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 application of a much smaller voltage in a linear accelerator by using time-varying fields In this way, a high particle beam energy could be attained by repeatedly applying voltage “kicks”

Widerøe or Sloan-Lawrence or Ising‘s idea interdigital structure SCHEMATIC OF WIDERØE’S LINAC ACCELERATION TECHNIQUES: DC FIELD

• The simplest acceleration method: DC voltage

• Can accelerate particles over many gaps: electrostatic accelerator

• Problem: breakdown voltage at ~10MV

DC field still used at start of injector chain ACCELERATION TECHNIQUES: RF FIELD

• Oscillating RF (radio-frequency) field

• Firstly introduced by Rolf Widerøe (The principle is known as “Widerøe accelerator” untill now .) • Particle must see the field only when the field is in the accelerating direction L = (1/ 2)vT • Requires the synchronism condition to hold: Tparticle =½T RF • Problem: high power loss due to radiation. ACCELERATION TECHNIQUES: RF FIELD

The particles gain energy by surfing on the electric fields of well-timed radio oscillations ACCELERATION TECHNIQUES: RF CAVITIES

• Electromagnetic power is stored in a resonant volume instead of being radiated;

• RF power feed into cavity, originating from RF power generators, like Klystrons

• RF power oscillating (from magnetic to electric energy), at the desired frequency

• RF cavities requires bunched beams (as opposite to coasting beams) – particles located in bunches separated in space RADIOFREQUENCY POWER GENERATION SYSTEM

• The radiofrequency power generation system produces the microwave radiation used in the accelerating waveguide to accelerate electrons to the desired kinetic energy and consists of two major components: – RF power source (magnetron or klystron) – Pulsed modulator FROM PILL-BOX TO REAL CAVITIES

(from A. Chao)

LHC cavity module ILC cavity ACCELERATION BY REPEATED APPLICATION OF TIME VARYING ACCELERATING FIELDS Two approaches for accelerating with time-varying fields: an electric field along the direction of particle motion with Radio- Frequency (RF) Cavities

Circular Accelerators Linear Accelerators Use one or a small number of Use many accelerating cavities Radiofrequency accelerating cavities through which the particle and make use of repeated passage passes only once through them. These are linear accelerators. This approach leads to circular accelerators: Cyclotrons, synchrotrons, and their variants FROM LINEAR TO CIRCULAR ACCELERATORS?

• Technological limit on the electrical field in an RF cavity (breakdown); • Gives a limited ∆E per distance. • ⇒ Circular accelerators, in order to re-use the same RF cavity • This requires a bending field F B in order to follow a circular trajectory CIRCULAR ACCELERATORS

• Circular accelerators: deflecting forces are needed

• Circular accelerators: piecewise circular orbits with a defined bending radius ρ – Straight sections are needed for e.g. particle detectors – In circular arc sections the magnetic field must provide the desired bending radius:

• For a constant particle energy we need a constant B field ⇒ dipole magnets with homogenous field. SYNCHROTRON

In synchrotrons acceleration is performed by RF cavities; particles are accelerated along a closed, circular orbit and the magnetic field which bends the particles increases with time so that a constant orbit is maintained during acceleration.

The bending field changes with particle beam energy to maintain a constant radius, so B ramps in proportion to the momentum. The revolution frequency also changes with momentum. SYNCHROTRON

• For an electron synchrotron, the injected beam is already relativistic, so only the magnetic field changes with beam energy. • For a proton synchrotron, the injected beam is not yet relativistic, so the RF accelerating frequency and the magnetic field both ramp with energy • RF frequency must stay locked to the revolution frequency of a particle

• Synchrotrons are used for most HEP experiments (LHC, Tevatron, HERA, LEP, SPS, PS)

The synchrotron concept was first proposed in 1943 by the Australian physicist Mark Oliphant. STRONG-FOCUSING SYNCHROTRONS

There were two nearly identical very large proton synchrotrons constructed at the same time 1959-1960:

At the European CERN laboratory At the Brookhaven National laboratory on in Geneva (28 GeV) Long Island (33 GeV).

Both of them are still in operation. SYNCHROTRON RADIATION

• Charged particles undergoing acceleration emit electromagnetic radiation • Main limitation for circular electron machines – RF power consumption becomes too high • The main limitation factor for LEP... – ...the main reason for building LHC ! • However, synchrotron radiations is also useful COLLIDERS

A collider can be thought of as a fixed-energy synchrotron. Beams of matter and antimatter particles counter-rotate, sharing the same beam pipe and are made to collide.

