~~Looking all around us in every direction, there appears to be nothing but empty space between the objects in our view and ourselves. Actually, untold numbers of molecules of nitrogen, oxygen and other gases traverse the seeming void. Even well beyond the Earth's atmosphere, cosmologists still attempt to explain the mysterious dark matter that pervades the universe at large. Though everything appears calm and serene, back here on Earth, even on a clear sunny day, it's raining. Not with droplets of water, but in numbers far exceeding those of air molecules, streams of tiny invisible particles continuously pass through everything we know as they shower down upon us.
For billions of years these "little neutral ones" have bombarded the Earth virtually undetected, as they take their journey from the sun and other stars, that is, until now. Before the construction of the first means of neutrino detection nearly fifty years ago, who would have thought that a bath essentially containing dry cleaning fluid or pure water would have been capable of revealing their presence? To make matters more difficult, only a small number of the anticipated solar neutrinos left their telltale signs of existence, all the while increasing the mystery of these elusive cosmic interlopers.
Recently, Dr. Raymond Davis, Jr. won the Nobel Prize in physics for his research on solar neutrinos. His initial interest in solar neutrinos began in the early 1950's when he was a chemist, and his research of their detection was the first of its kind deep underground at the Homestake Mine in South Dakota. Since then, we have learned that neutrinos actually seem to undergo changes in their identity as they travel from the sun. Hence, evidence has led to the belief that three different "flavors," or families of neutrinos go through a series of oscillations, and that neutrinos do indeed have some mass after all. This latter understanding literally revolutionizes some current models, extending to the entire universe.
What makes the Sun shine and has kept it doing so for the last 4.6 billion years? What fuels the Sun's enormous energy output? In order to answer these questions, we must look at the inside story of the Sun, and examine some of its properties. All of this information is based on the Standard Solar Model, which has widespread acceptance among the scientific community. The energy output of the Sun can be measured by dividing the Sun's luminosity by the Sun's mass. If the Sun's luminosity is 4 x 1026 Watts (W) and its mass is about 2 x 1030 kg, what is the energy output of the Sun? If this doesn't seem like a lot, you're right! In the same amount of time, a piece of wood in a burning fireplace gives off about a million times more energy than this. But keep in mind two things: the Sun is so large that well over a million Earth's can fit inside, and it has and will continue burning for billions of years.
This long, steady process of solar energy output is due directly to nuclear fusion, the joining of light atomic nuclei into a heavier nucleus. If we concentrate on where this is occurring, inside the inner 1/3 of the Sun - the core itself, where temperatures reach 27 million degrees Fahrenheit, we will need to understand the law of conservation of mass and energy. Albert Einstein taught us that matter can be converted into energy, and vice versa, with his famous equation E = mc2, or energy (J) equals mass (kg) times the speed of light (3 x 108 m/s) squared. Since the speed of light is so huge, if we square it, it's easy to see that a small amount of mass equates to an enormous amount of energy. Since all atomic nuclei have a positive charge, and like charges repel each other, this electromagnetic force must be overcome by collisions between the nuclei traveling at tremendous speeds. It takes a very high temperature to give these nuclei the kinetic energy they need to reach such speeds, and once they do, the strong nuclear force will bind the nuclei together.
The collision and subsequent fusion of two protons of ordinary hydrogen (1H) is the beginning of what is known as the proton-proton chain reaction, which initially results in a deuteron (proton + neutron, nucleus of deuterium or "heavy hydrogen" - 2H) plus a positron (anti-electron) plus a neutrino. The positron and neutrino are results of one of the protons being changed into a neutron by the weak interaction. Since the interaction of matter with antimatter results in complete annihilation, the positron reacts with an electron producing energy in the form of gamma rays. So the start of the proton-proton reaction can be written as:
1H + 1H → 2H + positron + neutrino
Now the deuteron particle collides with another proton and forms helium-3, an isotope of helium:
2H + 1H → 3He + energy (kinetic and photons)
Next, consider two of the former reactions occurring simultaneously producing two helium-3 nuclei, which collide together yielding a helium-4 nucleus and two leftover protons, which serve to continue the chain reaction on and on. So the net effect of all these reactions minus the two leftover protons is the production of a helium-4 nucleus along with gamma ray photons and two neutrinos:
4 1H + 2 electrons → 4He + 6 photons + 2 neutrinos
The energy produced by these reactions lessens as it travels to the Sun's surface, changing to photons in the infrared and visible range, giving us heat and light. The sun's energy, after traveling 93 million miles to get to Earth, hits the upper atmosphere at about the intensity of three 100-watt bulbs per square meter. A third is reflected back into space, two thirds warms the planet and drives its weather engine. The neutrinos react so weakly with matter that they just zip through the remaining outer layers of the Sun, travel through space at or near the speed of light, and pass unhindered through planets, moons, asteroids, people, etc.
