UNESCO and the International Astronomical Union (IAU) have declared 2009 the International Year of Astronomy, as a way to celebrate the 400th anniversary of Galileo Galilei’s first astronomical observations through a telescope.
Since Galileo’s days, telescopes have kept evolving, and now we are able to detect even the smallest amounts of energy emitted by distant astronomical sources, in a spectrum band that spans over 20 orders of magnitude in wavelength, from the most energetic gamma rays to the longest radio waves.
Astroparticle physics
A new window to the Universe has recently been opened. We are now starting to look at the skies using not just electromagnetic radiation, but subatomic particles as the carriers of valuable astrophysical information. The name of this new field is astroparticle physics, and it’s the result of the application of our current knowledge on the basic properties of elementary particles to the study of violent phenomena taking place in distant astrophysical objects.
However, not every particle can take the place of the photon in bringing new information to us; we have to be careful in picking the right messenger for the task. If we were able to design a custom particle for the job, these are some of the things that we’d ask for:
- – This particle should be either stable, or a have long decay time; otherwise it would not get to travel from the point of production before decaying.
- – It should interact very weakly with the medium through which it travels. If possible, it should also not be absorbed even after passing through considerable amounts of matter, so we can get an unobstructed view of the interior of the sources.
- – It should be electrically neutral; in this way they wouldn’t get deflected by magnetic fields, so they would still point to their origin at the time of detection.
Luckily enough, Nature (no, not that Nature, I mean Mother Nature) has provided us with such a particle: the neutrino. Unfortunately, as neutrinos interact very weakly with matter, the same advantages that make them reach us from their source will also make them go unnoticed through our detectors.
Just as an example of how sneaky neutrinos are, during any given year only 90 of them with energies around 100 keV interact with a proton or a neutron in your body, although there are 400000000000000 going through you every second. These neutrinos, by the way, are reaching you from the thermonuclear reactions happening at the very core of the sun.
Neutrino interactions
As neutrinos interact mainly with nucleons (this is, protons or neutrons), increasing the mass of the detector will increase the number of nucleons in the neutrino’s path, and hence its probability of interacting with matter. We are making the mesh of our fishing net finer.
In any given interaction with a nucleon, the neutrino will exchange one out of the two bosons that carry the weak force: either the neutral Z boson or the charged W boson. Although the name boson may sound cryptic, a more familiar member of the family is our good old friend the photon, which carries the electromagnetic force and it’s making the electrons on this screen interact with the electrons on your retinas as you read through the text.
The process of interchanging a Z boson is called “neutral current”, and its outcome is that the neutrino continues to travel along with the production of a hadronic shower. In the case of the charged current (the interchange of a W boson), a lepton is emitted (an electron, or one of its fat cousins: a muon or a tau) also accompanied with a hadronic shower.
Cherenkov radiation
Cherenkov radiation lights up a reactor core as energetic electrons move faster than light in water
Now we are in business, the elusive neutrino has finally produced secondary particles that are much easier to detect. If these charged secondary particles travel faster than the speed of light, they will emit light (mostly in the UV) called Cherenkov radiation.
But wait a minute… Particles traveling faster than c, the speed of light? Isn’t that supposed to be impossible? Well, it is true that Nature won’t let any massive particle travel at the speed of light, let alone go over it. But what Einstein postulated in his theory of relativity is that the particles can’t travel faster than c as measured in vacuum.
However, when light enters a medium (air, water, ice, the glass of a lens) it slows down, in an amount dictated by the medium’s coefficient of refraction. So, even if the particles are traveling at a speed lower than that of light in the vacuum, they will still be faster than the speed of light in that medium, setting the conditions for the production of Cherenkov light, and, trust me, Prof. Einstein would be completely OK with this.
So far, we know that neutrinos interact (although little) with matter producing secondary particles, which in their turn emit UV light. We only need to place sensitive light detectors to observe this light and that’ll be it!
In practice this is not so simple, first we need a huge volume of some highly transparent material, and this material should be placed somewhere dark, so that the dim Cherenkov radiation can be clearly distinguished from any background.
To be able to detect high energy neutrinos, we need volumes of the order of hundreds of thousands of cubic meters, so this leaves any kind of manmade tank out of the question. Thankfully enough, we can always count on Mother Nature; we have two places that are dark, spacious, and filled with a transparent medium: the bottom of the sea, and the deep ice in the Antarctic polar cap.
The completed IceCube detector as it will look in 2011 with all 80 strings deployed, click for a larger image
The IceCube Neutrino Observatory is the realization of one of these possibilities, and I’ve started to work in this project as a grad student.
At its completion, probably around 2011, the detector would have reached an instrumented volume of 1 km3, which will make it the first telescope with enough sensitivity to detect neutrinos from sources located beyond the Milky Way’s vicinity.
The detector itself is as awesome as the objects it is meant to observe. The baseline design consists of 80 strings with 60 digital optical modules (DOM) each, each DOM contains an extremely sensitive light detector, called photomultiplier tube (PMT), and all the associated electronics to do the data readout each time the PMT “sees” something. The strings are placed in a triangular grid with a 125 m spacing between neighboring strings. The 60 DOMs in each string are 17 m apart and are deployed between 1.5 and 2.5 km below the surface, because of the extreme transparency of the ice at those depths. To get down there, we use heat to melt the ice using specialized hot water drills; I’ll tell you more about drilling in a future post.
The great thing about this kind of experiment is that we can start taking data from the first day we deploy detectors. As the detector gets completed, its sensitivity and resolution increases but in the meantime we have data to analyze as the construction moves on.
Now that you’re familiar with some basics of the detector, during my coming posts I’ll be able to share with you some of the results we’ve got so far. In the meantime, I’ll leave you a little teaser with one of many neutrino events we have detected.
After its production in a neutrino interaction, an energetic muon goes through the IceCube detector. Colors indicate the time in which each DOM was triggered, increasing from red to blue. The size of each DOM in this representation is proportional to the amount of light detected at that point along the track.
