ACCELERATORS AND MEDICAL PHYSICS

Transcription

1 ACCELERATORS AND MEDICAL PHYSICS 1 Ugo Amaldi University of Milano Bicocca and TERA Foundation EPFL U. Amaldi 1

2 Short history of Medical Physics with radiations (*) In physics radiation is the transport of energy from one point of space to another without the need of macroscopic quantities of matter EPFL U. Amaldi 2

3 The beginnings of modern physics and of medical physics 1895 discovery of X rays Wilhelm Conrad Röntgen J.J. Thompson 1897 discovery of the electron EPFL U. Amaldi 3

4 The beginnings of modern physics and of medical physics Henri Becquerel ( ) 1896: Discovery of natural radioactivity Thesis of Mme. Curie 1904 α, β, γ in magnetic field 1898 Discovery of radium Marie Curie Pierre Curie ( ) ( ) EPFL U. Amaldi 4

5 Three types of radioactivity α radioactivity β radioactivity γ radioactivity Thesis of Mme. Curie 1904 α, β, γ in magnetic field EPFL U. Amaldi 5

6 The next magnificent three years for experimental physics and medical physics 1932 Discovery of the neutron James Chadwick ( ) EPFL U. Amaldi 6

7 The next magnificent three years for experimental physics and medical physics B Pontecorvo, E. Segrè E. Amaldi F. Rasetti E. Fermi Discovery of the effect of slow neutrons EPFL U. Amaldi 7

8 Fermi and collaborators: effectiveness of slow neutrons Chicago - E. Fermi Nuclear reactor based on slow neutrons EPFL U. Amaldi 8

9 Radioactivity in diagnostics: SPECT = Single Photon Emission Computer Tomography Emilio Segrè 1936: Discovery of technetium with Perrier 1938: discovery of 99m Tc with Ed McMillan Now extracted from fission products See lectures by Dr. S. Braccini EPFL U. Amaldi 9

10 Slown-down particle The next magnificent three years for experimental physics and medical physics 1932 C. D. Anderson Positron discovery Layer of lead inserted in a cloud chamber Fast positive particle coming from below Donald Glaser inventor of the buble chamber Carl D. Anderson discoverer of the positron EPFL U. Amaldi 10

11 How a e + e - pair is created? Cloud chamber 4 MeV photon (γ) 2 x 0.5 = 1 MeV spent to create the two masses Creation of a matter-antimatter pair (followed by an annihilation ) 2 MeV positron nucleus 1 MeV electron EPFL U. Amaldi 11

12 Particle accelerators: cyclotrons EPFL U. Amaldi 12

13 1930: invention of the cyclotron Spiral tajectory of an accelerated nucleus EPFL U. Amaldi Ernest Lawrence with a 0.1 MeV cyclotron ( ) 13

14 1930: invention of the cyclotron M. S. Livingston and E. Lawrence with the 27 inch cyclotron 14 Modern 30 MeV cyclotron for isotope production EPFL U. Amaldi

15 1929: Lawrence observation T = 2π r v = 2π m Q B z The rotation period does not depend on v and r While v is proportional to r = acceleration along a SPIRAL TRAJECTORY EPFL U. Amaldi 15

16 Lawrence cyclotron with flat poles 1931: H 2+ molecules accekerated to 80 kev (H 2 + very much used today...) Dee 1 Dee 2 Dee 1 Nord ion source Dee 2 RF generator Sud beam f = 2π 2π RF generator EPFL U. Amaldi 16

17 Relativistic particles f = 2π 2π m increases with energy/radius For isochronism B has also to increase but this defocuses the particles vertically Solution: pole made of Hills and Valleys Alternation of positive and negative B-gradients focuses the particles vertically EPFL U. Amaldi 17

18 The vertical focussing effect can be controlled by giving a spiral shape to the Hills Relativistic particles Focusing transition Valley Hill 3 Sectors Defocusing transition More details in the lectures by Dr. Saverio Braccini EPFL U. Amaldi 18

19 At PSI (Villigen) a sector cyclotron injects protons in a Isocronous cyclotron with separate sectros which accelerates them to 590 MeV Current = 1 ma EPFL U. Amaldi

20 Particle accelerators: linear accelerators = linacs EPFL U. Amaldi 20

21 Linacs for protons and other ions 1928 R. Wideröe (Aachen) Invention of the ion linac RF generator 1-7 MHz For slow ions β 0.05 EPFL U. Amaldi 21

22 Linacs for protons and other ions 1928 R. Wideröe (Aachen) Invention of the ion linac RF generator 1-7 MHz For slow ions β 0.05 Browsing a German journal in which this design by Wideröe was reproduced, Ernest Lawrence had the idea to bend the particles with a B-field: the cyclotron. EPFL U. Amaldi 22

23 Linacs for protons and other ions 1946 L. Alvarez (Berkeley) Drift Tube Linac (DTL) 200 MHz For intermediate velocity ions β 0.4 Quadrupoles β (and MeV/u) in each gap are fixed by the geometry Final energy fixed in MeV/u EPFL U. Amaldi 23

