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UNDERSTANDINGUNDERSTANDING

NUCLEAR PHYSICS NUCLEAR PHYSICS FIRST-HAND FIRST-HAND

Account of the excursion by the Physics

basic and advanced course - class 13

CERNCERN

Conseil Européen pour la Conseil Européen pour la

Recherche NucléaireRecherche Nucléaire

„Brandenburgische Schule für Blinde und Sehbehinderte“Physics basic and advanced course

Class 13

Foreword

Nuclear physic... One of the most interesting physical themes which tries to conclude from the smallest particle and its behaviour to the big bang theory and the further development of the universe.For a better understanding and to round off the theme, we, the pupils of the physics basic and advanced-course in the 13th class from the “Brandenburgischen Schule für Blinde und Sehbehinderte”, decided to visit one of the biggest centre of those nuclear researches, the CERN, near Geneva.The following account shall show what we’ve learnt and experienced there.

CERN

What is CERN and what happens there? European organisation for nuclear research (CERN, French: Conseil Européen pour la Recherche Nukléaire), international organisation for high energy and nuclear physics in Geneva. In 1954, CERN was founded with the goal of the basic elements research. Besides, it wants to bring an explanation about the origin of matter. The first members were Belgium, Denmark, Germany, France, Greece, Great Bri -tain, Italy, the Netherlands, Norway, Austria, Sweden und Switzerland. Finland, Poland, Slovakia, Spain, Czechoslovakia and Hungary joined later.Today, the CERN research centre is the biggest of its sort. There are particle accelerators which bring elementary particles (electrons, protons, positrons) to extreme high speeds. Particle detectors record the collision of those particles. Particle collisions in CERN are processes inside the matter, voyages back to the big bang und to the beginning of time. CERN’s research scientists analyse a million of unusual events, trying to find out why the universe developed 15 billion years ago like we see it today.In 1995, the budget was 970 million Swiss Franc. CERN’s uppermost authority is a council which con-sists of two representatives of each member country, which decides on the budget and the projects.About 6.500 guest researchers from more than 500 different universities and 80 countries work for CERN’s projects. And about 3.000 employees (scientists, technicians, engineers, craftsmen) support them. By-products of those researches are, among other things, maximum precision measuring instru-ments, detectors for medical radiology and the World Wide Web (WWW) for a faster information ex -change between its users.

CERN’s newest particle accelerator is the large electron-positron collider (LEP) with an accel-erator and collision circle of 27 km length. With LEP, positrons and electrons can almost be ac-celerated to the speed of light. DELPHI, one of the four LEP particle detectors, consists of a ho-rizontal cylinder with a diameter of ten metres and a weight of 3.000 tons. A large hadron col-lider shall be accommodated in the LEP-tunnel. In 1994, the member countries granted 3.1 mil-lion DM for it.In January 1996, CERN’s scientists led by the German physicist, Walter Oelert, produced atoms of antimatter for the first time. It was an anti-hydrogen atom consisting of an antiproton and an anti-electron.

CERN also plays a big role for the scientific and technical education. With a big number of education programs and scholarships, the laboratory attracts many young, talented scientists und engineers. Most of them continue their career in the industry, where their practical experiences in multinational high-tech-surrounding are high in demand.

CERN with its circular particle accelerator


Class 13

How does it work?

The particles are accelerated to very high energy. When particles collide or clash on “targets“, physicists can analyse the reaction and discover the forces, which exist between the particles.

There are two kinds of accelerators, linear and circular accelerators. CERN has got both types at its dis -posal. Accelerators use high electric fields in order to charge a particle beam with energy. Magnetic fields focus the beam and on a circular accelerator, they band it on orbit.

In a linear accelerator, the beam is charged with energy at the whole length of the machine. The longer the accelerator is, the higher is the energy.

