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Introduction, page 2 Particle Physics - Introduction 3 of 3 Fundamental forces

The construction of particle accelrators are complex and have to be very precise, yet the machinary and devices involved are very large. For example, in the case of one of the major accelerator, CERN, is 27 kilometers around and the detectors are the size of four-storey houses. There are generally two different types of collisional experiments.

(1) Fixed-Target Experiments
A charged particle is accelerated by an electric field and collides with a fixed target (can be a solid, liquid or gas). One of the earliest, most famous, classical example is the E. Rutherford (1871-1937) gold foil scattering experiment. The results of this experiment indicate the first modern view of the atomic structure.

Rutherford's scaterring experiment

Here, the particle source is a helium nucleus (a) travel in the vacuum (to minimize collision with gas molecules) and collide onto a very thin gold foil. The radiation produces flashes when it is struck on the detector screen. The results show that most a radiation passes through the foil unhindered, while a few been deflected at various angles. Even fewer were actually being deflected directly back to the radiation source. This shows that the nucleus of the atom is very small compare with the overall size of the atom.

(2) Colliding Beam Experiment
Illustration of this type of collision is shown from the diagram on the previous page. Basically, two beams of particles are made to cross and thus head-on collision can occur. This type of collision is more energetic than the fixed-target type and therefore the preferable choice (and popular) to study the production of higher mass particles and more favorable method to recreate the conditions of the early Universe.

There are, in general, two ways to accelerate the particle beam. The overall designs of the particle accelerators depend very much on the type of particle acceleration:

i. Linac
Abbrevation for linear accelerator. For example, the charged particles such as electron or proton can be accelerated in the presence of electric field and the beam starts at one end and exits at the other end, in a straight line.

ii. Synchrotrons
Here, particle beams are accelerated in a circular motion. The beam's goes round a circle due to a series of very powerful magnets. However, the beam is still accelerated by the presence of an electric field.

Usually synchrotron design is a favorite choice over linacs because the beam can go around many times, picking up speed along the way, while linacs only accelerate the beams in one pass. Of course, nothing is provided free: Linacs are generally easier and cheaper to build, while synchrotrons can be very large (in terms of radii) and requires expensive, powerful magnets.

Introduction, page 2 Fundamental forces


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