35C3 - Going Deep Underground to Watch the Stars

https://media.ccc.de/v/35c3-9913-going_deep_underground_to_watch_the_stars

Neutrino Astronomy with Hyper-Kamiokande

Neutrinos are “ghost-like” elementary particles that can literally go through walls. They can bring information from places that are impossible to observe through other means.
This talk provides a glimpse behind the scenes of a next-generation neutrino detector called Hyper-Kamiokande – a cylindrical water tank the size of a high-rise building. I will describe some of the problems you encounter when planning a subterranean detector of this size, and explain how this detector helps us figure out why the sun shines and how giant stars explode.

Neutrinos are tiny elementary particles that do not interact through the electromagnetic force. Almost like ghosts, they can literally go through walls and escape places that are inaccessible by other means, giving us a unique way of observing the interior of stars or nuclear reactors.

Hyper-Kamiokande – a cylindrical water tank that is 62 m high and 76 m in diameter – is a next-generation neutrino detector, which will be built inside a mountain 250 km northwest of Tokyo starting in 2020. The talk will give an overview on the process of designing and building a subterranean detector of this size, starting from preparations for cavern construction and ending with the design of photodetectors, electronics and data analysis.

In addition, the talk will cover selected areas of the physics programme of this detector. It will be explained how detecting neutrinos from our sun lets us figure out why the sun shines and how we can measure the temperature at its core to a precision of about 1%. Finally, I will explain how such a neutrino detector can help us watch, millisecond by millisecond, how giant stars explode in a supernova, creating many of the chemical elements that are necessary for life and computers to exist.

Jost Migenda

https://fahrplan.events.ccc.de/congress/2018/Fahrplan/events/9913.html

35c3 preroll music

Herald: Our speaker is Jost Migenda and he
is PhD student in astroparticle physics

from the University of Sheffield in the UK
and Jost is going to talk about going deep

underground to watch the stars. Please
give a huge round of applause for Jost

Migenda.

applause

J: Good morning everybody. I'm glad you
managed the first day of congress. Now

physics rarely makes highlight news. And
if and when it does it is often treated a

black box, where you pourd in money and
scientist on one end, you wait a while and

knowledge drops out. So today in this talk
I want to do this a bit differently. I

want to give you a glimpse behind the
scenes of an experiment, I have been

working on for over 4 years now. First as
part of my master's thesis and then as a

PhD student. Now earlier this year we
published a design report which is over

300 pages long and contains much more
detail about the experiment than you

probably want to know. So I'll focus on
just some of the highlights in this talk.

But before we actually talk about the
detector I'll have to introduce you to the

particles we're looking for. And that
story begins over 100 years ago with

radioactive beta decay. Now in radioactive
beta decay, you have a nucleus of one

chemical element that turns into a nucleus
of a different element and emits an

electron or in modern language we would
say a neutron decays into a proton and an

electron. Now after that was discovered
there were lots of experiments done to

measure the energy of the outgoing
electron and experiment after experiment

found that there was some variance in
energy but was always lower than expected.

And physicists at the time came up with
all sorts of possible explanations for

what might be going wrong with these
experiments but they excluded those

explanations very quickly as well. So
after a while physicists became desperate

and some pretty well-known physicists
actually thought: "Well, maybe we'll just

have to give up on conservation of
energy". So in this desperate situation a

guy called Wolfgang Pauli came up with
what he himself call "a desperate way

out". So in this letter to a group of his
colleagues which he addressed as "Dear

radioactive ladies and gentlemen", Pauli
suggested that maybe there's another

particle created in this beta decay. And
Pauli originally called this particle

neutron but of course two years later the
particle we nowadays know as Neutron was

discovered. So Pauli's particle was re-
named neutrino. Now you might be wondering

well why didn't they observe this particle
already. And the answer is very simple.

Neutrinos are like ghosts. So what I mean
by there is they can quite literally, you

know, go through walls or through your
body. And in effect we can do a little

experiment right now to try and detect
neutrinos. So to help me with this

experiment, please give me thumb's up.
Everyone? Okay so there's two things

happening right now. First thing of all
you're giving me a massive confidence

boost. But, you know, more importantly
somewhere out there the sun is shining and

it's producing a lot of neutrinos and
nuclear fusion. Now these neutrinos are

flying to Earth through the roof of this
building and then through your thumbnail.

