CC >> Welcome, everyone, to
Wednesday Nite at the Lab. I'm Tom Zinnen.
I work here at the UW Madison Biotechnology Center.
I also work for UW Extension Cooperative Extension, and on
behalf of those organizations and our other partners,
Wisconsin Public Television, the Wisconsin Alumni Association,
and the Science Alliance, thanks for coming to Wednesday Nite at
the Lab. We do this every Wednesday
night, 50 times a year. Tonight it is my pleasure to
introduce to you Dr. Mike DuVernois, who has bowled on all
seven continents, launched balloons in Antarctica, and is
interested in almost everything. He is the instrumentation
project manager for the Wisconsin IceCube Particle
Astrophysics Center, otherwise known as WIPAC.
He joined the UW Madison WIPAC team in 2010, and he currently
works on equipment for IceCube, the Askaryan Radio Array, and
The High Altitude Water Cherenkov Experiment projects.
Mike grew up in Connecticut. He attended Georgia Tech, and he
received a PhD in physics and astrophysics from the University
of Chicago. Following his PhD, he did a
postdoc at Penn State University.
He started a research group in cosmic ray and neutrino
astrophysics at the University of Minnesota, and was a visiting
professor at the University of Hawaii, and he has worked as a
freelance engineering consultant.
And tonight, in addition to Mike at the Science Cafe component,
he'll have his colleagues Jan Auffenburg, Laura Gladstone,
Naoko Neilson, Ben Riedel, Chris Weaver, Ian Wisher, Mark
Krasberg, and Gary Hill all here to talk with you.
So it's a particularly special Wednesday Nite at the Lab to
talk about a hundred years of cosmic ray research, how it's
done as far south as the Antarctic and the South Pole,
and then we get to have a science cafe, a cosmic ray
science cafe. So please join me in welcoming
Mike DuVernois to Wednesday Nite at the Lab.
[APPLAUSE]
>> Well, thank you very much.
I'm happy to be here and have such a great audience.
I've heard a lot about this audience.
And I meant that in a positive way.
[LAUGHTER] So yeah, I'm going to
talk just for a little while about cosmic rays, sort of a
very introductory and somewhat historically biased talk.
This is what cosmic ray physicists call the hundredth
anniversary of cosmic rays. We'll see why, where that comes
from, and look a little bit at sort of the prehistory of cosmic
rays as well. And then my colleagues and I
will entertain over coffee and questions.
Since there's no coffee allowed in here, we'll move over next
door. And I'm going to try to end the
talk with sort of a segue directly into IceCube.
All of our people here work on the IceCube experiment, and
we'll see, you can start asking about those connections between
the cosmic rays and the IceCube experiments.
So sort of the prehistory of cosmic rays.
There's a long-standing problem in physics.
And, actually, one of the attractions of the field of
cosmic rays is that it really does have this historic tie.
It's an old mystery, and it's not a fully solved mystery even
today. So this goes back to Charles
Coulomb and the discovery that if you charge something up, if
you charge up, if you think about our elementary school
projects rubbing fur on plastic rods and the like, charged up
objects discharge gradually. And there are charges moving
through the atmosphere. There's charges moving all
around us. And if you start shielding
against those charges, some of them are getting through.
So some of them are very high energy charges.
And that's normally associated with Rutherford around the turn
of the previous century, this discovery that there's very,
very penetrating charges out there.
And, of course, part of this then ties into really the
discovery of radioactivity. They, also in the same time
frame, right around 1900, discovery via blocking uranium
accidentally left on top of photographic plates.
The plates developed and even though this was in a sealed box,
light-tight box, the plate had an image of the crystal.
So there's no light involved, things were being emitted by,
basically, a piece of rock. So the Earth emits particles.
The Earth emits charged particles.
And so initially, okay that seemed like a clear solution.
The actual term of radioactivity comes from the Curies, and sort
of the early Nobel Prize history lies much in this story back
around the Curies and the other figures we'll see.
