Abstract:

Ernest Lawrence was an American nuclear physicist invented cyclotron machine in 1934; he won a Nobile price in 1939, Ernest Lawrence was born in Canton, South Dakota and educated at the universities of South Dakota, Minnesota, and Chicago and at Yale University. Lawrence founded the Lawrence Berkeley Lab, the oldest of the national laboratories.One of the most interesting applications of  motion of charged particles in an electric and magnetic field is the cyclotron, it is the machine which use for accelerate charged particles .

 

The invention that would rocket Ernest Lawrence to international fame started out modestly as a sketch on a scrap of paper. While sitting in the library one evening, Lawrence happened to glance over a journal article and was intrigued by one of the diagrams. The idea was to produce very high energy particles required for atomic disintegration by means of a succession of very small “pushes.” Ernest Lawrence told his colleagues that he had found a method for obtaining particles of very high energy, without the use of any high voltage. The idea was surprisingly simple, but Lawrence double-checked his theory with physicists from Yale to make sure he had not overlooked a critical detail.

Introduction

A cyclotron is an electrical powered machine that can accelerate the charged particles to high speed and beams them to the target, this can made a nuclear reaction that leads to producing radioisotopes.

Cyclotrons do not use uranium or produce difficult to dispose of fission product wastes. When cyclotron start to work, it will surrounded by strong radiation field, but this strong field will disappear immediately after turning off the cyclotron. To protect the operators and the environment, ANSTO’s National Medical Cyclotron is housed in a massive concrete vault with 2.3 metre thick walls.

 

Cyclotron can be classified as type of accelerator machines. These exist in two varieties, linear and cyclic. Both of them creates charged particles and affect the velocity of the charged particle and change it to high velocity after that the accelerated charged particle bombards target materials. The linear particles accelerators as the name could accelerate the charged particle linearly in straight line, but the circle particle accelerators could accelerate the charged particle in circular directions and makes the particle travel around the central point to achieve the highest acceleration possible than the linear accelerators.

 

• Type of accelerators:

 

1) Fixed Target accelerators

2) Colliding beam accelerators:

1.                a) Electrostatic
2.    b) Cyclic:                  linear

Circular

 

 

 

 

Theory:

 

How a cyclotron works?

 

Cyclotron is one of the most interesting applications of motion of charged particles in electric and magnetic field. To start with, we have an electromagnetic which continuously generates a magnetic field as it showed in figure 2, the magnetic field directed vertically up, inside this magnetic field we have two magnetic plates D1 and D2 , they are also known as “dees” as they are in shapes of latter D.

These magnetic field are connected to an alternating electric source which changes the polarity of the electric field periodically. If we have a charged particle such as a proton (p) for example is at the centre of the cyclotron, this proton will be attracted toward the negative plate D2. As it enters the plate it is acted upon by the magnetic field and we know that a charge moving with a velocity v perpendicular to the magnetic field B will more in circular fashion.

 

 

 

 

 

 

 

 

 

 

 

Figure 3: how a cyclotron works

 

As soon as this proton exits the plate and enters the gap, the polarity of the plates changes and D1 become negatively charged. Hence the proton will not get attracted toward D1 and will accelerate it. This increase the energy of the particle (p), and as it enters D1, due to the magnetic field it will start moving along a circular path, but this time as it got accelerated while entering the radius of the circular path will be larger.

Again, as it exits D1, the polarity gets reversed and now D2 gets negatively charged. Hence (p) will now get attracted towards D2 and will accelerate toward it. This will increase the energy of the particle (p) further. As it enters D2 due to the magnetic field, it will start moving along a circular path, but this time as it got accelerated while entering, the radius of the circular path will be more. This cycle continues and the particle (p) keeps on moving spirally.

Once the particles are accelerated to the required speed, they are then deflected using a magnetic field. They then leave the system via an exit slit.

This accelerated particle can then be used for bombarding other atoms or molecules or solids according to the requirement.

 

Cyclotron Frequency

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.

 

Figure 4: cyclotron frequency

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.

 

Charged particles (ions) created from a suitable source material are injected into the centre of the cyclotron. The ions are then forced to travel in a circular path around a central point and repeatedly accelerated by electrical fields.

