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B Few questions about radioactivity/radiation

  1. Jun 5, 2016 #1
    Hi, recently I have been using some dosimeters to measure radiation sources (No worries everything is done safely)
    apart from the confusion that arises due to the different measurement systems used across the world, some older ones use Roentgens while newer ones are mostly Sieverts , and when using an older dosimeter I have to constantly recalculate to sievert because I'm more used to that scale.

    Heres a few things I would love to know.I would like to state a few things and you could tell me where i'm wrong and where i'm correct.

    Overall there's two types of radiation (not including gravitational or other forms) , particle radiation like electrons , positrons , neutrons etc , and electromagnetic radiation which is photons and comes in all frequency ranges from very low radio to THz.The frequency of EM radiation also then divides it into non ionizing (lower frequency , less powerful per photon) and ionizing (higher frequency more powerful per photon)
    Now is particle radiation also ionizing at higher energies, or is only EM radiation (photons) ionizing ?

    From what I understand both photons and elementary particles with mass (like electrons for example) can be ionizing it all depends on their energy, the difference being that photons can penetrate far deeper into a medium but electrons cannot and I assume its because photons have no mass or charge so their almost transparent while high energy electrons for example get "trapped" soon due to their charge and mass which makes them interact with nearby atoms?

    Also I want to clear another confusion that I have had for a while.I read wikipedia and other sources and so i gather that the difference between X rays and Beta radiation is that X rays are EM radiation with a specific characteristic frequency range and Beta radiation is the radiation of energetic electrons as particles.or positrons in the Beta+ case.?
    Another confusion to me , is Beta radiation only coming from nuclear reactions , more specifically radioactive decay or can it also be made by bombarding a metal target with high energy EM radiation like X rays ?

    I know from the photoelectric effect that if a sufficiently high frequency photon hits a metal target it knocks out an electron whose energy corresponds to the input energy of the photon.So from this the question about Beta radiation arises for me.

    And the last question for this post would be like this.I was in a nuclear reactor while ago and from what i observed and understand is that alpha particles are of little concern if not directly swallowed or otherwise put inside , since they are easily absorbed by skin and other thin materials.
    Beta goes deeper and has higher energies I assume but can also be stopped by fairly thin and lightweight materials.
    The biggest concern is Gamma photons and neutrons I suppose , which would explain why the reactor has very thick lead treated glass window through which the crew looks at the moment the spent fuel assemblies are taken out of the core.

    Neutrons seem to be the only ones that can activate a material which itself wasn't radioactive before or cannot be such naturally.I assume this is because a neutron has the ability to change an atoms nucleus while other forms of radiation like Gamma no matter how strong can only interact with an atoms electrons.?

    One last question about gamma absorption , why it requires very dense and thick materials to stop it?Is it because of the massless and chargeless nature of the photon so it travels some distance through the material unaffected and then when it finally interacts with the medium it travels through, it causes some secondary emission and isn't easily stopped with the first time? Like it interacts with an electron through the photoelectric effect and then the ejected electron further interacts , only I suppose the secondary emitted electrons or electron positron pairs have a much higher chance and less time and distance to interact with the medium than the incoming photon that created them.

    Thank you for your time.
  2. jcsd
  3. Jun 6, 2016 #2

    Simon Bridge

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    Radiation is "ionizing" if it can result in ionisation of atoms or molecules that it interacts with ... so the short answer is "yes". The mechanism for the ionization is different.

    This is mostly correct, charged particles do not penetrate as far because of interactions with the electromagnetic fields. As a result, these particles slow gradually and come to rest at a distance related to their initial kinetic energy. This means the radiation levels do not decrease much before they have stopped.
    The kinds of particles you are thinking of would mostly interact with the electrons in matter. Higher energy particles may interact with the nucleus - and cause nuclear interactions and disintegrations.

    Photons are absorbed in an all-or-nothing fashion: either the atom can absorb it or not. If it can, it absorbs all the energy of the photon, maybe ionizing as a result, maybe leaving the atom in an excited state. This means EM radiation decreases exponentially as it passes through matter.

    But what about neutrons?

    This is correct.
    X-ays are made up of photons, beta radiation is electrons or positrons.

    Beta radiation is defined as coming from the nucleus of atoms.
    You can also get beams of electrons from other sources... for instance, cathode rays are made up of electrons as well.
    The label is historical.

    Electrons in the photo-electric effect come from the outer shell. Beta decay electrons come from the nucleus.

