Thursday, November 18, 2010

Cosmic Rays

Did you know that you are getting showered by particles as you read this? Even if you are inside…even if you are hundreds of feet underground!

Violent events in outer space (super novae, storms on the sun…ect) are constantly sending high energy particles into our neck of the galaxy. Sometimes they even come from outside of our galaxy.

Here’s what happens: a really speedy proton will come zooming in from an explosion in a distant part of space and reach our atmosphere. Our atmosphere has LOTS of stuff in it. Chances are pretty high that this proton is going to slam into one of those nitrogens or oxygens that we have hanging out up there. When this collision takes place some crazy stuff happens…

When a high-energy proton from outer space runs into an atmospheric molecule it creates a shower of secondary particles.

Turns out that if you want to know what something is all about, what you have to do is pummel it. Pummel the nucleus of an atmospheric molecule with a high energy proton and it will release all kinds of things that it’s been holding onto.

This is a diagram of the collision:


 See the things labeled with the pi symbol? Those are pions! They were a big freakin deal when they were discovered because they had actually been predicted a decade earlier by Japanese physicist Hideki Yukawa.

The reason why Yukawa thought these things needed to exist is because he was running with the idea that every force has a particle associated with it. There are four fundamental forces in nature: Gravity, the electromagnetic force, the nuclear strong force, and the nuclear weak force.  The nuclear strong force is the one that we care about here.

Here’s a run down on the nuclear strong force: We know that like charges repel each other, so protons are not going to want to live next to each other. YET, they do! They are tucked in very close together in the nucleus of atoms. So there must be an entirely different force holding them together that overcomes this electrostatic repulsion force. This is the nuclear strong force.

Yukawa argued that pions are the particle associated with the nuclear strong force. Since every particle can be modeled as a wave (wave-particle duality, heard of it?), and since pions have a wavelength that acts within the teeny dimensions of an atomic nucleus, and the wavelength of a particle depends on the mass of a particle… he was able to accurately predict the mass of the undiscovered particle! Bet he felt cool when a particle of exactly that mass was found 10 years later.

Ok, SO… pions are created in the upper atmosphere after protons from outer space disturb the nuclear force field of an atom in the atmosphere by smashing into it. But pions are very unstable and they decay within a tiny fraction of a second into other particles called muons and neutrinos. Then the muons decay into an electron (or a positron) and neutrinos. Here is a picture of it happening in a lab:

This is a particle explosion inside of a streamer chamber. All this chamber is doing is recording the tracks of charged particles as they move along. The scientists who took this picture launched a positively charged particle in from the left and watched as it collided with a neon atom. The starburst in the middle is where the neon atom released a bunch of pions because its nuclear force field was disturbed.

My favorite is the positively charged pion that sweeps up counter clockwise because we get to watch it decay into a muon. The muon is the one that makes the big spiral. When it got to the middle of the spiral, you can see that the path breaks off because the muon decayed into even smaller particles. Since it was a positively charged muon, it decayed into a positron (the anti-matter twin of the electron, it's labeled e+) which went off to the right, and a neutrino which went off somewhere unrecorded by the streamer chamber because neutrinos don’t have a charge.

So this is what is happening a few miles above our heads. (Except for the spiral pattern…that only happens because of the magnetic field inside of the streamer chamber, in the atmosphere these things go in straight paths).  But you get the jist… particles are careening in from space and smashing our atmospheric atoms into little bits and these little bits quickly decay into other little bits that rain down on us.

Muons are the evidence of cosmic rays that we detect down here near sea level. You can think of them as big fat electrons because they have electron charge and are about 207 times the mass of an electron.

I really like muons because I’ve spent a lot of time with them. Two years ago I did this experiment for a physics lab class at WWU where we (my lab partner and I) were trying to measure the lifetime of a muon. This was our apparatus:

And this is a schematic of what it looks like inside of that big shiny tube and in the box under the computer that says Muon Physics.

