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X Rays vs. Radio Waves - Danger to Humans.

  1. Nov 27, 2004 #1
    Ok, so we all know that the shorter the wavelength, the higher the frequency, and the higher the photon energy for EM radiation. How does this relationship explain why X rays are highly dangerous to living things, and radio waves are not?

    Thanks in advance.
  2. jcsd
  3. Nov 27, 2004 #2
    x-rays have sufficient energy to ionize matter (thus the term ionizing radiation). Ionizing radiation incident on biological material produces ion pairs which can disrupt molecular bonds, and create free radicals which go on to cause more damage to molecules.

    RF doesn't have enough energy to produce ion pairs.
  4. Nov 27, 2004 #3


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    There are dangers to some long wavelength radiation. For example, microwave and infrared will heat things up. This is a molecular effect, in contrast to x-rays. Also ultraviolet rays give you sunburn.
  5. Nov 27, 2004 #4
    Could you elaborate how these are molecular effects compared to atomic?
  6. Nov 27, 2004 #5
    ^ if I'm not wrong (my chemistry sucks) most cells and living tissues are made up of organic matter, which are basically carbon-hydrogen and sometimes oxygen compounds. Heck, even most "elements" in the body are molecules because except for any group 8 ones and some metals, almost everythng is basically molecular.
  7. Nov 28, 2004 #6


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    Molecular effect example: When you heat up something in a microwave oven, the energy makes the water molecules go faster ending up as boiling and steam.
  8. Nov 30, 2004 #7
    At the time I left Motorola no approved study had shown any ill effects from very low powered microwave radiation P<.1W. Since then I have heard of a single such study but have not seen it nor any possible explanation. I have been exposed to high power >10 W radio waves at medium wave frequencies generally 4 and 7 MHz. All I ever felt was the burning hot dummy load resistor that I was dumping the power into.
  9. Dec 1, 2004 #8
    The way that microwaves (and here I'm speaking of 500-5000Mhz or so) generate heat is actually a bit complicated. In general, it is due to electrical currents it generates in the molecules. Due to resistance, this generates heat. You will hear some people saying microwave ovens heat water by using one of water's rotational modes, but this is in general not true; these rotational modes are rarely excited if water is not in the gas phase, and there should be no doubt that microwave ovens can heat liquid water.

    Microwave radiation is usually absorbed in soft human tissue. This means that for the most part, low power (<.1w/cm^2) microwaves will not heat the skin, but pass through and affect tissues inside the body. Your eyes, testicles and some other soft tissues that you'd rather have are usually the first to be damaged.

    For the most part, microwaves are difficult to focus (excepting masers etc of course) and so the danger falls off very quickly with distance to the source. This is why tales of secret government programs to create crowd control devices using microwaves you'll hear from shady sources are mostly bunk; the masers would require great deals of energy to do awful little. On the other hand, due to the effects of microwaves on electronic systems, research into them continues for other battlefield effects.

    After going below a certain frequency these heating effects go away. I'm not sure what this frequency is.
  10. Dec 1, 2004 #9


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    gamma-rays - - - - - - - -highest energy
    ---------------- 1 A
    ---------------- 10 A
    "soft" X-rays
    ---------------- 100 A
    ---------------- 400 nm
    visible light
    ---------------- 800 nm
    near - IR
    ---------------- 2.5 um
    mid - IR
    ---------------- 25 um
    far IR
    ---------------- 400 um
    ---------------- 25 cm
    radiowaves - - - - - - - - -lowest energy

    Molecules are made of atoms 'chemically' bonded togther - the atoms share electrons. Molecules also have various modes of vibration, including translation, rotation, and vibration of the atomic bonds. Each is excited at a different frequency. Furthmore, assymmetric molecules like water have dipole moments.

    and Google on "dipole moment of water".

    Tissue is generally transparent to radiowaves which generally have low energy. The longest waves pass right through with no interaction.

    The vibration modes of water like those of other molecules can be stimulated primarily by microwaves (as in microwave oven or radar).

    "Infrared radiation is absorbed and emitted by the rotations and vibrations of chemically bonded atoms or groups of atoms ( and thus by many kinds of materials)", and is generated by atomic vibrations which can be induced by electrical resistance (as in heating elements). See next ref:

    www.spectroscopyeurope.com/IR_15_6.pdf (10-100 microns)

    Skin can absorb infrared to some extent - one can feel the warmth or heat. Too much IR can burn tissue.

    Light interacts by photoelectric effect (which has a threshold) or molecular interaction. Human skin is adapted to visible light, and is somewhat transparent to light. The eyes are specially adapted to visible light. Light can be concentrated (as with a convex prism) or focussed such that the energy flux could heat or burn a surface (like skin).

    UV, just beyond the visible spectrum penetrates the skin and can damage cells.

    ref: http://www.westonaprice.org/nutrition_guidelines/nutrition_vitaminD.html
    Ultraviolet (UV) light is divided into 3 bands or wavelength ranges, which are referred to as UV-C, UV-B and UV-A.6 UV-C is the most energetic and shortest of the UV bands. It will burn human skin rapidly in extremely small doses. Fortunately, it is completely absorbed by the ozone layer. However, UV-C is present in some lights. For this reason, fluorescent and halogen and other specialty lights may contribute to skin cancer.

