Published on February 12, 2008
Neutrinos: Little Neutrons. Not!: Neutrinos: Little Neutrons. Not! Discovery of Radioactivity: Discovery of Radioactivity In 1895 Roentgen discovered that when electrons accelerated by very high voltages struck hard surfaces, any photographic plate in the vicinity would get exposed and fluorescent materials in the region around would glow. Roentgen thus concluded that some radiation was being emitted and called it X-rays. Radiograph made by roentgen in 1895 of his wife’s hand For this discovery, he receives the first physics Nobel price in 1901. Today, those "X rays" are well known to be a particular type of light, that is photons of high energy Others (Bequerel and Rutherford) discover that uranium emits a kind of radiation called alpha and beta rays. Beta Decay: Beta Decay In certain types of radioactive decay, an electron or positron is emitted. The electron (or positron) was referred to as a “beta” particle before people knew what it was, and this type of process is referred to as “beta decay” Example: Copper decaying to nickel When this decay was first studied, it looked like one particle (the copper atom) decayed to just 2 other particles: the nickel and the positron. If this were the case, then the positron would have a very distinct energy. But when it is measured, the positron energy varies over a large range. This is not possible if energy and momentum are to be conserved! http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/beta.html Pauli & neutrino: Pauli & neutrino Wolfgang Pauli came up with a solution to save Conservation of Energy: he proposed a completely new particle, which as far as he knew, didn’t exist! He was so unsure of this that he didn’t even name his own particle. Enrico Fermi named it: the Neutrino (little neutral one) Dear Radioactive Ladies and Gentlemen, ….., how because of the "wrong" statistics of the N and Li6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, ….I agree that my remedy could seem incredible because one should have seen these neutrons much earlier if they really exist. http://www.ethbib.ethz.ch/exhibit/pauli/neutrino_e.html How to Find Neutrinos?: How to Find Neutrinos? Although Pauli’s neutrino was a good solution, no one knew if it was the right solution! The big problem is that neutrinos are very weakly interacting: A neutrino would not “notice” a lead barrier 50 light-years thick! But physicists started incorporating the neutrino into their calculations In 1945, the first atomic bomb explodes. Despite of the horror it inspires, it is for the physicists a remarkable powerful source of neutrinos (assuming they exist). Frederick Reines, who is working at Los Alamos, speaks to Fermi in 1951 about his project to place a neutrino detector near an atomic explosion. http://wwwlapp.in2p3.fr/neutrinos/anhistory.html Reines & Cowan: Reines & Cowan In 1952, Reines and Clyde Cowan decide to use a more peaceful source of neutrinos: the nuclear plant of Hanford, Washington. They use a target made of around 400 liters of a mixture of water and cadmium chloride. The anti-neutrino coming from the nuclear reactor interacts with a proton of the target matter, making a positron and a neutron. The positron annihilates with an electron of the surrounding material, giving two simultaneous photons the neutron slows down until it is eventually captured by a cadmium nucleus, implying the emission of photons some 15 microseconds after those of the positron annihilation. All those photons are detected and the 15 microseconds identify the neutrino interaction. The neutrino is detected in 1956! Reines receives Nobel prize in 1995 Observation of neutrinos: Observation of neutrinos http://hyperphysics.phy-astr.gsu.edu/hbase/particles/cowan.html What other kinds of neutrinos are there?: What other kinds of neutrinos are there? It turns out that Reines and Cowan discovered the anti-electron-neutrino. The electron-neutrino is discovered in 1957 by Goldhaber, Grodzins and Sunyar Muon neutrinos are discovered in 1962 by Leon Lederman, Mel Schwartz, Jack Steinberger and colleagues at Brookhaven National Laboratories and it is confirmed that they are different from electron neutrinos The tau lepton is discovered by Martin Perl and colleagues at SLAC in Stanford, California. After several years, analysis of tau decay modes leads to the conclusion that tau is accompanied by its own tau neutrino As far as we know, there are only these 3 neutrinos. The mass of the neutrinos: The mass of the neutrinos As each neutrino was discovered, physicists tried to measure their mass. But they only got so far as to say: they are very very light In fact, the Standard Model which describes all of the fundamental particles has the neutrinos as having ZERO mass An important implication: if neutrinos are massless, then they MUST travel at the speed of light! (Keep