Physics Neutrinos: Hunting ghosts This lesson is about neutrinos. In this lesson you will investigate the following: • Beta decay and the discovery of neutrinos • Neutrinos from the Sun • Neutrinos from space Find out how the ‘ghosts of matter’ may tell us the secrets of matter. This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmosforschools.com Neutrinos: Introduction This lesson is all about neutrinos – tiny ghostly particles much smaller than an atom. Billions of them have flewn through your body since you started reading this introduction. Most neutrinos come from space, travelling at the speed of light, and scientists are trying to find out more about them. To do so they have built an expensive neutrino detector deep in the ice near the South Pole. But before you read the article and look at the pictures, you will need to understand a few simple terms used there. The article talks about “subatomic particles”, which, as you might be able to guess, are the tiny particles that go together to make atoms. It also talks about “fusion”, this is the atomic reaction that happens when two atoms are joined, or fused, together. When they are, they release a lot of energy. The Sun and the stars give out light and heat energy because they are made up of billions of hydrogen atoms joining together to make helium. This is one way that neutrinos are formed, too. Another way neutrinos are created is when a large star explodes at the end of its life and becomes what we call a “supernova”. Now, go ahead and read the original Cosmos Magazine ​story to find out how the IceCube Neutrino Observatory is studying these elusive little particles. Read the full Cosmos Magazine article here​​ The neutrinos that we receive here on Earth come from the Sun and the deep reaches of space. Image credit: NASA Question 1 Interpret: The opening section of the Cosmos Magazine ​article is titled "Freezing a ghost". Why do you think the writer chose this catchy title? Question 2 Interpret: To help you understand the article, after reading each section try to think of another phrase for its heading. Cosmos heading Alternative heading Freezing a ghost Under the ice Captured Where in the Cosmos? Neutrinos: Gather Left to right: Lise Meitner, Wolfgang Pauli, Enrico Fermi & Wang Ganchang. Image credits: Getty Images & Wikipedia. Many people have contributed to our current understanding of neutrinos. Here is a little history of this tiny particle: 1911 Lise Meitner and Otto Hahn study beta decay and find evidence that the initial energy before decay is not accounted for by all of the decay products that they can detect. 1930 Wolfgang Pauli suggests that the missing energy is carried by a very small neutral particle. 1931 Enrico Fermi, an American born and brought up in Italy, names the particle "neutrino", Italian for "little neutral one". 1942 Wang Ganchang proposes a method for detecting the neutrino. 1956 Clyde Cowan and Frederick Raines detect the neutrino, receiving the Nobel Prize in 1995. Question 1 Imagine: The word "babyccino" describes a little capuccino. The word "neutrino" has its origins in Italian. Think up an English word to describe "a little neutral one". There may be students in your class from a non-English speaking background. You may wish to also ask them what word in their language could mean "a little neutral one". Symbols are used to represent elements. For example, C is the symbol for carbon and N is the symbol for nitrogen. The total number of particles in the nucleus, as well as the number of protons in the nucleus, can also be easily represented. For 14 example, a carbon atom with 14 particles in the nucleus, 6 of which are protons, can be written as 6C . Of course this means that there are 14 - 6 = 8 neutrons in the nucleus. Similarly there are symbols for the parts of atoms, including the neutrino. Particle Symbol 0 electron -1e proton p neutron n neutrino ν The symbol for the neutrino is ν, the Greek letter nu, which is the first syllable of neutrino. It is like the letter v but both lines are curved the same way. 14 14 0 A typical beta decay is 6C → 7N + -1e + ν Putting this into words it would read: A carbon nucleus decays into a nitrogen nucleus plus an electron and a neutrino. You will notice that the top numbers balance, i.e. 14 = 14 + 0, which means the total number of particles does not change. Also the bottom numbers balance 6 = 7 + -1, which means the total amount of charge does not change. These are called conservation rules. Question 2 40 40 0 ​​Calculate: