A gazillion neutrinos just passed through your body. They were completely untouched by the experience, and left you with--nothing. Feel violated? Feel used?
Well, that’s the problem with neutrinos. They just pass on through and hardly ever make any difference in our lives. At least until lately. Neutrino astronomy is here, and it’s apt to change the way we look at the universe.
A neutrino is a chargeless particle subject only to the weak nuclear force, which is another way of saying that neutrinos will bounce off a quark. Such collisions occur rarely. A neutrino of sufficient energy could pass through a light year of lead and not undergo any scattering. The neutrino was first predicted by Wolfgang Pauli as a way to balance the angular momentum of particle interactions. From the first it was clear that the neutrino was a very light particle (a lepton) and most physicists considered it to be without any mass at all. The particle process that caused Pauli to theorize neutrinos in the first place is that of beta decay, which is the spontaneous degeneration of a neutron. This process is responsible for the radioactive decay of certain heavy isotopes. In beta decay, a neutron (n) is transformed into a proton (p), an electron (e-) and a neutrino (n
):
n ®
p + e- + n
Actually, as it was later found, that’s not entirely accurate. Beta decay actually results in an antineutrino, a nicety your old Quantum Butcher won’t dwell on today. The principle remains the same: when a process involves the transformation of neutrons into protons (or vice-versa), then neutrinos are going to be involved. Another example is the fusion process that takes place in the heart of a star or the guts of a hydrogen bomb:
(2n + p) + (n + p) ®
(2n + 2p) + n + n
+ energy
(tritium nucleus) (deuterium n.) (helium-4 n.)
So you might begin to suspect why neutrinos are so important to astronomers and astrophysicists. For a start, observation of neutrinos under certain circumstances would go a long way toward verifying models of galactic evolution which call for the existence of Active Galactic Nuclei, or AGN. AGN are thought to be supermassive black holes lurking at the center of galaxies, where high concentrations of stars and interstellar dust have resulted in the formation of a sort of gravitational maw. The black holes that form the heart of AGN are true cosmic monsters, weighing in at 105-1010 stellar masses--up to several billions of times the mass of our sun.
Imagine such a monster, deep within the heart of a galaxy. This cosmic sinkhole is ripping matter from the surrounding galactic core, tearing apart entire suns and pulling interstellar gas from solar nurseries. But the hole also emits radiation as high energy particles. At a distance equivalent to about 75 times the radius of the hole itself, the radiation emitted from the hole collides with the incoming, superheated plasma of the accretion disk, forming a hellishly violent, supertubulent zone where particles undergo interactions of fantastic intensity. This shockfront is characterized by particle interactions attaining energies of up to 1018 eV (that is, up to 109 GeV)! This region of ultra high energies naturally pours out electromagnetic energy all over the spectrum, and several candidate AGN have been detected optically in all quadrants of the cosmos, including the hearts of ancient quasars and possibly the center of our own Milky Way galaxy. Unfortunately, the most characteristic high energy particles emitted by the shock front, gamma rays, protons and neutrinos, present difficulties for astronomers searching to verify the existence of AGN. Protons and gamma rays are absorbed by intervening matter. Neutrinos aren’t readily absorbed (remember?) but they aren’t easily detected, either, for precisely the same reason. Any experiment you set up to look for neutrinos must rely on some sort of detector, and your neutrinos will just as likely as not pass through your detector without so much as a how-do-you-do. So much for your funding.
Another source for neutrinos is the thermodynamic furnace that burns at the heart of any star. The immense energies of the fusion reactions in a stellar core generates neutrinos in vast quantities. So detecting neutrinos from the sun--in the right amounts and in the right "flavors"-- should go a long way toward verifying astrophysicists’ assumptions about stellar structure and evolution.
And speaking of stellar evolution, neutrinos play a significant role in the death throes of stars. When a star burns its nuclear fuel to the point where it has nothing left to fuse but iron, the gig is up. Fusing iron takes more energy than it releases, so the star is effectively out of fuel. For eons, thermonuclear reactions have generated the energy to resist the star’s tendency to undergo gravitational collapse. When those reactions cease, the star undergoes a catastrophic implosion. The result is a supernova, and modern models of supernovas call for huge showers of neutrinos to spill out of the explosion and across the cosmos. In fact, there’s reason to believe that without neutrinos, a supernova wouldn’t explode at all.
