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4. Particle Generators/Accelerators

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4. Particle Generators/Accelerators Joint innovative training and teaching/ learning program in enhancing development and transfer knowledge of application of ionizing radiation in materials processing 4. Particle Generators/Accelerators Diana Adlienė Department of Physics Kaunas University of Technolog y Joint innovative training and teaching/ learning program in enhancing development and transfer knowledge of application of ionizing radiation in materials processing This project has been funded with support from the European Commission. This publication reflects the views only of the author. Polish National Agency and the Commission cannot be held responsible for any use which may be made of the information contained therein. Date: Oct. 2017 DISCLAIMER This presentation contains some information addapted from open access education and training materials provided by IAEA TABLE OF CONTENTS 1. Introduction 2. X-ray machines 3. Particle generators/accelerators 4. Types of industrial irradiators The best accelerator in the universe… INTRODUCTION • Naturally occurring radioactive sources: – Up to 5 MeV Alpha’s (helium nuclei) – Up to 3 MeV Beta particles (electrons) • Natural sources are difficult to maintain, their applications are limited: – Chemical processing: purity, messy, and expensive; – Low intensity; – Poor geometry; – Uncontrolled energies, usually very broad Artificial sources (beams) are requested! INTRODUCTION • Beams of accelerated particles can be used to produce beams of secondary particles: Photons (x-rays, gamma-rays, visible light) are generated from beams of electrons; Neutrons are generated from beams of protons (spallation neutron sources). • Primary and secondary beams are used for radiation processing of materials and/or for analyzis of material (probe) properties. RADIATION GENERATORS Radiation generators are devices that produce energetic beams of particles which are used for: – Understanding the fundamental building blocks of nature and the forces that act upon them (nuclear and particle physics); – Understanding the structure and dynamics of materials and their properties (physics, chemistry, biology, medicine); – Medical treatment of tumors and cancers; – Production of medical isotopes; – Sterilization; – Ion Implantation to modify the surfaces of materials – National Security: cargo inspection, … There is active, ongoing work to utilize particle accelerators for – Transmutation of nuclear waste – Generating power more safely in sub-critical nuclear reactors RADIATION GENERATORS/ACCELERATORS X-ray set Linear accelerator Cyclotron Neutron generator X-RAY MACHINES • Coolidge in 1913 designed a “hot cathode” x ray tube and his design is still in use today. –The main characteristics of the Coolidge tube are its high vacuum and its use of heated filament (cathode). –The heated filament emits electrons through thermionic emission. –X rays are produced in the target (anode) through radiation losses of electrons producing characteristic and bremsstrahlung photons. –The maximum photon energy produced in the target equals the kinetic energy of electrons striking the target. X-RAY BEAMS AND X-RAY UNITS • X-ray beams are produced in energy range between 10 keV and 50 MeV when electrons with kinetic energies between strike special metallic targets. • In the target most of the electron’s kinetic energy is transformed into heat, and a small fraction of the kinetic energy is emitted in the form of x ray photons which are divided into two categories: – Characteristic X rays following electron - orbital electron interactions – Bremsstrahlung photons following electron - nucleus interactions X-RAY BEAMS AND X-RAY UNITS • Characteristic X rays result from Coulomb interactions between the incident electron and atomic orbital electrons of the target material (collision loss). • The orbital electron is ejected from its shell and an electron from a higher level shell fills the resulting orbital vacancy. • The energy difference between the two shells is: – Either emitted from the target atom in the form of a photon referred to as characteristic photon. – Or transferred to another orbital electron that is ejected from the target atom as an Auger electron. X-RAY BEAMS AND X-RAY UNITS CHARACTERISTIC X RAYS • Characteristic photon and Auger electron eKLM energies; following a vacancy in the atomic K shell. Energy of Kα photon: α Energy of eKLM Auger electron: X-RAY BEAMS AND X-RAY UNITS BREMSSTRAHLUNG (CONTINUOUS) X RAYS • Bremsstrahlung X rays result from Coulomb interactions between the incident electron and the nuclei of the target material. • During the interaction the incident electron is accelerated and loses part of its kinetic energy in the form of bremsstrahlung photons. • The