And right now as you're listening to me
around 60 billion neutrinos are flying

through your thumbnail. 60 billion
neutrinos flying through your thumbnail

every second. How does that feel? Hmm? You
don't feel any of them? Right, so that's

how ghost-like neutrinos are. And of
course physicists are clever and shortly

after Pauli had this idea some of them
estimated that how often neutrinos

interact with normal matter and they found
that there is no practically possible way

of observing the neutrino. And that
remained true for over 20 years

afterwards. So now that I have introduced
you to neutrinos. Let's talk about

building a detector to actually detect
them. And the original motivation for

building this detector was something a bit
different. I talked about beta decay and

over the next decades physicists slowly
discovered more particles. They discovered

that protons and neutrons are made up out
of quarks and in the 1970s theoretical

physicists came up was are some Grand
Unified Theories basically precursors to

string theory. And these theories
predicted that the proton should decay as

well. So of course you know we build
detectors to look for that and a group in

Japan built a detector near the town of
Kamioka which they called the Kamioka

Nucleon Decay Experiment or Kamiokande for
short. Now they didn't observe any proton

decay but shortly after they built it
somebody had a suggestion that if we

changed just a little bit above their
detector, if we modified just a little, we

would also be able to detect neutrinos
with that. So they modified the detector

switched it back on and just a couple of
weeks later they actually observe

neutrinos from an exploding star just
outside our Milky Way. And that was the

birth of neutrino astronomy. And for that
the then-leader of the experiment received

the Nobel Prize in 2002. Now after over a
decade of running physicists were

basically hitting the limits of what we
could do with a detector of their size. So

we needed to build a bigger detector and
that one was very creatively named Super-

Kamiokande. And it's about 20 times
bigger, started running in 1996 and still

running to this day. Now Super-K did not
discover proton decay but did detect a lot

of neutrinos and made very fascinating
discoveries. For example, they discovered

that different types of neutrinos can
change into each other back and forth as

they travel. That's like you buying a cone
of vanilla ice cream and then as you walk

out it suddenly turns into chocolate ice
cream. That's really weird. And for their

discovery just a few years ago they
received the Nobel Prize again. But today

we are again hitting the limit of you know
what we can learn from a detector that

size. So of course the next step is to
build an even bigger detector and we're

calling it Hyper-Kamiokande. By the way
"super" and "hyper" mean exactly the same

thing. Just one is Latin and one is Greek.
So we're currently getting ready the plans

to build Hyper-Kamiokande and we will
start construction probably in the spring

of 2020. Details of the Noble Prize are
still to be determined. Now I said that 60

billion neutrinos go through your
thumbnail every second. Of course Super-

Kamiokande which is running right now is
much larger than your thumbnail. So

there's not just 60 billion but 10000
billion billion neutrinos passing through

every day and only 10 or 15 of those get
detected. So let's look at what this

detection process looks like. Now this is
the water inside Super-K. And there's a

bunch of electrons in there but they'll
show just one. And there's neutrinos

flying through not just one, not just a
few, but loads of them. And most of them

go straight through without leaving a
trace. But every once in a while we're

lucky and one of those neutrinos will
actually hit the electron and give it a

little kick. And that little kick you know
like billiard balls basically and that

little kick accelerates the electron to
faster than the speed of light in water.

Still slower than the speed of light in
vacuum which is the absolute cosmic tempo

limit. But faster than the speed of light
in water. And then you get basically a

sonic boom, but with light, which is this
cone of light. And let's just show the

animation again. So you've got this cone
of light that hits the wall of the

detector you see a little ring, this ring
of light. Well we've got very sensitive

photo sensors all over the inside walls to
detect this flash of light and from how

bright it is we can tell the energy of the
neutrino. And we can also tell, you know

just like was billiard balls, we can
approximately tell what directions a

neutrino came from just based on in which
direction it pushed the electron. So

that's the basic idea how we detect
neutrinos from the sun. Now let's talk

about what it's actually like to build one
of these detectors. So this is a drawing

of Hyper-Kamiokande and you can see it's
78 meters high, 74 meters in diameter and

