Cyclotron can be defined as a type of particle accelerator in which charged particles accelerate outwards from the centre along a spiral path. These particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field.
It consists of a hollow metal cylinder that is divided into two sections D1 and D2. These sections are called Dees. They are enclosed in an evacuated chamber. The Dees are kept separated from each other and a source of ions is placed at the center. They are placed between the two poles of a strong electromagnet. The dees are connected to a high-frequency oscillator.
It works on a principle that a charged particle moving perpendicular to magnetic field experiences a magnetic force (Lorentz). This force makes the particle to move in a circular path.
Cyclotron involves the use of an electric field that is used to accelerate charged particles across the gap between two D shaped magnetic field regions. The magnetic field used in a cyclotron is perpendicular to the paths of the charged particles. When a positive ion of charge q and mass m is emitted from the source it is accelerated towards the Dee having a negative potential. Due to the perpendicular magnetic field the ion experience magnetic Lorentz force and moves in a circular path. When the ion arrives at the gap between the gap the polarity of Dees gets reversed. It again accelerate the particle and due to which it moves into the other Dee having greater velocity. The particle now moves in a spiral path that has an increased radius. When it comes near the edge it can be taken out with the help of deflector plate. The particle now having high energy is now allowed to hit the target.
USES OF CYCLOTRON:
For several decades, these were the best sources of high-energy beams for Nuclear Physics However, these are still in use for this type of research.
Treatment of cancer: Cyclotrons can be used in particle therapy to treat cancer, using the ion beams from cyclotrons can be used to penetrate the body and kill tumours by radiation damage.
A moving charge in a cyclotron will move in a circular path under the influence of a constant magnetic field. If the time to complete one orbit is calculated:
It is found that the period is independent of the radius. Therefore if a square wave is applied at angular frequency qB/m, the charge will spiral outward, increasing in speed.
Now by equating LORENTZ FORCE and centripetal force , we get,
When a square wave of angular frequency is applied between the two sides of the magnetic poles, the charge will be boosted again at just the right time to accelerate it across the gap. Thus the constant cyclotron frequency can continue to accelerate the charge (so long as it is not relativistic).
LIMITATIONS OF CYCLOTRON :
- Cyclotron cannot accelerate electrons because electrons are of very small mass.
- Neutral particles cannot be accelerated by a Cyclotron.
- It cannot accelerate positively charged particles with large mass due to the relativistic effect.
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