Detecting Anti-Neutrinos From Distant Fission Reactors Using Pure Water At SNO+

Although neutrinos are exceedingly common, their near-massless configuration means that their presence is rather ephemeral. Despite billions of them radiating every second towards Earth from sources like our Sun, most of them zip through our bodies and this very planet without ever interacting with either. This property is also what makes studying these particles that are so fundamental to our understanding so complicated. Fortunately recently published results by researchers behind the SNO+ neutrino detector project shows that we may see a significant bump in our neutrino detection sensitivity.

The Sudbury Neutrino Detector (Courtesy of SNO)
The Sudbury Neutrino Detector (Courtesy of SNO)

In their paper (preprint) in APS Physical Review Letters, the researchers describe how during the initial run of the new SNO+ neutrino detector they were able to detect anti-neutrinos originating from nuclear fission reactors over 240 kilometers away, including Canadian CANDU and US LWR types. This demonstrated the low detection threshold of the  SNO+ detector even in its still incomplete state between 2017 and 2019. Filled with just heavy water and during the second run with the addition of nitrogen to keep out radioactive radon gas from the surrounding rock of the deep mine shaft, SNO+ as a Cherenkov detector accomplished a threshold of 1.4 MeV at its core, more than sufficient to detect the 2.2 MeV gamma radiation from the inverse beta decays (IBD) that the detector is set up for.

The SNO+ detector is the evolution of the original Sudbury Neutrino Observatory (SNO), located 2.1 km below the surface in the Creighton Mine. SNO ran from 1999 to 2006, and was part of the effort to solve the solar neutrino problem, which ultimately revealed the shifting nature of neutrinos via neutrino oscillation. Once fully filled with 780 tons of linear alkylbenzene as a scintillator, SNO+ will investigate a number of topics, including neutrinoless double beta decay (Majorana fermion), specifically the confounding question regarding whether neutrinos are its own antiparticle or not

The focus of SNO+ on nearby nuclear fission reactors is due to the constant beta decay that occurs in their nuclear fuel, which not only produces a lot of electron anti-neutrinos. This production happens in a very predictable manner due to the careful composition of nuclear fuel. As the researchers noted in their paper, SNO+ is accurate enough to detect when a specific reactor is due for refueling, on account of its change in anti-neutrino emissions. This is a property that does not however affect Canadian CANDU PHWRs, as these are constantly refueled, making their neutrino production highly constant.

Each experiment by SNO+ produces immense amounts of data (hundreds of terabytes per year) that takes a while to process, but if these early results are anything to judge by, then SNO+ may progress neutrino research as much as SNO and kin have previously.

32 thoughts on “Detecting Anti-Neutrinos From Distant Fission Reactors Using Pure Water At SNO+

  1. Our physics prof. at Warsaw University of Technology told us that if we gathered all neutrinos existing in our universe they would fill a teacup. That’s how small they’re. I don’t know if it’s true but probably it is.

          1. Two out of the three need to be massive – the third state doesn’t necessarily have to have mass, since we only have evidence for differences at this point (so the third state could be zero).

            The neutrino masses *could* be from the Higgs mechanism, although that would be odd given the scale. It’s true that we only observe left handed neutrinos (and right handed antineutrinos) but that could be because the only interactions we see are weak interactions which force that handedness.

    1. I did the maths!
      diameter of a neutron is ~1.7×10−15 meters
      1 cup is equivalent to 250 milliliters (mL) 0.25 Liter (L)= 0.00025 cubic meters
      a cube can at most be 74% packed with spheres so 0.00025 x 0.74 = 0.000185 cubic meters of spheres

      Long story short this works out to be about 5×10^40 neutrons packed inside 1 cup

      The sun contains about 10^57 atoms of hydrogen
      And in the universe there is an estimated to be about 10^23 stars
      So about 10^80 atoms

      I’m going to disagree with your physics prof.

      1. Neutrinos aren’t neutrons. Neutrons aren’t fundamental, so you can calculate “size” a little more meaningfully because they’re already a bound state themselves, and you can measure that (just like an atom has size).

        For fundamental particles “size” is a little goofy, but you can imagine thinking of their “mass size” as the equivalent Schwarzschild radius of their mass. For neutrinos, that’s… mind-bogglingly tiny (and might be zero for one type!). But there are lots upon lots of neutrinos in the Universe.

        1. Wow, I was totally wrong, at a very early age was told the a Neutron was inside the atom and a Neutrino was outside the atom. Well today is a good day I unlearned something wrong and learned something new!

    2. That’s… certainly not true. Sounds cool though.

      The “fundamental limit” to the “size” of a particle is “how much can I compress these guys together?” For photons, for instance, you can compress them as much as you want. There’s absolutely no limit – they have no “size” whatsoever. Even if you glom together enough photons to create a black hole, there’s no lower limit on what individual energy each photon has, so you could still say there’s an infinite number of them, and the individual size is zero.

      For protons, electrons, etc. – they do have a size, but it depends on what interactions you’re talking about (how they’re bound). You can talk about their “charge radius” or their “mass radius,” etc.

      The issue with neutrinos is that *two* of them have to have mass (because they flavor oscillate) but technically… the third doesn’t *have* to. It… probably does, but it doesn’t *have* to, as of right now. And so if it’s really massless, and you gather all of *those* guys together, again, it’s the same as the proton.

      But if you gather *all* of them, it would absolutely not fit in a “teacup” – because we *know* that there’s a relic density of neutrinos which comes from known physics, and it’s directly related to something we can measure – the CMB. The Big Bang’s leftover neutrinos are around 100 per cubic centimeter of each type. And the *heaviest* of the three has to be at a minimum around a milli-electronvolt (1e-3 eV). This isn’t all the neutrinos! There are certainly more! But we *know* these exist, and we know the numbers very well.

      Now, this doesn’t sound like a lot, but when you integrate over *the observable universe* (around 50 Gly)… yeah, those numbers get way bigger. 1 ly is roughly 1e18 cm, so 1 Gly = 1e27 cm, now cube it and it’s 1e81 cubic centimeters, or around 1e83 neutrinos, with a *minimum* total mass-energy equivalent of 1e80 eV, or 1e44 kg. (I don’t care about pi, or factors, or whatever. Doesn’t matter, as you’ll see).

      Which is, oh, 100 trillion solar masses or so. And if I shove *that* together as close as I possibly can… I get a black hole. The size of which is roughly *thirty light years* in radius.

      To paraphrase Winston Zeddemore… that’s a big teacup.

      (Note: if I do the same thing for the *galaxy*, it’s still around two miles or so).

  2. At least we know why UAP are interested in Nuclear reactors now, they must be emitting some kind of particles we haven’t yet discovered, and maybe these particles affect the way UAP drives function.

  3. I visited SNO a few years back, I have a picture somewhere of the control room on top of the giant ball and the electronics racks. Getting to the underground part of the lab is quite a trip, SNOlab is a fairly impressive facility.

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