on the top left there is a truck for
comparison. But maybe a better size

comparison is to compare this to buildings
which you're familiar with. Like the

entrance hall which you just came in
through this morning. Or the Statue of

Liberty and it doesn't quite fit in there.
The arm still looks out. But you could

drown the Statue of Liberty in this
detector which nowadays is probably some

sort of political metaphor. So this is the
giant detector. And what's more we're

building it inside the mountain about 650
meters underground. So that all the rock

on top will act as a kind of a natural
shield against all sorts of other

particles, that are raining down on the
atmosphere from outer space so that all

other particles get stuck and only
Neutrinos can make it through. Now of

course to build such a huge cavern inside
the mountain - that's something that we

physicists can't do on our own. So we need
to talk to geologists who look at the rock

quality and tell us, you know, what's a
good place to build this cavern - where is

the rock stable enough to do that. And to
figure out the rock quality, they drill

bore holes in what's actually called a
boring survey. laughter Now, during my

years working on this experiment, I had to
listen to several hours of talks on these

geological surveys and I can tell you that
name is quite appropriate. laughter

though of course, there's a reason I'm not
a geologist, so, you know, take this with

a grain of salt. But okay, let's say, you
know, we talked to geologists, they told

us where we can build our detector. The
next step is: We need to actually excavate

the cavern. And something to keep in mind
is that we are building this somewhere in

the mountains of Japan, you know, pretty
far away from any major city. So we have

to think about stuff like lack of local
infrastructure like what's the electricity

supply like. Do we need to add the power
line. Or what are the local roads like.

And do they have enough capacity for, you
know, dozens of trucks every day to

transport away the excavated rock. And, by
the way, where do you store all that

excavated rock? Because we will be moving
something like half a million cubic meters

of rock. You can't just store that in your
backyard. You need to find a place where

all that fits. And, of course, if you've
listened to or watch the Lord of the

Rings, you'll know it's dangerous to dig
too deeply, to greedily. So we need a

Balrog early warning system as well. But
okay let's say we've got all those and we

managed to build a cavern, and now we need
to fill it. And as detector material we

use water. Both because it's actually
pretty good for detecting neutrinos, but

also because it's cheap and there's lots
of it. So you can afford to build a

detector of this size. A detector so big
that that little dot there is a scuba

diver. But even with water, you hit limits
of, you know, how much you can get. So to

fill Hyper-Kamiokande you need about as
much water as 5000 people use in a year.

And that's for drinking, for showering,
for washing their car and so on. Now

that's easy if you're near a big city. But
we are not, we're somewhere in the

mountains in Japan where the next biggest
town has far fewer than 5000 people. So

how do we get enough water to actually
fill our detector? And... we could use

rivers nearby, we could use springs. We
could wait for for the end of winter and

for the snow in the mountains to melt and
use that to fill our detector. But if you

use melting snow to fill the detector, you
can only fill it once a year. So, you

know, even "where do you get the water" is
is a pretty... pretty important question

that you need to solve. And then, we're
not just using any water but we actually

have, we will build our own water
purification system. So that we don't have

any, you know, traces of radioactivity in
there, any trace of dust and stuff in

there. And let me let me just tell you
just how pure this water will be. So, this

is my supervisor, who, when he was a PhD
student, worked in the detector on some

maintenance work, so he was working on a
boat doing the work, and then at the end

of his shift he leaned back on the boat,
and just the tip of his long hair fell

into the water, which he didn't know just
didn't think about too much until at the

end of his shift. He went home, you know,
went to bed, fell asleep, and then woke up

in the middle of the night, with his whole
head itching like mad. Now, what had

happened there? The ultra pure water had
sucked all of the nutrients out the tip of

his hair and then through osmosis over
time those had sucked the nutrients out of

the rest of his hair, and then his skin.
So that's how pure that water is. Now, I

said "all over the inside walls". And here
is, kind of, a photo of the inside of the

detector. And all these kind of golden
hemispheres, those are what we call

Photomultiplier tubes, of PMTs for short.
And those are, basically, giant very

sensitive pixels. And we will have 40,000
of those lining the inside wall of the

detector. Now, in smaller detectors, you
could just have from each PMT one cable

leading to the top of the detector and
then have your computers there to analyze

the signal. With a detector of this size,
you just can't do that because you would

need 40,000 cables some of which are over
100 meters long. That wouldn't work. So we

