Prompt Radiation Fields at Accelerators

Transcription

1 Prompt Radiation Fields at Accelerators Vashek Vylet, TJNAF HPS Professional Development School, Oakland, CA January 31 February 2,

2 Overview Introduction ti Prompt Fields at Electron Accelerators Prompt Fields at Proton and Ion Accelerators Some Specific Aspects of Accelerator Radiation Fields (if time permits ) 2

3 Introduction Primary particles: charged particles (CP) that are produced and accelerated Every primary CP ends up interacting with matter somewhere (where intended or not) Interactions of energetic primary CP generate prompt secondary particles our topic here Residual nuclei at interaction site may be radioactive residual activity 3

4 Introduction Prompt radiation stops as soon as accelerator is turned off (unlike residual activity) Source term: concept quantifying yield of secondary radiation: S = Q 2 /Q 1 Q 2 dose, dose rate, fluence, Q 1 particle, charge, current, power 4

5 Electron Facilities 5

6 Sources of Prompt Radiation Primary Electrons Bremsstrahlung Electromagnetic shower Neutrons Muons Synchrotron Radiation (covered elsewhere) 6

7 Source term for thick target: Dose Equivalent at 1 m per unit power 7

8 Primary Electrons Dose in primary beam D(Gy) Φ (S/ρ) col In beam irradiation (Swanson rule of thumb for MeV e ) H& [ rem] = [ cm s ] ϕ Range of e in air: R[m] 5.E[MeV] 8

9 Critical energy E c : de/dx col = de/dx rad Bremsstrahlung E c [MeV] = 800/(Z + 1.2) Rditi Radiative losses dominate for E > E c Ec [MeV] Pb Fe

10 Bremsstrahlung Highest radiation hazard near target Forward peaked: ( ) ( ) θ 1 / 2 º = 100/ E 0 MeV θ 1/2 = 1 for 100 MeV, 0.01 for 10 GeV / Two components: sharp small angles, mild variation iti at wide angles 1 dn E dω 0 = E 0 exp( θ ) 1.08exp( θ / 72) 10

11 Angular Brems. Distribution Dose Eq quivalent Rate [Sv.m 2.kW -1.h -1 ] 10 3 D~E 0 e -θ e - D~e -θ/72 17 X 0 Fe θ Radius=5 cm Angle θ [degrees] Thick target Forward ~E.P Wide angle only ~P A27 11

12 BREMS YIELD 12

13 Forward Brems Spectra Extend up to energy of electron beam Thin target: 1/k Thick target: ~1/k 2 13

14 Brems Source Terms (Gy.h 1.kW 1.m 2 ) at 0, E 0 < 20 MeV. D 20E 2 0 0, E 0 > 20 MeV. D 300E at 90, E 0 > 100 MeV. D

15 90 Bremstrahlung Of interest shielding often closest to beam at 90 degrees ectron 10 2 Copper Mao et al sr-inc. el GeV e - 50 MeV e - ton/mevrelatively soft spectra Phot 10-4 COMPTON Photon Energy [MeV] MULTIPLE e - SCATTERING 15

16 Photon Interactions 10 4 Cu Total 1. Giant Resonance 2. Quasi-Deuteron Production 3. Pion Production 10 2 ) S (b σ[b/atom]barn/atom 0 Photo- Pair-production (Nucleus) 10 Effect Pair-production Rayleigh- (Electron) Scattering 1 Compton- Effect E [MeV] 16

17 EM Cascade (shower) Brems pair brems 1 step ~1 X 0 for electons, ~9/7X 0 for photons X - 0 = radiation length (e energy reduced to 1/e) Multiplication li stops when E e drops below E c 17

18 Shower in W (E 0 = 10 GeV) Simulate showers online at electrons positrons photons 18

19 EM Cascade (shower) Distance and energy measured in units of X 0 and E t = x/x c: 0 y = E/E c ( bt) En. deposition: a 1 de e = E0b dt Γ(a) ( a 1 t = = 1.0 ( ln y + C ) j * C 05 max e =-0.5 C g =+0.5 b b varies, but 0.5, a obtained from * bt 19