24 Quadrupoles x : focusing direction y : defocusing direction A sequence of quadrupoles F D F D...in each plane keeps a beam focused. Example with two quadrupoles: Vertical plane focusing defocusing common focus Horizontal plane defocusing focusing EPFL U. Amaldi 24

25 The electron linac: β = 1 Sigmur Varian William W. Hansen Russell Varian 1939 Invention of the klystron 1947 first linac for electrons 4.5 MeV and 3 GHz EPFL U. Amaldi 25

26 The electron linac: β = 1 In a waveguide the waves with a longitudinal E-field (TM modes) have a phase velocity v ph > 1 Slow them down! Loaded structure cavity L disk 2.5 cm (approx.) EPFL U. Amaldi 26

27 Two types of structure Two waves propagate in opposite directions in a standing wave structure. The charges on the walls switch positions in synchronism with the electron bunches electron bunches Travelling wave structure EPFL U. Amaldi 27

28 space cut off in waveguide Technicalities... in a loaded structure EPFL U. Amaldi 28

29 Radiofrequency sources KLYSTRON With U o = KV a bunched beam of electrons excites a cavity. It is an amplifier RF efficiency : about 50% MAGNETRON A rotating beam of electrons excites the radial cavities It is an oscillator EPFL U. Amaldi 29

30 Klystrons Pulsed klystron Thales - Francia 3 GHz, 200 Hz Peak power : P = 7.5 MW Average power: 7 kw = 10-3 P TH duty cycle : typically 5 μs every 5 ms EPFL U. Amaldi 30

31 Evolution of electron linacs for cancer therapy EPFL U. Amaldi 31

32 Linac for Conventional radiotherapy e - + target X 1 linac every 250,000 inhabitants tumour Linac for electrons 3 GHz 5-20 MeV Varian is market leader In the world radiation oncologists use about electron linacs EPFL U. Amaldi 60% of all the existing accelerators 32

33 Radioactivity in cancer therapy gammas from Cobalt-60 still in use, in particular in the developing world Cobalt source 2000 Cobalt-60 (1 MeV gammas) is produced in reactors by slow neutrons EPFL U. Amaldi 33

34 Linacs for therapy Western producers: Elekta (Sweden and GB): Varian (USA) and Siemens (Germania) Travelling wave linac with magnetron Standing wave linac with klystron EPFL U. Amaldi 34

35 Linacs for therapy Multileaf collimator EPFL U. Amaldi 35

36 3 GHz linac for intraoperative radiation therapy = IORT EPFL U. Amaldi 36

37 Particle accelerators: synchrotrons EPFL U. Amaldi 37

38 The synchrotron Vertical magnetic field 1 GeV electron synchrotron Circular trajectory of the particles accelerated in a synchrotron Frascati - INFN RF cavity EPFL U. Amaldi 38

39 1945: E. McMillan and V.J.Veksler discover the principle of phase stability Three properties of a synchrotron 1959: Veksler visits McMilan at Berkeley EPFL U. Amaldi 39

40 1945: E. McMillan and V.J.Veksler discover the principle of phase stability Three properties of a synchrotron mv = Q B R R is constant T = 2πR /v = 2π m / QB Synchronism between T e m(e) Flat top variable time 1959: Veksler visits McMilan at Berkeley 2 s 1 s electrons Synchronous relativistic electron EPFL U. Amaldi 40

41 Strong focusing 1- Integrated functions: bending magnets Have a high-gradient 2. Separated functions: bending magnets + quadrupoles EPFL U. Amaldi 41

42 Advamntages of strong focusing used in the 50s In 1952 the strong-focusing method invented at BNL (USA) was chosen for the AGS (Brookhaven) and the PS (CERN) The PS in 1959 EPFL U. Amaldi 42

43 At CERN we have linacs and strong-focusing synchrotrons 8.5 km Large Hadron Collider (7+7) TeV 2009 EPFL U. Amaldi 43

44 Approximate numbers of accelerators in the world (E>1 MeV) CATEGORY OF ACCELERATORS High Energy acc. (E >1GeV) Synchrotron radiation sources Medical radioisotope production Radiotherapy accelerators Research acc. included biomedical research Acc. for industrial processing and research Ion implanters, surface modification NUMBER IN USE (*) ~120 >120 ~200 > ~1 500 ~2000 >9000 TOTAL > % of all the existing accelerators are used in medicine EPFL U. Amaldi 44

45 Current Energy The time structures of the beams are very different CYCLOTRONS (*) (Normal or SC) SYNCHROTRONS 15 1 sns BEAM ON BEAM ON OFF 1.5 s BEAM ON 1-4 s time The pulsed beam of fixed energy is always present time A cycling beam of variable energy has 1 second gaps (*) A synchrocyclotrons cycles at hundreds Hertz EPFL U. Amaldi 45

46 Beams from different accelerators 1-4 na na EPFL U. Amaldi 46

47 THE END EPFL U. Amaldi 47