And what's basically going on are some nuclei are unstable,
some of these uranium nuclei, and when the break apart, they
can emit particles, a few different types, at high
velocities. So these can be charged
particles. And this is a penetrating
radiation. So this seems to be a partial
solution. Professor Victor Hess, in 1912,
so here is going to be our hundredth anniversary event,
Victor Hess, very dapper gentleman, decided early
attempts at trying to get apparatus away from the Earth,
going up mountains, well, there's still a lot of rock
underneath you. You really want to try getting
away from the source of radioactivity to measure it
falling off. It's got to decrease as you go
up in altitude. And there were experiments where
they took electroscopes looking at, so you charge this
instrument up. Little metal foils charge up,
they repel, and if you nicely seal it, the rate at which the
foils discharge, come back together, is proportional to the
rate of particles going through your piece of equipment.
So Victor Hess took these and went on balloon flights, took
these up as high as he could before he started passing out,
then descended, and found that, in fact, the amount of
radiation, the rate of discharge of these electroscopes, the
amount of penetrating radiation increased as he went up in
altitude rather than decreased. So there was stuff coming in
from above, particles coming in from space, a space radiation, a
cosmic radiation. So these are the cosmic rays.
And this also is a Nobel Prize a little bit later.
And we can then ask, okay, since then what have we learned?
What are these cosmic rays? These cosmic rays are particles,
protons, electrons, heavier nuclei stripped of their
electrons that strike the Earth's upper atmosphere,
undergo collisions of the atmosphere and generate sort of
a cascade, a shower of what we would call secondary, so not the
original particles, secondary particles in the atmosphere.
And some of those are what are called muons, which are sort of
a heavy analog of the electron. And it's mostly those particles
that are observed down at the ground level down on Earth.
These incoming particles are traveling most of the speed of
light and come at very, very high energies.
It's very difficult to understand where they came from,
and we'll talk a little bit about that in the next few
slides. One of the nice connections,
actually, with the history of Victor Hess' balloon flights is
that one of the methods of looking at cosmic rays even
today is via scientific ballooning, stratospheric
ballooning. And that's because these
primaries, if they hit the atmosphere, break apart and
generate secondaries. So if you want to see the
primary, you can go into space, we've built spacecraft
experiments but that's typically timescales of 20 years between I
have a good idea and I have a rocket to put it on.
And another way of doing that are with these balloons.
These balloons are able to take a payload, so here we have
payload hanging on the crane, up to about 120,000 feet.
So that's above, literally, 99.9% of the atmosphere.
So you get above most of the atmosphere at maybe a thousandth
or so of the cost of going into space.
And so it's a nice historical connection, Victor Hess'
balloons and our balloons. This figure here sort of
compares the size. So what I have to explain here
is that this balloon at launch, when it looks like this, is open
at the bottom. It's zero differential pressure.
So you've got the 15 PSI down here on the Earth at sea level
pushing down on us, pushing in on all sides of this balloon,
and when it gets up to floating altitude, up 120,000 feet, it
still has the same amount of helium inside it, but it is now
at lower pressure, much lower pressure outside so it's
expanded and sort of gets you a scale size versus a 747 or a DC
phallic symbol. [laughter]
So these balloon launches, for a variety of reasons mostly having
to do with the upper atmosphere circulation patterns, take place
from Antarctica, sort of one of the first Antarctica
connections. And this is a launch from 2003,
I believe it is. Payload launched during the
austral summer. We run them with solar power.
The balloon is inflated here with the beautiful background of
Mount Erebus, the farthest south active volcano.
And, if I can go back, farthest south active volcano.
The crane holds the payload until the balloon is more or
less directly above it, and then the crane pulls away, the
payload ascends. You can launch about a two-ton
payload. And I was just showing my
daughters the annular solar eclipse this weekend.
We actually had this partial solar eclipse during this
balloon campaign, actually. So it was kind of a fun little
aspect of it. And then the payloads come back
on the parachute. We considered this one mostly in
tact. And the payload catches what are
essentially constant latitude winds at high altitude, which is
why we launch in the polar regions.
And about 10 days after you launch the balloon, it comes
flying back over head. Sorry about that.
So let me continue then with the history of cosmic rays.
The 1930s, Sir Arthur Holly Compton, who is my thesis
adviser's thesis adviser, this sort of interesting pyramid of
physicists, or relatively few physicists, just a hundred years
ago, or 80 years ago, found this radiation.
The cosmic rays coming in depending on magnetic latitude,
so depended on the Earth's magnetic field.
The Earth's magnetic field was acting as a spectrometer.
It we blocking particles at different energies.
And this led, then, to the beginnings of an understanding
of the Earth's magnetic field structure that the low energy
cosmic rays, the most numerous of them, are tied to the aurora.