have to put some electronics in the water
to digitize the signal and combine signal

from multiple PMTs into one, and then use
just a single cable, to bring it up

to the top where we analyze the signal.
But that means we have to put electronics

into the water, so that creates a whole
bunch of new problems. For example, we

need these electronics to be watertight.
And I'm not talking about the level of

watertight as you'd expect from your
smartwatch where it survives, you know,

you standing under the shower for five
minutes. I'm talking below 60 metres of

water for 20+ years. This electronics also
need to be very low power, because we

can't heet up the water too much.
Otherwise these pixels, the

Photomultiplier Tubes, would become noisy
and this would kill our signal. And then

also, because we don't want one defective
cable to kill a whole section of the

detector. We need to implement some sort
of mesh networking to introduce some

redundancy. Now each of that by itself is
not... not a heart problem. Each of these

problems can be solved. It's just a lot of
additional work you suddenly have to do,

because your detectors is that huge. And
it gets even worse: This is what one of

these PMTs looks like. It's about 50
centimeters in diameter, and inside that

glass bulb is a vacuum. So it's under a
lot of pressure. Plus we add 60 metres of

water on top of it, which adds additional
pressure. So you need to make absolutely

sure when you're manufacturing those that
you don't have any weak points in the

glass. And they don't just have to
withstand that water pressure, but there

will probably be some PMTs that have some
weak points, some air bubbles or something

in the glass. Some structural weakness.
And the neighboring PMT don't only have to

survive the normal water pressure. They
also need to survive whether their

neighboring PMT is imploding and sending
out a pressure wave. And that's not just a

hypothetical - that actually happened, you
know, 18 years ago. It's 17 years ago in

Superkamiokande. And that, you know,
within seconds killed more than half of

the PMTs we had in there and it took years
to restore the detector to full capacity.

So, lots of problems to solve, and, you
know, one group, one university can't

solve all these on their own. So we've got
this multinational collaboration with over

300 people from 17 different countries
marked in green here and across many

different time zones. So right here right
now, here it's about you know just before

noon. In Japan people have already had
dinner and they're going to bed soon. In

the U.S., people haven't even got up yet
in the morning. So good luck finding a

time for phone meetings which works for
all of these people. So, that's kind of a

glimpse behind the scenes of what it's
like to work on this detector and to

actually build it. But now I want to talk
about what we use the detector for. And

I've got two examples. But of course
there's a whole bunch more that we do, I

just don't have time to talk about it
today. So, first example: Why does the sun

shine? That seems like such a simple
question, right? And yet it turns out it's

really difficult to answer. So, in the
days of the Industrial Revolution, when,

you know, burning coal and steam power was
all the rage, people thought that, you

know, maybe it's, you know, a giant ball
of burning coal. But when you when you do

the math, it turns out that would burn for
a few thousand years maybe. So that

definitely doesn't work. A bit later,
physicists suggested that maybe the sun is

just slowly shrinking and shrinking and
it's that gravitational energy which is

released as light. And that would give you
a life time of a few million years. But

then you've got, you know pesky geologists
coming along and saying "no no, we've got

these rock formations or, I don't know,
fossils maybe, for more than a few million

years old on Earth. So the sun has to live
longer than a few million years. And the

arguments from the 19th century between
Lord Kelvin and the geologists back then

are just amazing to read. If you find
those somewhere. But, of course, nowadays

we know that the answer is nuclear fusion.
And here's, you know, a bunch of reactions

which lead you the energy generation of
the sun. But now the question is "how can

we check that? How can we check that this
is actually what's going on?" And the

answer is neutrinos, because many of these
reactions produce neutrinos, and we can

detect them. So the one we typically
detect in Super- and later Hyperkamiokande

is the one on the bottom right here,
called "Bore and eight neutrinos" because

those have the highest energy. So they are
easiest for us to detect. And the rate of

various of these processes depends very
much on the temperature. So by measuring

how many of these neutrinos we see, we can
measure the temperature inside the core of

the sun. And we have done that, and we
know that it's about 15.5 million Kelvin, plus or

minus 1 percent. So we know the
temperature in the core of the sun to less

than 1 percent uncertainty. That's pretty
amazing if you ask me. And, you know, I

said that we could detect the directions
the neutrinos were coming from. So we can

actually take a picture of the sun with
neutrinos. Now this is a bit blurry and

pixelated, not as nice as what you'd get
from a, you know, from the optical

telescope. But this is still a completely
different way of looking at the sun. And

what this tells us is that this, you know,
giant glowing orb in the sky. That's not

some optical illusion but that actually
exists. Okay. So onto our next topic.