Even though neutrinos have a neutral charge and can easily pass through a magnetic field without hindrance, as well as all forms of matter, then it may be difficult to accept that devices have been built that can detect the presence of these elusive particles. For his extensive research with solar neutrinos, Dr. Raymond Davis, Jr. recently received the Nobel Prize in Physics. The word neutrino literally means "little neutral particle." Since neutrinos are subatomic particles that are produced by reactions in the sun's core, are able to pass through the outer layers and continue their journey to the earth with very weak interactions with matter, many billions of these particles pass through our body, rocks, even lead each second. The oldest pioneering solar neutrino experiments were performed at Raymond Davis's chlorine radiochemical detector deep inside a South Dakota gold mine known as Homestake. It was built deep underground to shield out cosmic rays and other subatomic particles. Dr. Davis also worked with a 400,000-liter (100,000-gallon) tank of essentially dry cleaning fluid to detect the presence of these particles. The neutrinos would react with the chlorine-37 in the fluid and change it to argon-37. However, theory predicted a larger number of these detections than was observed, and this discrepancy puzzled scientists for years, commonly referred to as the solar neutrino problem.
A characteristic known as neutrino oscillation accounts for evidence those neutrinos have the ability to change from one form into another as they travel through space, alternating between one of the three known neutrino forms into another. They are known as the muon neutrino, electron neutrino, and the tauon neutrino because of their associations with these particles and their anti-counterparts. This understanding has major implications to conventional particle theory on a universal scale. According to quantum mechanics, there are three families, or "flavors," of quarks, the building blocks of protons and neutrons inside the nucleus of an atom. Similarly, it was guessed that neutrinos could oscillate between the flavors (families) of each type of neutrino.
You may also wish to read a paper from the Essays in Nuclear Astrophysics citing "An Account of the Development of the Solar Neutrino Problem," by Raymond Davis, Jr. and John N. Bahcall. The great value of this paper is that it gives the reader an appreciation of the human side of research, the challenge of acquiring project funding, and a chronological overview detailing the obstacles and slow progress often encountered in conducting real scientific experiments. It also illustrates well the common use of an acceptable range of values in science and cites historic suggestions of neutrino oscillations. The paper would also serve well in giving an historical perspective, comparing theory with observation, and as part of a timeline on solar neutrino research that students can develop, particularly in the first half.
Theory does not always agree with observations. According to quantum mechanics, there are three families, or "flavors," of quarks, the building blocks of protons and neutrons inside the nucleus of an atom. Similarly, it was guessed that neutrinos could oscillate between the flavors (families) of each type of neutrino. The three common flavors of Dippin' Dots® Ice Cream (chocolate, vanilla, and strawberry) could be used to represent these small elusive particles. Ray Davis's detector could only "taste" the chocolate neutrinos, but not the vanilla or strawberry ones. Therefore, methods must be devised to reveal the presence of the vanilla and strawberry neutrinos. If we could not use our eyes to see the colors of the three different Dippin' Dots®, then perhaps we could resort to our sense of taste. If our taste buds were not functioning, then we would need to come up with another method to tell the chocolate ones from the vanilla, and the vanilla from the strawberry, and the strawberry ones from the chocolate. If all the Dippin' Dots® had similar shape, size and mass (density), then our quest to differentiate among them becomes more complex. Other tools will be required for the task.
Originally published on October 31, 2016 by SpeakerMatch Speakers Bureau