Neutrinos are tiny, possibly massless, neutral elementary particles which interact with matter only via the weak nuclear force. The weakness  of the weak force gives neutrinos the property that matter is almost transparent to them.

Physicists believe there are 4 Fundamental Forces in Nature…..

In order of increasing strength :

Gravity : Gravity :

the attraction between masses

Electromagnetic :Electromagnetic :

the attraction or repulsion between charges

Weak Nuclear : Weak Nuclear :

the force that holds each proton or neutron together

Strong Nuclear :Strong Nuclear :

the force that holds protons & neutrons together in the

nuclei of atoms

Unlike the stuff we’re used to thinking about - protons, neutrons, & electrons, neutrinos interact with other types of matter through only the weak force (& maybe through gravity, though it’s still an open question how much mass they have).

To a physicist, this means they barely interact AT ALL…

The sun, and all other stars, produce neutrinos copiously due to nuclear fusion and decay processes within the core. Since they rarely interact, these neutrinos pass through the sun and the earth (and you) unhindered. Other sources of neutrinos include exploding stars (supernovae), relic neutrinos (from the birth of the universe) and nuclear power plants (in fact a lot of the fuel's energy is taken away by neutrinos).

The sun produces over two hundred trillion trillion trillion neutrinos every second, and a supernova blast can unleash 1000 times more neutrinos than our sun will produce in its 10-billion year lifetime. Billions of neutrinos stream through your body every second, yet only one or two of the higher energy neutrinos will scatter from you in your lifetime.

In recent years, theoretical models of the sun have permitted detailed calculations of the number (or flux) of neutrinos released from the sun. Several neutrino experiments have detected solar neutrinos and found the flux was too low. It appears that far too few neutrinos are detected than can be consistent with the known energy output of the sun.

This is known as the "Solar Neutrino Problem" (SNP).

The neutrino was proposed by Wolfgang Pauli in 1930; but it would be 26 years from then before the neutrino was actually detected. Pauli proposed the existence of the neutrino as a solution to a frustrating problem in a nuclear process called beta decay. It seemed that examination of the reaction products always indicated that some variable amount of energy was missing. Pauli concluded that the products must include a third particle, but one which didn't interact strongly enough for it to be detected. He liked to say: "I have done a terrible thing, I have postulated a particle that cannot be detected." Enrico Fermi called this particle the neutrino which meant "little neutral one".

In 1956 Reines and Cowan found evidence of neutrino interactions by monitoring a volume of cadmium chloride with scintillating liquid near to a nuclear reactor. Fred Reines was jointly awarded the Nobel Prize in physics in 1995 in part for this revolutionary work.

We know that the mass of the neutrino is approximately zero, but we are unsure how large the masses of the three individual neutrino types are because of the difficulty in detecting neutrinos. This is important because neutrinos are by far the most numerous particles in the universe (other than photons of light) and so even a tiny mass for the neutrinos can enable them to have an effect on the evolution of the Universe through their gravitational effects. There are other recent astrophysical measurements that provide information on the evolution of the Universe and it is interesting to seek complementary information by direct determinations of the masses of neutrinos.

Detecting neutrinos from the sun is very valuable as a way of seeing the sun's interior. The neutrinos produced in the core of the sun escape unhindered and a very small number may be detected with suitable apparatus on earth. The neutrinos act as a probe on the mechanisms in the solar core. Now since neutrinos only weakly interact with matter, neutrino detectors requires an enormous volume to compensate.

In the Sun

On the Earth

The flux of solar neutrinos has been measured by previous experiments; but the results have been inconsistent and perplexing. The pioneering experiment is Ray Davis's 600 ton chlorine tank (actually dry cleaning fluid) in the Homestake mine, South Dakota. His radio-chemistry assay, begun in 1967, found evidence for only 1/3 of the expected number of neutrino events. A light water Cherenkov experiment at Kamioka, Japan, upgraded to detect solar neutrinos in 1986, found 1/2 of the expected events for the part of the neutrino spectrum for which they are sensitive. 2 recent gallium detectors (SAGE and GALLEX), which have lower energy thresholds, found about 60-70% of the expected rate. The clear trend is that the measured flux is found to be dramatically less than is possible for our present understanding of the reaction processes in the sun.