 

Cyclotron-produced radioisotopes

 

Cyclotron-produced radioisotopes are used mainly to make radiopharmaceuticals for use in two diagnostic imaging systems – positron emission topography (PET) and single photon emission computed tomography (SPECT). Both methods involve the use of minute quantities of low-level radioactive chemicals that can be detected by the highly sensitive imaging equipment in hospitals. The radioactive materials decay rapidly and do not harm the patient. SPECT is a sophisticated camera system that produces images of slices of the body by photographing the low-energy gamma rays emitted from radioactive tracers introduced to pinpoint disease or organ function. The radioactive tracers used contain a radioisotope compound that is specific for the organ or disease being studied. If the body slices are added together, a three-dimensional image of the organ being studied is obtained. SPECT radiopharmaceuticals produced at the National Medical Cyclotron include: • gallium-67, which is used to diagnose soft tissue tumours and some inflammatory lesions. It has a half-life of 78 hours • thallium-201, which is used to assess heart conditions.

 

It has a half-life of 73 hours • iodine-123, which is used to diagnose certain thyroid diseases. It has a half-life of 13 hours. Iodine-123 labelled tracers are also commonly used to monitor neurodegenerative diseases and cancer.

 

PET is a highly sensitive system that uses positron-emitting radioisotopes. A positron is a positively charged electron particle. When a positron collides with an electron the two particles annihilate one another, releasing energy as two gamma rays which shoot off in exactly opposite directions. These two rays strike crystals in a ring of detectors around a patient, enabling sophisticated computers to then turn the information into an image. The only PET radiopharmaceutical currently routinely produced at the National Medical Cyclotron is fluorine-18. This is labelled onto a glucose molecule to form fluorodeoxyglucose (FDG). This is used to diagnose brain disease, heart viability, coronary artery disease and, increasingly, to assess the spread of cancers such as malignant melanomas. It has a half-life of 110 minutes.

 

 

 

 

Medical cyclotrons

 

A proton can be produced by medical cyclotrons, and we can these protons to create radioisotopes that are used in medical diagnosis. Radioisotopes produced in cyclotron decay by either positron emission or electron capture. Positron emission tomography (PET) and single photon emission computed tomography (SPECT), which utilises the gamma rays associated with electron capture, are two imaging techniques that rely on cyclotron-produced radioisotopes.

Figure 6: Medical Cyclotron              Figure 7:  Medical cyclotron

Why do we need both cyclotrons and reactors?

It depends on the radioactive properties required whether a nuclear reactor or a cyclotron is used to produce a radioisotope.

• Atoms with extra protons in the nucleus are called neutron-deficient and are produced in a particle accelerator such as a cyclotron.
• Atoms with extra neutrons in the nucleus are called neutron-rich and are produced in a nuclear reactor.

Neutron-rich and neutron-deficient radioisotopes decay by different means and hence have different properties and different uses. Radioisotopes made in cyclotrons complement those made in a reactor. Both types of radioisotopes are needed to service all of Australia’s nuclear medical needs.

More than 80 per cent of the radioisotopes used in medical procedures worldwide come from research reactors. Molybdenum-99 (Mo-99), which decays to form technetium-99m (Tc-99m) – the most commonly used radioisotope – is currently only produced in nuclear research reactors.

 

Conclusion

Based on above, a cyclotron is one of the most important application of the motion of charged particle in an electric and magnetic field, and there are two way to accelerate the charged particles, straight or linear acceleration which accelerates the particle linearly , and circular acceleration which accelerates the charged particles in circular directions as cyclotron machine doing this .

The cyclotron machine consists two plates in (D) laters that magnetic field goes through them and affect the charged particle and will accelerate it. The charged particle start to move and will have extra energy.

Cyclotrons can produce the medical isotopes that are used to make radiopharmaceuticals for use in two diagnostic imaging systems – positron emission topography (PET) and single photon emission computed tomography (SPECT).

Recently, Australian government has developed the nuclear reactors and nuclear medicine, and cyclotron one of the most important machines that use for medical purposes and  cyclotron facilities become more improve to provide best medical services in particular for cancer diseases .