Exploding stars or supernovae which is
what my own research is mostly about. So

supernovae are these giant explosions
where one single star, like in this

example here, can shine about as bright as
a whole galaxy consisting of billions of

stars. And the rule of thumb is this:
However big you think supernovae are

you're wrong. They're bigger than that.
Randall Munroe as an XKCD fan had this

excellent example of just how big
supernovae are so he asks which of the

following would be brighter in terms of
the amount of energy delivered to your

retina. Option 1: A supernova, seen from
as far away as the sun is from earth or

Option 2: A hydrogen bomb pressed against
your eyeball. So which of these would be

brighter. What do you think? You remember
the the rule of thumb we had earlier this

supernova is bigger than that. In fact
it's a billion times bigger than that. So

supernovae are some of the biggest bangs
since the original big bang. They also

leave a neutron star or a black hole which
are really interesting objects to study in

their own right and the outgoing shockwave
also leads to the creation of lots of new

stars. But maybe most importantly
supernovae are where many of the chemical

elements around you come from. So whether
it's things like the oxygen in your lungs

or the calcium in your bones or the
silicon in your favorite computer chip.

Life as we know it, and congress as we
know it could not exist without

supernovae. And yet we don't actually
understand how these explosions happen...

well I have no idea what's happening....
life as we know it could not exist without

supernovae and yet we don't actually
understand how these explosions happen.

And even observing them with telescopes
doesn't really help us because telescopes

can only ever see the surface of a star.
They can't look inside the core of the

star where the explosion actually takes
place. So that's why we need neutrinos.

And we have tens of thousands of
supernovae with optical telescopes, with

neutrinos we've observed just one. This
one in February of 1987. And we've seen

two dozen neutrinos which you see here on
the right. That's what we know. So we know

basically that there were many neutrinos
emitted during the first one second or so

and then fewer and fewer for the next 10
seconds. We know that neutrinos make up

most of the energy of the explosion of the
supernova, with the actual energy of the

explosion and the visible light making
just up a tiny fraction. We know that the

neutrinos arrive a few hours before the
light. And that's all, that's all we know.

And still about these two dozen events,
these two dozen neutrinos more than

sixteen hundred papers are written. That's
more than one paper a week for over 30

years. So this gives you an idea of just
how important this event was. And how

creative physicists are. Or I guess, you
could call it desperate.

Laughter
But you know, I prefer creative. In fact

this, you know, this one Supernova, we
observed was such big deal, that 30 years

later February of last year we had a
conference in Tokyo on supernova neutrinos

and we had a 30th anniversary celebration.
So there we were about 40 or 50 physicists

looking over the skyline of Tokyo having
dinner, there's the now leader of the

Super-Kamiokande experiment who was a PhD
student back then when it happened, and in

his hand he's holding the original data
tape with the events that he himself

analyzed back then. So there we were, and
at one point that evening we actually

started to sing Happy Birthday. So you
know how it goes - happy birthday to you,

happy birthday to you, happy birthday dear
supernova 1987-A. You know it absolutely

doesn't work, but it was still amazing. So
that's all we know, and then there's what

we think we know, and most of that comes
from computer simulations of supernovae.

But the problem is, those are really
really hard. You know it's one of these

extremely rare situations where all four
fundamental forces: gravity,

electromagnetism, and the weak, and the
strong nuclear force, all play a role. You

know normally, in particle physics you
don't have to worry about gravity, and in

pretty much all other areas of physics you
only have to worry about gravity and

electromagnetism. Here all four play a
role. You've also got nonlinear

hydrodynamics of the gas and plasma inside
the star. You've got the matter moving

relativistically at 10 or 20 percent of
the speed of light and you've got extreme

pressures and extreme temperatures that
are sometimes beyond what we can produce

in the laboratory on earth. So that's why
these simulations even in 2018 are still

limited by the available computing power.
So we need to do a lot of approximations