Furthermore, the neutrino deficit appears to depend on the energy of the neutrino. The sun produces neutrinos with a range of energies, and the different detectors are sensitive to different energy ranges. We’re looking at 2 ways to address the Solar Neutrino Problem:

* The structure and constitution of the sun, and hence the reaction mechanisms are not correctly understood. This would be a real blow for models that have otherwise been very successful. Many modifications have been proposed, yet none can satisfactorily remedy the large neutrino deficit. * Something happens to the neutrinos in transit to earth; in particular, they might change into another type of neutrino. This idea is not as crazy as it sounds, as a similar phenomenon is well known to occur with the meson particles.

You could argue that all the experiments are simply wrong, but this is highly unlikely. The different experiments all use diverse detection techniques, overseen by large collaborations, and have been calibrated with a variety of sources.

Here’s another fact about neutrinos; there are actually 3 types of neutrinos (6 types if you count the anti-neutrinos). The 3 types (called flavors) are the electron-neutrino (ne), the muon-neutrino (nu) and the tau-neutrino (nt); they correspond to the 3 known "generations" of particles that make up the known roster of elementary particles. Normal "earthly" matter is made from 1st generation particles, protons, neutrons and electrons. The higher generation particles can be created in particle accelerators (that’s how they were discovered), but they rapidly decay back to the 1st generation due to their larger mass.

Now the sun only produces electron neutrinos, and, to date, detectors on earth have only been sensitive to electron neutrinos. So if the neutrinos were undergoing a "flavor" oscillation then the probability of detection would be reduced. There is a proposed scenario, called the MSW effect, where the large mass densities in the sun could greatly enhance this oscillation effect. This turns out to be a very attractive possibility for the solution to the solar neutrino problem.

Using heavy water, the Sudbury Neutrino Observatory (SNO) can detect all three flavors of neutrinos. So the SNO detector will be able to observe separately the number of electron neutrinos and the number of all neutrinos. This allows a determination of the probability for these flavor oscillations to occur. From the neutrino flux and shape of the energy spectrum SNO will be able to determine how strongly the neutrino flavors mix together, and determine information about the neutrino masses. SNO started collecting the first data in April 1999, and this will be a very exciting time for neutrino physics.

The Sudbury Neutrino Observatory (SNO) is taking data that has provided revolutionary insight into the properties of neutrinos and the core of the sun. The detector was built 6800 feet under ground, in a mine near Sudbury, Ontario. SNO is a heavy-water Cherenkov detector that is designed to detect neutrinos produced by fusion reactions in the sun. It uses 1000 tons of heavy water, on loan from Atomic Energy of Canada Limited (AECL), contained in a 12 meter diameter acrylic vessel. Neutrinos react with the heavy water (D2O) to produce flashes of light called Cherenkov radiation. This light is then detected by an array of 9600 photomultiplier tubes mounted on a geodesic support structure surrounding the heavy water vessel. The detector is immersed in light (normal) water within a 30 meter barrel-shaped cavity (the size of a 10 story building!) excavated from rock. Located in the deepest part of the mine, the overburden of rock shields the detector from cosmic rays. The detector laboratory is extremely clean to reduce background signals from radioactive elements present in the mine dust which would otherwise hide the very weak signal from neutrinos.

Sources of neutrinos far beyond the sun are expected to be much dimmer. It is generally believed that one must construct kilometer-scale detectors to scrutinize the skies for neutrino sources beyond the sun. It is a daunting task but worth it. Theorists have indeed already anticipated an impressive wish-list of missions for such an instrument ranging from the observation of neutrinos from bright, far away galaxies to those from the annihilation of cold dark matter particles inside our own galaxy's halo. The hope is that a particle that is almost nothing may tell us everything about the

Universe.