this in mind: it will become important later!) Neutrino sources: Neutrino sources Solar neutrinos From the process of thermonuclear fusion inside a star. Also produced copiously by supernovae. Our sun produces about 2x1038 per second total. Neutrinos from nuclear reactors and accelerators A standard nuclear power plant radiates about 5x 1020 neutrinos per second) and their energy is around 4 MeV. Neutrinos from natural radioactivity on the earth The power coming from this natural radioactivity is estimated at about 20,000 Giga Watts (about 20,000 nuclear plants!) and the neutrinos coming from this radioactivity are numerous: about 6 millions per second and per cm2. Neutrinos from cosmic rays When a cosmic ray (proton coming from somewhere in space) penetrates the atmosphere, it interacts with an atomic nucleus and this generates a particles shower. They are called "atmospheric neutrinos". Neutrinos from the Big-Bang The "standard" model of the Big-Bang predicts, like for the photons, a cosmic background of neutrinos. There are about 330 neutrinos per cm3. But their energy is theoretically so little (about 0.0004 eV), that no experiment, even very huge, has been able to detect them. http://wwwlapp.in2p3.fr/neutrinos/ansources.html Importance of Neutrinos: Importance of Neutrinos In the universe, there are: about 1 billion photons per cubic meter About 100 million per cubic meter of neutrinos of each type (electron,muon, tau), or 300 million total About 0.5 protons per cubic meter Solar Fusion: Solar Fusion The evidence is strong that the overall reaction is "burning" hydrogen to make helium: 4H + 2 e --> 4He + 2 neutrinos + 6 photons Why do we think that this is what goes on? Energy output of millions of eV per reaction is needed if the Sun has been producing energy at the observed rate over billions of years. The reactions exist. (They have been studied in the laboratory.) There is a consistent step-by-step theory for the reaction. http://ideaplace.org/Why/FusionE.html Solar Neutrinos: Solar Neutrinos We know how many of these reactions happen per second in the Sun because we know how much energy each reaction releases and we know the solar luminosity. Thus we know how many neutrinos the Sun is producing per second: about 2x1038 Then we can calculate how many neutrinos are arriving at Earth. The answer is about 10^14 per square meter per second - all moving away from the Sun at the speed of light. Wait one second: a thousand trillion solar neutrinos just went through your body! Ouch! http://zebu.uoregon.edu/~soper/Sun/solarneutrinos.html Homestake Gold Mine: Homestake Gold Mine Ray Davis decides to “see” neutrinoes from the sun. To do this he filled a huge vat with cleaning fluid. I am not making this up! The pioneering experiment in this direction was performed deep in the Homestake Gold Mine in South Dakota starting in the early 1970's. The experiment is deep underground to protect it from high energy particles from outer space called cosmic rays. The detection method was based on the reaction 37Cl + neutrino --> 37Ar + electron. Chlorine, Cl has 17 protons while argon, Ar has 18 protons. Thus one neutron got converted into a proton. After a few days, the argon decays back to chlorine: 37Ar --> 37Cl + neutrino + antielectron . Result: About 1/3 of the expected number of reactions occurred. http://zebu.uoregon.edu/~soper/Sun/solarneutrinos.html Kamiokande: Kamiokande Masatoshi Koshiba followed up on the measurements made by Ray Davis by developing a large water-filled detector, called Kamiokande, in a Japanese mine. Kamiokande was direction sensitive and could confirm Davis' discovery that neutrinos came from the sun. The Kamiokande water tank was lined with photomultipliers. When neutrinos enter the tank, they can interact with electrons. These produce flashes of light, which are registered by the photomultipliers. Result: Neutrino reactions detected, but not as many as expected based on the theoretical calculations. But Kamiokande also saw something else even more surprising! http://www.nobel.se/physics/laureates/2002/illpres/kamiokande.html Seeing a SuperNova with Neutrinos!: Seeing a SuperNova with Neutrinos! Kamiokande was operating on 23 February 1987 and detected 12 neutrinos emitted by supernova 1987A when it exploded 170,000 light years from the earth – the first clear observation of neutrinos produced outside our galaxy. If you were in a Jupiter-type orbit a billion kilometers from SN1987A when it exploded and were protected from the other effects of the supernova, you would be killed by the radiation damage from neutrinos streaming through your body SN1987A probably produced 1058 neutrinos Based on the number and energy of the neutrinos, the energy released by the SN was about 10^53 ergs/sec compared to sun 10^33 erg/sec But the neutrinos don’t all arrive at the same