First electron-positron collider, ADA, at Frascati which was built by Bruno The large hadron collider at Touschek in 1960 (eventually reached 3 CERN (13 TeV) GeV) LARGE HADRON COLLIDER

The evidence of Higgs boson was experimentally aproved on 4.07.2012 Protons collide at 14 TeV in this simulation from CMS, producing four muons. Lines denote other particles, and energy deposited is shown in 15.07.2015 The LHCb experiment at blue CERN’s Large Hadron Collider has reported the discovery of a class of particles known as pentaquarks CYCLOTRON PRINCIPLE

Ernest Lawrence recognized that the revolution period and frequency are independent of particle velocity:

Lawrence’s Application of Wideroe’s Idea: The Cyclotron Uniform circular motion is maintained via centripetal acceleration: Therefore, a particle in resonance with a time varying field applied to the Dees with frequency given as above will be accelerated. The The radius is: particle is in synchronism with the time-varying field. • Such cylcotrons can accelerate proton energies up to 20-30MeV CYCLOTRON

• In a cyclotron the particles are accelerated along a spiral trajectory guided inside two evacuated half-cylindrical electrodes (dees) by a uniform magnetic field produced between the pole pieces of a large magnet (1 T). THE CYCLOTRON PRINCIPLE

A vertical B-field provides the force to maintain the electron’s circular orbit
The particles pass repeatedly from cavity to cavity, gaining energy.
As the energy of the particles increases, the radius of the orbit increases until the particle is ejected CYCLOTRON

– constant B field – constant RF field in the gap increases energy – radius increases proportionally to energy – limit: relativistic energy, RF phase out of synch – In some respects simpler than the synchrotron, and often used as medical accelerators THE FIRST MILLION VOLT CYCLOTRON

08/01/32

“... we were concerned about how many of the protons would succeed in spiralling around a great many times without getting lost on the way." Lawrence and Livingston at Berkeley PARTICLE ACCELERATORS BETATRON

• Betatron is a cyclic accelerator in which the electrons are made to circulate in a toroidal vacuum chamber (doughnut) that is placed into a gap between two magnet poles. • Conceptually, the betatron may be considered an analog of a transformer:

Primary current is the alternating current exciting the magnet. Secondary current is the electron current circulating in the doughnut. PARTICLE ACCELERATORS MICROTRON

• Microtron is an electron accelerator that combines the features of a linac and a cyclotron. • The electron gains energy from a resonant wave guide cavity and describes circular orbits of increasing radius in a uniform magnetic field. • After each passage through the wave guide the electrons gain an energy increment resulting in a larger radius for the next pass through the wave guide cavity. OTHER ACCELERATOR TYPES

Linear accelerators for linear colliders

- LINEAR ACCELERATORS

Whereas a circular accelerator can make use of one or a small number of RF accelerating cavities, a linear accelerator utilizes many (hundreds to thousands) individual accelerating cells. Again, accelerators for protons or ions “look” quite different from those that accelerate electrons, because electron beams are already relativistic at low energy. Modern proton linear accelerators are based on the Alvarez Drift-Tube Linac. Alvarez was awarded the 1968 Nobel Prize in Physics for his contributions to elementary particle physics. The two largest proton linear accelerators are the LANSCE linac at Los Alamos (800 MeV) and the Spallation Neutron Source Linac at ORNL (1000 MeV). LINEAR ACCELERATORS FOR ELECTRONS

Most electron linacs utilize a structure known as the Disk-Loaded Waveguide. Geometry looks somewhat different from that used for protons since electrons quickly become relativistic ACCELERATING CAVITIES

Modern machines use a time-dependent electric field in a cavity to accelerate the particles LINACS

Medical linacs are cyclic accelerators that accelerate electrons to kinetic energies from 4 to 25 MeV using microwave radiofrequency fields: – 10 3 MHz : L band – 2856 MHz: S band – 10 4 MHz: X band

In a linac the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides. LINACS

Schematic diagram of a modern fifth generation linac LINACS INJECTION SYSTEM

Two types of electron gun producing electrons in linac: LINACS ACCELERATING WAVEGUIDE

Two types of accelerating waveguide are in use: – Traveling wave structure – Standing wave structure ELECTRON ACCELERATORS/IRRADIATORS

Direct Linear (transformer) Single cavity (microwave) accelerator accelerator accelerator DIRECT ACCELERATORS: TRANSFORMER TYPE

Capability for DC power supply (for transformer accelerators) Addopted from the lecture of dr. Z.Zimek (ICTJ) • Single cavity accelerators

TYPES OF INDUSTRIAL IRRADIATORS

• Gamma-irradiators
Sources: mainly Co-60 and Cs-137
Categorization: from I to IV, specific design based on safety requirements

• Electron-beam irradiators – Electron energy < 10 MeV – also use of bremstrahlung GAMMA IRRADIATORS: CO-60 SOURCE

• Activities ranging from 1 TBq to 100 PBq. • Relatively long half-life of 5.26 years (source changing frequency is minimised). • Co in its solid form is insoluble and non-friable and has a high melting point (1492 oC) - reduced risk of the spread of radioactive contamination beta minus decay with emission of 2 high energy gamma photons (1.17 MeV and 1.33 MeV) beta particles are absorbed in the source capsule GAMMA IRRADIATORS: CS-137 SOURCE