to actually get our code to run in a
reasonable time, but that gives you, some,

that produces some problems, and in fact
the week I started my PhD one of those

groups doing the supernova simulations
published a paper saying that there is a

long list of numerical challenges and code
verification issues. Basically we're using

this approximations and we don't know
exactly how much error they introduce, and

the results of different groups are still
too far apart, and that's not because

those people are dummies. Quite the
opposite they're some of the smartest

people in the world. It's just that the
problem is so damn hard. In fact, in many

of these simulations the stars don't even
explode on their own, and we don't know

whether that means that some of these
approximations just introduce numerical

errors which change the result, or whether
it means that there are some completely

new physics happening in there which we
don't know about, or whether that is

actually realistic and some stars you know
in the universe don't explode but just

implode silently into a blackhole. We
don't know. We just don't know. So take

any, you know, any results of the
simulations with a grain of salt. That said

here's our best guess for what happens. So
we start out was a massive star that's at

least eight times the mass of our sun, and
it starts fusing hydrogen to helium, and

then on and on into heavier elements until
finally it reaches iron, and at that point

fusion stops because you can't gain energy
from fusing two iron nuclei. So there the

iron just accretes in the core of the star
while in the outer layers - here in orange

- nuclear fusion is still going on, but as
more and more iron accretes and that core

reaches about one-and-a-half solar masses
it can't hold its own weight anymore and

it starts to collapse, and inside the core
in nuclear reactions you're starting to

form neutrinos which I'm showing us ghost
emoji here. Now let's zoom in a bit. The

core continues to collapse until at the
center it surpasses nuclear density, and

at that point it's so dense that the
neutrinos are actually trapped in there.

So even neutrinos which literally can go
through walls can not escape from there

because matter is so dense, and the
incoming matter basically hits a wall

because the matter in the center can't be
compressed any further, so just hits the

wall and bounces back. So from that
collapse you suddenly get an outgoing

shock wave, and in the wake of that
shockwave suddenly you get a whole burst

of neutrinos which escape the star
quickly. Now that shockwave moves on and

slows down and as it slows down the matter
from outer layer still falls in, and in

this kind of collision region where
neutrinos in the center are still trapped

in that collision region, neutrinos are,
you know, being produced at a relatively

steady rate and the shock wave has pretty
much stopped and just wavers back and

forth, and we see some neutrino emission.
Now after about half a second, maybe a

second, neutrinos from the center are
slowly starting to escape. You know most

of them are still trapped but some are
making their way outside and some of those

actually manage to leave the star while
others interact with matter in this

shockwave layer and give that matter a
little energy transfer a little push and

heat it back up. So the shock wave gets
revived and the star actually explodes,

and all of that took just one second, and
then over the next 10 seconds or so the

neutrinos remaining at the core slowly
make their way outwards and then travel

away at the speed of light hopefully to
Earth to our detector. While that shock wave

moves much slower than the speed of light,
you know, slowly makes its way outwards

and only a few hours later when that shock
wave reaches the surface of the star do we

actually see something with telescopes.
So, remember earlier the neutrinos signal

we saw was something like a bunch of
neutrinos in the first second and then

fewer and fewer neutrinos for 10 seconds.
Not a lot of detail but what we might see

is something like this: a brief and
intense burst when the matter hits the

wall and is thrown back in this first
shock wave, then as the shock wave

stagnates we might see some wiggles
corresponding to the shock wave, you know,

sloshing around aimlessly until the shock
wave is revived. The explosion starts and

then over the next 10 or so seconds we
would see fewer and fewer neutrinos as a

star cools down and as neutrinos escape.
So if we have good neutrino detectors we

should be able to watch, you know,
millisecond by millisecond what exactly

happens inside the star. Now luckily we've
got many more neutrino detectors by now.

