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 throughyour 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 sothey 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 makeatoms.

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 hydrogenatoms 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 theseelusive 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 thiscatchy 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 byall 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 wordto 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 theirlanguage 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. Forexample, a carbon atom with 14 particles in the nucleus, 6 of which are protons, can be written as C . Of course this means thatthere are 14 - 6 = 8 neutrons in the nucleus.

Similarly there are symbols for the parts of atoms, including the neutrino.

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Particle Symbol

electron e-10

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 arecurved the same way.

A typical beta decay is C → N + e + 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 thebottom numbers balance 6 = 7 + -1, which means the total amount of charge does not change. These are called conservation rules.

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Question 2

Calculate: Write the following decay in words: K → Ca + e + ν1940

2040

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Question 3

Calculate: Find the unknown numbers, x and y, in each of the following:

Equation x y

Pb → Bi + e + ν82210

x210

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Th → Pa + e + νy234

91x

-10

events in the Sun, by supernovae and when matter falls into ablack hole. In fact, billions of neutrinos are passing throughyour body every second, mainly from the Sun.

This animated video looks at a neutrino called Nino that comesfrom the Sun and his travels from the Sun to the Earth, throughthe Earth and beyond. It is a six-minute cartoon with Englishsubtitles, while the characters speak in Italian.

Nino Neutrino. Video credit: AGI / YouTube

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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 NeutrinoObservatory. This was in the first two years of operation. Calculate the average time from detecting one neutrino to detecting thenext.

Question 7

Recall: In "Under the ice" the article refers to neutrinos hitting subatomic particles in hydrogen and oxygen nuclei. What subatomicparticles exist in these nuclei?

Neutrinos are not only produced in beta decay but in nuclear

Question 8

Calculate: The section "Captured" mentions the light detectors. How many light detectors are there? If all the money (see openingsection) 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 isthis 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 10 , to give 50,000metres. Similarly you may have heard about megahertz and gigabytes. "kilo", "mega" and "giga" are all multipliers. T stands for"Tera" and is another multiplier.

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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

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 ofzeroes.

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"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 whentalking 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 theneutrino 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 the28 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 around 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 theNorth 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: IceCubecollaboration

Question 3

Compare: In the map showing where the neutrinos came from (Figure 3) there is one area where many neutrinos seem to comefrom, that is, numbers 25, 24, 2, 14, etc. Look at the map of the constellations (Figure 2) and work out from which constellationsthese 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. When

comparing the two maps make sure to keep this in mind.

Question 4

Compare: Where do numbers 3 and 6 appear to come from?

Question 5

Compare: Which neutrinos seem to come from Orion?

Question 6

Speculate: The centre of our galaxy, the Milky Way, is located at 0° on Figures 2 and 3. The map showing where the neutrinos camefrom (Figure 3) is blank at 0°. So far, no neutrinos have been observed coming from the centre of our galaxy, which is thought tohave a large black hole.

Suggest some reasons why this might be the case.

Neutrinos: Apply

Popcorn, neutrinos and Nobel prizes

Image credit: iStock

This activity is divided into two parts: a short experiment and some additional Cosmos questions. You may complete the tasks inwhichever order suits you best.

To learn about beta decay, neutrinos and the IceCube Project by measuring the mass of popcorn before and after popping.

Popping corn: Some popping corn can be purchased already bagged for the microwave oven. It is also available in largerquantities and you will need to bag it yourself.

Measuring cup, if needed.

Paper bag, if needed.

Microwave oven.

Top loading balance accurate to at least 0.01 g. Note: the balance is very sensitive and should be used with care.

1. Do you have pre-bagged popping corn? If so, go to step 3.

2. Put ¼ cup of popping corn in a paper bag and fold over the top, tight enough to keep the popping corn in, but not so tight thatthe popping corn may tear the bag or prevent all the corn from popping.

3. Put the bag of popping corn on the balance and record the mass.

Part 1: Experiment - The missing energy of popping corn

Aim

Materials

Method

are using pre-bagged corn. When the sounds of the popping corn are about five seconds apart, the oven can be turned off.

5. Remove the bag from the oven and record its mass again on the balance. Note: the bag may be hot, so take care removing itfrom the oven and you may need a petri dish to support the bag before putting it on the balance.

Question 1

Did you find a difference in the mass? If so, calculate its value.

Question 2

Express this difference as a percentage of the initial mass.

Question 3

What factors or variables might affect the outcome of this experiment?

Question 4

How could you re-design this experiment to be more accurate?

The change in the popcorn could be explained either as a physical change or a chemical change. A physical change explanationwould be that water, as a liquid, was converted to steam and expanded inside the kernel, causing it to burst. A chemical changeexplanation would be, as for a rising cake, a chemical substance such as sodium bicarbonate breaks down releasing carbon dioxidewhich causes the kernel to burst.

4. Put the bag in the microwave oven and put the oven on for just over two minutes, or as per the instructions on the bag if you

Question 5

How would you test these two explanations?

Question 6

If this experiment is an analogy for beta decay, such as C → N + e + ν614

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what does an unpopped corn kernel represent?

what does a popped corn kernel represent?

what represents the neutrino?

Part 2: Additional Cosmos questions

Question 7

Interpret: The section "Under the ice": what do you think is the purpose of the round objects in the image?

Note: This is the same image as the one at the top of "Process".

Question 8

Interpret: What do you think the black lines are for?

Question 9

Speculate: It has been suggested that the discovery of neutrinos from space may be awarded a future Nobel Prize. However therules for the prize limit the number of people who can share the prize to three.

Who should get the prize ? The people who thought up the idea of the IceCube, the people who designed and built it or the peoplewho used it to discover the neutrinos?

Decide who should get the prize and give your reasons.

Neutrinos: Career

Geoffrey Taylor is a physicist from Melbourne who hunts down neutrinos in the search for clues to the origins of ouruniverse.

What is our universe made of? And how did it begin? These arequestions that physicist Geoffrey Taylor hopes to answer.Growing up in Western Australia, Geoffrey’s high school mathsteacher told him to study physics at university.

Geoffrey’s teacher must have had an eye for talent - Geoffreynow works with the world-renowned Large Hadron Collider atthe CERN laboratory in Switzerland. As its name suggests, theLarge Hadron Collider is a massive machine that acceleratesparticles to incredible speeds before it smashes them together.Geoffrey’s job is to sift through the “debris” of the collision tosearch for elusive subatomic particles like neutrinos and quarks.The discovery of these particles could answer many questionsabout our universe.

It is both inspiring and challenging to try to understand theorigins of the universe with experiments here on Earth, Geoffreysays. And there’s no better place to do it than CERN. More than3,000 physicists from 38 countries work alongside Geoffrey onthe Large Hadron Collider, and he loves being surrounded byintelligent, enthusiastic scientists. He fondly recalls CERN’scafeteria, which he describes as an amazing hub whereeveryone from Nobel Prize winners to young PhD students canmeet and discuss many topics.

While his job takes him as far as Japan and Switzerland, Geoffreycalls Melbourne home. He enjoys participating in racingtriathlons and meeting friends over dinner or coffee.

Image credit: University of Melbourne.

Question 1

Imagine: You are sitting at a table in CERN's cafeteria for lunch. Sitting at the same table are Nobel prize winners and youngaspiring PhD students. What would you wish to talk to them about? Why?

Cosmos Lessons team

Education Editor: Bill CondieArt director: Robyn AdderlyProfile author: Yi-Di NgLesson authors: Dan O'Keeffe and Daniel Pikler