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How did you make the detectors<br>work in Halls A, B and C?

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How can you tell if a new type of element has been created, especially if only a few atoms of it were created and if they only existed for a millisecond?

Well it's about time someone asked this question! Seeing three atoms of something does seem kind of unlikely. The question "How did they do that?" should have been asked right away. Believe it or not there are whole armies of people (and I am one of those people) working on expanding human senses. This has been going on since shortly after humans stretched out and stood up on our hind legs. The first hairy guy... oh, excuse me... person who crawled up a tree to see farther in the distance is my direct professional ancestor. The benefits of this work is all around us. Eyeglasses, magnifying glasses, binoculars, telescopes, microscopes, cameras, x-ray machines, CAT, PET, SPECT, and MRI scanners, radar, sonar, radio telescopes, sonograms and clocks are all examples of devices invented to expand the capabilities of our senses. The list could go on and on. For particle physics we build special instruments, cleverly called particle detectors. There are several branches of experimental physics. The experiments can be quite different, but the detectors themselves typically have a lot in common.

It is somewhat ironic that the devices designed to see the smallest, simplest pieces of our universe tend to be huge and extraordinarily complex. They do use only a few different physical phenomena to help us "see" the particles.

In a typical experiment there are two general classes of detectors. The first is called a tracking detector. These tell you the path that a particle has taken.

The second type of detector is a calorimeter. These measure the energy that the particle has. It's the nature of these devices that make it very difficult to measure a particle's track and its energy at the same time. Therefore, the two detectors are often used in tandem.

Tracking detectors have to have very little material in them to prevent them from interfering with the natural path of particles. Calorimeters have to have as much material in them as possible to interfere with and stop the particle. Usually trackers are in front of calorimeters.

There are two naturally occurring phenomena that we take advantage of to amplify our senses to the point where we can detect sub-atomic particles. These two phenomena make up the way most of the detectors work. The first is called scintillation. There is a property of some materials that when particles go through them, they give off a tiny flash of light. We can catch that flash with a very sensitive light sensors which tells us a particle just went through that material. The other technique is called ionization. This happens when a charged particle passes through a material and rips off electrons. We can collect the electrons or the ions left behind after the electrons are ripped off. The electrons/ions can illustrate something about the particle that went through our detector.

At Jefferson Lab, combinations of these two types of detectors are assembled in clever ways to allow us to learn the most from our machine. So you see, even though these devices are huge and seem enormously complex, they are actually based on fairly simple processes. It should be clear though, that no one actually "sees" the particle. We only see the tracks it left, but we can still learn a lot from the tracks.

By putting a magnetic field around the instrument, we cause the path the particle takes to curve. By measuring the radius of the curve we can calculate the momentum of that particle. If we know how fast it is moving, which we can also get from the detector, we can use that with the momentum to find its mass.

The lifetime of the particle is easily determined by knowing how fast it is moving along with how far it has traveled. A millisecond lifetime is not a problem because many particle detectors can measure timing to nanoseconds. Milliseconds are like hours to some of our detectors.

To know if you've succeeded at making a particular element, you need to know what properties to look for. That's where the periodic table helps out. That wonderful chart tells us a lot about the properties of particular elements - like if it doesn't exist! Discovering that pattern was one of mankind's greatest leaps in understanding the nature of matter.

Author:

Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)