Summer in Japan (Tentaibutsurigaku): Super-Kamiokande

Following the morning in Kenrokuen Garden that formed the first part of our (me, the other student, and our adviser) week traveling around Japan, we headed back to Toyama in order to get a ride up to Kamioka for our tour. Our tour guide (and more importantly cool scientist!), Mark Vagins, was a total boss all-round, and helped make the entire thing both fun and informative. I found the entire experience both inspiring as a young prospective researcher and as a feat of engineering. It definitely helped give me an appreciation to just how much work goes into some of these huge astronomy/physics collaborations and experiments nowadays!

Some small things to note before I begin:

Mark is a professor at the Kavli Institute for the Physics and Mathematics of the Universe, and is part of the Super-K collaboration. Super-K is owned and operated by Kamioka Observatory. KamLAND is owned and operated by the Research Center for Neutrino Sciences at Tohoku Univisersity.

Note: I apologize in advance if some of the physics/history/explanations are slightly incorrect. All of this is done from memory, and you know how that is.

Edit: Made a few corrections/changes to the article after getting in touch with IPMU.

So on the way up to Kamioka we encountered several interesting signs that show just how far 1. north and 2. rural/mountainous we were.

First:

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So that’s pretty awesome. Sadly, we didn’t spot any on the way up. The second sign takes the cake though:

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I especially enjoy the baby monkey for the extra guilt factor.

After driving out of Toyama for about an hour, we arrived at the main office building/ground base, where we had to check in before we drove down to the mine (since Super-K is under ~ 1 km of rock to shield it from picking up too many unwanted particles, since it’s sensitive as eff).

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It also offered a spectacular view of the mountains that were all around us. I hadn’t seen a similar sight (including the mist and everything) in – wow, I think at least a decade – since I tend not to go out to these types of places much. So I spent some time admiring the view.

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Now, the mine itself is in the middle of nowhere. It used to be somewhere, back when it was active, but sadly nowadays the town (well, about 10 elderly families who are left) is pretty much gone. It actually reminded me a little bit of Space Bros – or at least I ended up wondering if the town Pico and Vince grew up in would’ve ended up like this one. If all past mining towns are like this…

But enough brooding. After driving through, we headed off into the mine.

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The pipe on the right is actually used to pump clean air into the mine from the outside. This used to be a mine for radioactive materials, and breathing the air inside (without this fresh outside air pumped in) was supposed to on average take 15 minutes off your lifespan for each hour you spent down there. So obviously the miners didn’t really have a good time back when it was active.

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After arriving and checking in, we got to watch a great (not sarcastic – it was actually quite good) informational video about the construction of the experiment filmed back in the 90s. The best part was how quickly I managed to pinpoint the decade of production based on the background music alone. The little things you take pleasure in.

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It then turns out that the Super-K experiment keeps several cool “whiteboard”-esque doors, which they try and get famous visitors to sign. If you recognize any names, let me know in the comments and fill me in on the backstory to how you learned about him/her/them. I’m always interested to know how people learn and/or become interested in science (especially astronomy/physics), plus having someone else with whom to geek out with about physics celebrities is always a good thing!

Also, speaking of astro/physics things, have you seen the new COSMOS trailer? SO EXCITED.

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One signature not on this list though: the Emperor. It seems he (and the Empress) came through a while back, but the Super-K members couldn’t convince either of them to sign one of the doors. There is a Prime Minister on there though I think.

So after I was done geeking out over the doors (and I was legitimately geeking out over several of the names), we moved on to some quick overviews of the science being done at Super-K.

So what does Super-K actually do? It looks for neutrinos, the “ghost particles” of the Universe.

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Why are we interested in neutrinos? Plus, what the heck are they anyway? Well, about a half-century or so ago, scientists came up with a theory to describe most of the ways matter interacted with each other and what everything was made of. Namely, there were four fundamental forces: electromagnetism (that thing), gravity (THAT thing) , the weak force (it’s what makes things radioactive), and the strong force (how fusion works). (Fun fact – this pick up line will always work: “Baby, our love makes the strong force look weak.”) And each of the forces were governed by a set of unique conservation laws. In mechanics, this means such things like momentum and energy are conserved. In electromagnetism, this is essentially charge. But in the weak and strong forces, you have a bunch of things like “lepton number” and whatnot, including such great things as antiparticles (like that makes sense) and other weird stuff.

But essentially what it boils down to was that when physicists wrote down a bunch of equations, they noticed they couldn’t make these conservation laws match up perfectly. So this guy, Enrico Fermi (who I took the inspiration for my Back of the Envelope posts from), just pretty much goes and invents a new particle to make the equations work. It’s pretty much (but ultimately not!) massless and has no charge, so you essentially have this tiny thing that interacts with absolutely nothing unless it hits an atom directly.

Amazingly enough, this particle actually exists. However, it was a incredibly difficult to discover and remains incredibly difficult to detect.

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So that answers the what. Why then do we care about these particles? Well, they actually tell us some cool thing about fundamental physics, so that’s pretty cool. But in terms of astronomy though, they actually tell us a lot about stars.

First, neutrinos are created in nuclear reactions deep in the heart of stars (e.g. our Sun), and since they’re such “ghosts” they pretty much just whiz out soon after they’re created (don’t ask how – nobody actually knows how particles are created). So somehow detecting neutrinos from the Sun would tell us a lot about the process.