The accelerator physicist's method for building a neutrino detector will typically use lead absorber to filter out all particles but the neutrinos, wire chambers and associated electronics with a combined price tag of roughly 10,000 US dollars per square meter. A 1 kilometer-square detector would cost 10 billion dollars. Realistically, we are compelled to develop methods which are more cost-effective by a factor of one hundred. The only known solution is to use a "natural" detector consisting of a thousand billion liters (a teraliter) of instrumented natural water, as in the ANTARES array, (pictured) or ice. Such instruments should be able to study our Universe far beyond the sun and watch cosmic cataclysms without having to wait for a one-in-a-century

miracle like a nearby supernova.

As shown in the AMANDA logo, the array depends on the immense mass of the Earth to filter out other types of particles, leaving only the weakly acting neutrinos, able to travel unimpeded through the planet, to create signals within the array.

An additional array(IceTop) must be constructed on the surface of the ice to collect information about particles arriving from that direction. Fast computers will sift out signals from non-neutrinos.

High energy neutrinos interacting with ice produce a burst of light. Some of High energy neutrinos interacting with ice produce a burst of light. Some of these photons will knock electrons out of atoms and produce a spark, a few meters these photons will knock electrons out of atoms and produce a spark, a few meters in length, which may occasionally contain a million electrons or more. The in length, which may occasionally contain a million electrons or more. The AMANDA idea is, in principle, very simple: sink an array of photomultiplier AMANDA idea is, in principle, very simple: sink an array of photomultiplier tubes (PMTs) deep into the polar ice cap where the pressure of the overburden tubes (PMTs) deep into the polar ice cap where the pressure of the overburden squeezes all the bubbles out. Light from neutrino interactions will be transported squeezes all the bubbles out. Light from neutrino interactions will be transported over large distances in this clear ice to illuminate the array of PMTs. over large distances in this clear ice to illuminate the array of PMTs.

It did not take long to realize the advantages of this technique. Polar ice is a totally sterile medium, free of radioactive material such as potassium which is ubiquitous in sea-water. Its decay products are responsible for ten thousand false signals every second in the sensitive PMTs looking for the faint neutrino-light. In the absence of radioactivity off-the-shelf commercial electronics can be used to identify the footprints of neutrino interactions in the array, a great simplification and cost-saver. Also, you can walk on your experiment. Only the PMT is deployed. It is connected by a simple kilometer-long coax cable to the data acquisition which remains at the surface, accessible for repair and updating. In deep ocean experiments the deployment of active electronics in an unfriendly environment is inevitable because the raw signals would be totally degraded after transmission over twenty kilometers of cable from experiment to shore. Conveniently, glaciologists had already invented the technology to cheaply deploy equipment in ice: hot water drilling.

In summary, the experiment can be made technologically simple and cheap, critical factors when one has the ultimate ambition to instrument kilometers of relatively inaccessible

Antarctic ice.

It turned out that there were more advantages to the ice. Ice is transparent as It turned out that there were more advantages to the ice. Ice is transparent as diamond in the blue wavelength region where the PMTs operate; more about that diamond in the blue wavelength region where the PMTs operate; more about that later. The National Science Foundation operates a research station at the South later. The National Science Foundation operates a research station at the South Pole with an infrastructure reminiscent of a national laboratory. With Raytheon, Pole with an infrastructure reminiscent of a national laboratory. With Raytheon, the contractor for polar support services, it has created an effectivethe contractor for polar support services, it has created an effective and surprisingly friendly and surprisingly friendly environment to operate an environment to operate an experiment like AMANDA experiment like AMANDA whose natural milieu is the whose natural milieu is the large high energy physics large high energy physics laboratories of the US or laboratories of the US or Europe. A fifty foot crane, Europe. A fifty foot crane, heavy-duty vehicle or heavy-duty vehicle or snowmobile is only a phone snowmobile is only a phone call away.call away.