time! Based on the direction, they came from Large Megellenic cloud 1987 all growed up!: 1987 all growed up! Atmospheric neutrinos: Atmospheric neutrinos Kamiokande, and other experiments like it (like IMB) also looked for “atmospheric neutrinos”, which come from cosmic rays – not the sun. All of these experiments looked for electron neutrinos, and muon neutrinos. Problem: they did not see as many muon neutrinos as expected: this is the “anomaly” When physicists have a problem like this, there is only one thing to do: build a bigger experiment! And give it a snappy name: SuperK! SuperKamioka: SuperKamioka In 1990, in order to make more progress int hese fields of research, construction was started on the 50,000 ton water Cerenkov detector, Super-Kamiokande (Super-KAMIOKA Nucleon Decay Experiment or Neutrino Detection Experiment). Super-Kamiokande is bigger and has greater photocathode coverage than Kamiokande. Construction was completed in 1995 and observation began in April of 1996. SuperK Event: SuperK Event 481 MeV muon neutrino (MC) produces 394 MeV muon which later decays at rest into 52 MeV electron. Size of PMT corresponds to amount of light seen by the PMT. PMTs are drawn as a flat squares even though in reality they look more like huge flattened golden light bulbs. http://www.ps.uci.edu/~tomba/sk/tscan/pictures.html Muon neutrino muon electron Events point at the sun: Events point at the sun Super-K detects Boron-8 neutrinos when they scatter off of atomic electrons in the water. The recoil electron direction is oriented along the direction of neutrino travel (as in the banner at the top of this page). The electron makes a weak Cherenkov ring in the detector- only 40-50 PMT hits are expected for a 8 MeV electron (in a narrow time window, shown as bright green hits in this event display). At this low energy, there is considerable random background, mostly from radon gas in the water. So we count solar neutrinos by making an angular distribution with respect to the sun's known direction. This is shown if the figure below; the sharp peak near cosine equals one is due to solar neutrinos. The area under the peak, after subtracting background, is the measured number of solar neutrinos. http://hep.bu.edu/~superk/solar.html Pointing at sun Pointing away from sun SuperKamioka: SuperKamioka Only(!) 500 days worth of data was needed to produce this "neutrino image" of the Sun, using Super-K to detect the neutrinos from nuclear fusion in the solar interior. Centered on the Sun's postion, the picture covers a significant fraction of the sky (90x90 degrees in R.A. and Dec.). Brighter colors represent a larger flux of neutrinos. The little blue dot is what the size of the sun would look like in the visible spectrum (using photons) http://antwrp.gsfc.nasa.gov/apod/ap980605.html Credit: R. Svoboda and K. Gordan (LSU) Jun 5, 1998 What else did SuperK do with Neutrinos?: What else did SuperK do with Neutrinos? Also looked at “Atmospheric Neutrinos” Predictions exist for how many they should see SuperK discovered a deficit in muon neutrinos! They “disappeared”! And discovered that muon neutrinos which come “upward” (through the earth) are more likely to “disappear”. Hmmm… Disappear is not quite right: they “oscillate” into something else: an electron neutrino! This can only happen if neutrinos have Mass! Clinton on Neutrinos: Clinton on Neutrinos “[W]e must help you to ensure that America continues to lead the revolution in science and technology. Growth is a prerequisite for opportunity, and scientific research is a basic prerequisite for growth. Just yesterday in Japan, physicists announced a discovery that tiny neutrinos have mass. Now, that may not mean much to most Americans, but it may change our most fundamental theories -- from the nature of the smallest subatomic particles to how the universe itself works, and indeed how it expands. This discovery was made, in Japan, yes, but it had the support of the investment of the U.S. Department of Energy. This discovery calls into question the decision made in Washington a couple of years ago to disband the Super-conducting Supercollider, and it reaffirms the importance of the work now being done at the Fermi National Acceleration Facility in Illinois. The larger issue is that these kinds of findings have implications that are not limited to the laboratory. They affect the whole of society -- not only our economy, but our very view of life, our understanding of our relations with others, and our place in time.” Meanwhile…: Meanwhile… BATAVIA, IL--President Bush met with members of the Fermi National Accelerator Laboratory research team Monday to discuss a mathematical error he recently discovered in the famed laboratory's "Improved Determination Of Tau Lepton Paths From Inclusive Semileptonic B-Meson Decays" report. "I'm somewhat out of my depth here," said