Detecting Cosmic Rays with the Birmingham QuarkNet Project

Cosmic Ray Shower

What is QuarkNet?

QuarkNet is an international collaboration of schools which is investigating cosmic rays. Cosmic Ray Muon Detectors (CRMD) are used to detect muons which are produced when primary cosmic rays interact in the upper atmosphere. Data from these detectors are then uploaded to the Cosmic e-lab and can be accessed by all schools in the collaboration. The website can also be used to analyse the data, and pupils can use it to perform their own cosmic ray experiments. It is a great opportunity for pupils to create their own research proposal and see it through from start to finish.

What are Cosmic Rays?

Cosmic rays are particles which originate in outer space. They are principally protons, with a small fraction of alpha particles and some heavier nuclei. There are many different sources of cosmic rays in the universe: for example, supernovae. When a large star comes to the end of its life, it collapses in on itself, which causes a huge explosion, and a shockwave which accelerates nearby particles to high energies and flings them off in all directions. Cosmic rays which originate from astrophysical sources like this are usually called primary cosmic rays. Since the universe is mostly empty, a lot of these particles will just travel through space without ever encountering another particle. Some will actually hit the Earth. Luckily, we are protected by our atmosphere. When the primary cosmic rays approach the Earth, they collide with nuclei in the atmosphere. The energy of these collisions will then make new particles, called secondary cosmic rays. These particles will then also interact with the upper atmosphere to produce yet more particles, and so for each primary cosmic ray, a shower of secondary cosmic rays will be produced, as illustrated in the picture below. Some of these secondary comsic rays will then travel down to ground level where we can detect them.

How a shower is produced

But why are we interested in cosmic rays? Some high energy primary cosmic rays produce collisions in the atmosphere with an energy of 140TeV, 10 times higher than the LHC can produce. Studying the collisions of these ultra high energy cosmic rays is therefore extremely useful for particle physics. The only problem is that it is impossible to control the collisions, and difficult to put detectors in the atmosphere where the collisions take place. Also, most cosmic rays have a much lower energy than this, and so it is quite rare to see these ultra high energy cosmic rays. However, studying the secondary cosmic rays which reach sea level can give us valuable information about the sources of the primary cosmic rays.

How do we detect Cosmic Rays?

QuarkNet Setup

Within the shower of secondary cosmic rays there will be a lot of muons. These are leptons, like electrons, but much heavier, and we can detect muons using scintillation counters.


Scintillation Counter
They consist of large squares of a plastic material called scintillator. When a charged particle, like a muon, passes through the plastic, it uses some of its energy to remove electrons from the atoms, or ionises it. The electrons then try to join back up with the atoms, but they can only do this if they give off some of their energy by emitting photons, or a flash of light. The light then bounces around the scintillator until it reaches one corner, where we have placed a photomultiplier tube, or PMT. This can take the tiny flash of light, and convert it into an electrical signal which tells us that a charged particle has passed through.


A photomultiplier tube
A diagram of the PMT is shown above. When a photon strikes the photocathode surface, it releases an electron. This electron is then attracted to a series of dynodes. A negative high voltage is applied across the dynodes so that the electron is accelerated as it approaches them. This means that when it strikes the dynode, 3 or 4 electrons are then emitted, so that at each stage the signal is amplified. The electrons eventually strike the anode at the end which is held at 0V and produces a negative pulse for a signal. The size of the signal is determined by the high voltage across the dynodes. The PMTs we use do not require a high voltage. We apply a small control voltage using the power distribution unit (PDU), and the base of the PMT converts this into a high voltage to apply across the dynodes.

Four scintillation counters are then placed one on top of the other. Since muons produced by collisions of cosmic rays are travelling at nearly the speed of light, they will pass through all detectors at essentially the same time. So we can look at the electrical signals from all of the PMTs and use a data aquisition board (DAQ board) to analyse the signals and decide if they arrived at the same time. If all the counters produce a signal at the same time then we call this a coincidence, and that is our evidence that a cosmic ray muon has passed through. We can then use the DAQ board to work out how many muons are passing through the counter each second, or the flux of muons, and move our detectors to investigate what factors affect the flux.

The Birmingham QuarkNet Project in Schools

At the University of Birmingham, we currently have a cosmic ray telescope which we would like to loan to schools so that they can run a QuarkNet project. After an initial trial period we will have two sets of detectors which we can loan out to two schools simultaneously. Along with the detectors we will also provide worksheets which have been written by the University of Birmingham. These will provide full details of how to use the detector and will take students through all the different experiments they can carry out. Links to these worksheets can be found at the bottom of this page. The length of the loan will depend on how long the school would like to run the project for. There are 3 different possibilities which we invisage for schools borrowing the detectors. Of course this is flexible so if you would like to run the project a different way then do please get in touch.