    UV-A, known as the "tanning ray," is primarily responsible for darkening the pigment in our skin. Most tanning bulbs have a high UV-A output, with a small percentage of UV-B. UV-A is less energetic than UV-B, so exposure to UV-A will not result in a burn, unless the skin is photosensitive or excessive doses are used. UV-A penetrates more deeply into the skin than UV-B, due to its longer wavelength. Until recently, UV-A was not blocked by sunscreens. It is now considered to be a major contributor to the high incidence of non-melanoma skin cancers.7 Seventy-eight percent of UV-A penetrates glass so windows do not offer protection.

    The ultraviolet wavelength that stimulates our bodies to produce vitamin D is UV-B. It is sometimes called the "burning ray" because it is the primary cause of sunburn (erythema). However, UV-B initiates beneficial responses, stimulating the production of vitamin D that the body uses in many important processes. Although UV-B causes sunburn, it also causes special skin cells called melanocytes to produce melanin, which is protective. UV-B also stimulates the production of Melanocyte Stimulating Hormone (MSH), an important hormone in weight loss and energy production.

    UVA 400 nm - 320 nm
    UVB 320 nm - 290 nm
    UVC 290 nm - 100 nm

    X-rays (Roentgen rays, or Roentgenstrahlung) lie beyond UV. X-rays are generated normally by bombarding heavy metals with electrons accelerated to several KeV. The electrons 'knock out' an electron preferrably in the K-shell (closest to nucleus) or L-shell. An electron falling into this shell will emit a UV photons. Anyone who has had an X-ray knows first hand of the penetrating power of X-rays. The shorter the wave length (higher frequency) the more penertrating the X-ray.

    Gamma-rays originate from decay of the radioactive nuclei and some subatomic particls. They are the most penetrating of radiation.

    Gamma rays and X-rays can cause chemical reactions with the cells, in a process called "radiolysis". This means breaking atomic bonds within molecules (DNA, RNA, proteins, etc), thus damaging or destroying the vital chemicals within the cell. In addition, power X-rays and gamma-rays may deflect electrons from atoms (Compton effect) and the electrons ionize the water and compounds inside the cells. With all of the water present in cells, one common product from radiolysis is hydrogen peroxide (H2O2), which is a powerful oxidizer. H2O2 can oxidize and destroy important compounds in the cell - a good reason to take antioxidants.
  11. Dec 2, 2004 #10
    X-rays generated by this method (accelerating electrons towards a metal target) are produced by two mechanisms.

    The primary method of x-ray production is via interaction with the atomic electric field. Accelerated electrons passing near an atom experience forces that cause it to slow down and lose energy. In the process x-rays given off by the decelerating electron. This is known as Bremsstrahlung radiation (from the German word for 'braking'). This produces a continuous x-ray energy spectrum.

    The second method described above is a photoelectric interaction with the target material. An inner shell electron is knocked out by the incoming electron. When outer shell electrons cascade down to fill the vacancy, radiation is emitted. The radiation energy will depend on the atomic number of the target material but since we're talking about x-ray production, the target will almost always be a high Z material and the resulting radiation will be x-rays. This is commonly called 'characteristic radiation' and will have discrete energies.

    There is no difference between the penetrating ability of x-rays and gamma rays (of the same energy). The only distinction between the two is the origin (x-rays are generally atomic in origin, gamma rays are nuclear in origin). Other than that, there is no way to tell the difference between a gamma ray and an x-ray.
  12. Dec 2, 2004 #11


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    Gamma rays are higher energy (=shorter wave length=higher momentum=higher frequency) than x-rays. This is the difference between them, not modes of production.
  13. Dec 2, 2004 #12
    Gamma rays are not always hgher energy than x-rays. You can get gamma rays all the way froma few keV to tens of MeV and higher. You can also easily produce X-rays in the same energy range.

    Gamma rays and x-rays are both photons. Mode of production is the primary distinction between the two.
    Last edited: Dec 2, 2004
  14. Dec 2, 2004 #13


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    X-rays basically start just above UV, with H (13.6 eV), and have max energy of about 145 keV (excluding) Z>100.

    Gamma rays (emitted from radionuclides) have energies as low as 8 keV with the highest around ~6.1 MeV, ~7.1 MeV (N-16). There are a number of gamma rays emitted following neutron capture with energies between ~0.08 - 8 keV.

    See post on X-rays and gamma-rays - https://www.physicsforums.com/showthread.php?p=389835#post389835

    However, just as rose by any other name is still a rose, an X-ray and gamma-ray of the same energy are indistinguishable.
    Last edited: Dec 2, 2004
  15. Dec 3, 2004 #14
    if you are only considering x-rays produced by atomic electrons cascading down to lower energy levels, then I would agree with you there. But x-rays can be produced with much higher energies than that by increasing the electron beam accelerating potential to higher voltages. kV therapy machines commonly operate at 200-250 keV range to produce high energy x-rays for radiation therapy.

    I just noticed this thread is in the Quantum Physics forum...maybe it should be moved to a more relavent forum?
    Last edited: Dec 3, 2004
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