Bush, a longtime Fermilab follower…… Above: Bush shows Fermilab scientists where they went wrong in their calculations. Are Neutrinos Dark Matter?: Are Neutrinos Dark Matter? Neutrinos don’t “shine”. And now we know they have mass. And there sure are a lot of them. Dark Matter!? This mass difference, coupled with absolute neutrino mass measurements and the Kamiokande's measurements, indicates that the combined mass of all the neutrinos in the universe is about equal to the combined mass of all the visible stars. That means neutrinos cannot account for all the "dark matter" known to make up most of the mass of the universe. Summary: Summary What we know: There are 3 “light” neutrinos The sun is a copious source of neutrinos Supernovae produce a lot of neutrinos Neutrinos have mass What we don’t know: What are the masses of the 3 neutrinos reallY? How do we find out? Would be great if there was a way to control the neutrinos to study them in more detail But wait! There is! Fermilab can make a lot of neutrinos too! Making a Beam of Neutrinos: Making a Beam of Neutrinos 120 GeV protons hit target (1020/Protons per year!) p+ (“pions”) produced at wide range of angles Magnetic horns to focus p+ p+ decay to m+n in long evacuated pipe Left-over hadrons shower in hadron absorber Rock shield ranges out m+ n beam travels through earth to experiment But the experiment is hundreds of miles away! Numi-MINOS from the Air: Numi-MINOS from the Air So the neutrinos start out at Fermilab, and are aimed through the earth at Minnesota. Why Minnesota? NUMI: Neutrinos at the Main Injector MINOS: Main Injector Neutrino Oscillation Search MINOS Experiment: MINOS Experiment Two Detector Neutrino Oscillation Experiment (Start 2004) Near Detector: 980 tons Far Detector: 5400 tons Beam and “Near” Detector: Beam and “Near” Detector Decay tunnel before installation of decay pipe Near detector hall Tunneling completed Detector elements built Installation starts later this year First beam December 2004 The “Far” Detector: The “Far” Detector Minos Plans: Minos Plans The basic plan of MINOS is to use the controlled source of neutrinos from Fermilab to really show that muon neutrinos can oscillate into electron neutrinos Compare interactions in the near detector with the far detector Both detectors will be able to determine the type of neutrinos Basic measurement: the mass difference between the two neutrinos (not the actual masses) Will the experiment soar to great heights? Or will it come crashing down to earth? Slide35: Solar neutrinos are produced by the nuclear reactions that power the Sun. The fusion of proton plus proton (pp) to deuterium plus positron plus neutrino is responsible for 98% of the energy production of the sun. Therefore these pp-neutrinos are the most plentiful, and the most reliably estimated. About 60 billion pp-neutrinos pass through a square centimeter at the Earth each second. They are relatively low energy, however, with a continuous spectrum that ends at 420 keV. In addition, there are several rarer reactions which also produce neutrinos. The electron capture on Beryllium-7 produces a sharp line of Beryllium-7 neutrinos at 861 keV. A small fraction of the time, Beryllium-7 captures a proton instead of an electron, to form Boron-8. The beta decay of Boron-8: 8B -> 8Be + e+ + nu_e produces a continuous spectrum of neutrino energies that extends to 15 MeV. Super-K is sensitive to these rare but high energy Boron-8 neutrinos. Super-K detects Boron-8 neutrinos when they scatter off of atomic electrons in the water. The recoil electron direction is oriented along the direction of neutrino travel (as in the banner at the top of this page). The electron makes a weak Cherenkov ring in the detector- only 40-50 PMT hits are expected for a 8 MeV electron (in a narrow time window, shown as bright green hits in this event display). At this low energy, there is considerable random background, mostly from radon gas in the water. So we count solar neutrinos by making an angular distribution with respect to the sun's known direction. This is shown if the figure below; the sharp peak near cosine equals one is due to solar neutrinos. The area under the peak, after subtracting background, is the measured number of solar neutrinos. http://hep.bu.edu/~superk/solar.html Did Kamioka See the Sun?: Did Kamioka See the Sun? That bright thing in the sky!: That bright thing in the sky! Kamioka: Kamioka Brief History Kamioka Underground Observatory, the predecessor of the present Kamioka Observatory, Institute for Cosmic Ray Reserch, University of Tokyo, was established in 1983. The original purpose of this observatory was to verify the Grand Unified Theories, one of the most impenetrable matters of elementary particle physics, through a Nucleon Decay Experiment. Thus, the water Cerenkov detector which was used for this experiment