neutrinos are incredibly hard things to
detect from the Sun at this moment I
think it's about a hundred billion
neutrinos per second is part are passing
through your little finger and so it's a
truly ludicrous number of these
particles if you actually want to stop
one neutrino you need about four
light-years of lead to have a good
chance of making it stop
yeah so this is kind of an oozy one so
an article just appeared in the journal
Nature where they have detected
neutrinos from the Sun and a rather
different kind of neutrino than have
been detected previously from the Sun I
guess we need to back up a little bit
and talk about the neutrinos and where
they come from
okay so stars like the Sun are powered
by nuclear fusion in their cores
what's happening in the middle of the
Sun is by a series of nuclear reactions
you're turning hydrogen into helium and
hydrogen is one proton and the helium
you make consists of two protons and two
neutrons and so it's not just a matter
of sticking protons together because you
can't make two protons and two neutrons
by just taking protons together you have
to transmute some of those protons into
neutrons and so somewhere the nuclear
processes has to change protons into
neutrons and that's a thing called a
weak force interaction but a by product
of those weak interactions is you always
end up throwing out a neutrino as part
of the process the fact that the son is
turning hydrogen into helium means that
you have to be making neutrinos in in
the process for a good number of years
now the beam experiments to look for
solar neutrinos indeed we've been
detecting solar neutrinos but up until
now the only solar neutrinos that have
been detected are the very high energy
once they're all and and it turns out
the vast bulk of neutrino is a much
lower energy neutrinos the main reaction
that produces neutrinos is in the Sun is
where you take two protons smack them
together and end up creating the thing
called deuterium which is heavy hydrogen
which is one proton and one Neutron so
you can see you started with two protons
you've turned it into one proton and one
Neutron so therefore you must have done
one of these weak interactions and
therefore there has to be this byproduct
of a neutrino and it's sort of the first
step of turning hydrogen into helium is
to take two hydrogen atoms smack them
together create eutteum in fact it's
sort of the rate limiting step if you
just keep smacking protons into each
other turns out it would take you about
ten billion years of smacking them into
each other to actually convince them to
gun to go this reaction so it's a very
very slow process but fortunately
there's an awful lot of hydrogen in the
Sun and the Sun has a very long lifetime
and therefore actually this is you know
the fact that this is a long time scale
process actually it's probably what
dictates the lifetime of the Sun is the
fact that this this reaction is so slow
so you also have to create another
particle so you have to create a
positron in the process so actually the
charge is conserved as well because the
actual reaction is proton plus proton
goes
to proton the neutron in deuterium plus
a positron which carries away the extra
bit of charge plus a neutrino I've
always been aware that the Sun is
showering us with neutrinos is it not
showering us with positrons as well then
no positrons don't last very long
because a positron will very soon meet
up with an electron and then they'll
annihilate and turn into a couple of
gamma rays and that's this is what
remember this is all going on right in
the core of the Sun and all that is
opaque to us because actually any
positrons or gamma rays or anything else
that gets created down there scatters
around and takes literally thousands of
years to make its way out of the Sun
whereas neutrinos because they interact
so little with anything once you've
created the middle they stream straight
out of the Sun so the neutrinos we're
detecting now were made very recently in
the Sun whereas the rest of the energy
we see from the Sun has taken this long
part to get out that's the key reaction
that's going on in the center of the Sun
it's the thing that's creating most of
the energy in the Sun but those are the
neutrinos that have been previously
undetectable because they're too low
energy the previous generations of
neutrino detectors have not been able to
detect them and now with this new
generation a neutrino detector they're
finally reaching a point where they can
actually start detecting these sort of
core neutrinos being produced by the Sun
the ones that they were detecting the
high-energy ones where were they from so
in additions I mean there were lots of
reactions as I say you have to somehow
turn your protons into helium and
there's lots of different channels you
can go go down to actually do make that
reaction happen some of which involve
creating your heavier things than helium
which then split up again and so on so
this quite a complicated set of a
channels you can go down and some of
those more obscure channels produce very
high-energy neutrinos so those are the
ones that were being detected before if
you actually want to stop one neutrino
you need about four light-years of lead
to have a good chance of making it stop
okay they reel because they just don't
interact they interact so weakly with
matter they'll just happily go streaming
they'll go streaming through the earth
they'll go streaming through you through
your little finger through several
light-years of lead you need an awful
lot to stop them but of course because