People were going out to try looking at the aurora, was that
last night, Naoko? Last night, without any luck
here, but the aurora is caused by these charged particles
coming through the Earth's magnetic field.
And then also, 1937, I already mentioned the muon, this sort of
heavy analog electron. So the early history of cosmic
rays is also the early history of particle physics.
Our understanding of elementary particles comes from measuring
the cosmic rays. And another sort of piece of the
cosmic ray business is actually some very early of what we would
consider detector electronics coincidence circuits.
So that is a circuit that depends on a condition here,
logical or/and to something going on over here in the analog
though; in analog, not in digital electronics.
Goes to Pierre Auger, who's looking at these showers and is
trying to understand the extent of these showers.
So if he had a detector sitting here and every time it sees a
particle, it makes a little click, makes an indication, he
then wanted to see how often it was related to a detector a
little ways away, farther away, farther away, farther away.
And eventually he got these half a mile apart and was still
seeing lots of these showers. So these incoming particles had
phenomenally high energies, and at the time it was very, very
hard to understand. We understood uranium, it could
produce particles of a certain amount of energy.
But he was seeing particles coming in with billions of times
more energy. Very difficult to understand.
It needed some mechanism to accelerate them.
This is before particle accelerators, or right at the
beginning of building particle accelerators, so it was hard to
conceive of how you get particles of these energies.
Well, Fermi figured out a way of getting particles up to higher
energy. It was a famous 1949 paper,
Fermi, the sort of normal version of the story is that
there was a very, very bad snowstorm in Chicago, and he was
stuck in his apartment for a week and a half, and during that
time, he figured all of this out.
I assume that that's probably not a true story.
Although, I imagine Fermi would very much approve of us
retelling this story. And the basic model was that if
you had a shock wave, multiple crossings of this shock wave
can accelerate particles. And probably the best example,
using this room, would be if the walls of this room were closing
in on us, if we start throwing tennis balls in this room with
moving walls, some of them will pick up speed.
Will be able to take a little bit of the energy of the walls
moving and go up to higher energies.
A statistical process, so not every particle gains a lot of
energy, but some particles gain a fair amount of energy, and a
smaller number gain very large amounts of energy.
And so this process is observed experimentally, not in 1949, but
in the spacecraft era at shock waves in the Earth's magnetic
field structure and in the heliosphere as a whole.
So this has been observed sort of in situ.
And for the cosmic rays, this is thought to occur in supernova,
actually. I'll show that in a minute.
At the very, very high energy end, there are particles that
Auger had sort of hints at when he was looking half a mile apart
of particles that come in with macroscopic energies.
So 10 to the 20th electron volts, but let's not worry about
that. The highest energy particles
that have been seen have about 50 joules of energy.
So that's me kicking my bag pretty hard.
It's a good tennis serve. It's getting your hand stuck in
a door. It's that kind of energy, but
it's stuck on a single subatomic particle.
Very, very hard to understand amounts of energy.
The first person who really observed cosmic rays of that
energy is John Lindsay. Here he's poking for
rattlesnakes, actually, in the straw he uses to cover his
detectors at Volcano Ranch experiment, 1962.
And something that will almost certainly come up in our
discussions if we start talking about neutrinos and IceCube is
that there is a limit to the energy of the cosmic rays due to
their interactions with the notification, due to their
interactions with the microwave background radiation, this sort
of echo, microwave echo, of the big bang.
So the very far away particles, very high energy particles can't
get to us. So Hess' flights have been
recreated over the last few years.
People tend to take supplement oxygen along with them.
And so our basic discoveries are cosmic rays, so there's
energetic particles coming in from above, these primary
nuclei, unknown source, protons, heavier elements, electrons, and
then these cascades, these showers.
If we put sort of all of what we know about the cosmic rays, sort
of the fluxes, the number of these intersecting and area on
the ground, as a function of energy, details here aren't
what's important. What's important is, in fact, 32
orders of magnitude in the numbers of particles over 12
orders of magnitude in energy. Just this phenomenal range of
energies, phenomenal range of numbers of particles which then
tell you that you are certainly using different techniques to
measure them where these are abundant.
These are spacecraft experiments that are roughly this size.
As you go up in energy, we would come to a region where we have
these balloon experiments, size of a minivan, a couple tons.