was named Kamiokande (KAMIOKA Nucleon Decay Experiment). The 4,500 ton water Cerenkov detector was placed at 1,000 m underground of Mozumi Mine of the Kamioka Mining and Smelting Co. located in Kamioka-cho, Gifu, Japan. The original purpose of Kamiokande was to investigate the stability of matter, one of the most fundamental questions of elementary particle physics. An upgrade of Kamiokande was started in 1985 to observe particles called neutrino (Solar, Atmospheric and other neutrinos) which come from astrophysical sources and cosmic ray interactions. As a result of this upgrade, the detector had become highly sensitive. In February, 1987,Kamiokande had succeeded in detecting neutrinos from a supernova explosion which occurred in the Large Magellanic Cloud. Solar neutrinos were detected in 1988 adding to the advancements in neutrino astronomy and neutrino astrophysics. In 1996, Kamioka Observatory which belongs to the Institute for Cosmic Ray Research(ICRR), University of Tokyo was established. Kamiokande had been world famous for its achievements on the observation of supernova neutrinos, solar neutrinos and atmospheric neutrinos and also the study of the Grand Unified Theories of particles. In 1990, in order to make more progress int hese fields of research, construction was started on the 50,000 ton water Cerenkov detector, Super-Kamiokande (Super-KAMIOKA Nucleon Decay Experiment or Neutrino Detection Experiment). Super-Kamiokande is bigger and has greater photocathode coverage than Kamiokande. Construction was completed in 1995 and observation began in April of 1996. Neutrino sources: Neutrino sources Solar neutrinos From the process of thermonuclear fusion inside the stars (our sun or any other star in the universe). Some other neutrinos could come from very cataclysmic phemomena like explosions of supernovae or neutron stars coalescences. Neutrinos from nuclear reactors and accelerators These are high energy neutrinos produced by the particles accelerators and low energy neutrinos coming out of nuclear reactors. The first ones, whose energy can reach about 100 GeV, are produced to study the structure of the nucleons (protons and neutrons composing the atomic nuclei) and to study the weak interaction. The second ones are here although we did not ask for them. They are an abundant product made by the nuclear reactions inside the reactors cores (a standard nuclear plant radiate about 5x 10^20 neutrinos per second) and their energy is around 4 MeV. Neutrinos from natural radioactivity on the earth The power coming from this natural radioactivity is estimated at about 20.000 Giga Watts (about 20.000 nuclear plants!) and the neutrinos coming from this radioactivity are numerous: about 6 millions per second and per cm2. But those neutrinos, despite of their quantity, are often locally drowned in the oceans of neutrinos coming from the nuclear plants. Neutrinos from cosmic rays When a cosmic ray (proton coming from somewhere in space) penetrates the atmosphere, it interacts with an atomic nucleus and this generates a particles shower. They are called "atmospheric neutrinos". Neutrinos from the Big-Bang The "standard" model of the Big-Bang predicts, like for the photons, a cosmic background of neutrinos. Those neutrinos, nobody has never seen them. They are yet very numerous: about 330 neutrinos per cm3. But their energy is theoretically so little (about 0.0004 eV), that no experiment, even very huge, has been able to detect them. http://wwwlapp.in2p3.fr/neutrinos/ansources.html Timeline: Timeline A NEUTRINO TIMELINE The following is a short history of neutrinos as it relates to neutrino oscillation studies. 1920-1927 Charles Drummond Ellis (along with James Chadwick and colleagues) establishes clearly that the beta decay spectrum is really continuous, ending all controversies. 1930 Wolfgang Pauli hypothesizes the existence of neutrinos to account for the beta decay energy conservation crisis. 1932 James Chadwick discovers the neutron. 1933 Enrico Fermi writes down the correct theory for beta decay, incorporating the neutrino. 1946 Shoichi Sakata and Takesi Inoue propose the pi-mu scheme with a neutrino to accompany muon. 1956 Fred Reines and Clyde Cowan discover (electron anti-) neutrinos using a nuclear reactor. 1957 Neutrinos found to be left handed by Goldhaber, Grodzins and Sunyar. 1957 Bruno Pontecorvo proposes neutrino-antineutrino oscillations analogously to K0-K0bar, this is the first time neutrino oscillations (of some sort) are hypothesized. 1962 Ziro Maki, Masami Nakagawa and Sakata introduce neutrino flavor mixing and flavor oscillations. 1962 Muon neutrinos are discovered by Leon Lederman, Mel Schwartz, Jack Steinberger and colleagues at Brookhaven National Laboratories and it is confirmed that they are different from electron neutrinos. 