Build Your Own Particle Accelerator

Home , Collider, Particle accelerator, Particle beam

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Issue 30 : Autumn 2014 Autumn Issue 30 : I . Teaching activities Teaching w1 Metadata, citation and similar papers at core.ac.uk at papers similar and citation Metadata, at the LHC: creating a particle beam, a particle at the LHC: creating changing the path of the particles and altering their speed. All four and altering their speed. activities could occupy a class for at but they could also least half a day, be used separately in individual lessons. For all activities, the particle accelerator needs to be set up as outlined in the worksheet that can be the Science in School downloaded from website Science in School Science in School Particle accelerators like CERN accelerators Particle are huge, but smaller ones can be controlled in the classroom

The activities described below found in old-fashioned computer is a CRT A and television monitors. linear particle accelerator that creates by screen an image on a fluorescent accelerating and deflecting a beam 1). in a vacuum (figure of electrons many orders are And although CRTs of magnitude less powerful than the LHC, the principles of operation are similar (table 1). the same enable students to control as scientists do parameters in a CRT

hen students think of a particle likely to they are accelerator,

W your classroom. your Andrew Merkert, By Julian Wilson and Becca Brown Here’s how to explore the principles of a particle accelerator in of a particle accelerator to explore the principles how Here’s our understanding of what happened just after the Big Bang. after the Big Bang. happened just of what our understanding The world’s largest particle accelerator, the LHC, is deepening the accelerator, largest particle world’s The accelerator Build your own particle particle own your Build imagine the world’s largest ‒ CERN’s imagine the world’s largest Collider (LHC). Hadron Large accelerators not all particle However, investigate the origins used to are they in a 27 of the Universe, nor are an tunnel that crosses km circular Much closer to international border. home is the cathode ray tube (CRT) www.scienceinschool.org

CORE Image courtesy of CERN of courtesy Image Provided by Newcastle University E-Prints University Newcastle by Provided E

A B C

D Image courtesy of CERN

Figure 1: The cathode ray tube is a vacuum tube in which electrons are produced by a heated filament (the cathode, A), focused into a beam as they pass through the aperture of the control grid (Wehnelt cylinder, B) and accelerated by the voltage (VA) between the cathode and the anode (C). The electrons can then be deflected by a magnetic (or in the case of oscilloscopes, an electrical) field (D) before they strike the phosphorescent screen (E), creating an image. The image could be, for example, electrical waveforms (on an oscilloscope), radio wave echoes of aircraft or ships (on a radar screen) or pictures on an old-fashioned television screen or computer monitor.

Characteristic CRT LHC

Pressure 10-6–10-10 atm 10-9–10-15 atm (For comparison, a vacuum cleaner has pressure of 1–10-3 atm, and outer space has a pressure of <10-15 atm

Distance travelled by a particle 0.1–100 mm 1–105 km between collisions

Particle types and source Electrons produced by thermionic · Protons produced by ionisation of emission at the cathode (a heated hydrogen atoms filament) · Lead nuclei

Mode of accelerating particles A potential difference between the Electronic fields and radio frequencies, anode and cathode synchronised with particle speed

Mode of directing particles Electrical or magnetic fields Strong magnetic fields achieved using superconducting electromagnets (4 T in strength)

Mode of focusing particles Wehnelt cylinder and anode hole Quadrupole magnets

Ultimate aim To cause a beam of particles to form an To collide the beam of particles with a image on a fluorescent screen second beam and observe the result

Table 1: A comparison of the classroom particle accelerator (the CRT) and CERN’s LHC

22 I Science in School I Issue 30 : Autumn 2014 www.scienceinschool.org Teaching activities

lower than the energy that binds them Physics to the metal nuclei, sometimes called Ages 14+ the work function. Consequently, no We have all heard about CERN and the particle acceleration experi- electron beam is observed and no spot ments conducted there. However, for some it may seem like a place appears on the screen. that is very far from the classroom. Despite this physical distance, the How does this compare to the LHC? project described in this article succeeds in reducing the barrier be- Instead of electrons, the LHC acceler- tween the scientists’ working place and the students’ classroom. ates beams of protons or lead nuclei (table 1). The protons, however, are The procedure for setting up the apparatus is very detailed, hence mak- produced using a similar technique ing it accessible to teachers. While ensuring that every part of the pro- to the CRT – in this case with an ion ject is explained in terms of the physics theories involved, the authors source known as a duoplasmatron. have also compared the LHC with the CRT throughout the article. This A cathode filament emits electrons makes it extremely interesting, apart from being highly instructive. into a vacuum chamber containing This article can give rise to a discussion about the work being done small amounts of hydrogen gas. The at CERN, linked with the origin of the Universe, the progress we have electrons ionise the hydrogen gas, made so far in the exploration of this theory, and the certainties and forming a plasma of hydrogen ions uncertainties surrounding it! (protons) and free electrons. The

REVIEW Catherine Cutajar, Malta protons are then confined by magnetic fields and accelerated to become a beam.

