*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*
*35c3 postroll music*
subtitles created by c3subtitles.de in the year 2019. Join, and help us!