Detectors are like cylindrical onions. Collisions take place close to the middle, and different layers of de-tectors measure different properties of the emerging particles. The precision tracking devices are closest to the beam, which pinpoint particle tracks with a thousandth-of-a-millimetre precision. These are sur-rounded with less exact, but vastly bigger trackers to keep tubs on the particles as they fly away from the collision. Further out there are energy measuring devices, calorimeters, in which most particles finish their journey. The final layer consists of trackers again, this time to identify the only detectable particles, which get this far, the weakly interacting muons. A magnet embedded within the detector bends the tracks of charged particles, helping to identify and measure individual ones.

In case of particle collisions in accelerators or their clashes on “targets” outside the accelerator, new particles come into being. At this time matter converts into energy, and energy converts back into matter again, appropriate to Einstein’s famous formula E=mc².However, it’s important that in a particle collision energy is concentrated on tiniest room. From such ex-tremes energy concentration, new particles come into being which we need to analyse to get new in-sights into the secrets of nature.

What is a particle accelerator?

Particle accelerators are physical and technical facilities used for accelerating charged elementary particles. Particle accelerators belong to the biggest and most expensive physical devices which bring particles to a high speed. On the whole, these facilities consist of three components: a source from which the elementary particle beams start, a largely evacuated and cylindrical railway in which the particles can move freely and a unit to accelerate the particles. Usually, accelerators are combined with other facilities, for instance particle detectors.Charged particles can be accelerated with the help of electric fields.

What are particle detectors?

Particle detectors are devices or facilities used to prove and study element-ary particles. Detectors can either be small and simple, or they can be very big and complex. They can even reach the size of a room, a flat or a house.

A look in L3


Class 13

LEP

What is LEP?

The large Electron Positron Collider, LEP, is the world’s largest particle accelerator. Built inside a circular tunnel, it is 27 km round und buried 100 metres underground. At four points around the accelerator, huge de-tectors, called ALEPH, DELPHI, L3 and OPAL, study what happens when electrons and their antimatter counterparts, positrons, collide at high energy.

How do accelerators work?

Particle accelerators like LEP work by exploiting the way charged particles move in electric and mag-netic fields. Electric fields accelerate them. Magnetic fields bend and focus them into beams.All particle beams start from a particle source. The simplest source is a hot wire, like the filament inside a light bulb. This is the kind of source used by televisions. A similar filament is also used in the Linear Injector for LEP, LIL. LIL is a linear accelerator, linac for short, which prepares LEP’s beams. In a linac, particles accelerate from one electrode to the next, gain-ing energy with each one they pass. LIL’s filament produces LEP’s electrons, but producing positrons is a little trickier. To provide the positrons, electrons are accelerated through a foil, where they cause pairs of electrons and positrons to be created. The positrons are selected by magnets and stored until they are enough of them to form a beam. All of CERN’s beams begin their lives in linacs, but to reach the ener-gies that physicists need would require extremely long accelerators. For this reason, CERN’s big ma-chines are circular. Particle beams travel round and round gaining energy with each lap. In LEP, 3368 magnets bend the particle beams and keep them on orbit.

What does LEP do?

LEP was designed to study one of nature’s fundamental forces, the weak force which fuels the sun, and is responsible for some forms of radioactivity.One of the first results to come from LEP is also one of the most profound. LEP has shown that matter comes in three distinct “families” of particles. All of the things we see around us, ourselves included, are made from particles belonging to the lightest of these families. The other two are just heavier copies of the first. Why there are three families, instead of just one, is still a mystery.

How do we use the four detectors?

The four LEP detectors work on one and the same basic principle, but each is optimised with a different goal in mind. OPAL is based on well-understood techniques, to guarantee results right from the start. DELPHI is at the other extreme, packed with innovative tech-nology. ALEPH takes the middle line, whilst L3’s design is optim-ised for muon detection. All four have worked impeccably, and friendly rivalry has spurred on the collaborations of physicists who built them to the dominant place they hold in particle physics today.

The LHC

The LEP-tunnel

L3


Class 13

The large hadron collider, LHC, is a particle accelerator, which will probe deeper into matter than ever before. Due to switch on in 2005, it will ultimately collide beams of protons at an energy of 14 TeV. Beams of lead nuclei will also be accelerated, smashing together with a collision energy of 1150 TeVThe LHC is the next step in a voyage of discovery which began a millennium ago.With the help of such an accelerator, scientists hope to get, among other things, new knowledge about the connections between matter and antimatter.

