cyclotron

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basics and operational aspects

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Medical Cyclotron:

Medical Cyclotron Rajeev Kumar Medical Physicist Department of Nuclear Medicine AIIMS rajeevraj_aiims@yahoo.com

Accelerator:

Accelerator Linear accelerator - Electron Cyclotron – Proton/Ion

Types of cyclotron:

Types of cyclotron Ion Shielding Energy Direction

Ion:

Ion Positive ion Cyclotron Negative ion Cyclotron

Advantage of –ve ion Cyclotron:

Advantage of –ve ion Cyclotron Efficiency Compact Dual Bombardment Extractor system smaller Less current No activation of internal parts

Shielded vs. Bunker:

Shielded vs. Bunker Shielded Bunker Space Less More Accessibility Less More Shielding Less More Radiation Less More

Energy:

Energy Low Energy Medium Energy Higher Energy

Direction :

Direction Horizontally Vertically Mounted

Basics of Cyclotron:

Basics of Cyclotron

Cyclotron:

Cyclotron The principle of cyclotron was first conceived by Lawrence in 1929 The first cyclotron was made operational by him and Livingston

PET Cyclotrons – Technical Considerations Siemens RDS-111 PET Cyclotron (newer) :

PET Cyclotrons – Technical Considerations Siemens RDS-111 PET Cyclotron (newer) 1 Power Supply Cabinet 1 Control Cabinet 1 Water System Cabinet Cyclotron - 22,000 pounds Shields - 60,000 pounds Cabinets - 3,200 pounds Total = 85,200 pounds

Components of Medical Cyclotron :

Components of Medical Cyclotron Medical Cyclotron mainly has six components Ion source system RF system Vacuum system Magnet system Extractor system Targetry

Ion source:

Ion source Negative ion source is preferred over positive ion beam The negative ion beam gets accelerated in the “dees” of the cyclotron and then passed through a thin carbon foil The carbon foil removes both the electrons from beam and proton beam leaves the foil The proton beam is directed towards the target where it bombards stable target and produces radioisotope Dual targets possible

Radio Frequency System :

Radio Frequency System Most important & sensitive components supplies an alternating high voltage potential to the Dee structure structure is composed of four pie-shaped electrodes The ion beam gains energy by being attracted in to, and repelled from, each of the four Dees once per orbit, yielding eight accelerations per beam orbit. With each acceleration, the beam gains energy and its orbital radius increases until it reaches the desired energy of 11 MeV before being extracted with carbon foil.

Vacuum system:

Vacuum system vacuum system consists of a vacuum chamber contained with in the magnet yokes One Mechanical and 4 Diffusion pumps vacuum is necessary in the main acceleration region of the cyclotron to maintain the desired acceleration to the ion beam with out any loss of ion sand their energy

Magnet system:

Magnet system Magnet system provides the bending force so that beam can move in circular orbit between the upper and lower magnet poles Each magnet pole consists of four pie shaped wedges extending from the yoke toward the opposing pole. These pie-shaped wedges are called hills Each of the recessed regions between adjacent hills is termed a valley region

Extractor system :

Extractor system The extraction system strips the loosely bound electrons from the hydrogen nuclei Extractor carousels are located in the pass position to allow beam extraction to the alternate target on systems with the dual extraction option. The dual beam option allows two extractor foils to be moved in to the path, thus splitting the beam between two targets .

Targetry :

Targetry There are four clinically useful positron emitting radionuclides such as F-18, O-15, N-13, and C-11are produced with RDS-111 target system consist four parts: - Target material - Liquid or gaseous material Target body, Target changer and Collimator assembly

Targetry:

Targetry

Targetry:

Targetry Target body – hold target material during irradiation usually made of silver As the beam passes through the windows it loses energy. This energy loss is converts into heat which is remove from the windows through helium jet cooling. Target changer -This includes a collimator and single vacuum window assembly . It controls the rotation of the selected target into bombardment position Any ports not occupied by an isotope production target are filled with Faraday cups. Water and helium are circulated through the target changer to remove heat from the targets.

