October 6, 2009
This is my first post and I am quite excited about it. I am an assistant level professor at Drexel University in Philadelphia. We study neutrinos, their properties, and especially features related to the fact that neutrinos oscillate – peculiar feature that we learned about more in the last decade. In short, neutrinos come in three different types that are clearly distinct from each other. Nevertheless, they oscillate between different types as they fly. And neutrinos, just to make it clear, always viz around close to the speed of light. Neutrino oscillations are as striking as if you would see an eagle soaring over the sky rapidly turn into a parrot, continue flying, then turn into a duck, and after more flying turn BACK into an eagle soaring in the sky! Neutrino oscillations also told us that neutrinos have mass, and even better, that they maybe hold the key to the question of why we have more matter than antimatter in the universe (and we know we have!). So it is a puzzle that many, many people would like to see solved. However, it is also a tough one, and luckily there are quite a few physicists that are on the enthusiastic quest for it. And as of recently, I am one of them too.
This year I got involved in the project called Long Baseline Neutrino Experiment (LBNE – boring name, but temporary; we are about to name it). Along looking to see if protons can decay (major prediction of Standard Model of particle physics) it will observe (detect) neutrinos and anti-neutrinos from various sources like: accelerator neutrinos and antineutrinos sent from Fermi lab 1300 km away (to search for neutrino-antineutrino asymmetry), neutrinos from the Sun, neutrinos and antineutrinos from galactic supernovae, neutrinos and antineutrinos from past supernovae streaming through our Universe and maybe even antineutrinos from the Earth. That is A LOT OF different neutrinos and it will be FUN, BUT we need to build our neutrino detector first.
So, I am just out of the meeting for the LBNE that is all about planning this giant 300 kton Water Cherenkov neutrino detector . It will be built in the abandoned old goldmine called Homestake in the town of Lead in South Dakota. 300 ktons is a lot of water and no one has ever built a water Cherenkov detector of this size. The largest up to date is SuperKamiokande water Cherenkov neutrino detector in Japan and that one is 30 kton (still huge, but significantly smaller). The detector is envisioned to be made out of three 100 kton tanks or maybe even two 150 kton tanks (take a look at the picture).
Long Baseline Neutrino Oscillation Experiment at Homestake mine in South Dakota
Did I mention that they will be placed very deep in the mine at 4850 feet (1479 m) depth? This is to use all the 1.5 km dirt above, as a sort of umbrella to shield the detector and get much fewer cosmic ray muons that interact in the detector and make it noisy. Just the size of the caverns one needs to place the detectors is enormous. A cavern that can fit a single tank needs to be 55 m high and around 60 m wide cylindrical hole (like a 20 floor high-rise building). So we need to dig out a total of 60 floors high buildings of rock! Therefore, at this meeting, there was a whole set of reports and plans presented how to excavate such large caverns, evaluate the rock hardness in the mine at that depth, make sure that there is enough electricity, air, working elevators, wide enough, completely vertical shafts and many of other things. Huge work! And expensive!
The whole plethora of detector elements and aspects of detector building were addressed. There were people talking about producing a neutrino beam at Fermi lab, small near to the beam detector, alternative liquid argon detector and accompanying details. There were more than 100 people at the meeting, while whole collaboration has more than 180 members which is huge for the neutrino experiment. But this time I will focus on one important part of Water Cherenkov detectors and these are PMTs, and also something that I got involved in.
To “see” neutrinos we will use very sensitive photomultiplier tubes (PMTs) that can observe a single visible photon that hits, and produce measurable electric signal on the order of mili volts. And here is a picture of one PMT.
Single photomultiplier tube
Such sensitivity to light is crucial for us. Using a large number of PMTs mounted on detector walls, floor and ceiling and looking inward we can observe light coming from the particles interacting inside the detector to help us determine what type of particle interacted in the detector, how much energy it carried and where the interaction took place. In this way we detect neutrinos as well! For such huge detector modules we will need about 150,000 PMTs that are 25 cm (10 inch) in diameter. Not only that the PMTs are expensive, but that many PMTs will require 13,000 km of cables and accompanying electronic modules which makes it twice as much expensive. So is there a way around that? This is a question that my group at Drexel will try to answer. So, at the meeting we discussed using light concentrators which are basically non-imaging light collectors that gather the photons that would otherwise miss the PMTs. So, we can collect even more light or use fewer PMTs and keep the light level unchanged! Light concentrators look like bottomless, shallow, shiny metallic bowls that are placed on the PMT face (top part) to funnel the photons efficiently to the PMTs. So, we discussed how to optimize their shape, what to make them from and compare to experience of other experiments like SNO and Borexino where similar devices produced 50%-60% increase in light collection. It is promising that with a right design we can make a real difference. So, I am looking forward to some simulation work with my postdoc Karim and collaborators at Duke. I will also offer a new graduate student Frank to work on it since he expressed a strong preference to simulation work. We also choose materials and see if they can withstand ultrapure water that these kind of detectors use. A lot of interesting work ahead!