Producing free particles up and its electrons escape in the form Deflecting an electron beam with

of thermionic emission. The high posi- an electrostatic field Physics Materials tive potential of the anode relative to See the list of the necessary materi- the cathode pulls the electrons into a Materials als in the downloadable documentw1. narrow beam that strikes the fluores- See the list of the necessary materi- cent screen, appearing as a spot. als in the downloadable documentw1. Procedure When the power is disconnected Procedure and the cathode is not heated, the 1. On the CRT power supply unit, electrons cannot escape from the sur- disconnect the lead that supplies 1. On the power supply unit for the face because their thermal energy is the voltage to the cathode (see the deflection plate, alter the voltage circuit diagram in the attached worksheet). 2. Set the voltage of the auxiliary anode – the anode of the control grid or Wehnelt cylinder – to 10 V. 3. Set the voltage of the anode to 30–50 V. 4. Set the cathode voltage to 200–300 V. 5. Connect the power unit to a source of electricity. Can you see a spot on the fluorescent screen? 6. Reconnect the voltage lead to the cathode and repeat the previous step. Now can you see a spot? About what happens Image courtesy of CERN A spot is only visible on the fluorescent screen when the cathode is Figure 2: The completed CRT setup connected. The metal filament heats www.scienceinschool.org Science in School I Issue 30 : Autumn 2014 I 23 Image courtesy of Jfmeler/ Wikimedia Commons between the cathode and the anode. The electrons released by the cathode are repulsed by the control grid and focused towards the anode, resulting in a fine electron beam.

Deflecting the beam with magnetism If you do not have access to a CRT, you could try a comparable dem- onstration using an old television screenw2. Procedure 1. Bring one pole of the bar magnet close to the side of the CRT, beside the path of the beam. What happens to the spot? 2. Power up some electromagnetic coils and bring them close to the side of the CRT. What happens to the spot?

About what happens

Figure 3: Fleming’s left hand rule: using your left hand, your thumb indicates the direc- When electrons in the beam pass tion of motion, your first finger represents the magnetic field (north to south) and your through the magnetic field, they second finger shows the current (from positive to negative). experience a force at right angles both to the direction in which they are trav- elling and to the magnetic field. This deflects the electron beam. You can first to the left and then to the right which they left the field. The greater work out the direction of force using deflection plate (between -80 V and the potential applied to the plate, Fleming’s left hand rule (figure 3). +80 V). What happens to the spot the wider the deflection angle of the Electromagnets produce a stronger on the screen? beam. magnetic field so the beam is deflect- 2. Vary the voltage to the auxiliary an- Increasing the voltage to the control ed more than by the bar magnet. ode of the control grid. How does grid brightens and sharpens the spot How does this compare to the LHC? the spot on the screen change? on the screen because the potential The LHC uses superconducting quad- difference between the control grid rupole magnets to focus the particle About what happens and the anode is greater than that beam. A quadrupole magnet consists When the voltage to the left deflec- of four magnetic poles, positioned so tion plate is greater than the voltage to Image courtesy of K. Aainsqatsi/Wikimedia Commons that the field lines cancel each other the right plate, the spot will be to the out at the centre (figure 4). When a left of the screen and vice versa. particle beam passes through the very This is because an electrostatic field centre, where there is no magnetic is created when a potential is applied field, it feels no force. The quadrupole across the plates. The negatively magnet, therefore, pushes the beam charged electrons are deflected to- into a small cross-section, akin to a wards the positive plate, which makes lens focusing light. However, each them follow a curved path within the quadrupole magnet only focuses the electrostatic field. beam in one direction, so to produce a Once the electrons leave the elec- Figure 4: A quadrupole magnet consists fully focused beam, successive quad- trostatic field, they travel in a straight of four magnetic poles positioned so the rupole magnets at 90° to each other line towards the screen, at the angle at field lines are cancelled at the centre. are used.