1964 John Bahcall and Ray Davis discuss the feasibility of measuring neutrinos from the sun. 1965 The first natural neutrinos are observed by Reines and colleagues in a gold mine in South Africa, and by Goku Menon and colleagues in Kolar gold fields in India, setting first astrophysical limits. 1968 Ray Davis and colleagues get first radiochemical solar neutrino results using cleaning fluid in the Homestake Mine in North Dakota, leading to the observed deficit now known as the "solar neutrino problem". 1976 The tau lepton is discovered by Martin Perl and colleagues at SLAC in Stanford, California. After several years, analysis of tau decay modes leads to the conclusion that tau is accompanied by its own neutrino, nutau, which is neither nue nor numu. 1980s The IMB, the first massive underground nucleon decay search instrument and neutrino detector is built in a 2000' deep Morton Salt mine near Cleveland, Ohio. The Kamioka experiment is built in a zinc mine in Japan. 1985 The "atmospheric neutrino anomaly" is observed by IMB and Kamiokande. 1986 Kamiokande group makes first directional counting observation solar of solar neutrinos and confirms deficit. 1987 The Kamiokande and IMB experiments detect burst of neutrinos from Supernova 1987A, heralding the birth of neutrino astronomy, and setting many limits on neutrino properties, such as mass. 1988 Lederman, Schwartz and Steinberger awarded the Physics Nobel Prize for the discovery of the muon neutrino. 1989 The LEP accelerator experiments in Switzerland and the SLC at SLAC (Stanford) determine that there are only 3 light neutrino species (electron, muon and tau). 1991-2 SAGE (in Russia) and GALLEX (in Italy) confirm the solar neutrino deficit in radiochemical experiments. 1995 Frederick Reines and Martin Perl share the Physics Nobel Prize for discovery of electron neutrinos (and observation of supernove neutrinos) and the tau lepton, respectively. 1996 Super-Kamiokande, the largest particle detector ever, begins searching for neutrino interactions on 1 April at the site of the Kamioka experiment, with a Japan-US team. 1998 After analyzing more than 500 days of data, the Super-Kamiokande team reports finding oscillations in atmospheric neutrinos and, thus, neutrino mass. 1999-2000 The Chooz and Palo Verde reactor experiments report no oscillations, concluding that electron neutrinos are not the dominant participant in the atmospheric neutrino oscillations. 2000 The DONUT Collaboration working at Fermilab announces observation of tau particles produced by tau neutrinos, making the first direct observation of the tau neutrino. 2000 Super-Kamiokande announces that the oscillating partner to the muon neutrino is not a sterile neutrino, but the tau neutrino. 2001-02 SNO announces observation of neutral currents from solar neutrinos, along with charged currents and elastic scatters, providing convincing evidence that neutrino oscillations are the cause of the solar neutrino problem. 2002 Masatoshi Koshiba and Raymond Davis win Nobel Prize for measuring solar neutrinos (as well as supernova neutrinos). 2002 KamLAND observes neutrino oscillations consistent with the solar neutrino puzzle using, for the first time, man-made neutrinos. Modified by Giorgio Gratta from an original by John Leaned and Sandip Pakvasa. This page is maintained by [email protected] Slide42: It appears established beyond reasonable doubt, through the success of the standard solar model, that the sun shines from nuclear fusion in its core. A fusion reaction involves the merging of two atomic nuclei into one. In the sun, a chain of several different fusion reactions along any of about four different pathways, leads from four hydrogen nuclei (single protons) to one helium nucleus (two protons and two neutrons). In this process, two protons have to be converted into neutrons through beta decays. In each beta decay, a neutrino is emitted (an electron-flavored neutrino, that is). So it is straightforward to calculate that, if the sun shines through hydrogen fusion, it ought to emit two neutrinos per fusion chain. And in our standard theory of particle physics, the neutrinos will zip straight out from the sun, without interacting with the intervening material. The total flux of neutrinos from the sun ought to be some 200 000 000 000 000 000 000 000 000 000 000 000 000 per second, corresponding to a flux of about 6.5 × 1010 neutrinos per square centimeter per second hitting the earth. Most of those neutrinos come from the main energy-producing reaction chain in the sun: proton-proton fusion. Unfortunately, the neutrinos from proton-proton (pp) fusion have a very low energy. Energy in this context in measured in electron-volts (1 eV = 1.6 × 10-19 Joule), or millions of electron-volts (MeV), and the energy of the pp neutrinos is less than 0.42 MeV, making them difficult to detect. Smaller (but still enormous) numbers of