Our visit in CERN

After a lecture by Mr. Schöneich at our school – Mr. Schöneich is a physicist in DESY Zeuthen –, many discussions about universe, matter, infinity, speed of light etc. and a seventeen-hour long ride by train, the most important day of this trip started. On Monday, the 24 th of January 2000 at 9 o‘clock a.m., our guided tour began with a lecture by Dr. Schäfer, an employee of L3. With the help of some basic ques-tions, for instance: “What is CERN?” or: “How do scientist work in this facility?” he explained CERN’s functions and aims to us. Like in the Mr. Schöneich’s lecture, his report was very interesting, detailed and comprehensible. And of course, it was very good that it was perfectly arranged also for our blind classmates. During this lecture, in the follow-ing guided tours and exhibitions he answered all our ques-tions. Our complicated questions led to interesting, and in-structive discussions with Dr. Schäfer. After the lecture we visited the exhibition Microcosm,

which was informat-ive, too. Here we found a very good graphic description about the big bang theory and the development of the universe, a Calorimeters, which is used for the measurement of impulses, different detectors mock-ups and some separate accelerator components, for instance the resonator.We only noticed how quickly time had passed because we suddenly noticed that we were hungry. So Mr. Schöneich led us to the canteen, where we could get new power for the rest of the day. The meal there was great. And even

while having the meal, the discussions went on. We were fascinated by the things we heard a moment ago, but most of us had nevertheless our own opinions, which we discussed.The bus arrived on the parking place, and we went off to the ex-periment L3. When we arrived there, we were very impressed because many preparations had already been done. First we looked at a model of the experiment. Dr. Schäfer explained its necessities and essentials. This model is traced in a scale of one to ten. Our blind classmates could also get a good impression of the experiment because they had the opportunity to touch the model.

But now another great part started. We went along a railing from which we could look down into a depth of fifty metres. We were approximately twenty-five people in the elevator which brought us into the

The exhibition Microcosm

The exhibition Microcosm

The model of L3


Class 13

depth. Down there, we went through a hall outside L3 and were divided into five groups. The guides of the groups said some introducing words to each group and then we went into the experiment. Around L3 there is a wall which is six metres thick. Fascination and astonishment are no words to describe our im-pression there. We were speechless. The guides showed and explained us L3’s several components, the way it worked in detail. And in spite of our speechlessness we went on asking questions which were answered by the guides with much patience. The stay on the fourteen-metre high door of the magnet was also very great. Normally, it’s not part of a guided tour and it was done only for us. It was exceptional that there was so much security personnel and that many security measures were taken only for us.

Another outstanding experience waited for us. After having seen almost everything of L3, we were allowed to enjoy a ride with the service rail which was organ-ised only for us. We climbed into the rail – they had set up a scaffold platform especially for us – and we went 300 metres into the LEP-tunnel. Usually, visitors mustn’t do something like this. It was also for our guides something new. Here in the LEP-tunnel, we saw the several magnets and the very, very big number of cables, pipes and lines.After the last questions we went into the quite big elev-ator again. When we arrived on the surface, we went out into the fresh air. Now we thanked the several

guides and the bus brought us back to our accommodation, where this really nice and interesting day came to its end.So one of our greatest experiences finished. And till late in the evening we debated and discussed the whole day in the canteen.Now we can say, that our knowledge and understanding increased very much because of this excursion with all the special events and preparation for us. And for the blind among us, it was as interesting and as easy to understand as for the rest of us. For this and everything else we are very grateful to the several organisers. Special thanks to Mr. Schöneich who had organised this trip for us, and especially to Dr. Schäfer who accompanied us all day with much patience and greatest understanding for our questions, comments and problems.

Thomas Schrowe (Physics advanced course, 13th class)Monique Gartmann, Marko Weber (Physics basic course, 13th class)

In the service rail

das cern - desy · web viewour visit in cern after a lecture by mr. schöneich at our school –...

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