there's so many of them the fact that
the probability of stopping in the
individual one is very low really
doesn't matter because you know soon if
you've got 100 billion of them passing
through your little finger every second
if you think about building a nice big
detector then there'll be trillions and
trillions your neutrinos going through
per second even though the chances of
any one of them interacting is very
small
or a few will and so this particular
detector is a uses an organic
scintillator scintillator is just
something which gives out light and what
happens is a neutrino goes through very
occasionally one of these neutrinos will
bump into an electron as it passes
through give the electron a little bit
of a kick so suddenly the electrons got
some energy and the and then the nature
of these scintillators is that the
electron then dumps that energy into the
scintillator makes a little flash of
light and you have a whole array of
photomultiplier tubes of very sensitive
light detectors all around it that just
look for these little flashes of light
and so they essentially detect the
neutrinos by these very occasional
interactions leading to these
interaction in the scintillator which
produces a tiny flash of light what does
it take for a neutrino electron
collision to be sufficient for this to
happen does it have to hit it smack in
the face or at a certain angle or an
electron that set in a certain state
itself or it really it's just the very
rare interaction I mean the there's no
particular threshold for this it's just
that the vast majority of the time the
the neutrino and the electron take no
notice of each other at all and this is
once in a while you get close and you
know if you like you can think about
them as a close enough collision that
actually it will bump into it and dump a
decent amount of energy into the
electron so it is just proximity it is
just smacking it in the face that's
really the way that people that I mean
you know you can think about them in two
ways you can think about it as a
probability or you can think about the
the neutrino sort of having a
cross-section and if you happen to get
within that cross-section it's a hit and
you get an interaction but it's not you
it's probably not right to think of the
neutrinos a little hard ball in that
sense I can see how previously the
high-energy ones were easier to find
they were more likely to shove an
electron what did they change that made
it possible to find these lower energy
ones so there's a couple of things
firstly the particular kind of
scintillator they're using is capable of
actually its will scintillate even with
these lower energy interactions and
secondly they've gone to enormous
lengths to push the background down
because as well as these neutrinos
creating these little flashes of light
other things can create little flashes
of light in your detector as well for
example if you've got cosmic rays
hitting the detector so other particles
from space nothing did you do with
neutrinos they can cause scintillations
in the detector so they stick the whole
thing a mile underground any radioactive
material
in the detector itself in the body of
the detector in the scintillator or
whatever it is then the radioactive
decays of that will also create little
flashes of light so they've gone to
enormous lengths for example there's a
radioactive isotope of carbon that
decays and produces you know will also
produce these dumps energy into the
scintillator produces flashes of light
the way they've dealt with that is they
built the scintillator is made of
organic molecules which have lots of
carbon in them so that's a bad news so
they used extremely old oil deposits to
make this inflator
so you get the carbon from very very old
oil deposits where almost all the
carbon-14 is already decayed even with
that being the case it's still one of
the dominant sources of background in
this thing it's brighter than the
neutrinos are so it's actually quite a
subtle measurement you have to make in
order to dig out from all these possible
sources of confusion where those
neutrinos from the Sun from this basic
reaction are coming from at some level
we've got these two measures as to the
the luminosity of the sunlight one is
how bright the Sun appears to be in the
sky and remember so it takes tens of
thousands of years for that light to
actually get to the surface and the
other is the neutrino flux is telling us
how much energy is being produced right
now so on least on that we can compare
the two to see whether the sun's
brightness has changed on there ten
hundred thousand year timescale by
comparing what's going on now in the
center of the Sun from what was going on
many tens of thousands of years ago so
in that sense it's a sort of it is at
least to check that the Sun is sort of
behaving itself on that time scale which
we've never been able to measure before
I guess it would have been a bit of a
surprise if we found anything weird that
way and it does turn out that actually
the Sun seems to behave in more or less
the way we'd expected at the bay so
neutrinos are like real time Sun and
light is the Sun thousands of years ago
exactly so if ever the neutrino flux
stops from the Sun then we know that in
you know thirty thousand years time
we're going to be in deep trouble it's
so difficult to detect these so the way
that they tend to be detected is deep
underground
so the original experiment the first one
for example that was there to detect
neutrinos from the Sun was actually a
big tank of cleaning fluid carbon
tetrachloride