Solar Neutrinos

From South Dublin Astronomical Society

Needs to be formatted correctly --Albertw 21:50, 17 March 2006 (GMT)

The Sun is powered by Nuclear Fusion, the nuclear combination of lighter elements into heavier elements. Under the extreme temperatures and pressures that exist at the core of the Sun, Hydrogen Nuclei (Protons) fuse to form Helium in what is known as the Proton-Proton (p-p) chain. This chain has three distinct steps (see the diagram just opposite also). 1. Two protons combine to produce deuterium releasing a positron and a neutrino. P+P = 2H + positron + neutrino 2. The deuterium fuses with another proton to produce a helium nucleus and releases a gamma ray. 2H + P ¦ 3He + gamma 3. Finally two helium nuclei fuse to produce `normal` helium and release two protons. 3He + 3He ¦ 4He + 1H + 1H This process can be summed up by 4(1H) ¦ 4He + neutrino + neutrino + energy The neutrino produced in the initial proton fusion is the dominant neutrino produced in the Sun with an energy of 0.0 to 0.4 MeV. This process accounts for over 99% of the neutrinos produced in the initial stage of the p-p chain reaction. The other 1% is produced via the production of deuterium from 2 protons and an electron. p + e- + p ¦ 2H + ve The neutrino in this reaction has an energy of 1.4MeV Also if the p-p chain continues we get the following: 4. 3He + 4He ¦ 7Be + gamma 5. e- + 7Be ¦ 7Li + ve 6. 7Li + p ¦ 4He + 4He 7. p + 7Be ¦ 8B + gamma 8. 8B ¦ 8Be + e+ + ve The diagram overleaf shows the flux and energy of the neutrinos produced in the reactions above. You will notice that some of the lines are at distinct energies while some are curves indicating that the neutrinos from these reactions are emitted with different energies. The graph also shows the neutrino energies for Nitrogen and Oxygen reactions, however these reactions are not believed to contribute much to the neutrinos produced in our Sun. The top of this diagram also indicates the types of detectors that are capable of detecting the neutrinos at the corresponding energies. The physics of these reactions is fairly well understood. So once we know the temperature of the core of the sun we can predict how many neutrinos should arrive at the earth. Current evidence suggests a core temperature of 15.7 million Kelvin. Neutrinos are very difficult to detect. They very rarely interact with other matter, in fact of the billions that are passing through your body every second, it is estimated that only 2 or 3 will interact with an atom in your body in your lifetime. Given this lack of interaction the detectors used to search for these particles are rather special. The earliest detectors were developed in the 1950's however the breakthroughs came in 1968 when the Homestake Neutrino Observatory came on line. This detector consisted of a 100,000 gallon tank that was located deep within the Homestake mine. The detector was based on Chlorine. Diagram of the entire proton—proton chain A total of six protons (and two electrons) are converted into two protons, one helium-4 nucleus, and two neutrinos. The two leftover protons are available as fuel for new proton— proton reactions, so the net effect is that four protons are fused to form one helium-4 nucleus. Energy, in the form of gamma rays, is produced in each reaction. — 14 — Neutrinos can occasionally interact with Chlorine to produce an Argon isotope, this argon is filtered off and and is decays it emits an electron with an energy of 2.8keV, this electron can be easily detected and allows a count of the neutrinos to be calculated, however this can only be done after enough argon has been accumulated, so the test run typically ran for weeks. These detectors are placed deep within mines to provide shielding from cosmic rays which could also produce similar chemical interactions and contaminate the results. Despite little funding being available for such projects, they received funding by promising key new data into the understanding of nuclear processes that may be of use in the nuclear industry which in te 1960's was a hot topic. They didn't have enough money however to fill the detector with pure Chloirine, instead they used perchloroethelene in the form of industrial cleaning fluid! This detector succeeded in detecting 2.56 ± 0.16 Solar Neutrino Units (SNU). 1 SNU is defined as 10-36 captures per target atom per second. The total theoretically predicted number for a chlorine based detector is 7.6 ± 1.3 SNU, The majority coming from the 8B source at 5.6 MeV. The detector was only detecting about 1/3 of the predicted number of neutrinos, and test after test showed the same result. Other experiments were later devised around Gallium, which has an energy threshold of 0.233MeV, so was capable of detecting the predicted P-P neutrinos. The predicted value for these detectors was 128 ± 9 SNU. However the detectors at (Soviet American Gallium Experiment) SAGE, (Gallium Experiment) GALLEX and GNO, only managed to detect about 75 SNU. An improvement but still well short. From these experiments it would appear that either the Solar model is wrong or the detectors are wrong. However the Solar theory did not appear to have any flaws, and the results from the various detectors were consistently showing a reduced number of detected neutrinos. Two Russian Scientists in 1969 suggested that since these detectors were only capable of detecting electron neutrinos, perhaps the electron neutrinos were changing into mu or tau neutrinos which the detectors could not detect. To test their theory a completely different detector would need to be designed. The Sudbury Neutrino Detector is located over 2km deep in Creighton mine in Ontario, Canada. The detector contains 1 kilotonne of pure D20, heavy water in which deuterium is used instead of Hydrogen. Deuterium is an isotope of Hydrogen which has two neutrons and 2 protons in the nucleus. This