20 Shower Maximum GeV (x100) rb. units] de/dt [a GeV (x10) 10 GeV t [cm] 20

21 EM Shower radial spread Molière radius E S = 21.2 MeV X m = X E 0 E c S 90% of energy deposited by the shower within radius r 1 X 99% for r 35X within radius r = 1 X m, 99% for r = 3.5 X m 21

22 EM Shower radial spread E c and X m estimated from earlier equations, X 0 from Particle Data Booklet 6C 13Al 26Fe 29Cu 74W 82Pb 92U E c [MeV] X 0 [cm] X m [cm]

23 Neutrons σ [b b/atom] Cu Total 1. Giant Resonance 2. Quasi-Deuteron Production 3. Pion Production 10 0 Photo- Pair-production (Nucleus) Effect Pair-production Rayleigh- (Electron) Scattering 1 Compton- Effect Much lower yield compared to photons, but more penetrating important behind shielding E [MeV] 23

24 Giant Resonance Neutrons Two step process: excitation by γ absorption, followed by n emission isotropic distribution (λ γ = hc/k ~ size of nucleus) Peak σ at MVf MeV for light (A<40) nuclei, li MeV for heavier ones: k max 80A 1/3 MeV Two components: Evaporation (Maxwell dominant) and direct emission (higher E tail) 24

25 GNR spectra for Pb and U (3X 0 target) 25

26 Pseudo Deuteron Production Beyond GR, photon more likely interacts with proton neutron pair (pseudodeuteron); neutron produced by pseudodeuteron breakup For 50<k<125 MeV σ~1/k ; this low energy part is most heavily weighted for 5 MeV < E n < E 0 /2 : α = 1.7 to ~3.6 dn (increase with Z) de n E α n 26

27 Photopion Production For k > ~140 MeV pions can be produced; neutrons produced as companions and in subsequent reaction Largest resonance ~300 MeV, σ const in GeV region (again, 1/k or 1/k 2 weighting) Most penetrant, t generate evaporation following in their path equilibrium spectra behind bhidthick shielding 27

28 N. Yield as f(e) & thickness (for Cu) 28

29 N. Yield as f. of thickness (Pb) 29

30 N. Yield as f(e,z) 30

31 Muon Pair Production Possible for photon energy k > 211 MeV σ(e +,e )/ σ(μ +, μ ) (m μ /m e ) Important above E 0 ~ 1 GeV Energy loss only by ionization (<100 GeV); very penetrating & forward peaked Yield ~ E 0 (per unit beam power) Problem mainly at ~0 behind beam dumps 31

32 Muon Source Term at 0 1Sv ~ E 0 32

33 Muon Range

34 Proton and Ion Facilities 34

35 Protons High LET, high w R compared to electrons Interactions: Coulomb with atomic electrons range Nuclear, producing secondary hadrons At high enough energies protons initiate nuclear interactions before ranging out Radiative losses negligible till close to TeV 35

36 Pelliccioni: h φ for protons dose coe ef [Sv.cm 2 ] Fluence e-to-eff. 1.E-7 1.E-8 1.E-9 1.E-10 1.E-11 Protons AP PA LAT ISO 1E 1.E+0 1E 1.E+2 1E 1.E+4 1E 1.E+6 1E 1.E+8 E [MeV] 36

37 Probabil ity [%] In nteraction Be C Al Fe Cu Pb Proton Energy [GeV] 37

38 Neutrons: E p < 10 MeV Several (p,n) reactions with threshold < 10 MeV some materials ( 3 H, 7 Li) used as targets for neutron production Reaction 3 H(p,n) 3 He Li(p,n) 7 Be S( Sc(p,n) 45 Ti V(p,n) 51 Cr Threshold energy [MeV] 63 Cu(pn) Cu(p,n) 63 Zn Cu(p,n) 65 Zn

39 Note: d-t reaction is exoenergetic no threshold Q 17.5 MeV 39

40 Neutrons: 10 MeV <E p < 200 MeV Evaporation neutrons (at lower energies): equilibrium and pre equilibrium emission Equilibrium emission: proton absorbed b din nucleus excitation i multiple energy transfers among nucleons Proton direction forgotten, stat. equilibrium One neutron boils off Neutron emission in uncorrelated direction to p + 40

