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Sunday, 15 March 2015

Q and A with a cosmic ray hunter...

Nature usually manages to outdo humankind's best achievements: We have the tallest building (Burj Khalifa in Dubai at 830 meters), but nature has Olympus Mons on Mars (27,000 meters). We have electricity, but nature has giant lightning on Jupiter and Saturn.

Above: Jupiter and Saturn are giant planets with nothing but giant terrifying weather, did anyone think the thunder and lightning would go 'meow'? Yet these scientists seem quite surprised.... Courtesy of NASA
We even built the atom bomb, but the Sun is a nuclear explosion so big its own gravity keeps it all in one place. It's like playing cards as a child, against your mean uncle: You never win, except when he lets you - and then he makes damn sure you know he let you win....

Above: Olympus Mons, the biggest volcano in the solar system. It's showy if you ask me. Courtesy of ESA
You wouldn't think nature would try to compete with us over particle accelerators though. But it has - and to show us puny humans what it can do nature has given us 'cosmic rays' - ultra high speed subatomic particles from space. And, yes, I know that they should be called 'cosmic particles' - the name is a historical accident. It's not my fault, I swear.
Look there's nothing I can do, ok? Everyone's used to calling them rays now.
Ahem. To get all technical: Cosmic rays are ionised atomic nuclei, flung into space at immense speeds by some of the most powerful things known in the universe - solar flares, supernova, quasars - and perhaps by undiscovered events even more powerful.

Above: A model of an atom, showing the nucleus and the surrounding electrons.The simplest atom, hydrogen, has just one proton in the middle, and one electron. 'Ionised' means that one or more electrons are missing, which gives the atom a positive electric charge. Courtesy of
These tiny, tiny, pieces of matter are nature's extreme racers, but only since 1991 have we realised that some are bizarrely fast - so fast that it's hard to imagine what could have thrown them so hard across the universe. So fast that, had you told a physicist prior to 1991 that cosmic rays went that fast they'd have laughed.

Above: Our imaginary physicist wouldn't have laughed like Mr Burns of course. Unless he knew something you didn't about where the cosmic rays come from, and how they held the secret of UNLIMITED POWER....
But in 1991 Earth was hit by a cosmic ray that astronomers the Oh My God particle*: It was only a single proton, but it was travelling so close to lightspeed that it had the same momentum as a fast pitched baseball - 40,000,000 times more than a proton from the LHC accelerator. Time had slowed down for it** so much that the whole life of the universe would have lasted only 16 days from its point of view.

[By the way: Particle energy is measured in 'eV' -  1 eV is the amount of energy gained by an electron passing between the terminals of a 1 volt battery. Remember that, it'll help you make sense of some of the absurdly huge energy numbers you're about to meet....]

After the OMG particle the hunt was on, and more extreme particles were found - we now know that cosmic rays with such ridiculously high energies are rare, but real. And because things travelling impossibly  fast are worrying and fascinating, we began building observatories to study them.

Above: This weird looking thing is the Milagro cosmic ray observatory - it has several meters of water over the top to screen out lower energy particles. Looks like it'd fun to bounce up and down on, doesn't it?
One such observatory hunting ultra high energy cosmic rays is the Telescope Array Project, a collection of specialised telescopes and scintillator detectors out in the quite and dark of the desert. Recently they found evidence that the highest energy cosmic rays might be coming from the same part of the sky. I had the chance to ask John Matthews, a University of Utah research Professor who works on the project, a few questions by e-mail on what its like working with some of the most extreme particles in the universe...

Thank you ever so much for talking with me. Could you explain to us why the array was set up to study cosmic rays, and what you’re hoping to learn about the universe from them?

Professor Matthews:
At the lowest energies, where they are most frequent, they are passing through us all the time.  However, as you start to move up in energy they become rarer and rarer. In fact, for every order of magnitude you move up in energy, the rate of arrival drops off by about a factor of 1000.
There are some ideas on how to generate cosmic rays at lower energies, but as you move to higher and higher energies, it becomes harder and harder to explain just how it happens that a cosmic ray of these ultra high energies were accelerated. When one is [artificially] accelerating a charged particle, like a proton, one must contain it in a magnetic field while one acts to accelerate the particle over and over again. Thus, we need a large confinement space where there is repeated acceleration of the particles. Here on Earth, we call these particle accelerators.One is at Fermilab near Chicago, more recently, one hears more and more about the LHC (Large Hadron Collider) near Geneva, Switzerland. The limits on the technology allow us to accelerate particles to about a few times 1000,000,000,000 eV.
Out in the "Wild", we observe cosmic rays with as much as 300,000,000,000,000,000,000 eV of energy. How were they accelerated to such energies? What kind of objects are these that are capable to do this? They must be extremely violent: How large are they?
We hope to find sources of ultra high energy cosmic rays and if we are able to do that, we would like to further study these objects to better understand our universe.

There are low energy, high energy, and ultra high energy cosmic rays, as well as cosmic rays that are different types of ionised nuclei. How do you detect the differences between different energies and types?

