BYEmmanuel W. FiagbedziMaster In Medical Physics StudentICTP-UNITSTrieste-Italy

04/15/23 1

A medical cyclotron is a type of compact particle accelerator used to produce quantities of radioactive isotopes called positron emitters.

In general, cyclotron is a particle accelerator that Sends charged particles, which constantly accelerate through a ‘Dee’, through a circular path until they are (most likely) directed towards a designated target for a specific purpose.

The cyclotron was invented by Ernest Lawrence and M.S Livingston of the University of California, Berkeley, where it was first operated in 1932.

Cyclotron are source of a large number of high-energy (MeV) range charged particle

04/15/23 2

It consists of two D shaped metal boxes (about 21″ in diameter) with their straight sides facing each other.

There is a very small gap between these Dee’s faces across which a high potential difference (of order of 105 volts) is applied by a high frequency (10-15 mega cycles per second) oscillator.

Thus an alternating electric field is established between Dee’s 

during one half cycle, one dee is positive and the other is negative while during the next half cycle, the other is positive and one is negative.

A source of particle (heavy hydrogen for producing deuterons or a proton) is placed between the faces. The whole apparatus is placed between the pole pieces a very strong magnet.

04/15/23 3

04/15/23 4

Cyclotron Component Major Exterior Component Shielding

Major Interior Component Vacuum system Ion Source Magnetic system Radiofrequency system Dees Targets

04/15/23 5

04/15/23 6

Cyclotrons accelerate charged particle beams using a high frequency alternating voltage which is applied between two "D"-shaped electrodes (also called "dees").

D1 and D2 are hollow evacuated metal chambers. The dees are connected to a high frequency oscillator.

The two dees are enclosed in a evacuated steel box. The box is placed in a strong magnetic field produced by two pole pieces of electromagnet.

The positive ion to be accelerated is placed in between the dees.

04/15/23 7

Now suppose a deuteron is at the centre of the gap and D1 is at negative potential compared to D2 then deuterons will be accelerated by the electric field between the dees towards D1.

Since magnetic field B is perpendicular to the direction of motion of deuteron, a force will act on the particle given by

F = qv x B (where v and B are vectors) = qvB

Due to perpendicular magnetic field it describes a semi circular path inside D1 of radius r=mv/Bq (Centripetal force=Magnetic force)

04/15/23 8

The time period and frequency is independent of speed of charge and radius of circular path. If this frequency is equal to the frequency of the oscillator then by the time particle complete half revolution the polarity of the field will change.

Deuteron will again be accelerated across the gap, its velocity will increase so that now it will move in a circular path of greater radius but its frequency will not be affected and the process of acceleration continues.

The particle is thus accelerated to gain high velocity. Its kinetic energy increases and when radius of the path reaches up to the value of the radius of the dee (R), beam is deflected by a negatively charged plate.

Therefore with r=R, its kinetic energy is maximum.

04/15/23 9

So the necessary condition for accelerating the deuterons is f= fo where f0 is oscillator frequency. This is called resonance condition

As velocity attained is high, we choose heavier particles e.g., protons, deuterons for acceleration in cyclotron. The reason is that at such high velocities, mass varies with velocity and f will then depend on velocity, upsetting the resonance condition.

This variation in mass is more marked in the case of lighter particles. So cyclotron is suitable only for accelerating heavy particle like protons, deuterons a - particles etc.

04/15/23 10

04/15/23 11

Electrons can not be accelerated by cyclotrons because the mass of electron is very small and a small increase in energy of electron makes the electrons move with very high speed.

As a result of it the electron go quickly out of step with oscillating electric field, it is for these reasons that other accelerating machines such as synchrotron, betatron (for accelerating electrons) have been developed.

Cyclotron can not be used for accelerating uncharged particle like neutrons.

04/15/23 12

Cyclotrons are a clean nuclear technology that create very little radioactive waste as a result of their operation.

Cyclotrons are helpful since its products are used in PET for most advanced medical diagnostic imaging technology like early and accurate detection of cancer and detecting certain diseases of the heart and brain.

04/15/23 13

En.wikipedia.org/wiki/cyclotron www.icbse.com/topic/cyclotron www.lawsonreach.com

Thank you

04/15/23 14