Inside of the aluminum tube is a scintillator and a photomultiplier tube (that’s the thing that says PMT). The muons come in and hit the scintillator, which is just a luminescent material that converts the energy of the muon into light. The photomultiplier amplifies that light and sends the signal off to the rest of the stuff that I don’t really care about.

All I know is that the first signal (a muon hitting the scinitillator) will start a timer, if a second signal occurs within 20 micro seconds, then we know that the muon that hit the scintillator also decayed in the scintillator. This is what we want! Both of those flashes are sent off to the read out software on the laptop and the time between when they occurred is measured. This is the lifetime of the muon. Zing!

If you look at what you are getting from the photomultiplier tube with an oscilloscope it might look like this:

The first blip is the muon hitting the scintillator and the second blip is the muon decaying.

Do this enough and you can find the average life time of a muon. It works out to be something like 2 microseconds (split a second into a million pieces and take two of those pieces…pretty short). It’s an easy experiment!

Using this lifetime and some assumptions, we can figure out how many muons should be able to make it from their birth place in the upper atmosphere to sea level. We can do this because we know how fast the muons are traveling. We know how fast they are traveling because we know how far a charged particle of a given mass and speed can go through matter. We know the mass, we know how far it goes through the scintillator…we’re in good shape. We found that muons have speeds of .9950-.9954 the speed of light. In case you don’t know…light is really fast! 99% of the speed of light is haulin ass.

But here’s the catch: the lifetime of a muon is so short that they don’t have time to make it from the upper atmosphere to the surface of the earth before they decay…even if they are traveling at the speed of light. BUT they DO make it to Earth. Lot’s of muons make it to sea level. A muon goes through an area the size of my finger nail every minute. Sit still long enough and thousands of muons will go through you.

So they don’t have time to make it to earth, but they do… How are they doing this?

This is Einstein’s theory of relativity at work. Because the muons are traveling so fast, they are experiencing time at a slower rate then we are. This is called time dilation. It’s where the fast moving clock runs slower. If you had two clocks that were perfectly synched up and then accelerated one of them to near light speed and let it cruise around for a while, when it comes back to earth you would find that years have passed on the stationary clock while only a few minutes have passed on the moving clock. This happens in Planet of the Apes, remember?  

I like to talk about the muon time dilation experiment because I get the feeling there are lots people that think relativity is just some hypothetical, unsubstantiated, frou frou, magic, pretend physics. But no, it’s not. Time dilation happens and the fact that so many muons make it to earth is proof. I think people have a hard time relating to it because relativity only really matters when you are traveling at speeds close to the speed of light. We can’t speed ourselves up to a significant fraction of the speed of light, we are too massive. It’s pretty easy to accelerate a dainty little thing like a muon to near light speed, however. Therefore it’s pretty easy for them to experience time dilation. And they do!

In fact, if you divide the observed muon lifetime by the lifetime we think we should be observing if there were no effects due to time dilation, you get 1/9. This means that muons are keeping time at 1/9 the rate that we are. Nine years in your life is one year in a muon’s life.  Neat, huh?

Other cool things about muons:

You can do astronomy with muons! Remember, they are the grandchildren of a short fling between an extraterrestrial proton and an atmospheric atom. If you trace back the trajectory of these incoming muons you can figure out where these high energy proton grandparents came from in the first place. Turns out lots of them are coming from the direction of a cluster of galaxies in the constellation Virgo. Inside this cluster is a giant galaxy called M87 which is believed to have a super massive black hole at its center. 

You can even see the shadow of the moon in muons!  

Also, muons can cause lightning! As they are making their trek from their birthplace in the upper atmosphere to the surface of the earth they strip away electrons from atoms in the lower atmosphere. This creates a separation of charge and thus, a strong electric field. When the field strength becomes too high, a discharge occurs. This is lightning!

Well, that’s enough nerding out for now. See ya next time!

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