higher-energy neutrinos are expected from various side reactions, notably boron and beryllium decays. There is also an alternative energy-producing chain, CNO-fusion, where the fusion of hydrogen to helium is catalyzed by carbon. This CNO-chain is expected to be the main energy source in larger, hotter stars, but it should only give a modest contribution in the sun. The CNO neutrinos are otherwise easier to detect than pp-neutrinos, having three to four times more energy each. Number of interactions/person/lifetime from solar neutrinos: 1. http://www.talkorigins.org/faqs/faq-solar.html Slide43: Most physicists and astronomers believe that the sun's heat is produced by thermonuclear reactions that fuse light elements into heavier ones, thereby converting mass into energy. To demonstrate the truth of this hypothesis, however, is still not easy, nearly 50 years after it was suggested by Sir Arthur Eddington. The difficulty is that the sun's thermonuclear furnace is deep in the interior, where it is hidden by an enormous mass of cooler material. Hence conventional astronomical instruments, even when placed in orbit above the earth, can do no more than record the particles, chiefly photons, emitted by the outermost layers of the sun. Of the particles released by the hypothetical thermonuclear reactions in the solar interior, only one species has the ability to penetrate from the center of the sun to the surface (a distance of some 400,000 miles) and escape into space: the neutrino. This massless particle, which travels with the speed of light, is so unreactive that only one in every 100 billion created in the solar furnace is stopped or deflected on its flight to the sun's surface. Thus neutrinos offer us the possibility of ``seeing'' into the solar interior because they alone escape directly into space. About 3 percent of the total energy radiated by the sun is in the form of neutrinos. The flux of solar neutrinos at the earth's surface is on the order of 1011 per square centimeter per second. Unfortunately the fact that neutrinos escape so easily from the sun implies that they are difficult to capture. Slide44: Neutrinos were first suggested as hypothetical entities in 1931 after it was noted that small amounts of mass seemingly vanish in the radioactive decay of certain nuclei. Wolfgang Pauli suggested that the mass was spirited away in the form of energy by massless particles, for which Enrico Fermi proposed the name neutrino (``little neutral one''). Fermi also provided a quantitative theory of processes involving neutrinos. In 1956 Frederick Reines and Clyde L. Cowan, Jr., succeeded in detecting neutrinos by installing an elaborate apparatus near a large nuclear reactor. Such a reactor emits a prodigious flux of antineutrinos produced by the radioactive decay of fission products. For purposes of demonstrating a particle's existence, of course, an antiparticle is as good as a particle. In the late 1930's Hans A. Bethe of Cornell University followed up Eddington's 1920 suggestion of the nuclear origin of the sun's energy and outlined how the fusion of atomic nuclei might enable the sun and other stars to shine for the billions of years required by the age of meteorites and terrestrial rocks. Since the 1930's the birth, evolution and death of stars have been widely studied. It is generally assumed that the original main constituent of the universe was hydrogen. Under certain conditions hydrogen atoms presumably assemble into clouds, or protostars, dense enough to contract by their own gravitational force. The contraction continues until the pressure and temperature at the center of the protostar ignite thermonuclear reactions in which hydrogen nuclei combine to form helium nuclei. After most of the hydrogen has been consumed, the star contracts again gravitationally until its center becomes hot enough to fuse helium nuclei into still heavier elements. The process of fuel exhaustion and contraction continues through a number of cycles. The sun is thought to be in the first stage of nuclear burning. In this stage four hydrogen nuclei (protons) are fused to create a helium nucleus, consisting of two protons and two neutrons. In the process two positive charges (originally carried by two of the four protons) emerge as two positive electrons (antiparticles of the familiar electron). The fusion also releases two neutrinos and some excess energy, about 25 million electron volts (MeV). This energy corresponds to the amount of mass lost in the overall reaction, which is to say that a helium nucleus and two electrons weigh slightly less than four protons. The 25 MeV of energy so released appears as energy of motion in the gas of the solar furnace and as photons (particles of radiant energy). This energy ultimately diffuses to the surface of the sun and escapes in the form of sunlight and other radiation.