But the story doesn’t end there. A supernova explosion may leave a remnant consisting solely of neutrons crammed into a superdense, spinning stellar corpse. This neutron star is spinning rapidly, and if it also has a magnetic dipole the spin may create a sort of cosmic beacon, sending blips of radio energy at intervals that correspond to the period of the neutron star’s spin. The spin of such pulsars in particular and neutron stars in general had long been assumed to derive from the spin of the parent star as it collapsed into a supernova, but researchers from the Max Planck institute and Caltech have recently challenged this assumption. They assert that the spin of neutron stars cannot be accounted for by the spin of the collapsing parent star. They speculate that neutrino jets generated by the supernova blast may set the residual neutron star spinning.
There’s still more. Because theory predicts that neutrinos exist in the universe in such abundance, they might, if they had mass, account for much of the missing "dark matter" necessary to account for discrepancies in calculations of the cosmological constant. Perhaps enough to "close" the universe. Even a very low mass for the neutrino--say, as small as one millionth the mass of the proton--would be enough to have a tremendous impact on the ultimate shape and destiny of the cosmos.
Okay, so it’s important to be able to look for neutrinos. But how? Remember, you’re looking for a particle that doesn’t seem to want to interact with anything. When Pauli dreamed up the neutrino to balance the books on beta decay, he lamented that he had committed a great sin--theorizing a particle that couldn’t be detected. Actually, this problem was overcome by Frederick Reines in 1956, who used an elaborate setup to detect neutrino emissions from nuclear reactors. This work, which scored Reines the 1995 Nobel Prize in physics, showed that you could do neutrino physics. But that is not the same thing as doing neutrino astronomy. It’s one thing to put together a setup that can detect a few neutrinos out of the trillions spilling from a nearby point source--that is, to detect neutrinos in an area of high neutrino flux. But scanning the sky for neutrinos is an entirely different matter.
Okay, then, so how to catch neutrinos coming from astronomical sources? I first got interested in neutrino astrophysics in 1984, while I was in college. At about that time, John Learned and a lot of other folks were involved in the DUMAND project, which used the Pacific Ocean as a neutrino detector. The idea was that by using such a huge volume of matter, you could statistically increase the likelihood that a detectable neutrino interaction would occur. Researchers suspended thousands of ovoid, glasslike detectors in the ocean in the hopes of spotting the signature of neutrino interactions.
The idea is simple: when an atomic nucleus interacts with a high energy neutrino, it kicks out an energetic charged particle, typically a muon, sometimes an electron. These charged particles go on to interact with other particles, generating scattered photons of characteristic blue Cerenkov light. This pulse of light is generated along a vector that is determined by the vectors of the muon and the neutrino, and emerges as a cone of radiation that projects a ring or ellipse on a detector.
Unfortunately, DUMAND was less than optimal as a neutrino telescope (in part because it was looking primarily for proton decay). Impurities in seawater and problems with the detectors themselves brought the project to its demise. Besides, others were showing that there were better ways to catch neutrinos. This time and next, we’ll talk briefly about two: the Super Kamiokande reactor and AMANDA.
The Super Kamiokande reactor is a project built and operated by Japanese and Americans. Essentially, it’s a hollowed out mountain filled with very pure water and lined with photomultiplier tubes. But it’s got nothing on AMANDA, a project headquartered in Antarctica. AMANDA researchers sink huge strings of photomultipliers into the ultra-pure, ultra-clear Antarctic ice to look for Cerenkov emissions. But whereas Super-K researchers are most interested in solar and atmospheric neutrinos, and in catching protons in the act of decaying, the guys at AMANDA are interested in catching neutrinos from the far reaches of the universe. And that’s a problem.
When gamma rays strike the earth’s atmosphere, they generate a spray of high energy neutrinos that in turn generate muons, that in turn generate readings in detectors like Super-K and AMANDA. This happens all the time, and if you’re looking for atmospheric neutrinos, it’s okay. But what if you want to look only at "astronomical" neutrinos? The folks at AMANDA solve this problem by setting their photomultipliers to look only for Cerenkov light from below. Remember, Cerenkov light is emitted when the muon spray from a neutrino interaction travels through water or ice, and the vector of the emission corresponds to the vector of the particles that generated it. Cerenkov light generated by muons from skyward might be due to neutrinos from space, or to neutrinos from gamma rays striking the atmosphere. But muon sprays from below can only be due to high energy neutrinos passing through the earth. In other words, AMANDA uses the entire earth as a neutrino detector!
Figure 1. A highly simplified and schematic diagram of AMANDA.
Using these massive Cerenkov detectors, physicists have opened a new door on the universe, and raised as many questions as they've answered--that's the delicious part of science. But we'll talk more about that next time, when we'll explore neutrino oscillations, heavy neutrinos, and other tasty stuff. Until then.....
[ NW ISSUE #01 ] Immortality: A Primer
[ NW ISSUE #02 ] Nano, Right Under Our Noses
