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Can Gamma Rays Make Plasma in a Vaccum

  1. Oct 28, 2010 #1
    Alright, If you had a vaccum chamber and put in a gamma ray producing material (say Radium) would it produce plasma? I have seen someone do this with microwaves, so why wouldnt it work way gamma? Heres the site that uses microwaves to produce plasma.

  2. jcsd
  3. Oct 28, 2010 #2


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    Radium decays by alpha emission, not gamma.

    If you put radium in a vacuum chamber, after a period of time you would generate helium and radon gas, but it would not be a plasma. The decay products of the radioactive atoms quickly neutralize themselves when colliding with the walls of the container.
  4. Oct 28, 2010 #3
    Gamma rays and alpha particles produce ionization in air, and the ions and electrons quickly recombine. If there is a voltage across the volume, the electrons will gain energy and create more ionization. If there are enough ionization electrons, you get electron multiplication (and plasma) like in a Geiger tube. In air at 1 atmosphere, the mean free path of electrons is too short to make more electrons (e.g., plasma) unless the voltage is very high. This is why the required voltage in an automobile gas engine (spark plug) is so high. At pressures like one millitorr, the mean free path is very long, and the electrons gain enough energy at low voltages (volts/cm) to create ionization, like in a microwave oven. At very low pressure, there are insufficient atoms per cm3 to create a plasma. Read about Crookes tube at


    Also read about the Frank-Hertz experiment:


    So in the end, you need ionization, some (but not excessive) residual gas or vapor, electric field (volts/cm), adequate mean free path, etc. to get plasma. Maybe an ac electric field (microwave oven) is best.

    Bob S
    Last edited: Oct 28, 2010
  5. Oct 28, 2010 #4
    So you are saying that the lower the pressur is the easier it is to get plasma? Can you please explain? But thanks anyway that helped a lot.
  6. Oct 28, 2010 #5
    At high pressures, the mean free path x of electrons (which varies inversely with pressure) is too short to gain enough energy in an electric field E to ionize gas molecules and create more free electrons, unless x·E exceeds required ionization voltage. Read about the Frank Hertz experiment at

    http://en.wikipedia.org/wiki/Franck%...rtz_experiment [Broken]

    [added] See table of ionization potential of elements at


    See mercury (vapor) at 10.437 volts.

    Bob S
    Last edited by a moderator: May 5, 2017
  7. Oct 28, 2010 #6
    The problem with producing a plasma with gamma rays, is that gamma rays will very rarely collide with air molecules. If you reduce the pressure there will be even less collisions.
  8. Oct 29, 2010 #7
    Most of the ionizing radiation from gamma rays in vacuum chambers is due to Compton-scattering recoil electrons coming from the vacuum chamber walls. At lower gamma energies, the electrons are from deep core photo-ejection electrons from the vacuum chamber walls.

    Bob S
  9. Dec 5, 2010 #8
    so if i were to ionize a chamber with helium as the medium in a microwave, would it be an ionization of one or both electrons?
    Does a microwave have enough energy to remove both electrons?
  10. Dec 6, 2010 #9
    Dont make questions on other peoples threads! Go make your own so you can get more answers.
  11. Dec 6, 2010 #10


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    I'm going to intervene here because this thread is going in all different directions, and I'm seeing that you are being given rather strange responses.

    First of all, let's correct one misconception that you have based on your original question above.

    1. You cannot make a plasma in a vacuum, and certainly, you cannot make one in a perfect vacuum. A 'plasma', by definition requires that presence of charge ions and/or electrons, so already, you need neutral gasses to be present.

    2. The microwave source you gave is an example where the EM field produces electrons via field emission. These electrons then gain energy from the EM field. If the chamber where this is produced is sufficiently evacuated, but still have some neutral atoms (as is the case for may low vacuum chambers), then two things will occur: (i) the mean free path of the field electrons is sufficiently long that they will acquire quite a bit of energy from the external EM field and (ii) they can collide with the neutrals in such a way as to ionize these atoms/molecules, thus, generating a plasma.

    So now, hopefully, you've understood the mechanism for such plasma generation in the reference you gave, and why asking if such a thing can be made in a "vacuum" is rather misleading and inaccurate.

    The question on whether replacing microwave with gamma rays will do the same thing isn't as trivia to answer. Gamma rays have such high frequency. One has to figure out if an oscillating E field at such a rapid rate will produce (i) sufficient field-emitted electrons and (ii) produce sufficient time for the electrons to gain energy and collide with the neutrals before the field reverses direction

    Note that in the microwave case, the whole chamber (or waveguide) is flooded with the EM field. Using just a "source" is no where near the same situation, and one is expected that small amount of gamma photons would collide with an atom to cause ionization. This is not a very efficient way to make a plasma (and different than the microwave case), and certainly the high the vacuum level, the less likely one would get such an ionization.

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