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Does Beta Plus (positron) not cause ionisation? watch

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    In my textbook there is a table with properties of alpha, beta and gamma radiation. It says alpba causes intense ionisation, beta minus weak and gamma very weak. However the table is left blank for beta plus. Does this mean it doesn't ionise air particles at all?
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    (Original post by Freerider101)
    In my textbook there is a table with properties of alpha, beta and gamma radiation. It says alpba causes intense ionisation, beta minus weak and gamma very weak. However the table is left blank for beta plus. Does this mean it doesn't ionise air particles at all?
    It certainly does ionise gas molecules. However, positrons also annihilate on interaction with electrons (they are the antiparticle equivalent of the electron), of which there are a lot around. Annihilation produces two 511 keV gamma rays (the energy equivalent of two electron masses). So the lifetime of a positron won't be very long.
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    Most positrons dont make it out of the atom when they are emitted from a nucleus.
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    (Original post by teachercol)
    Most positrons dont make it out of the atom when they are emitted from a nucleus.
    Hmmm. Not sure about this. Whenever I've worked with positron emitters you get a hefty beta dose rate close to and can detect them some metres away (a metre or two for F-18, 3 or 4 for C-11 and 5 or 6 for O-15). They are moving at a substantial fraction of the velocity of light when emitted, and are accelerated by repulsion from the nuclear charge, so they are out of the electron cloud around the nucleus in 10-18 s or so, I guess. Unless annihilation works on that timescale, then I'd have thought they would get out.
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    I guess it depends on how big the source atom is and how much of it you've got.

    I'm suprised they get several metres. It would make PET scans pretty iffy.
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    (Original post by teachercol)
    I guess it depends on how big the source atom is and how much of it you've got.

    I'm suprised they get several metres. It would make PET scans pretty iffy.
    That's range in air. Water (which is basically what most human tissue is) is much more electron-dense so range in water is typically 5-15 mm, so that's really the limit on spatial resolution of a PET scan
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    (Original post by Cora Lindsay)
    That's range in air. Water (which is basically what most human tissue is) is much more electron-dense so range in water is typically 5-15 mm, so that's really the limit on spatial resolution of a PET scan
    This is all really intresting. But on an A level paper, if it asked the range of a beta plus particle what would be a sensible answer?

    I know I would need to range varying distances since the energy of the beta plus is split randomly with a neutrino however I'm just wondering if there is anything else I would need to add. (Anhilation with electron maybe)
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    (Original post by Freerider101)
    This is all really intresting. But on an A level paper, if it asked the range of a beta plus particle what would be a sensible answer?

    I know I would need to range varying distances since the energy of the beta plus is split randomly with a neutrino however I'm just wondering if there is anything else I would need to add. (Anhilation with electron maybe)
    Most neutrinos only annihilate after coming to rest, otherwise the energy from annihilation wouldn't equate to two electron rest mass equivalents. Because positrons are emitted at relativistic velocities, their masses are much greater when they are moving, so if they annihilate while moving, you don't get two 511 keV photons. Some events like this are seen, but most annihilations give you two 511 keV photons, which tells you the event happens when the positron isn't moving fast.

    The range of a beta plus particle won't be any different from the range of a beta minus particle of the same energy, but its eventual fate will be. Range will depend on the energy with which it is emitted, and the maximum range will be found when the particle is emitted with energy = Emax (when all decay energy is associated with the beta and none with the neutrino). Roughly, beta particle range is 4 m/MeV in air.

    Minor point- energy partitioning between beta particle and neutrino is not actually random. Among other things, it depends on the momentum of the beta and the neutrino in a complex way, wave functions for initial and final states, and interaction of the outgoing particle with the nuclear charge. Check out Fermi theory of beta decay if you want a headache
 
 
 
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