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Like what the title says, my question is if a beta particle is just an electron, what causes it to be dangerous?

I understand that a beta particle is not only an electron but it is also one with a lot of energy.

I wish to understand what happens when this electron hits a living cell, the effects, and why it is dangerous.

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    $\begingroup$ Its energy. Anything is dangerous given enough energy, including ordinary nitrogen and oxygen molecules. $\endgroup$ – Oscar Lanzi Oct 3 '19 at 1:14
  • $\begingroup$ @OscarLanzi What does Tornado Wikipedia page have to do with beta-particles? $\endgroup$ – andselisk Oct 3 '19 at 1:17
  • $\begingroup$ "Anything" (not just electrons) is dangerous given enough energy, in that case air. $\endgroup$ – Oscar Lanzi Oct 3 '19 at 1:19
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    $\begingroup$ @QuesoPez It looks like you've already answered the question from the title, and the remaining part regarding interaction with living tissue looks like a homework question. $\endgroup$ – andselisk Oct 3 '19 at 1:19
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    $\begingroup$ Not really the same but one can as well ask " if a book [thrown towards you] is just a bunch of paper sheets, what causes it to be a blunt body/dangerous? ". This is not to make fun of you, but rather to let you think and likely understand the analogy. $\endgroup$ – Alchimista Oct 3 '19 at 8:51
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We should be able to answer questions at the level of the questioner.

A beta particle is described as an electron with a lot of energy. We generally use this term to describe electrons emitted by a nuclear transformation. Different atoms emit electrons with different energy: the energy of the electrons emitted by tritium are so low (5.7 keV) that it is difficult to observe them with a Geiger counter (they won't penetrate the walls of the counter) https://en.wikipedia.org/wiki/Common_beta_emitters , but some nuclei emit very high energy electrons: Cesium-137 emits particles with an energy of 661 keV, so high that the energetic electron can eject more electrons from matter - that is, they can ionize lots of molecules in the human body and cause major changes in DNA - and that could lead to serious effects, like cancer https://en.wikipedia.org/wiki/Beta_particle .

For a reference, x-rays (photons, not electrons) have energies between 100 eV and 100 keV https://en.wikipedia.org/wiki/X-ray . While the energies of x-rays can be high enough to cause damage, they are used at low doses and for such a short time - just enough to be caught on film or CCD devices - such that the damage to the body is small enough to be repairable. Radioactive elements might be in contact with the body for a much longer time and have a much larger total effect.

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Danger is a relative term. In general for beta-emitters it's not dangerous to be in the same room. Even a relatively hard beta, like P32 (around 1.7 million electron volts) will not penetrate its container. If ingested and absorbed into the body, however, the beta-emitter will be in close proximity to the other molecules of the body. So its energy can cause disruption of those molecules. Even a relatively soft beta, like tritium's (around 15 thousand electron volts) can cause problems if enough is ingested. One damage mechanism is mutations. Another is formation of poisonous compounds such as free radicals.

Nucleic acids (RNA and especially DNA) carry information regulating the functioning of the cell. Radiation-incduced mutation of the DNA could cause the cell to behave badly, such as multiplying without control, forming a cancer.

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