    Sort of. Neutrons are convenient for this purpose because they are neutral, and so get past the outer electrons easily. They are also relatively cheap.
    But other things can make a nucleus unstable, and nuclear fusion is used to build big atoms that are naturally unstable and decay quickly.
    The nuclear industry just happens to use neutron capture to make unstable atoms

    It is a mixture of those things as well as the way that it interacts... which I touched on above.

    Since the removal of a gamma from the overall radiation is a chance event, there is always a possibility that a gamma can get through even the thickest shielding... while charged particles slow down more deterministically.
  4. Jun 8, 2016 #3
    Oh I see , so an unstable nucleus that has too many neutrons to be stable undergoes a beta decay emitting a proton , electron and another particle i'm less familiar with , I read this in wiki.Now I understand why you said it comes from the nucleus , like it wasn't there but it emerged from it as the transition happened.

    Does beta decay happen in the naturally unstable/radioactive elements like Uranium or does it emerge only after we reach critical mass and the fission has made it's first transition elements from the main element Uranium?

    P.S. what did you mean when you said , What about neutrons ?
  5. Jun 8, 2016 #4


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    It doesn't emit the proton, the proton stays in the nucleus. The "another particle" is a neutrino.
    Yes, although uranium is not a good example because most of its isotopes mainly decay via alpha radiation.
    You don't need a critical mass of anything for that. Uranium naturally decays to other radioactive elements, and sometimes it also fissions spontaneously. There are also many radioactive isotopes that are completely unrelated to uranium.
  6. Jun 8, 2016 #5
    The Roentgen and the Sievert are units for different quantities. The Roentgen is a measure of x-ray exposure in terms of the charge liberated per unit mass of exposed air. i.e. Coulombs /kg in fact 1R is 2.54x10-4 C/Kg.

    The Sievert is a measure of biological equivalent dose in terms of Joules/Kg of tissue. Equivalent dose is a quantity that takes into account the biological effects of different ionizing radiations which increase from x-ray/gamma ray to beta, to protons, alpha particle to neutrons. For x-ray/gamma rays the biological equivalent dose is the same as the physical dose which is measured in Grays measured also in Joules/Kg of tissue.

    With regards to the interaction of gamma rays with matter the Compton effect is the predominant mode of interaction for light elements. shifting to the PE effect as the material becomes heavier. But at about 700Kev for heavy elements the Compton effects predominates again. Above 1.02 Mev pair production begins to come into play for heavy elements becoming significant at about 3 MeV for Lead. In any event there is strong energy dependence for these interactions.

    For heavy particles as protons and alpha particles at high energies like 100Mev they loose energy at an almost constant rate until the energy drops to about 10% of the original whereupon the rate of energy loss increases dramatically until fully dissipated.
  7. Jun 8, 2016 #6


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    You need energies of several GeV (1GeV=1000MeV) for this almost constant rate ("minimally ionizing particles"). Radioactive decays with protons or alpha particles are way below that energy.
  8. Jun 8, 2016 #7


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    Neutrons can make a material radioactive, because the nuclei will then have an additional neutron, and therefore can be in an unstable state.
    High energy photons -such as gamma radiation- can make a material radioactive, because they can knock off protons-neutrons from the nuclei, the resulting nuclei may be unstable. (If you put something in a high energy linear accelerator, it will become radioactive)
  9. Jun 8, 2016 #8
    My definition is a little more loose than yours i should have said that the energy loss varies slowly.

    Bragg Curve for 205 MeV protons. Range in HDPE is 26.100 cm where the peak of the curve occurs. The LET at the entrance point is 0.4457 keV/micron in water.

    This shape is characteristic of heavy particle energy loss with depth.
  10. Jun 9, 2016 #9
    Many kinds of high energy particles can make a material radioactive. Protons, electrons, gammas, etc. But note that the required energy is above energy range typical for nuclear reactors (below 10 MeV). For energies typical for nuclear reactors and radioactive decays, only neutrons are activating IIRC.
  11. Jun 10, 2016 #10
    So the Beta decay produces an outgoing electron and neutrino , now I wonder why the electron and neutrino goes outside the nucleus but the proton stays in the nucleus , has that to do with the protons larger mass and charge , or other forces ?
  12. Jun 10, 2016 #11


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    The whole nucleus is involved in a beta decay, the final product is really the new nucleus plus electron plus neutrino. You cannot say "this proton came from the decay", there is no such specific proton.

    There is not enough energy to have a proton fly away from the nucleus.
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