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I Neutrino Experiment Question

  1. Aug 15, 2016 #1
    I am completely unschooled here, so go a bit easy, if you would.

    My question(s) involves the actual mechanics of the T2K neutrino/antineutrino experiment that was written up here on Aug. 8.

    I have a hard time understanding the process described here:

    "To explore the (anti)neutrino flavour changes, known as osciallations, the T2K experiment fires a beam of (anti)neutrinos from the J-PARC laboratory at Tokai Village on the eastern coast of Japan.

    It then detects them at the Super-Kamiokande detector, 295 km away in the mountains of the north-western part of the country. Here, the scientists look to see if the (anti)neutrinos at the end of the beam matched those emitted at the start."

    1. How is the beam of (anti)neutrinos physically fired?

    2. How does the detector differentiate the "fired" (anti)neutrinos from regular(?) (anti)neutrinos?

    With thanks,


    Read more at: http://phys.org/news/2016-08-evidence-mounts-neutrinos-key-universe.html#jCp
  2. jcsd
  3. Aug 15, 2016 #2


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    I don't exactly know how are doing that, but in my opinion "firing (anti)neutrinos" is simply achievable just by putting some beta decaying radioactive material in the other side.
    Beta decaying radioactive material decay by emitting an electron or a positron, and an electron neutrino or an electron antineutrino, depending on which isotope they are.
    For example, the Na22 isotope decays by emitting a positron and an electron neutrino. And Sr90 decays by emitting an electron and an electron antineutrino.

    Neutrinos, by nature, oscillate into other flavours by time. In one end you fire electron neutrinos or antineutrinos, in the detector you count the number of electron, muon and tau neutrinos coming from the lab.

    Again, I don't really know how the detector works, but;
    The Super-Kamiokande detector is positioned deep under earth, so that other particles (cosmic radiation etc.) don't interfere with the experiment. In my opinion, the high energetic neutrinos coming from the space are more reactive than the low energy neutrinos, and can maybe be blocked by the Earth's surface. And most of the low energy neutrinos just pass through the whole earth, so that not many of them are detected in the detector.
    On the other hand, the detector might help us differentiate between the experiment neutrinos and the cosmic neutrinos by comparing their kinetical energies and coming directions.
  4. Aug 15, 2016 #3


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    A Google search for "j-parc neutrino beam" led me to this page:


    The mechanism described there is basically the same as the one Fermilab used when I participated in a (anti)neutrino experiment 35-40 years ago as a graduate student.

    1. Fire a beam of high-energy protons into a target, which produces a lot of particles, mostly pions.
    2. Use a magnetic field to select positive or negative pions and focus them so they're going more or less in the same direction. (the "horns" indicated on the diagram)
    3. The pions decay into muons plus neutrinos (or antineutrinos, depending on whether the pions/muons are positive or negative)
    4. Put something in the beam (a "beam dump") to absorb the muons and let the neutrinos or antineutrinos pass through.
  5. Aug 15, 2016 #4
    Many thanks to both of you for your responses- I appreciate your help.

    Best regards,

  6. Aug 17, 2016 #5


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    That would not give a beam, the neutrino production rate would be completely negligible, and the neutrinos would be too low-energetic to distinguish them from other sources. The beam is produced via accelerators, as jtbell explained.
    Earth does not provide any relevant shielding against neutrinos. The detectors are underground to reduce the flux of other particles, in particular muons produced in the collisions of cosmic rays with our atmosphere - the rate of muons goes down significantly if you go underground.

    The neutrinos from Tokai have a higher energy than most other neutrinos, they all come from the same direction. Both together allow to distinguish between those and neutrinos from other sources.
    In addition, the accelerator does not run 24/7, while it is shut down you can count the neutrino rate to estimate the background from other sources, and subtract it from the rate measured while the accelerator is running.
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