[HN Gopher] Physicists produce neutrino images of Milky Way galaxy
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Physicists produce neutrino images of Milky Way galaxy
Author : _Microft
Score : 70 points
Date : 2023-06-29 19:22 UTC (3 hours ago)
(HTM) web link (drexel.edu)
(TXT) w3m dump (drexel.edu)
| oblib wrote:
| This link is to the image shown in the article but it's a lot
| larger and it's an animated gif.
|
| https://drexel.edu/news/~/media/Drexel/Core-Site-Group/News/...
| sbierwagen wrote:
| If you want to see the actual diagram, rather than the composite
| overlaying it with a visible light image of the milky way, here
| is figure 4 from the paper:
| https://res.cloudinary.com/icecube/images/q_auto/v1671570509...
| (From the media gallery here:
| https://icecube.wisc.edu/gallery/high-energy-neutrinos-from-... )
|
| As you'd expect, the resolution from a 86x60 neutrino detector
| array is not great.
| causality0 wrote:
| Right. That doesn't seem to line up super well with the
| galactic disc. Even those diagrams look like they've had an
| awful lot of smoothing.
| _Microft wrote:
| > _" In the Milky Way Galaxy, cosmic rays (high-energy
| protons and heavier nuclei) interact with galactic gas and
| dust to produce both gamma rays and neutrinos."_, from the
| article
|
| The detected neutrinos were not necessarily produced in the
| vicinity of the source of the cosmic rays as I understand it.
| Imagine it like a bank of fog lighting up in the night
| because a car with headlights is moving towards it.
| NoMoreNicksLeft wrote:
| I don't think I understand any of this. You have to bury a few
| zillion tons of water in a pitch-black salt mine a mile
| underneath the surface, and run detectors that see a dim flash of
| light when one of these hits a water molecule, right?
|
| How is that in any way directional? Or is there a way to compile
| an image from this without directionality?
| _Microft wrote:
| The "dim flash of light" is called Cherenkov radiation and
| there's directional information encoded in it. Depending on
| which (or when a?) particular detector saw that light, the
| direction of the incoming particle can be calculated.
|
| https://en.wikipedia.org/wiki/Cherenkov_radiation
| SpectralName wrote:
| In addition to the other answers already posted, the neutrino
| may hit multiple water molecules along its path, or its decay
| products may hit other molecules themselves, so you get many
| flashes if you're lucky.
|
| But another category of detector [1] adds additional signal by
| applying a strong, constant electric field vertically across
| the entire detection chamber (heavy noble gas, not ice, in this
| case). Then whatever charged particles are produced drift up to
| the top of the tank, are annihilated there, and you get a flash
| that gives you extra good localization in the z direction since
| you know how long it took them to get there.
|
| [1] https://en.m.wikipedia.org/wiki/Time_projection_chamber
| Taniwha wrote:
| Icecube is at the south pole - the zillions of ton s of water
| is just ice
| sjackso wrote:
| The detector is made of thousands of light sensors arranged in
| a a km-scale 3D grid. When a neutrino interaction causes a
| flash of light inside this volume, multiple sensors detect the
| photons with nanosecond-level time resolution. So a 3D map can
| be made of where the light started and how it moved over time.
|
| For a macroscale analogue, imagine a large 3D grid of
| microphones all recording sound. If you fired a cannon from
| inside this grid and looked at the waveforms from all the
| microphones, you could work out where the cannon was, and also
| form a pretty good guess of what direction it was pointed.
| 0PingWithJesus wrote:
| The only neutrino detector placed in a salt mine was the IMB
| detector. That detector was ~10kt of water observed by ~2
| thousand photo-detectors, it was located ~600meters
| underground. The only neutrino detector that's a mile
| underground was the SNO detector, which was ~1kt of water,
| observed by ~10 thousand photo-detectors. The SNO detector is
| still running today as the rechristened SNO+ experiment.
|
| Both the IMB and SNO detectors used electron scattering to
| observe neutrinos, a neutrino comes in and bumps into an
| electron orbiting an atom, the electron & neutrino both then go
| flying off. The electron will usually go off in the same
| approximate direction that the neutrino was traveling,
| conservation of energy and momentum requires that. The
| electron, if energetic enough, emits Cherenkov radiation as it
| goes. Cherenkov radiation is just the light equivalent to a
| sonic-boom, it is emitted in a cone centered around the
| electrons direction of travel. The light from that cone is
| detected by the photo-detectors. Crucially, both the
| interaction process (electron-scattering) and the detection
| process (Cherenkov radiation) will preserve the directionality
| from the original neutrino (for the most part). The pattern of
| photo-detectors that gets hit by the Cerenkov light can be
| analyzed to reconstruct the Cerenkov cone and estimate the
| original neutrinos direction. Here's an example of an observed
| Cerenkov ring at the Super Kamiokande detector, although this
| example is very clear, the Cerenkov rings aren't always so
| obvious. https://cerncourier.com/wp-
| content/uploads/2016/07/CCthe1_06...