Professor Matthews:
To study different energy cosmic rays, we need different types of detectors. At low energy, one can build magnetic spectrometers, such as the Alpha Magnetic Spectrometer on the space station,  which measure the energy and momentum of the cosmic ray particles.....By measuring both of these, we also measure the mass or chemical composition of the particles. However, as you move up in energy, these cease to work due to inability to measure momentum - since the track of the particle no longer bends significantly in the magnetic field and the energy is no longer measured since the particles leak out of the back of the calorimeter. In addition, as energy goes up, the rate of particles drops dramatically, and one needs a larger and larger detector.
For particles above about 1000,000,000,000,000 eV, we resort to indirect measurements of cosmic rays. The Telescope Array uses the Earth's atmosphere as part of the detector. When cosmic rays hit the Earth's atmosphere, they collide with the protons and neutrons in the Oxygen and Nitrogen etc in the atmosphere. They break up that nucleus and a bunch of secondary particles come flying out of that collision.  These secondary particles still have a huge amount of energy. They collide with other nuclei and generate still more particles. This continues with kinetic energy being converted to mass energy as more and more particles are created. We soon can have a billion or more secondary particles depending, of course, on the energy of the primary cosmic ray's energy....

The Telescope Array observes this shower in two ways: Many of the secondary particles make it to the Earth's surface. We sprinkle the Earth's surface (more than 300 sq miles of it anyway) with scintillation detectors. Each scintillation detector is 3 sq m in area and they are placed on a 3/4 mile square grid. These detectors sample the density of secondary particles on the Earth's surface. They also measure the precise time of arrival of these particles. One can use this to determine the energy of the primary cosmic ray  as well as its original direction when it hit the atmosphere....
In addition to the hard collisions which add to the secondary particles in the shower, there are also soft collisions which simply excite the gas molecules in the atmosphere. The gas molecules want to get back to the ground state and do this by emitting  UV light. Therefore, the entire extensive air shower is glowing in the UV. We place telescopes on the periphery of the scintillation detector array and these observe the UV light from the air showers as they pass through the atmosphere. By looking at the signal size as the shower develops, we can determine the chemical composition (at least roughly) of the primary cosmic ray.. The proton has a much smaller cross-section than an Fe nucleus. So it is likely to penetrate the atmosphere further before its first interaction and to have more fluctuations in its shower maximum. We can also determine the pointing direction and energy of the primary cosmic ray.  One can fit a line through the track as measured by the camera and then knowing the centre of curvature of the mirror, one can fit the line and point to a plane. Using the timing info, one can turn the plane into a line within the plane and thus know the pointing direction. Integrating the area under the shower profile curve gives a good estimate of the primary particle's energy.....

The 'Oh My God' particle shocked the scientific community. I know we don't see such extreme high energy particles often. How close are we to understanding where the come from?

Professor Matthews:
The Telescope Array recently published a paper with evidence of a preferred direction of ultra high energy cosmic rays. The paper indicates that there is a 3 sigma excess of cosmic rays with energy greater than 57,000,000,000,000,000,000 eV coming from a 20 degree area south of Ursa Major. With an additional year of data, that excess has become about 4 sigma [John's note: '4 sigma' is a measure of reliability - the higher the sigma number the less likely it is to be caused by chance].We are hoping that when we add yet another year of data, we will have a 5 sigma result which would allow us to announce a discovery. Of course, once we discover a source in a 20 degree cone, we will want to collect more data to see the finer structure and sources within that......

Above: The cosmic ray hotspot.

Do you collaborate with other projects, and could you give us an example?

Professor Matthews:
We collaborate (at least loosely) with the Pierre Auger (Argentina)  and Ice Cube (South Pole) projects. We do some joint analyses where we combine data  to make whole sky maps and to try to better understand cosmic ray sources...

Above: The Ice Cube Neutrino observatory in Antarctica - at least the little bit of it that's above the surface. Or possibly a supervillian's B-string base, where they keep the broom closet and the mutants that didn't work out... Courtesy of physicsworld.

Could you tell us about some of the research you're working on at the moment, and the plans to expand project with a low energy section?

Professor Matthews:
At the moment we have expanded one of our telescope stations and we are attempting to get funding to add a more densely packed scintillator array to study lower energy cosmic rays. Our current turn-on threshold is about 1000,000,000,000,000,000 eV.  The new telescopes look higher in the sky since lower energy cosmic rays develop higher in the atmosphere. Lower energy cosmic rays also have a smaller footprint on the Earth, hence the additional scintillation detectors. By studying these lower energy cosmic rays we hope to sort out which cosmic rays are coming from inside the galaxy and which ones are extra-galactic.
In addition, we are trying to expand the area of the main array by a factor of 4 in order to collect more of the ultra high energy cosmic rays. This will help us to study the developing source(s) which we are seeing evidence of.....

Are there any special challenges to working out in the desert?

Professor Matthews: 
We work about 3 hrs drive south of Salt Lake City in the Western Utah desert. It is a long drive any time you need to go to work or find parts to repair something - the reward is that it is very dark and quiet. We have lots of wildlife.... crows and cows which chew on our cables, birds which "do their business" on our solar panels, scorpions, black widow spiders, tarantulas, rattle snakes, etc.... It all keeps life interesting...

Thank you ever so much taking the time to answer professor.

Above: If cosmic ray strikes were made visible they might look a bit like this. You can't see it here but all the Londoners in the picture are shrugging - weirder things happen in London every day. Sometimes the traffic jams even move.... Courtesy of the University of Birmingham
Elsewhere in the universe:
A 15 meter asteroid flew within 100,000 km of Earth.

Bucky ball molecules form in space:
Buckyminsterfullerene, a gigantic molecule of 60 carbon atoms in a football shape, was a breakthrough when it was first manufactured. Now it seems that  Buckyballs form naturally in space - how exactly is a bit of a mystery, but nature is full of surprises and the linked paper explores some of the possible ways they could form.

*Seriously, google it.

 ** This is something that happens to things that get close to lightspeed. Hey, don't blame me, blame Albert Einstein.

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