• Half life – 30.7 years. • Caesium-137 has become less popular in recent years because its solubility makes it more difficult to contain for long periods. • It also emits lower energy which means that much larger amounts of radioactive material are required to produce an output equivalent to that from cobalt-60. beta minus decay with emission of gamma photons (0.66 MeV, 0.036 MeV and 0.032 MeV) beta particles are absorbed in the source capsule TYPES OF GAMMA IRRADIATORS

• Category I – self contained, dry storage irradiator • Category II – panoramic, dry storage irradiator • Category III – self contained, wet storage irradiator • Category IV – panoramic, wet storage irradiator CATEGORY I SELF-CONTAINED IRRADIATORS

• Sealed source (Co-60 or Cs-137) is mounted round a tube and completely enclosed in a dry container constructed of solid materials and is shielded (lead-steel containement ) • Activity – several Ci (1 Ci = 37 000 000 000 Bq) • Applications: • Preservation (food, seeds, etc.) • Radiation Effects (biological research) • Chemical synthesis (chemical research) CATEGORY I SELF-CONTAINED IRRADIATORS

Sample goes in here

Sample lowered

Sources in here (do not move) CATEGORY II PANORAMIC SELF-CONTAINED IRRADIATORS

• A controlled human access irradiator in which the sealed source (Co-60 or Cs-137) is enclosed in a dry container constructed of solid materials, is fully shielded when not in use and is exposed within a radiation volume that is maintained inaccessible during use by an entry control system (maze, concrete wals) • Activity – several dozens of Ci • Application: • Sterilization (medical supplies) • Preservation (food) CATEGORY II PANORAMIC SELF-CONTAINED IRRADIATORS

Open Beam Port CATEGORY III SELF-CONTAINED WET STORAGE IRRADIATORS

• Sealed Co-60 source is contained in a water filled storage pool and is shielded. • Human access to the sealed source and the volume undergoing irradiation is physically restricted in the design configuration. • Activity – from several hundreds to thousands of Ci Sterilization (small medical supplies) CATEGORY IV SELF-CONTAINED WET STORAGE IRRADIATORS

A controlled human access irradiator in which the sealed Co-60 source is contained in a water filled storage pool, is fully shielded when not in use and is exposed within a radiation volume that is maintained inaccessible during use by an entry Source rack control system

Activity – from dozens of thousands of Ci CATEGORY IV SELF-CONTAINED WET STORAGE IRRADIATORS

Co-60 source construction

Co-60 source pencil CATEGORY IV SELF-CONTAINED WET STORAGE IRRADIATORS

Rack Pencils

Source Storage CATEGORY IV SELF-CONTAINED WET STORAGE IRRADIATORS

• Water pool for storage • Concrete walls and maze during operation

Sterilization of big packs with medical suppliers, pharmacy or cosmetic products ELECTRON BEAM IRRADIATORS (EBI)

• Beam energy up to 10 MeV • Advantage – no induced radioactivity • Bremstrahlung from electron beam interactions with exposing items is sometimes used but should be protected • Possibility of neutron production should be taken into account

Schematic view of electron generator ELECTRON BEAM IRRADIATORS (EBI)

• Categorization • Category I – integrally shielded irradiator unit with interlocks • Category II – Irradiator unit housed in shielded room maintained inaccessible during irradiation • Applications • Preservation (food, caught fish, etc.) • Sterilization • Chemical synthesis (polymerization) EBI: CATEGORY I

• An integrally shielded unit with interlocks, where human access during operation is not physically possible owing to the configuration of the shielding • Concrete walls against electrons and neutrons • Some lead from bremstrahlung, • Aluminium for shielding against electron beam EBI: CATEGORY II

A unit housed in shielded rooms that are maintained inaccessible during operation by an entry control system

– Concrete walls – Maze for item supply X-RAY MASCHINE

Forward Peaked Emission of 5.0 MeV X-rays COMPARISON: BEAM PENETRATION

• The attractive features of X-ray processing for industrial applications, Greater depth of penetration, allowing for treatment of products with large volumes. • Controllable dose rates, which can facilitate monomer polymerization. • Not a thermal process which eliminates adverse effects on materials due to the heat COMPARISON: IONIZING RADIATION SOURCES X-RAY PROCESSING THROUGH-PUT POTENTIAL ENERGY EVOLUTION

Exponential growth of energy with time • Increase of the energy by an order of magnitude every 6-10 years • Each generation replaces previous one to get even higher energies. • The process continues… • Energy is not the only interesting parameter. – Intensity – Size of the beam DEVELOPMENT IN RADIATION PROCESSING

Continuous increase and development of irradiation facilities: ∼ 250 high activity 60 Co gamma and ∼ 1000 EB machines (0.1 – 10 MeV). Prospect for X-ray application. NEW DEVELOPMENT IN RADIATION PROCESSING EQUIPMENT ADVANCED EB EMITER MODULE LOW ENERGY EB ACCELERATOR

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