Neutrino astronomy: Neutrino telescopes
 

The most publicized neutrino detection experiment has been operating since 1968 in a disused gold mine at a depth of 1·5 km. The detector is in the form of a large tank (over 600 tonnes) of tetrachloroethylene (C2Cl4) commonly known as cleaning fluid. One in four of the chlorine atoms is the isotope ³⁷₁₇Cl and hence, on average, one of the molecules contains one atom of the necessary isotope. Within the tank, there are of the order of 2 × 10³⁰ atoms of ³⁷₁₇Cl. The argon (³⁷₁₈Ar) that is produced by neutrino capture has a half-life of 35 days, decaying back to ³⁷₁₇Cl by capturing one of its inner orbital electrons, at the same time ejecting a 2·8 keV electron. The detection process involves sweeping out the argon with helium, separating the gases through a charcoal trap and counting the number of 2·8 keV electron events.
Placing the tank some 1·5 km below ground shields it from cosmic rays and from natural and artificial radioactive sources. A thick water jacket surrounding the tank acts as a shield to absorb neutrinos. The efficiency of extracting the argon is monitored by introducing known quantities of ³⁷₁₈Ar. The collected gas containing any ³⁷₁₈Ar is shielded during the monitoring of its decays and anticoincidence techniques are applied.
As with five other different observatory experiments established since the early 1970s using three different detector techniques, significantly fewer neutrinos are detected from the Sun than predicted by the theory associated with the thermonuclear generation of energy within its core. The ‘missing solar neutrino problem’ has been very significant in developing our ideas about the nature of the neutrino itself. The question as to whether the particle carries mass is a very fundamental one, it not being supported by the Standard Model of Particle Physics.
Neutrinos are known to carry one of three flavours, there being ‘electron’, ‘muon’ and ‘tau’ varieties. The Sun’s thermonuclear reactions are sufficiently energetic to produce electron neutrinos but not muon or tau ones. A mooted possibility is that neutrinos, once produced, can oscillate from one flavour to another as a result of a tiny but non-zero mass. To explore the issue, a detector that can differentiate between the neutrino flavours is required. Such a capability has been established at the Sudbury Neutrino Observatory (SNO) in Canada.
The detector core comprises a chamber 34 m high and 22 m wide of 1000 tonnes of heavy water (D₂O) some 2 km below ground in the Creighton nickel mine near Sudbury, Ontario. The volume is surrounded by 10 000 photomultiplier tubes to detect ˇCerenkov radiation events. The system has greater accuracy than all previous measurements and provides approximately one event per hour.
By using D2O, the detector responds to three types of neutrino interaction referred to as charged current (CC), neutral current (NC) and elastic scattering (ES). In the CC interaction, only electron neutrinos are detected with the production of two protons and an electron; the NC interaction is equally sensitive to all flavours with the production of a proton and a neutron. The ES process is ×6·5 more sensitive to electron neutrinos than other neutrino flavours. By detecting the different decay products, it is possible to measure both the electron neutrino flux and the total neutrino flux. By looking at the ratio of reactions specific to ES and NC, precise information can be obtained as to whether neutrinos change flavour on their way between the Sun and Earth.
The first phase of the experiments, in collaborationwith a measurement of the ES rate, as obtained by the Superkamiokande Observatory in Japan, has provided strong evidence that electron neutrinos disappear but are replaced by neutrinos of alternative flavour. Further evidence that electron neutrinos produced in the Sun oscillate into other neutrino flavours on their way to Earth and, hence, have mass has now emerged from differences in the signals during day and night. The SNO results suggest that, although the Standard Model is not destroyed, it is somewhat tarnished and requires modification.