Neutrino observatories

In this section we present the instruments of neutrino astronomy. Neutrinos are chargeless and massless (or almost so) and interact with matter with extremely low probability. Thus their detection requires detectors of large mass and hence volume. The field has had two major successes, (i) the discovery of neutrinos from the reactions ongoing within the sun and (ii) the spectacular discovery of neutrinos created in the collapse of a star in the relatively nearby Large Magellanic Cloud which resulted in supernova 1987A. Here we describe the solar work and describe several types of detectors, namely chlorine (the Homestake mine experiment), gallium (e.g., GALLEX), and water Cerenkov (e.g., Super-Kamiokande).

Neutrinos from the sun

Electron neutrinos ve are emitted in the power-generating nuclear interactions that occur at the center of stars such as the sun. There are two other flavors of neutrinos, the muon neutrino v ^ and the tau neutrino vT but these are not emitted in the nuclear reactions within the sun. (Each of the three flavors has a counterpart antineutrino; there may also be a sterile neutrino.) Neutrinos are like photons in that they are chargeless particles with zero mass, or possibly with a very small mass (mc2 < few eV). They travel at, or very close to, the speed of light.

Unlike photons, neutrinos interact with matter only very rarely, i.e., with incredibly small cross sections (Section 10.4). For absorption on hydrogen the cross section is only ~ 10-48 m2. This corresponds to a mean free path of ~2 LY in material of density 105 kg/m3, the density of the solar core. Since the sun is only a 2 light-seconds in radius, essentially all the neutrinos escape the sun without interacting. Large numbers must be raining copiously down onto the earth, roughly 1015 s-1m-2!

Detection of neutrinos from the sun is important because it would permit a good cross check on the nuclear physics that astrophysicists believe is taking place within it. However, detection is difficult because the same low cross sections that allow the neutrinos to escape the sun make difficult their detection. A very large detector mass is required to detect even a tiny proportion of the incident neutrinos. All others pass right on through the detector and nearly all pass right on through the earth!

The primary energy generation process in the sun is the conversion of hydrogen to helium. This occurs through several series of nuclear reactions that take four protons and convert them into one helium nucleus and two positive electrons (positrons). This conserves both the number of baryons (protons and neutrons) and charge. Additional products are gamma rays and electron neutrinos. The initial steps involve the interactions of two protons, so this is called the pp chain. There is also a cycle of nuclear interactions involving carbon, nitrogen and oxygen as catalysts (the CNO cycle) that has the same effect, namely the conversion of four protons to a helium nucleus, and two positrons, again with the emission of gamma rays and neutrinos.

The mass of the final products of one of these nuclear cycles is a bit less than that of the input products so a bit of excess energy is released. It is in the form of kinetic energy of the products of the reactions, including the neutrinos. The neutrinos freely escape the sun while the other particles contribute to the energy content of the sun. The standard solar model indicates that 98% of the energy is generated by the pp chain and the remaining 2% by the CNO process. The CNO cycle is more important in more massive stars which have higher central temperatures.

The spectrum of solar neutrinos from the pp process at the earth as expected from the standard model is shown in Fig. 1. The reactions or decays that give rise to each component are indicated. If the final state consists of only two particles, one of which is a neutrino, the neutrino will always have the same energy in the center of mass (CM) frame of reference. This follows from momentum conservation and the



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