In the 1992 Antarctic summer the embryonic AMANDA-collaboration deployed an odd collection of PMTs at the South Pole, positioning them at various depths to try out the technology. The equipment was deployed using a hot water drilling technique pioneered by glaciologists. The drill has been compared to a rather heavy bathroom shower head. Gushing out hot water, it melts its way down the ice steered only by gravity. In its free fall it will deviate by less than 1 meter from vertical when reaching the 1 kilometer mark. The melted ice is not removed from

the hole; the hot water is continuously recirculated in order to keep the hole from refreezing. This task is not too difficult --- the surrounding solid ice is a great insulator. After the stream of hot water is interrupted, the water will not refreeze for several days, leaving plenty of time for deployment of the equipment. All of the equipment deployed at the time is still working, ticking away like Swiss watches frozen in time in ten thousand year old ice. That is the good news. The drilling itself was a nightmare. The method turned out to be time- and fuel-intensive and "plateau-ed" at 800 meters, a depth too shallow for our ambitious project.

The first string of 80 PMTs was successfully deployed on Christmas eve 1993. It took 10 people 17 hours in minus 30 degrees to assemble the string. Some tasks, involving connectors and fiber optic cable, have to be handled without the protection of gloves. It does help to have heaters and to shield the open working area against wind by a suspended parachute.

The AMANDA strings are assembled in-situ by attaching the PMTs to the cable carrying the signals. But 2 days later the ambient pressure in the hole, about 100 atmospheres at 1 kilometer, would exceed five times that value or more; the dynamic range of our pressure meters was insufficient. The water column breaks the symmetry and is responsible for a nasty overpressure just before the ice finally turns solid. One PMT broke and there were a few connectors damaged. More heat was pumped into the bottom of the next holes which refroze very gently reaching only twice ambient pressure. The last 2 strings were essentially flawless. In the end, 73 tubes were operating as expected and only 3 were fatally damaged.

Any science was of course held hostage to calibrating the detector, i.e. one has to quantitatively understand how light propagates through kilometer-deep polar ice. At this depth the absorption length, which measures how far a typical photon travels, was supposed to be 8 meters and the scattering on remnant bubbles predicted to be negligible. Bubbles of air should be squeezed by pressure to sub-micron size at a depth of 1 kilometer. Detector simulation programs, based on these expectations, totally failed to reproduce the observations of the well-understood beam of cosmic ray muons which we used for calibration purposes. For instance, observations recorded over one hundred cosmic ray events in coincidence with the Bartol-Leeds

air shower detector at the surface, when only 2 might be expected.

Resolving this puzzle required disentangling the combined effects of two complete surprises: the bubbles were not small, more like 50 microns, and the absorption length was not 8 but over 300 meters! Shortly after the deployment Gregorii Domagatzky, the leader of an experiment in Lake Baikal which is chasing similar science, described the observation of large bubbles as deep as 1 kilometer in an ice core extracted at the Russian Vostok station not too far from the South Pole. This helped, but did not resolve the problems. It became a private joke among the people doing the detector simulation that all problems would go away if only light traveled hundreds of meters in ice...

QuickTime™ and aGIF decompressor

are needed to see this picture.

It does. With every PMT a small plastic ball had been deployed which is connected to the surface by a fiber optic cable. Using a laser, pulses of light can be pumped into the deep ice and its propagation studied with the PMT array. It turned out that the absorption length of deep South Pole ice had the astonishingly large value of ~310 m for the blue light to which the PMTs are sensitive. A value of only 8 m had been anticipated from laboratory measurements. For many applications, such as the detection of supernovae, the detector volume scales linearly with the absorption length. The discovery turned AMANDA into a supernova detector. It has been monitoring our galaxy

since February 1995.

A schematic diagram shows the AMANDA array, alongside a scale that shows ice depth and, for comparison, the Eiffel Tower.

Eventually, astrophysicists hope to build out the detector array to collect data from a cubic kilometer of ice : IceCube !

Why study neutrinos ?

About 200 years ago, the British Prime Minister Lord Gladstone was touring the laboratory of Micheal Faraday, who discovered many of the fundamental properties of electricity. Gladstone asked,”But what good is this electricity ?” Faraday shrugged and smiled and said he didn’t really know.

Faraday’s discoveries led directly to electric motors, generators, radio, television, and a few thousand other inventions we now take for granted.