isotope occurs naturally, but rarely, about 1 molecule in 20 million would be deuterium. This isotope is used in atomic reactors as moderator and a heat transfer agent, and the deuterium used in the SNO is on loan from the Atomic Energy of Canada Ltd. and is valued at $300million Canadian. The SNO Detector The Kamiokande experiment was started in 1983 to investigate the stability of matter. The observatory is located 1000m below the surface in the Mozumi mine in Gifu, Japan. The central detector was filled with 608 tonnes of D2O, this detector is surrounded by a larger tank containing the remainder of the 4500 tonnes. So how does a very big tank of water manage to detect neutrinos? When a neutrino comes close to a molecule of d2o it can interact with an electron and cause the electron to be fired out of the molecule in the direction the neutrino came from at a speed faster then the speed of light in water. This is not a violation of the laws of physics, nothing can go faster then the speed of light in a vacuum, but you can overtake it in a different medium! When this electron zips past the other atoms in the medium it causes them to radiate, and this radiation spreads out in a cone along the path of the electron, this type of radiation is referred to Cherenkov radiation. To detect this radiation photoelectric cells are placed around the detectors, the SNO detector has 10,000 placed around the walls. An accident in 2001 in the upgraded Super-Kaminokande detector led to almost all of the 11,200 photomultiplier tubes being destroyed, work is underway to get the detector operational again. This technique allows some neutrinosto be detected in real time, without having to run lengthy test runs, this means that neutrino outbursts can be detected quickly, and in 1987 both detectors detected neutrinos from the 1987A supernova. Unfortunately this was only noticed by going back through the data, however the detectors are now monitoring the data more closely in real time to hopefully give optical telescopes — 15 — a chance to train their eyes on supernovas as they are about to happen. In this reaction the heavy weak force particle exchanged (called the Z boson) is not charged, hence the name "neutral current reaction". The net reaction is just to break apart the deuterium nucleus; the liberated neutron is then thermalized in the heavy water as it scatters around. The reaction can eventually be observed due to gamma rays which are emitted when the neutron is finally captured by another nucleus. The gamma rays will scatter electrons which produce detectable light via the Cherenkov process discussed above. This diagram illustrates the process. This reaction is the only detection reaction that is equally sensitive to all families of neutrinos. The NC results, released in 2001, almost exactly matched the prediction 100.8±12%. This confirmed the suspicions that had been raised in 1968, and the results that had came from the Super Kaminokande observatory three years earlier. The Solar Neutrino problem was solved. All expected neutrinos were now accounted for, but they were oscillating between the different families of neutrinos. An electron neutrino for example can change into a muon neutrino, and back again. In fact it appears to oscillate back and forth between the two on its way from the Sun to the Earth. It does this though interference. Normal interference occurs when 2 waves meet, where the resultant wavelength is the sum of the original 2 waves, however in the case of neutrino oscillation, there are not two waves, only one. It turns out that this wave is capable of interfering with itself and so even though it starts out as say an electron neutrino it can oscillate and be come both an electron and muon neutrino. In order for this to occur there are some preconditions set down by quantum mechanics. Primarily that the electron neutrino must have a mass. It is likely to be a very small mass, but it must be nonzero. The discovery of neutrino oscillations was the key point in a paper describing that electron neutrinos had mass in experiments from Super Kamiokande in 1998. The mass is calculated at being < 15x10-6 MeV. Astronomically this has some interesting implications. Diagram of the neutral-current reaction — a neutrino of any type interacts with the deuteron to break it apart so that there is an outgoing neutrino plus a free neutron and proton. The neutron wanders within the detector and will eventually get captured by another nucleus. If the capturing nucleus is for deuterium, a 6.25 MeV gamma-ray is produced; the gamma-ray will scatter off of an atomic electron (Compton scatter) and the scattered electron will create Cherenkov light. If the capturing nucleus is 35Cl, a series of gamma-rays with a total energy of 8.6 MeV is produced; some of these gamma-rays will Compton scatter and cause Cherenkov light. Although the electron neutrino is almost massless there are vast numbers of them spread throughout the universe. And their combined mass could significantly contribute to the mass of the universe. This is not at present thought to explain the problem of `dark matter` as dark matter models to date require the matter to be clumped into groups which would not happen with neutrinos. Research is being carried out to design new detectors capable of detecting lower energy neutrinos below about 2MeV, this is important as we currently are not observing the most common neutrinos produced in the Suns core, in fact we are only capable of detecting about 9% of the neutrinos produced. Research is also ongoing at improving the efficiency of the detectors so that results may be determined better in real time, this should allow astronomers to have advance warning of supernova explosions as the neutrinos will reach Earth before the visible light will, enabling optical telescopes to be trained on the supernova as it starts.