interaction is also referred to as radiation loss producing braking radiation. X-RAY BEAMS AND X-RAY UNITS BREMSSTRAHLUNG (CONTINUOUS) X RAYS • In bremsstrahlung interaction X rays with energies ranging from zero to the kinetic energy of the incident electron may be produced, resulting in a continuous photon spectrum. • The bremsstrahlung spectrum produced in a given X ray target depends upon: – Kinetic energy of the incident electron – Atomic number of the target – Thickness of the target X-RAY BEAMS AND X-RAY UNITS X-RAY TARGETS • The range R of a charged particle in a particular absorbing medium is an experimental concept providing the thickness of the absorber that the particle can just penetrate. • With regard to the range R of electrons with kinetic energy EK in the target material of atomic number Z two types of targets are known: – Thin targets with thickness much smaller than R. – Thick targets with thickness of the order of R. X-RAY BEAMS AND X-RAY UNITS X-RAY TARGETS • For thin target radiation and electron kinetic energy EK: – Intensity of emitted radiation is proportional to the number of photons N times their energy EK. – Intensity of radiation emitted into each photon energy interval between 0 and EK is constant. – The total energy emitted in the form of radiation from a thin target is proportional to ( Z*EK). X-RAY BEAMS AND X-RAY UNITS X-RAY TARGETS • Thick target radiation may be considered as a superposition of a large number of thin target radiations. • The intensity of thick target radiation spectrum is expressed as: • In practice thickness of thick x-ray targets is about 1.1 R to satisfy two opposing conditions: – To ensure that no electrons that strike the target can traverse the target. – To minimize the attenuation of the bremsstrahlung beam in the target. X-RAY BEAMS AND X-RAY UNITS CLINICAL X-RAY BEAMS • A typical spectrum of a clinical x-ray beam consists of: – Continuous bremsstrahlung spectrum – Line spectra characteristic of the target material and superimposed onto the continuous bremsstrahlung spectrum. The bremsstrahlung spectrum originates in the x-ray target. The characteristic line spectra originate in the target and in any attenuators placed into the x-ray beam. TYPES OF PARTICLE ACCELERATORS A wide variety of particle accelerators is in use today. • The types of machines producing particles are distinguished by the velocity of particles that are accelerated and by the mass of particle accelerated. • Accelerators for electrons differ from accelerators for protons or heavy ions. GENERATORS/ACCELERATORS Example: A typical method for generating electrons utilizes a thermionic gun at a potential of about 100 kV. This gives a beam of 100 keV electrons. Comparison of the velocities of different particles generated at 100 keV kinetic energy shows: – Electrons: v/c = 0.55 – Protons: v/c= 0.015 – Au1+: v/c= 0.001 This has important implications for the type of acceleration scheme PROTON AND ELECTRON VELOCITIES vs KINETIC ENERGY THE DEVELOPMENT OF ACCELERATORS • Accelerators have gone through a long development process, including – Electrostatic accelerators – The Van der Graaf accelerator – The Cyclotron – The Synchrotron DIRECT ACCELERATORS: TRANSFORMER TYPE Direct accelerators are machines in which accelerated particle moves in a constant electric field gaining the energy (eV) which is equal to the potential difference (V) applied. This applies for acceleration of electrons, protons and ions DIRECT ACCELERATORS: TRANSFORMER TYPE Earliest particle accelerators/generators also called potential drop generators were the Cockcroft- Walton generator and the Van der Graaf generator • Highest voltage achieved is 24 MV • It is difficult to establish and maintain a static DC field of 20+ MV VAN DER GRAAF GENERATORS • Van der Graaf generators (electrostatic generators) are direct accelerators. • Generated energy is from the range 0.5- 5.0 MeV • Proton current 50 µA, in pulses - 5 µA. • Electrostatic generators are energy stable, accelerated particles are monoenergetic . VAN DER GRAAF GENERATORS VAN DER GRAAF GENERATOR It was a hit ! Many labs could easily obtain a Van der Graff. - Low currents - High precision ☺ COCKCROFT-WALTON GENERATOR COCKCROFT & WALTON GENERATOR • The 1 st stage of Fermilab’s huge accelerator is a Cockcroft-Walton Machine • 750 keV (Upper limit) PARTICLE ACCELERATORS • R. Widerøe (1929) proposed an accelerator by using an alternating voltage across many alternating “gaps.” • It was not without a myriad of problems • - Focusing of beam • - Vacuum leaks • - Oscillating high voltages • - Again, imagination • His professor refused any further work because it was “sure to fail.” • - Widerøe still published his idea in Archiv fur Electrotechnic ACCELERATION BY REPEATED APPLICATION OF TIME-VARYING FIELDS Ising and Widerøe suggested the repeated
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