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Trouble For Dark Energy Hypothesis?
In article , jacobnavia
writes: Le 23/01/2018 22:33, Steve Willner a écrit : In article , Gary Harnagel writes: The neutrino flux would be red-shifted by z ~ 1100 also, This ignores the part about the neutrinos having decoupled long before the photons. One source, which seems to be a textbook by Daniel Baumann at Cambridge: http://www.damtp.cam.ac.uk/user/db27...y/Chapter3.pdf gives a neutrino decoupling redshift of 6E9. That corresponds to an energy of about 1 Mev and a time about 1 s after the Big Bang. Cosmological neutrinos should therefore have a kinetic energy today of about 1/6 meV (i.e., milli-, not mega-). As the OP wrote, that's very far from detectable. It depends on your antena's neutrino sensitivity. I'm no expert here, but the energy of neutrinos in the cosmic neutrino background have energies orders of magnitude below that needed for the reactions used in conventional detectors. Why do neutrinos react with some Chlorate compounds? Isn't it a consequence of the geometry of the collision? I'm not sure what you mean here, but even if it were, the energies are too low. I'm sure some expert will weigh in on the "direction" stuff. |
#12
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Trouble For Dark Energy Hypothesis?
On 24/01/2018 22:55, jacobnavia wrote:
Le 23/01/2018 22:33, Steve Willner a =C3=A9crit : In article , Gary Harnagel writes: The neutrino flux would be red-shifted by z ~ 1100 also, This ignores the part about the neutrinos having decoupled long before the photons. One source, which seems to be a textbook by Daniel Baumann at Cambridge: http://www.damtp.cam.ac.uk/user/db27...y/Chapter3.pdf gives a neutrino decoupling redshift of 6E9. That corresponds to an energy of about 1 Mev and a time about 1 s after the Big Bang. Cosmological neutrinos should therefore have a kinetic energy today of about 1/6 meV (i.e., milli-, not mega-). As the OP wrote, that's very far from detectable. It depends on your antena's neutrino sensitivity. Which is so poor that they evaded detection for quite a while. It took bulk tanks and exquisite experimental technique to see anything at all. When they did it was about a third of what was expected. Why do neutrinos react with some Chlorate compounds? Neutrinos and anti-neutrinos barely interact with anything but when they do if they hit the right thing with enough energy they drive the proton to neutron conversion that underpins fusion backwards. Proton proton fusion is : p + p + e- = pn + e-neutrino + energy Neutron to proton is: n + e-neutrino + energy = p + e- The significance of chlorine and gallium is mainly that they are easily purified liquids and the product of a neutrino interaction can be isolated and identified from the bulk unchanged material (as argon gas). I think there may be a little bit of resonance enhancement in some nuclei too and in a tight spot every little helps. The crucial point about the whole exercise is that you have to be able to detect the absolutely miniscule amount of product which limits your choices to something where the resulting species is radioactive with a suitable half life and separable from the bulk material. If they are lucky they get a dozen or so atoms converted in many tonnes of material in each experimental run. See: http://www.slac.stanford.edu/econf/C...rib/hahn_r.pdf Isn't it a consequence of the geometry of the collision? It takes a direct hit and a lot of luck for anything to happen. It also depends critically on the energy of the incident neutrino - higher energy ones giving you more options for detection a la super Kamiokande. What about putting a ring of neutrino sensitive atoms orientable with an outer magnetic field and just trying to point to the sun? Neutrinos mostly pass right through the earth without hindrance. You can't point a detector at anything. The best you could hope for would be to arrange a series of big detectors so that they partially self shadow and use timing details to do indirect imaging. Supernova neutrino detection where there is a good pulse at high energy is now realistic: https://arxiv.org/pdf/1205.6003.pdf And with a bit of cunning they can get some directionality out of it. We could turn around the chlorine with its ring until we see what direction and position should the chlorine have to intercept at best neutrinos coming from a specific direction. Why does underground chlorine detectors work? Enough deep shielding to prevent confusing side reactions of protons and muons from masquerading as neutrino events. Together with the right combination of properties in the Cl37 atom and Ar37 product to allow detection of a reaction which is happening to neutrons in everything. If the product of a neutrino reaction is stable, too radioactive or not radioactive enough you have no way of distinguishing it. Bceause among the millions of atoms, one has the right orientation to exactly trap a neutrino. Because the size nucleus is tiny compared to the volume of an atom and the probability of a neutrino reaction occurring is even smaller. Neutrinos mostly go straight through the Earth as if it wasn't there. Having an array of neutrino detectors at a molecular level would increase sensitivity and positioning. Or not? Not. There might be something in forcing their *nuclear* spins to align with a strong magnetic field but I doubt if it is realistically possible or would give any worthwhile advantage in sensitivity. -- Regards, Martin Brown |
#13
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Trouble For Dark Energy Hypothesis?
In article ,
jacobnavia writes: Why do neutrinos react with some Chlorate compounds? Isn't it a consequence of the geometry of the collision? Not that I'm aware of. The chlorine-37 experiment has an energy threshold of 814 keV. Neutrinos with lower energy cannot be detected by that type of detector. Other detectors have lower thresholds but still in the many-keV range. I can't imagine any hope of detecting milli-eV neutrinos, though perhaps some very advanced civilization might find a way. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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