2010 #2
    These are good questions.

    First, visible light covers the wavelength range ~6500 Angstroms (~ 2 eV) to ~4000 Angstroms (~3 eV).
    UV light extends from ~4000 Angstroms (~3 eV) to ~ 100 Angstroms (~120 eV). See chart in


    X-rays cover the range 100 Angstroms (~120 eV) to 0.1 Angstroms (~120 KeV)

    Photons above this energy are generally called gamma rays.

    A photon exceeding the work function energy of a material can eject a photoelectron. Work function energies range from ~ 2 eV to ~ 6 eV. (Photoelectrons of a lower energy can knock electrons from the valence band to the conduction band in semiconductors.). See table in


    For isolated atoms, the ionization energy (energy to knock out an electron) ranges from ~ 3 eV (cesium) to 24 eV(helium). See table in


    X-rays historically were the rays coming from atomic electron transitions, the highest being a K-shell x-ray from uranium. These were first generated by accelerating electrons and hitting targets made of elemental materials. The x-ray energy from a bare uranium nucleus capturing 1 electron is 13.6·Z2 = 13.6·922 = 115 KeV.

    In the photoelectric effect, a bound electron absorbs 100% of the photon energy. The electron must be bound to satisfy kinematic equations (conservation of energy and momentum). For deeply bound electrons in atoms (e.g., K-shell) this now called deep core photoejection.

    For free electrons (and also bound electrons), a photon can inelastically scatter off an electron. At low energies this is called Thomson scattering; at higher energies Compton scattering.

    X-ray sources include electron bremmstrahlung and x-ray targets and synchrotron radiation. Any photon from a nuclear decay is a gamma ray, irrespective of its energy.

    X-rays from particle (e.g., electron) accelerators (including electrostatic machines) can be extremely dangerous. They can cause severe burns, organ damage, and chromosome damage.

    Bob S
  4. Mar 6, 2010 #3
    Thank you very much.

    It would appear that producing x-rays is an energy intensive process that requires a lot of specialised equipment.

    I have heard of oil molecules being ionised by x-rays, and you have pointed out that individual isolated atoms can be ionised, but that they will not need the energy of an x-ray (.12-120 keV) to lose an electron (ie. helium 24eV).

    If i wanted to ionise an O2 molecule, for instance, would i really need x-rays, or could i use a lower energy photon to do the same job?

  5. Mar 6, 2010 #4


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    X-ray Photoemission Spectroscopy (XPS) is a common technique. So yes, x-ray can eject electrons from non-metals, as well as metals, of couse.

    Depends on what you want to use it for. If you want x-ray to study other things, and the resolution, energy, etc. is very important to you, then you use a synchrotron center. The x-ray that you can get from, say, a doctor's office for diagnostic purposes can come from a particle accelerator that bombards some crystals.

    It can kill you.

  6. Mar 6, 2010 #5
    The voltages in the regular cathode-ray-tube (CRT) TV sets is ~ 10 kV, maybe more for color tV. The front window is thick lead glass to attenuate x-rays from the electrons hitting the back side of the glass. I have an old capacitor-discharge ignition circuit (Mark Ten B, sometimes available on eBay) that can generate 30,000 volt sparks from a 12 volt battery.
    A 13.6 eV photon is enough to ionize an oxygen atom. I think about 5 eV is required to break up an oxygen molecule into two neutral atoms.

    Bob S
  7. Mar 7, 2010 #6
    Zz, when you say 'particle accelerator', are you referring to an electron gun or equivalent?

    I've always wondered how capacitor-discharge ignition circuits work. I'll start a new thread on it, please contribute.
    Can you use them to produce short bursts of high energy photons?
  8. Mar 7, 2010 #7
    CD ignition circuits (like the Mark Ten B circuit I have) have a high frequency toroidal coil driven by two NPN push pull transistors that charge a capacitor to about 300 volts. A signal from the points in the distributor triggers an avalanch transistor in series with the capacitor, which discharges into the primary of the automobile ignition coil. The 1:100 primary-secondary ratio gives a 30-kV spark. This will give a short burst of high-energy photons if discharged in vacuum. The photons are a combination of bremsstrahlung and characteristic x-rays from the material used in the discharge circuit. The characteristic K-shell x-ray of copper is 13.6 x 292 = 11.4 kV max, the characteristic K-shell x-ray of zirconium is 13.6 x 402 = 21.6 kV max.

    Bob S

    Correction: The Mark Ten B schematic shows the capacitor being discharged by a common-emitter NPN transistor firing a SCR.
    Last edited: Mar 7, 2010
  9. Mar 9, 2010 #8
    Well that answers all my questions on this subject. Thanks again :)
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