Second, they tell us about what happens when massive stars die. When stars reach the end of their lifetimes, they are no longer able to get more energy out of fusion than they put in. The graph below shows the amount of energy you get out relative to the amount you put in for elements and as you get higher and higher in terms of mass, you get less and less out of it thanks to the weak force slowly overpowering the strong force.

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At this point the star collapses under its own weight and then explodes in a supernova. When it collapses, all the heat and pressure it generates at the core pretty much spurs an explosive chain reaction of fusion (this normally would not occur since you lose energy, but at this point there’s just so much stuff in the core anything goes). And this forms a shit ton of neutrinos (as in they constitute 99% of the energy of a supernova.). So much so that they often heat the surrounding material and ultimately are what cause the star to “bounce” back outwards in this spectacular explosion. This outburst of neutrinos tells us a lot about stellar evolution and super-high-energy physics, and so by trying to observe them we hope to learn more about this kind of stuff.

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Now we get to the cool stuff – how do we actually observe these mofos? Well, first if they only interact by hitting stuff, then you just want a lot of possible stuff they can hit. That seems like a logical first step. Second, when they hit something you want it to give off some clear signal that is unique from say other things hitting said stuff. So you need a large volume of something that is pretty “pure”, both for the quality of the signal and for consistency. Then you’d probably want to minimize the amount of other crap possibly hitting your detector, so you’d want to somehow shield it from other particles.

The most elegant solution for this as of now is to get a giant rig of water that is ultra-pure (as in so pure that it becomes a perfect insulator, a perfect acid, and a perfect base, AKA the absolutely last thing you’d ever want to drink) and somehow stick it as far underground as possible to avoid bad signals from other particles that aren’t as un-interactive as neutrinos are.

Detecting them is actually pretty cool, and it’s done by detecting this thing called Cherenkov radiation. As light travels through materials its effective speed, c, changes. Light travels much faster through air than it does through water since it’s not hitting as many particles and bouncing around all the time (this also is sort of why light bends between surfaces). And although nothing can travel faster than the speed of light in a vacuum, stuff can travel faster than light through a medium. Which is what happens when a neutrino smacks into a water molecule. It actually is traveling faster than the speed of light in water, and so this reaction releases a “light boom” very similar to a sonic boom. The direction this cone of light is emitted, it’s size, and other properties tells you information about the collision.

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So essentially, if you get a bunch of sensitive-ass detectors and a humongous tank of ultra-pure water, put them ~ 1 km underground, and stare at the thing 24/7, you can hope to observe these events and do some cool neutrino physics.

Speaking of observing things, that’s what these photomultiplier tubes are for.

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They’re absolutely enormous, and sensitive enough to technically be able to detect the light if you lit a match on the Moon.

Seeing as this ultra-pure water is the foundation for the experiment, you can expect that the last thing you want to do is touch random knobs that affect anything to do with the water processing system. But just in case, they make sure to emphasize it.

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After all of this, we finally got to set foot inside the detector itself (well, on top of it, but still!). It’s damn impressive, so I’ll let my photos do the talking.

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After feeling puny relative to the size of this thing, we got a tour of some of the rooms around the area. I gotta say I was particularly impressed with one of the electronics sheds.

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Just imagine if a single cable got loose.

Mark also showed up us one of many good acronyms he had come up with during his time there.

IMG_0929The best part about all of that was it made perfect scientific sense, although I’m forgetting exactly what it stood for at the moment…

The main control room also has a cool monitor where you can see neutrino detections happening in real time! Gotta love the pretty colors.

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After the main Super-K tour we got a chance to visit two other experiments: KamLAND-Zen and Mark’s own pet project. The former involved some more interesting neutrino physics, which I regrettably have largely forgotten about in the weeks since I’ve visited. It was cool though, since they had a huge balloon as part of the experiment!

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Mark’s pet project was trying to figure out a way of injecting gadolinium into Super-K. But wait, you might ask, wouldn’t that ruin this super-mega-pure water you’ve talked about? Yep. Plus trying to filter out everything except for gadolinium turns out to be pretty hard. HOWEVER. Gadolinium it seems has this monster ability to detect neutrinos for some reason I don’t understand, so actually injecting it into Super-K would increase its sensitivity enormously. Right now, it’s pretty much only able to detect neutrinos from a supernova that goes off in (or really near) our own galaxy. With gadolinium though, it’ll be able to detect supernova from other galaxies much further away. Which is good, since on average the Milky Way only gets one supernova every hundred years.

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Our arrival in Mark’s lab turned out to be very fortunately timed, since it was the one day that the tank was empty. So we got to take a peek inside!

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And topside.

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After the visit, we got a chance to bask in the afterglow of our physics journey by going to a kaiten (conveyor belt) sushi restaurant. I managed to eat quite a bit, and even ordered an entire platter of squid sushi. (Speaking of squid, while there are a lot of Japanese words I forget from time to time, I never forget this one thanks to Shinryaku! Ika Musume. I haven’t even watched the show, but for some reason it just really sticks in my mind. *shrugs*.)

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Oh, and to cap off our visit, we got a glimpse of a particularly well-named love hotel on the way back to Toyama.

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Given that I’d spent the morning in Kanazawa, all in all I’d say this was a pretty awesome day.

 

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