Write the following decay in words: 19K → 20Ca + -1e + ν Question 3 Calculate: Find the unknown numbers, x and y, in each of the following: Equation x y 210 210 0 82Pb → xBi + -1e + ν N/A 234 x 0 yTh → 91Pa + -1e + ν Neutrinos are not only produced in beta decay but in nuclear events in the Sun, by supernovae and when matter falls into a black hole. In fact, billions of neutrinos are passing through your body every second, mainly from the Sun. This animated video looks at a neutrino called Nino that comes from the Sun and his travels from the Sun to the Earth, through the Earth and beyond. It is a six-minute cartoon with English subtitles, while the characters speak in Italian. Loading Nino Neutrino. Video credit: AGI / YouTube Question 4 Recount: Describe Nino's journey mentioning the different characters he meets on the way. Question 5 Describe: ​What happens to the protons and photons that Nino meets? Question 6 Calculate: The ​Cosmos Magazine article mentioned that 28 high energy neutrinos have been detected by the IceCube Neutrino Observatory. This was in the first two years of operation. Calculate the average time from detecting one neutrino to detecting the next. Question 7 Recall: ​In "Under the ice" the article refers to neutrinos hitting subatomic particles in hydrogen and oxygen nuclei. What subatomic particles exist in these nuclei? Question 8 Calculate: The section "Captured" mentions the light detectors. How many light detectors are there? If all the money (see opening section) had been spent on light detectors, how much would each have cost? ​ Neutrinos: Process Artist's impression of some of the IceCube Neutrino Observatory neutrino detectors. The section "Captured" also describes the detection of a very energetic neutrino. Its energy was measured at 1040.7 TeV. What is this unit of energy, eV, and what does the prefix, "T", stand for? You will have seen distances measured in kilometres, written as 50 km, the "k" means multiply by 1000 or 103, to give 50,000 metres. Similarly you may have heard about megahertz and gigabytes. "kilo", "mega" and "giga" are all multipliers. T stands for "Tera" and is another multiplier. Question 1 Infer: Complete the table below. Name Prefix Symbol Power Number Thousand Kilo k 103 1,000 Million Mega M 106 1,000,000 Billion (US) Giga G 109 1,000,000,000 Trillion Tera T 1012 Quadrillion Peta P 1,000,000,000,000,000 Googol N/A N/A 10100 Too long for this box! Question 2 15 ​Infer:​ The neutrino's energy is 1040.7 TeV, which is 1040.7 x 10 eV in power form. Write this value in number form with lots of zeroes. "eV" stands for electron volt. It is a unit of energy like Joules or kilowatt-hours, except that it is a very small value that is used when talking about electrons and protons. Where are these neutrinos coming from? After a neutrino impact, the amount of light that each detector receives can be used to work out the point in space from which the neutrino came. The oval image in the article (see Figure 3 below) is a map of the sky showing the part of the sky where each of the 28 detected neutrinos came from. But it looks very complicated. So, let us start with something familiar. Figure 1: The familiar map of the Earth. The flat map of the Earth is this peculiar shape because the Earth is a round ball but it is impossible to put the complete surface of a round ball on a sheet of paper, so this shape is a way of showing the curved surface of a sphere on a flat surface. The sky above us is also a bit like a round ball, so a similar map can be used. In the map of the sky in the ​Cosmos ​Magazine article (Figure 3, below), the line through the middle from left to right is the sky above the earth's equator. The top is the sky above the North pole and the bottom is the sky above the South pole. Figure 2 below is another map of the sky, this one showing the different constellations. Figure 2: A map of the night sky, also known as the atlas of the Universe. Image credit: Richard Powell Figure 3: A map showing where in the Universe the neutrinos detected by IceCube came from. Image credit: IceCube collaboration Question 3 Compare: In the map showing where the neutrinos came from (Figure 3) there is one area where many neutrinos seem to come from, that is, numbers 25, 24, 2, 14, etc. Look at the map of the constellations (Figure 2) and work out from which constellations these neutrinos came from.​ Careful! Figure 2 has the centre of our galaxy (marked 0°) at the centre of the map, where as Figure 3 has it on the far right.