Accelerators In order to probe the nucleus - just as Rutherford probed the atom with alpha particles - we need to use more energetic particles. These are not easily found in naturally occurring radioactive samples so we have to accelerate them artificially.
In order to probe the nucleus - just as Rutherford probed the atom with alpha particles - we need to use more energetic particles.
These are not easily found in naturally occurring radioactive samples so we have to accelerate them artificially.
Particles are accelerated by the electromagnetic force - this implies that only charged particles can be accelerated.
Due to the small size/mass and strong ionising power of the particles, a very high degree of vacuum is required.
Large superconducting electromagnets are frequently used to produce strong magnetic fields and radio frequency (rf) fields within cavities along the accelerator, which accelerate bunches of charged particles.
The Van de Graff accelerator:
The Linear Accelerators (Linacs)
An accelerator, which consists of a long straight tube, is called a linear accelerator. The particles are fired from a VDG , into the linac and then into a stationary target.
It should be remembered that upon collision of two particles, the centre of mass (C of M) of the system will remain in a motion such that the momentum of the C of M remains unchanged.
Energy will consequently be “soaked up” in maintaining this motion so that the energy left to make up new particles will be limited.
If the target were moving towards the projectile, the momentum of the centre of mass of the system would be reduced and hence more energy would be available for the production of new particles.
This is not possible with the linac.
Drift Tubes - a type of Linac
Each RF cavity accelerates particles as they pass between them. The electric field changes when the particles are in the cavity and that has no effect. However, as they reach the spaces between the cavities, they are accelerated along to the next.
The first RF cavity goes positive
Negative particles will be attracted to it - accelerated
They will pass through it unaffected - constant velocity
The cavity in front goes positive
The cavity behind goes negative
The electron accelerates across the RF cavity
Adjacent cavities are always of opposite polarity
Accelerating Voltage (RF)
This was the first of the circular accelerators and was made from two Dees (semicircular boxes).
This was fine at low speeds. At high speeds, m increased and they became unsynchronised. For this reason the synchrocyclotron was invented. It uses drift tubes in a large evacuated ring.
The degree of bending is exaggerated here
The main ring in Geneva (CERN) is 27km in diameter
The Synchrocyclotron Accelerator
This type of accelerator consists of a large diameter ring in which the particles are accelerated by rf cavities. The bending is obtained by using electromagnets - use Flemming’s left hand rule (FLHR) to convince yourself. This is useful as the tubes do not need to be too long and the particles can be stored.
Note that it is not really a ring but a series of straight lines (RF cavities) and curves (bending electromagnets).