©CERN Geneva	CERN The European Laboratory for Particle Physics         High School Teachers 2000 Program                July 2, 2000 to July 21, 2000
Sister Mary Catherine Burns, S.U.S.C      Physics/Mathematics Teacher      Coyle and Cassidy High School      Taunton, MA 02780      USA	PURPOSE       The purpose of the program is to bring together teachers from all members countries as well as a few from the United  States to learn about particle physics, about teaching  it to young people and about the contributions the field of physics makes to society in general.

DETECTORS USED IN PARTICLE PHYSICS
 
 

                                                                                                                                                            ©CERN Geneva
 
 

QUESTIONS AND ANSWERS ABOUT DETECTORS

What is a detector used for in particle physics?

Are there different kinds of detectors?

What are the other ways you can categorize them?

What are the names of the kinds of detectors used at CERN in the CMS ?

What is the CMS?

How do all the different kinds of detectors get arranged in the CMS?

Is there any everyday thing that shows how all of these different detectors work together to solve the problem of identifying particles?

How does a tracking detector work?

Where is there more informtion about detectors on the web?
 
 

ANSWERS - Long and Short

What is a detector used for in particle physics? TO IDENTIFY THE VARIOUS PARTICLES

          The detector is a device that allows scientists to interpret signs left by the particles as they pass through it. Different particles have different "signatures" or tracks by which they can be recognized. As a result of using the detectors they can identify different particles and discover properties of these particles.

                   The W boson                              The Top Quark                               The Z0 boson                                Neutral current

©CERN Geneva

©CERN Geneva

©CERN Geneva

©CERN Geneva

Are there different kinds of detectors? YES - FOR MEASUREMENT OF POSITION AND OF ENERGY

         Yes, there are. One way to categorize them is by their function. Detectors do one of two things. They may measure the position of passing particles, allowing scientists to reconstruct the path they took and thus calculate the momentum of the particle or they may measure the energy of the particle. This assists the scientists when they try to identify the particles and may add to the information already known about the particle.

The table below gives you an idea of how the different detector layers tell the scientist which particle left the track.  If you would like to test your understanding of this chart and how it works click here.
 
 

What are the other ways you can categorize them? BY DESIGN AND FUNCTION

         Detectors can also be categorized by their design and function. Properties such as charge, energy and momentum need to be measured in different ways, hence the need for a variety of design in building detectors.
 

What are the names of the kinds of detectors that have been used or are presently being used at CERN ?
 
 

The very first detector was the bubble chamber. The scientists could use it to measure the position and the energy of the different particles.      In the middle picture various particles are identified next to the track they made.     In the picture at the far right is Donald Glaser, inventor of the bubble chamber. He received the Nobel Prize for his invention in 1970.

©CERN Geneva

©CERN Geneva

Many new ones have been designed since then.

Tracking detectors include:
proportional wire chambers,
pixel detectors,
drift chambers,
silicon detectors,
    gas microstrip chambers (or MSGCs),
and muon chambers.
 
 

SOME TRACKING DETECTORS

©CERN Geneva	©CERN Geneva	©CERN Geneva
Silicon Vertex detector	Pixel detector readout chip	Scintillator for particle detection

Energy measuring detectors are called calorimeters.
There are two kinds: electromagnetic and hadron.

They are often named ECAL and HCAL.
 

SOME CALORIMETERS

©CERN Geneva	©CERN Geneva
One half of the electromagnetic calorimeterof the OPAL detector.	Aleph Hadronic Calorimeter

How does a tracking detector work?

      Very simply - the curvature of the path can be measured and from that the momentum can be determiined. Here is how it works:

            The track is circular is circular so the force required to keep the particle moving on a circular path is given by

F = mv² / r

            The force experienced by a particle in an electric field is given by

F = qvB

            The field CAUSES the circular path, the EFFECT; so here it means that

qvB = mv² / r

            which simplified is mv = qBr = p

            and the conclusion is that the MOMENTUM, p, can be calculated since the charge, q, is known, the
            induction of the electric field, B, is known and the radius, r, can be measured.
 
 

How do all the different kinds of detectors get arranged in the CMS?

LIKE AN ONION IN LAYERS GOING AWAY FROM THE CENTER

 If you think of an onion you have a good idea of  this answer. The particle comes in at the center of  the detector. The first layer wrapped around the beam line (as the opening left for the particle is called), are the pixel detectors.  Going away from the center the silicon detectors come next and then the gas microstrip chambers.  Continuing out from the center, the electromagnetic calorimeters are placed next followed by the hadron calorimeters.
 
 

Is there any everyday thing that shows how all of these different detectors work together to solve the problem of identifying particles?

COIN SORTERS and SCREENS WITH DIFFERENT SIZE HOLES

 In fact there are a few. Consider a coin sorter. You drop a handful of coins into the top of  the sorter and they go each into its own cylinder.  Another way you might think about this is to put pieces of screening  with different size holes above one another. The screen with the largest holes goes on top and the one with the smallest holes goes on the bottom. When a handful of stony soil is poured on top, the largest stones are not able to get through the first screen. Each successive screen filters out the next size stones.

Test of your ability to use the particle identification chart:
 

ANSWER THE FIVE QUESTIONS - CLICK ON ANSWERS TO CHECK

1.    This particle does not leave any track in the tracking chamber but loses all of its energy in the electromagnetic calorimeter.
 

2.    This particle goes through every layer of the detector and comes out of it still with some energy.
 

3.    This particle does not leave any track in the tracking chamber; it loses very little energy in the electromagnetic calorimeter;
        but it deposits the rest of its energy in the hadronic calorimeter.
 

4.    This particle  leaves a track in the tracking chamber and deposits all of its energy in the electromagnetic
        calorimeter
 

5.    This particle leaves a curved track in the tracking chamber and deposits all of its energy in the electromagnetic calorimeter.

ANSWERS

Where is there more informtion about detectors on the web?

CHECK OUT THE FOLLOWING SITES

NOTE:  Links are active only if the site reference is colored blue; otherwise you must type in the address yourself.

INFORMATION ABOUT DETECTORS:

a)      http://www2.slac.stanford.edu/vvc/detectors/vertex.html
            This is a large site with a lot of information on many different aspects of detecting particles

b)      http://public.web.cern.ch/Public/SCIENCE/lepcolexp.html
            This is a great site to get a close look at how a detector works

c)      http://www.ph.rhbnc.ac.uk/publicity/masterclass/events/taunuqq.html
            Visualization of significant "hadronic" events, with links to others

GENERAL INFORMTION ABOUT PARTICLE PHYSICS:

e)   http://www.fnal.gov/

f)   http://www.fnal.gov/pub/fermilab_intro.html

g)   http://www.fnal.gov/pub/hep_descript.html

h)   http://www.fnal.gov/pub/electronic_game.html
 
 

ANSWERS

                                         1.    the photon
                                                        The photon is not charged so leavesno track in the tracking
                                                        chamber
 

                                        2.     the muon
 
 

                                        3.     the neutron
 
 

                                        4.    either the electron or the positron
                                                    By the direction of the curvature the correct one can be identified
 
 

                                        5.    a pion
 

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