I was studying cathode ray experiment and there the electrons were obtained by ionisation of the gas inthe tube by the high voltage on the electrodes. So how does this happen?

  • 2
    $\begingroup$ Search for Townsend avalanche and Paschen curve. $\endgroup$
    – Paul
    Commented Oct 1, 2021 at 5:54

1 Answer 1


There is always a small amount of ions and electrons in gases due omnipresent radiation ( cosmic radiation and radioactive decays. Depending of scenario, there is possible an electron injection by thermo-emission ( heated cathodes ) or by pin-point electrode geometry ( various DIY ozone or light negative ion generators ).

When voltage is applied on the tube electrodes, ions/electrons are accelerated and collisions create other ions. At normal pressure, the path between collisions is too short (mean free flight path for air molecules is typically 70 nm), so enormous potential gradient is needed for high enough collision energy.

Low pressure allows longer acceleration paths along higher potential difference, leading to higher collision energy. Finally there is established equilibrium between ionization and ion recombination, including hitting the electrodes.

  • $\begingroup$ @Poutnik, Is there a solid reference which states that ionization only occurs because of the omnipresent cosmic background radiation. If we shield the cathode ray tube, will ionization not occur? $\endgroup$
    – ACR
    Commented Sep 30, 2021 at 13:48
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    $\begingroup$ @M.Farooq It is not limited to CBR, but radioactive decays, mostly 222Rn - typically 50-x00 Bq/m3 of air , or 50 Bq/L water, then 226Ra in buildings, 14C + K40 in living bodies. Not that sensitive detectors are built more than 200m below surface because of highly energetic and penetrating cosmic radiation. Note that top ever detectedenergy of a flying proton is energy of a flying baseball - about 50 J. another contribution can be static electricity and water evaporation. $\endgroup$
    – Poutnik
    Commented Sep 30, 2021 at 14:20
  • $\begingroup$ @ Poutnik, Sorry, I was not clear. I understand the background radiation concept. I was specifically asking about the initiation step of a highly evacuated gas discharge tube. Does the discharge in the gas tube start because of background radiation in a CRT? $\endgroup$
    – ACR
    Commented Sep 30, 2021 at 14:47
  • $\begingroup$ @M.Farooq No, it is rather sufficient yet not essential initial condition. Without external voltage there is no discharge. :-) // In CRT specifically, there is cathode thermo-emission, the same as in a vacuum diode. $\endgroup$
    – Poutnik
    Commented Sep 30, 2021 at 15:02
  • $\begingroup$ This is what I was wondering that a cosmic event may not be a necessary condition. $\endgroup$
    – ACR
    Commented Sep 30, 2021 at 15:06

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