41 Neutrons: 10 MeV <E p < 200 MeV Evaporation neutrons: isotropic emission, Maxwell energy spectrum (T few MeV) Pre equilibrium: neutron emitted only after few energy transfers emission i forwardpeaked, somewhat correlated to p + direction 41

42 Neutrons: 10 MeV <E p < 200 MeV Intranuclear cascade at higher energies proton collides with individual nucleons, these collide further cascade of nucleons Multiple l nucleons may be emitted Excited nucleus may de excite by evaporation 42

43 Neutron spectra from targets bombarded by 30 MeV protons Solid line: evaporation Maxwell spectrum component 43

44 Angular distribution of neutrons from targets bombarded by 52 MeV protons. Pre-equilibrium equilibrium and intranuclear cascade enhance the forward component. 44

45 Neutrons: 200 MeV <E p < 1 GeV Intermediate energy morereactions reactions possible, more particles emitted; # of protons equals # neutrons at higher energies Hd Hadronic cascades + evaporation neutrons Distribution of secondary particles more forward peaked than before 45

46 Neutron spectra below E p not very sensitive to E p 46

47 Neutrons: E p > 1 GeV High energy range: many morereactionsreactions possible Hadronic showers may extend beyond initial target complicated source term Sullivan: production of hd hadrons with ihe>40 MeV MV by protons 1 GeV < E p < 1000 GeV for thin Fe or Cu targets (< 1 removal mfp for hi E E proton) 5000 Φ ( θ ) = [m 2 proton 1 ] 2 ( θ + 35 E 0 ) 47

48 1000 Hadro on fluence e at 1 m [m -2 ] GeV 10 GeV 1 TeV Angle [deg] 48

49 Total neutron yield per proton Neutron dose equivalent per proton at 90 at 1 m from a copper target 49

50 Muons Generated by decay of charged pions and kaons m π c 2 = 140 MeV, important for E p > 300 MeV; decay quickly to muons: π ± μ ± + ν μ m Κ c 2 = 494 MeV, important for E p > 1 GeV; decay quickly ikl to muons: Κ ± μ ± + ν μ Extremely forward peaked emission At very high eneries: so called direct muon production 50

51 Hadronic Cascade Extranuclear cascade: propagated by hi en secondary nucleons mostly neutrons up to ~450 MeV; protons, pions important above this energy (and kaons even higher) Neutral pion π 0 decays into photon pair, initiates i i elmag cascade this generates a leading hadron spike at very high E p (>100 GeV) 51

52 52

53 Ion Accelerators Radiation fields in many aspects similar to proton machines Primary ions shorter range (de/dx ~ z 2 ) Low energy light ion machines: use specific exoenergetic (d d, d T) and endoenergetic reactions to produce neutron beams 53

54 Ion Accelerators Energetic ions generate secondary nucleons; from there follow processes described before Neutron yield by heavy ions (Kurosawa): A Y = W A + A N [neutrons.particle 1 ] ( ) 2 P 13 P P T P 2 NT ZP P projectile, T target Early shielding calculations for ion machines used scaled source terms for protons 54

55 THE END 55

56 But wait, there is more: Skyshine Neutron Spectra 56

57 Neutron Skyshine q H ( r ) = e π 2 4 r r / λ Spectrum hardens with distance At close distance 1/r 2 not good More complex models exist

58 Equilibrium Neutron Spectra Most penetrating part: neutrons >150 MeV (for E>150 MeV σ inel reaches a constant t minimum: σ inel = 43.1 A 0.70 ) CERN SLAC 10 7 concrete iron.φ(e) E E.Φ(E) ENERGY [MeV] ENERGY [MeV]

59 SLAC SSRL Pulsed e, thin shielding: gamma flash Neutrons come later (up to tens of μs) ) MCA LiI Pulse height spectra (with γ flash) 250 Counts gate off gate on Channel 59

60 SLAC SSRL Gamma flash with non equilibrium i spectra 60