24 I Science in School I Issue 30 : Autumn 2014 www.scienceinschool.org Teaching activities

Image courtesy of Nicola Graf

Combining lenses Lens 1 Lens 2 extends the focal length F f1

O I

Optical lenses can be used as an About what happens Answers analogue for quadrupole magnets. When the anode voltage is low, 1. Kinetic energy of the electrons: Just as a series of quadrupole magnets there is no electron beam. As the E = 250 eV = 4 × 10-17 J Physics at 90° to each other focuses the elec- voltage is increased, the spot becomes But E = ½ mv2, which can be re- tron beam in the LHC, combining two visible and then brighter. arranged to give the speed of the lenses of the same focal length (one Increasing the potential difference electrons as: converging/focusing and one diverg- between the anode and the cathode ing/defocusing) results in an overall (by increasing the voltage to the increased focal length. anode) increases the velocity of the The total focal length F of a system electrons towards the screen. of two lenses with focal lengths f and 1 How does this compare to the LHC? f separated by the distance d is given 6 -1 2 The first electrostatic accelerator of = 9.38 × 10 ms by: the LHC (located inside the proton Energy of the protons: source) accelerates protons using a po- E = 90 keV = 1.44 × 10-14 J tential difference of 90 kV. However, these protons do not reach the velocity Speed of the protons: that the electrons in the CRT reach Because the first lens is focusing and with a lower potential. This is due to the second defocusing, while their fo- the higher mass of the protons. Proton cal length is the same, f = -f . Substi- 2 1 accelerators like the LHC, therefore, tuting this into the formula gives: need more energy to accelerate parti- = 4.15 × 106 ms-1 cles to high speed. Comprehension questions Web references The total focal length is increased 1. What is the speed of electrons that w1 – For the list of materials and when two lenses are combined. have been accelerated by a poten- instructions on how to set up the tial difference of 250 V in the CRT? apparatus, please download the Changing the speed of particles 2. What is the speed of protons that worksheet in either Word or PDF form at www.scienceinschool. Procedure have been accelerated by a potential difference of 90 kV at the org/2014/issue30/accelerator#w1. 1. Alter the voltage of the anode. How first electrostatic accelerator of the w2 – An alternative to activity 3, using does the spot on the screen change? LHC? an old-fashioned television screen, www.scienceinschool.org Science in School I Issue 30 : Autumn 2014 I 25 More about CERN Acknowledgement These activities were developed in Julian Merkert’s thesis during his study at the University of Karlsruhe, The European Organization for Nuclear Germany, and a two-month stay at Research (CERN)w3 in Meyrin, Switzerland, is one CERN. The initial inspiration came of the world’s most prestigious research centres. It from an idea from Prof. Dr Günter aims to understand fundamental physics – finding Quast at the University of Karlsruhe out what makes our Universe work, where it came from and where it is to “explain particle physics with going. school experiments”. CERN is a member of EIROforumw4, the publisher of Science in School. ______See all CERN-related articles in Science in School: www.scienceinschool. org/cern Andrew Brown is a molecular and cellular biology graduate of the Uni- versity of Bath, UK. After working for Science in School, he returned to is described on the website of the Resources the UK and is now at the Royal Institution. Oxford University physics depart- To learn more about CERN, see: ment. See: www.physics.ox.ac.uk Julian Merkert is a secondary-school Hayes E (2012) Accelerating the (search for ‘cathode ray tube’) or use physics and mathematics teacher pace of science: interview with the direct link: http://tinyurl.com/ working at St. Dominikus-Gymnasi- CERN’s Rolf Heuer. Science in School alq4hgl um Karlsruhe, Germany. During his 25: 6–12. www.scienceinschool. academic studies at the University w3 – Find out more about CERN: org/2012/issue25/heuer www.cern.ch of Karlsruhe, he produced teaching Landua R (2008) The LHC: a look materials about the LHC at CERN. He The CERN education website offers inside. Science in School 10: 34–45. has run several teacher programmes, resources for schools and infor- www.scienceinschool.org/2008/ both at CERN and in Germany. mation about CERN’s residential issue10/lhchow Dr Rebecca Wilson is a planetary courses for teachers. See: https:// Landua R, Rau M (2008) The LHC – scientist working on public and education.web.cern.ch a step closer to the Big Bang. Science business engagement projects at the w4 – Learn more about EIROforum at in School 10: 26–33. www.sciencein- Space Research Centre, University of www.eiroforum.org school.org/2008/issue10/lhcwhy Leicester, UK. She is a project scientist If you found this article useful, you for the UK’s National Space Acad- may like to browse the other teach- emy, collaborating with scientists ing activity articles on the Science and educators to develop secondary- in School website: www.scienceinschool revision materials based on school.org/teaching space science. She also works for the Space IDEAS Hub, giving small local businesses access to the university’s space-derived expertise.

To learn how to use this code, see page 53.

26 I Science in School I Issue 30 : Autumn 2014 www.scienceinschool.org