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