Short Experiment/Demonstration:
The first option is to just use the setup as a demonstration experiment in one lesson. It could be shown collecting data and it should be explained exactly how it works. Then students could use the cosmic e-lab website to analyse data which already exists and has been uploaded on the website. Each worksheet in the student pack includes details of some example data to use for each study. These are data which have been recorded by the University of Birmingham and should provide good plots which are easy to understand. The disadvantage of this is that students will not get a chance to take their own data, or to gain any hands on experience. However, it can easily fit into one hour so is ideal if there is only one lesson available.
Short Project: possibly to run as a classroom excersise

Another option is to run the project over about 4 weeks. The idea here is to split up your class into groups of 2 or 3, depending on how large the class is. The first 2 weeks will be the same for all students. Then they will have a chance to use the detectors to take data for their own research topic, which they suggest themselves. It could be investigating a factor which affects the flux of cosmic rays, calculating the lifetime of a muon or investigating air showers. They may need to take data with different settings, and so will need to work together to decide who can use the detector and when. Finally, they can make a poster and present their results to their class mates.

  • Week 1: Introduction to the project. Teach them the theory behind it and how the detector works. They can play about with the detector and teh website to get used to using it. Before they can use the website, they will need to take the preliminary test. This will allow you to find out what they know before the project starts. They can also write a short research proposal including what they will want to investigate and how they plan to investigate it. Work for this week is covered in Worksheet 1 of the student pack.
  • Week 2: Calibrating and testing the counters. Worksheet 2 teaches how to plateau the counters to find their optimum operating voltages. Worksheet 3 describes how to use the cosmic e-lab website to test the counters to ensure they are working well.
  • Week 3: This is when they use the detector to answer the research question they posed in week 1. They may need to carry out their experiments at different times as each group will probably need to take different data. This will encourage collaboration to organise who can have the detector when and to see if some data can be reused for different groups. Different methods of collecting data are described in Worksheet 4 - 6.
  • Week 4: They can use the e-lab website to create a poster displaying their results. This is described in Worksheet 7. They can end the week by presenting their results to their classmates, by showing their poster and preparing a short talk describing how they obtained those results. This is a nice way to round off the project and means that all students learn from the other groups.
Long Project: possibly to run as an after school activity
This project might perhaps run for 7 or 8 weeks as an after school activity. There are 7 worksheets in total, and it shouldn't take more than an hour or two to work through them (except Worksheet 2: plateauing the counter, this takes a few hours). The worksheets are as follows, and links to all of them can be found at the bottom of the page:
  • Worksheet 1: Introduction
  • Worksheet 2: Plateuaing the Counters
  • Worksheet 3: Performance Study
  • Worksheet 4: Flux Study
  • Worksheet 5: Measuring the Lifetime of a muon
  • Worksheet 6: Shower Study
  • Worksheet 7: Making a Poster
The project should therefore last about 7 weeks, unless you aim to get through 2 worksheets each week. This time does not include the time it takes to collect data, but once the data is being recorded, the experiment can be left to run. You may need to organise it so that you split one worksheet into 2 sessions. The first session will be an introduction to the worksheet and using data already uploaded to learn how to run the analysis. They can then set up the detector, leave it recording data and analyse the data in the next session. The last worksheet is about making a poster to present their results. This is simple to do with the Cosmic e-lab website, and gives them the opportunity to present the conclusions they have drawn from their results.The could also be accompanied by a 5 minute presentation.

Contact

For more information about the Birmingham QuarkNet project, please contact Dr John Wilson

Dr John Wilson

School of Physics and Astronomy

University of Birmingham

Edgbaston

Birmingham

B15 2TT

email: jaw@hep.ph.bham.ac.uk

telephone: 0121 414 4654

Group Secretary: Mrs M Hobbs 0121 414 4625

Resources

Note: You can explore the Cosmic e-lab website by logging in as a guest.

Topic attachments
I Attachment Action Size DateSorted ascending Who Comment
jpgjpg cosmic_rays.jpg manage 61.6 K 01 Sep 2011 - 15:21 NigelWatson  
pngpng shower.png manage 21.0 K 01 Sep 2011 - 15:29 NigelWatson  
pngpng counter.png manage 5.0 K 01 Sep 2011 - 15:32 NigelWatson  
pngpng pmt.png manage 31.3 K 01 Sep 2011 - 15:32 NigelWatson  
jpgjpg 4counter_setup.jpg manage 162.1 K 09 Sep 2011 - 12:00 NigelWatson  
jpgjpg Quarknet_setup.jpg manage 162.1 K 09 Sep 2011 - 12:01 NigelWatson  
Topic revision: r10 - 15 Feb 2016 - 12:29:33 - NigelWatson
 
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