Monday, May 7, 2012

The hunt for cosmic rays

This past week included three days of holidays: two federal and one provincial. It was suggested that if I were interested in travelling, this would be a good week to do it. So... after visiting Buenos Aires I headed west to the town of Malargue, home to Pierre Auger Observatory. This facility is massive- 1600 tanks (which detect cosmic ray showers) each separated by 1.5km. The total size is about the size of Rhode Island! Auger doesn't own the land, but leases it from the owners, who are mostly ranchers and farmers.

A friend of mine through QuarkNet had arranged for me to visit the site, and I was given the red carpet treatment when I arrived. Analia, the Outreach Coordinator, started the visit in classic Argentina style: café and pastries with some of the staff! Over the food we chatted a bit about what their individual roles are and how they came to be involved with Auger. Afterwards, Ricardo, one of the senior staff members, gave me the standard tour presentation, and was kind enough to give it in Spanish so I could hear the technical vocabulary. 

After the presentation he took me on the behind the scenes tour of the main building.
The Pierre Auger Control Center.
This is the room where the staff collects all of the data being sent in by the 1600 detectors. They also keep tabs on atmospheric conditions and watch the weather- high winds can be a problem for some of the equipment.

Access vehicle for tricky sites (and no, I didn't get to ride in it)
Virtually all of the tanks are placed in sites that are hard to access and do not have power available. The solution to run is equipment is to rely on solar power, which is abundant here. The solar panels have to replaced periodically though- this one was damaged by vandals throwing stones.

The solar panels only generate power during the daytime (thanks captain obvious), but the tanks detect particles continuously and need to transmit the data they take back to the control center immediately. The reason for this is because this site generates an immense amount of raw data- too much to store. So the system has to do a quick scan of the raw data to see if an event is "interesting" enough to warrant keeping. Hence the need for the immediate transmission. Anyway, the solution is to use car batteries; two for each tank. With 1600 sites, they keep a lot of spares on hand just in case!

Lots o' batteries
The tanks themselves are quite large and are filled with water. There are three photomultiplier tubes on the top of the tank which can measure Cherenkov Radiation, which is produced when an eletcromagnetic particle hits the water. For any of you PPG members reading this, it's worth noting that this is a different mechanism than what our muon detector uses. Rather than water, ours uses scintillating plastic, and our PMT's are a little bit different. The concept is similar, but Auger does things on a much larger scale!
Ricardo posing with a tank that was damaged by a brush fire and brought back from the field.
After my morning tour I went to the hotel for lunch and then came back in the afternoon to tag along on a site visit with Primo. He was headed out to replace some equipment in one of the large buildings that measures fluorescence. These buildings each house 6 identical parabolic sensors, each pointing in a slightly different direction out over the array. 

Schematic from the observatory
In the image above each red dot is a tank. The four sites with the green lines radiating outward are the fluorescence detectors. These gather different data than the tanks- not so much events themselves, but related atmospheric data that can be used to help interpret the results of different showers. 

We took a truck out to the site: complete with two spare tires and a radio. While in the field the staff are required to check in with the central office periodically to stay abreast of changing weather.

Inside the fluorescence buildings are the 5 fluorescence detectors (FD's). Each one consists of a massive parabolic mirror (constructed in segments) and a camera at the focal point to collect the data. 
Yours truly as reflected in the mirror. This magnificent shot only took 20 or 30 attempts.

The FD's are protected from sunlight and harsh weather by automated doors. Primo cranked one of the doors open so I could see what the filtering system looks like.

In general, the FD"s only run during the night. If wind speeds rise about 15 km/hr or so, the doors shut to keep the equipment from getting damaged.

Primo was also working on another instrument, which is called the Photometric Robotic Atmospheric Monitor (FRAM, since photometric is spelled with an F in Spanish). It's housed in a separate building, which also opens up at night. He was kind enough to open it up so I could see it in action- it was pretty incredible. 
Primo with FRAM

The final image I have to share is from the top of the Fluorescence building. I used a timer to take this and was really impressed with how well it turned out, especially given the bright sunlight. You can see how remote this site is- we're pretty far out from Malargue, which is back toward the mountains.

My pictures don't do this place justice. If you want to see more, check out:

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