|
| Also the Super Kamiokande experiment used this sort of analysis
| to produce a "neutrino picture of the sun", which is kind of a
| predecessor to the OP image. https://www-
| sk.icrr.u-tokyo.ac.jp/en/sk/about/research/
|
| The IceCube detector is somewhat different. Their photo-
| detectors are buried in the Antarctic ice at various depths
| from ~1-2km and spread out over a roughly 1-cubic km volume,
| which is ~1Gt of water. I'm not exactly sure how many PMTs in
| total they have, I reckon its probably around 5-10 thousand.
| Since their PMT array is so much less dense than the previously
| mentioned experiments, they can only observe very high energy,
| very bright, light flashes. So neutrino sources that are low
| energy, like the Sun, are invisible to them. But, they can see
| sources that are very high energy, and Ice Cube's extraordinary
| size lets them observe interactions that are rare/infrequent,
| such as those from very far away galaxies.
|
| High energy neutrinos will almost always interact via "Deep
| Inelastic Scattering" (DIS), which is basically the neutrino
| hitting the protons & neutrons within an atomic nucleus. Since
| DIS is a scattering process, conservation of energy/momentum
| requires the scattered particles will preferentially travel in
| the same direction that the incoming neutrino was traveling in.
| After that Cerenkov radiation is produced from the scattered
| protons & neutrons, and that Cerenkov radiation still is
| emitted in a cone pointing in the direction of travel. So once
| again, the interaction (DIS) and detection (Cerenkov radiation)
| preserves directional information. So the pattern of which
| photo-detectors observe the light can be used to reconstruct
| that direction, and point back to the neutrinos source
| (approximately).
| wthomp wrote:
| As has been pointed out elsewhere, this is the first image of our
| galaxy in something other than light (radio, infrared, x rays,
| gamma rays are all photons).
| __MatrixMan__ wrote:
| Odd timing, given that only a few days ago we were talking
| about a gravitational wave background
| (https://astrobites.org/2023/06/28/drop-the-bass-evidence-
| for...) not that we're in a position to render that as an
| image, but it's close.
| nonameiguess wrote:
| Unfortunately, the actual paper seems to be paywalled, but
| stories like this often seem to do a poor job of motivating why
| research like this is interesting.
|
| For both this and all of the articles coming out about
| gravitational wave detection, these technologies allow us to
| sense things that can't be seen with light. Gravitational waves
| are produced by binary black hole systems and mergers, which
| don't give off any detectable radiation, and neutrinos can be
| produced by spin down of neutron stars we don't have any other
| easy way of detecting.
|
| But, these also potentially give us a window into the deep past.
| The cosmic microwave background represents the furthest back in
| time we can ever see with light, and it happened during the first
| formation of neutral hydrogen atoms when the universe first
| cooled enough to allow that, and thus light could travel without
| being immediately scattered by free electrons, which was 378,000
| years after the Big Bang. Seeing anything before that is
| impossible.
|
| Neutrinos, however, first decoupled from matter 1 second after
| the Big Bang. The possibility of being able to detect a cosmic
| neutrino background from this event would allow us to detect the
| early universe much earlier than we can with light. And if
| gravity decoupling from the strong and electroweak forces is ever
| detectable in a cosmic gravitational wave background, that would
| have happened even earlier, and represents the earliest possible
| viewing of the universe by any means whatsoever.
|
| I'm not a cosmologist and have no idea what usable data would
| ever come from being able to see these things, but keep in mind
| at least one reason we've had so much difficulty developing a
| grand unified theory and theory of everything even after
| conquering electroweak, is the inability to produce the enormous
| energies required to recouple the forces in a lab. A particle
| accelerator that could do it for strong force recoupling to
| electroweak would have to be the size of Pluto's orbit. But there
| is at least one event in nature where the necessary energy
| existed, which is the early universe. We've just never been able
| to see it.
| _Microft wrote:
| The paper is here, for those who directly want to go there
| instead: https://www.science.org/doi/10.1126/science.adc9818
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