Target Volume :

Target Volume Low Target Volume – 1.25 ml, 1.4 ml High Target Volume – 2.2 ml , 2.5 ml, 5ml etc.

Collimator assembly:

Collimator assembly A collimator is mounted at the target changer entrance to ensure a relatively uniform beam profile across the target diameter The collimator is a carbon disk with a 0.8 cm central hole through which the beam passes and bombards the target

Principle of Cyclotron:

Principle of Cyclotron A particle of mass m , charge q moving with velocity v in a magnetic field B experiences a force Bqv in a direction perpendicular to the direction of its motion and the magnetic field This force is balanced by centrifugal force equal to mv2/r, where r is the radius of circular trajectory of the ions Thus mv2/r = Bqv or r = mv2/Bqv mv/Bq Ampere’s right hand rule

PowerPoint Presentation:

The p.d changes each time the particle crosses the gap between the “dees” Thus time taken by ion in a semicircular path in a dee: t= r/v = m/Bq The RF frequency (f) of oscillation of accelerating p.d must be equal to the frequency of revolution of the ion in the magnetic field Thus f = v/2 r = Bq/ 2 m f will be independent of R provided “q/m” and “B” do not change Energy (MeV) = mv2/2 = (Brq)2/2m

Dee voltage:

Dee voltage Max energy gained by ion in a cyclotron is given by: Energy = (Brq) 2 /2m (independent of applied dee voltage) Small dee voltage means large no. of turns to acquire desired energy Modern cyclotrons: large dee voltage (30-100 kV) is generally used At 35 kV dee voltage, energy acquired by proton in each revolution with 1.8 tesla is nearly 0.1 MeV so to reach an energy of 11 MeV the proton will have to pass through 11/o.1 = 110 revolutions

Problem due to relativistic increase in mass:

Problem due to relativistic increase in mass Mass of the accelerating particle increases according to: m = m 0 /  (1-v 2 /c 2 ) Where m 0 - rest mass of particle v - relativistic velocity of particle c - velocity of light The relativistic increase in mass of the accelerating particle may be such that it reaches the gap behind time. To overcome this problem magnetic field should proportionately increase

Axial focussing:

Axial focussing Charged particles generally orbit in a plane (median plane) between the poles Particles that deviate from median plane experience a restraining force from the magnetic field For this force to prevail, magnetic field should decrease at larger radius as it has a component of magnetic force that directs any orbiting ion above or below to median plane

Axial focusing and Magnetic field B:

Axial focusing and Magnetic field B To maintain phase relationship, any increase in mass of charge particle due to acceleration would require an appropriate increase in magnetic field strength For axial focussing the magnetic field must decrease at larger radius Thus, the relativistic mass increase and axial focussing impose mutually contradicting requirements on the magnetic field B

OPERATIONAL ASPECTS:

OPERATIONAL ASPECTS

Yield :

Yield Beam Energy Current Time Target volume Target shape Purity of O-18 Water Beam Optics Cooling System

PowerPoint Presentation:

Fleming’s Left hand motor rule

PET Radionuclide Characteristics:

PET Radionuclide Characteristics 396 72 0.511 (200) 1.2 (99.8) 10 min. 307 72 0.511 (200) 0.96 (99.8) 20.5 min. 622 176 72 70 0.511 0.511 (200) 0.511 (193) 1.90 1.73 (99.9) 0.63 (96.7) 275 days 2 min. 110 min. Nuclide F-18 C-11 N-13 O-15 Ge-68 Half-Life Energy-  + (Mev) (Intensity) Energy-  (Mev) (Intensity) @ 10cm (mrem/hr/mCi) Range-air (cm)

Manufacturer :

Manufacturer Company Model Proton Energy (Mev) No. of Target Siemens RDS112 11 4 Siemens RDS111 11 2 ×8 Siemens HP 11 2 ×4 IBA Cyclone 10 8 IBA 10/5 18 8 GE PET trace 16.5 6 Ebco TR-13 13 2 ×4 NKK Oxford 12 7

Siemens RDS 111 Negative Ion Machine Cyclotron :

Siemens RDS 111 Negative Ion Machine Cyclotron

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