Probably the biggest one is the Super-
Kamiokande in the Mozumi Mine which would

see about 4000 events from an average
supernova in our Milky Way, and then we've

got a bunch of other detectors which was
typically you know hundreds of events, and

some of these detectors are part of
something called the supernova early

warning system or SNEWS, and snooze is
meant to act as a wake up call to

astronomers. So when when these detectors
observe neutrinos which are probably from

a supernova they will send out an alert to
astronomers to get their telescopes ready

to be able to see that supernova from the
very beginning and then of course just in

the past few years we've also had
gravitation wave detectors like LIGO in

the U.S, Virgo in Italy, and in just a few
years we will get another one called KAGRA

which is located in Japan actually inside
the same mountain as Super-Kamiokande. So

they're literally next door neighbors, and
then we might get another detector in

India, maybe one China in the future. So
that's three completely different ways of

looking at supernovae. So when we observe
a supernova it will be headline news, and

now you know what's behind those
headlines. So I've introduced you to

neutrinos, I've told you a bit about what
it's like to work on on such a detector

and, you know the challenges of building a
detector of this scale and I've showed you

how with neutrinos we can observe things
but we can't directly observe otherwise

like the interior of exploding stars, and
with that I want to thank you for your

attention and please let me know if you
have any questions.

applause

Herald: Thank you Jost, it was an amazing

talk. We have plenty of time for questions
and there are two microphones. Microphone

one is on the left side of the stage,
microphone two is in the middle so queue

up and we're going to take some questions.
First question from microphone two.

Q: Yeah, thank you, I do have a question -
I come from a mining area and I just

looked up how deep other mines go and I'm
wondering why do you dig into useless rock

if you can just go to some area where
there are mines that are no longer used,

my area they go as deep as 1,200 metres I
think. I just looked up and was surprised

that the deepest mine on Earth is almost
four kilometres in South Africa - an

active goldmine - so why don't you use
those?

A: So, part of why we're using that
particular location is because we used it

for Super-K and Kamiokande before, and the
mountain that Kamiokande was in actually

is a mine. So we had some previous
infrastructure there, and then there's I

guess some tradeoff between the benefits
you get from going deeper and deeper and

the additional cost I think.
Herald: Thank you, we have a question from

the Internet, that's going to be narrated
by our wonderful Signal Angel.

Jost: Hello Internet. So the question, I
didn't understand then whole question, but

something about earthquake. Okay.
Q: Does the earthquake affect the

detector.
A: Well there's two parts of the answer:

Part one I'm not a geologist. Part two I
think the earthquakes are mostly centered

in the cavern on the east coast of Japan
and we're about 200-300 kilometers away

from there, so the region we're in is
relatively stable, and in fact we've been

running, since 1983 and we haven't had
problems with earthquakes, and during the

Fukushima earthquake our detector was
mostly fine but we've actually had, so in

addition to what I was talking about we're
also producing a beam of neutrinos at an

accelerator which we shoot at the
detector, and that accelerator is right at

the east coast. So, the only damage from
the Fukushima earthquake was to that

accelerator, not to the detector itself. I
hope that answers your question.

Herald: Next is from microphone two.
Q: Hello, thanks for an interesting talk.

Do you, or does science, have any theory
if the neutrinos who hit the electron are

affected themselves from this hit, are
they like directed in another direction or

lose some sort of energy themselves or
just hit the electron and pass through.

A: So, conservation of energy and of
momentum still holds, so they would lose

some energy as they give the electron a
little kick.

OK. Thank you.
Herald: Thank you, one more question from

mic two.
Q: Hello, thanks for your talk. My

question is you said that the only
supernova where we detected some neutrinos

from is from the 80's. So what is so
special about that supernova that with all

the new detectors built there was never
another detection?

A: So, the special thing about that one is
that it was relatively close. So it was in

the Large Magellanic Cloud about 150,000
light years away which is, you know, on

cosmic scales our next door neighbor,
whereas other supernovae which we observe

can be you know millions of light years
away. We can easily see them at that

distance but we can't detect any
neutrinos, and we expect about between 1

and 3 supernovae in our Milky Way per
century, so we're in this for the long

term. Okay.

Herald: Thank you, microphone two again
please.

Q: Hi, thanks for your talk. My question
is you said that changing the water once a

year is not often enough, how often do you
change the water?

A: How often do we change the water in the
detector? - Yes - So we completely drain

and refill the detector only for repair
work which, you know, happens every

depending on what we want to do but
typically every couple of years to, you

know, 10 plus years and apart from that we
recirculate the water all the time to

purify it because there will always be
some traces of radioactivity from the

surrounding rock which make the way in the
water over time.

Mic 2: Thank you.
Herald: Thank you, and that would be all,

that was a wonderful start to the
Congress, thank you Jost.

applause

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