Going still higher in energy, ultimately we're looking at
particles that as only one of them comes in for every square
kilometer on the ground per year.
Go up another factor of 10 in energy, it's one particle per
century per square kilometer. So you have to build very, very
large experiments to be able to see this.
And so we then try to tie this to the sky, to the universe.
At lower energies, at the lower energy, 90% of that plot, the
cosmic rays are confined within the galaxy by the galaxy's
magnetic field, but they're bent so they do not point.
They're not like light. We go outside, we can look
through a telescope, we can see a star, it points in that
direction, that star is really at that direction.
We see a cosmic ray coming from that direction, it has been
bent. It has taken a circuitous route
to get to us. So we don't have the ability to
point them back to their sources.
And I had mentioned that we can get low energy cosmic rays from
events on the sun. So if we go up in energy, we
have these solar events, typically measured by
spacecraft, interesting and important for the aurora and
electrical interference on Earth.
Going up to higher energies, we understand these galactic cosmic
rays to come from supernova. So this is a massive star that
explodes, and it ejects material with enough speed to be those
initial tennis balls. We've got to send the tennis
balls out with a little bit of speed, and then the shock waves
from the supernova can accelerate them like the moving
walls, bringing them up to higher energies.
So this has almost been directly observed but not quite.
So this is sort of the schematic of these particles that are
scattering off the magnetic field and crossing the shocks.
Every time they cross the shock, they can gain some energy.
There's no direct evidence. The model seems to work, and
there's some close to direct evidence.
And this is an image, actually, from that same supernova, 87A,
showing the shock wave hitting material and producing a lot of
x-rays and, presumably in that shock, also accelerating cosmic
rays. Going up to higher energies,
these particles are no longer trapped by the galaxy.
These are extragalactic particles.
There's several models for where these might be coming from, but,
once again, no direct evidence of this.
Active galaxies, very, very large black holes at the centers
of these galaxies generate jets of particles, and you can have
acceleration along the shocks again.
Gamma ray bursts. You can ask the IceCube folks
about why this may not be a very good model anymore.
Gamma ray bursts, very mysterious, very distant objects
that light up many times brighter than usual for a few
seconds, sort of occurring randomly over the sky.
And just to give sort of a scale and a little bit of how this
measurements of these very high energy cosmic rays are done,
I'll just show a few slides about the Pierre Auger
Observatory, once again named after Pierre Auger, and try to
show a little bit of the hardware.
I'm very much a hardware person. This experiment was built up out
of surface detectors. These are detectors that sit out
on the grasslands of the Pampas of Argentina.
And when particles travel through them, there's light
emitted in the detectors and signals are sent back.
These are spread out over 3,000 square kilometers to see those
particles that only occur once per square kilometer per century
in sort of human timescales or graduate student timescales.
Some of the construction of the experiments.
And then there's other ways of looking at these signals.
A technique called fluorescence. This is the sort of predecessor,
an early experiment trying to look at fluorescence which is
really just mirrors and light detectors that watch the trail
of all of these shower particles through the sky.
So really just the glow of them passing through the sky.
And there's the fluorescence detectors at the Auger
Observatory. And the data comes in and the
different techniques have sort of different tradeoffs.
Fluorescence, looking up at the sky, only works when it's dark,
for example. But it's a measurement that ties
only to electricity and magnetism.
Sort of a side thing. So, sort of to wrap up,
basically if we think about this in astronomy terms, we're used
to thinking of astronomy in terms of light, and so that
effort goes back over 800 years. But there are other particles.
There are other ways of looking at the universe in astronomy,
including the cosmic rays, the charged particles, and also
neutrinos that come. Once again, wherever you have
these nuclear reactions, there must be neutrinos as well.
And every time a new photon window opens up, radio
astronomy, microwave astronomy, gamma ray astronomy, there's
always been sort of an intent for gamma ray astronomy
originated with the US military trying to look for nuclear
detonations on Earth. And instead they found that
there were much, much larger nuclear detonations above them
instead. So, going into this field,
looking at the charged particles, looking at the
neutrinos, there may be huge discoveries that we don't know
about. And another way of looking at
the cosmic rays, another way of looking at this physics,
involves a long trip to the South Pole that I think our
IceCube folks would be happy to talk about.
So I'm just going to let these slides go and I'm done talking,
and we'll break away into smaller groups and talk.
Thank you very much. [APPLAUSE]