Resolution Studies of inorganic Scintillation Screens for high energetic and high brilliant Electron Beams

Transcription

1 Resolution Studies of inorganic Scintillation Screens for high energetic and high brilliant Electron Beams Gero Kube, Christopher Behrens (DESY), Werner Lauth (IKP, Mainz) Introduction Results of Test MAMI Outlook

2 Standard Diagnostics in Linacs: OTR transition radiation: electromagnetic radiation emitted when a charged particle crosses boundary between two media with different optical properties visible part: Optical Transition Radiation (OTR) beam diagnostics: backward OTR (reflection of virtual photons) typical setup: image beam profile with optical system beam image and measurements of beam shape and size angular distribution Intensity (a.u.) E = GeV θ = /γ =.5 mrad courtesy: K. Honkavaara (DESY) θ (rad) Workshop on Scintillating Screen Applications, 5..

3 OTR Diagnostics: Pitfalls Linac Coherent Light Source SLAC Gun OTR L QB LS LX OTR OTR Screen OTR courtesy: H. Loos (SLAC) DL 5 MeV BC 5 MeV L-linac BC 4. GeV L-linac BSY 4 GeV OTR monitor observation with BC, BC switched on OTR OTR y (Pixel) OTRS:LI5: x (Pixel) measured spot is no beam image interpretation: coherent OTR (COTR) emission strong compression in bunch compressors in the meantime COTR also at FLASH long bunch (λ<σ z ) short bunch (λ>σ z ) Workshop on Scintillating Screen Applications, 5..

4 Consequences & Alternatives LCLS: coherent emission compromise use of OTR as reliable beam diagnostics wire scanners for transverse beam diagnostics instead of OTR profile diagnostics based on transition radiation reduce coherent effects: observation at smaller wavelength EUV/XUV transition radiation imaging (in collaboration with Tomsk Polytechnic University, Russia and Institut für Kernphysik, Mainz University).) spectral range of coherent emission?.) EUV/XUV optics expensive and difficult to handle profile diagnostics based on different physical processes - wire scanners in preparation for dedicated XFEL - luminescent screen monitors widely used at hadron accelerators motivation for test experiment nearly no information for high-energy electron machines Workshop on Scintillating Screen Applications, 5..

5 Inorganic Scintillators properties radiation resistant high stopping power short decay time widely used in high energy physics, astrophysics, dosimetry, high light yield reduced saturation generation of scintillation light energy conversion (characterstic time -8-9 sec) Formation of el. magn. shower. Below threshold of e + e - pair creation relaxation of primary electrons/holes by generation of secondary ones, phonons, plasmons, and other electronic excitations. thermalization of seconray electrons/holes ( -6 - sec) Inelastic processes: cooling down the energy by coupling to the lattice vibration modes until they reach top of valence resp. bottom of conduction band. transfer to luminescent center ( - -8 sec) Energy transfer from e-h pairs to luminescent centers. photon emission (> - sec) radiative relaxation of excited luminescence centers Workshop on Scintillating Screen Applications, 5..

6 Implication on Transverse Resolution Which effects may affect transverse resolution? light generation: energy conversion light propagation transverse range of ionization total reflection at scintillator surface energy conversion thick target : formation of electromagnetic shower (thickness in the order of radiation length X ) transverse shower dimension: Molière radius as scaling variable R M containing 9% of shower energy.65 X ( Z +.) F. Schmidt, "CORSIKA Shower Images", X : radiation length, Z : atomic number Workshop on Scintillating Screen Applications, 5..

7 Implication on Transverse Resolution energy loss Bethe-Bloch (collision) Bremsstrahlung (radiative) energy deposition in thin target ignore radiative contribution - thickness / X - - small amount of re-absorption in material ionization: interaction of particle em. field with lattice particle field virtual photons, in classical picture transverse evanescent waves relativistic rise incease of transverse field extension Fermi plateau cancellation of incoming particle field by induced polarization field of electrons in medium saturation range as scaling variable R δ Stopping Power / MeV cm g - collsion stopping power radiative stopping power total stopping power de dx - - v e y β kinematical term βγ 4 (MIP) Energy /MeV r E y e de dx YAG / y ln β γ relativistic rise with y = Dβ β m = β Fermi plateau m γ m ε( ω) Workshop on Scintillating Screen Applications, 5..

8 Implication on Transverse Resolution extension radius limiting value: Rδ = c ω ε ( ω ) ε(ω): complex dielectric function approximation as free electron gas (Drude model) hc Rδ = hω p ω p : plasma frequency hω p = 8.86 ρ Z A ev light propagation light generated inside scintillator has to cross surface BGO crystal λ = 48 nm refractive index n inorganic scintillators high n, i.e. large contribution of total reflection Workshop on Scintillating Screen Applications, 5..

9 Scintillator Material Properties 4.8 YAG YAP R δ / nm LSO LuAG GSO LuAP refractive index BGO PWO NBWO CWO scintillators under investigation BGO:.5 mm PWO:. mm LYSO:.8 mm,.5 mm (Prelude 4) YAG:. mm,. mm, phosphor ρ [g/cm ] ħω p [ev] R M [cm] λ max [nm] yield [/kev] λ max R δ [nm] BGO PWO LSO:Ce YAG:Ce Workshop on Scintillating Screen Applications, 5..

10 Mainz Microtron MAMI Institute of Nuclear Physics, University of Mainz (Germany) cascaded Racetrack Microtrons: E max = 855 MeV double-sided Microtron (HDSM): E max =.5 GeV % duty cycle polarized electron beam (~ 8%) Workshop on Scintillating Screen Applications, 5..

11 Experimental Setup target YAG mm YAG phosphor OTR Wire Scanner LYSO.8mm LYSO.5mm YAG.mm PWO.mm BGO.5mm Al O.5mm observation geometry -.5 w.r.t. beam axis camera: BASLER Af 659 x 494 size 9.9μm x 9.9μm - e BM 5m X beamline Workshop on Scintillating Screen Applications, 5..

12 Beam Images measurement and analysis: I = 46 pa 5 signal and background frame LYSO:Ce (.5mm) hor. BGO (.5mm) hor vert. pixe l ve rt. p ixel 4 LYSO:Ce (.8mm) hor. 4 PWO (.mm) hor ve rt. p ixel ve rt. p ixel YAG:Ce (powder) YAG:Ce (.mm) hor. 4 6 vert. p ixel hor. 4 YAG:Ce (mm) Al O (.5mm) hor. 5 4 vert. p ixel hor. different scale! vert. p ixel 4 vert. p ixel Workshop on Scintillating Screen Applications, 5..

13 Results vertical beam size 6 mean values σ y / µ m BGO.5mm PWO.mm LYSO.5mm LYSO.8mm YAG phosphor YAG.mm YAG.mm YAG phosphor YAG,.mm YAG,.mm LYSO,.8mm WS 5 LYSO,.5mm horizontal beam size I / na Wire na PWO,.mm BGO,.5mm σ y / µ m YAG phosphor WS σ x / µ m BGO.5mm PWO.mm LYSO.5mm LYSO.8mm YAG phosphor YAG.mm YAG.mm YAG,.mm YAG,.mm LYSO,.8mm LYSO,.5mm PWO,.mm BGO,.5mm σ x / µ m Wire na - - I / na dependency on observation geometry Workshop on Scintillating Screen Applications, 5..

14 Observation Geometry beam diagnostics popular OTR-like observation geometry: scintillator tilt versus beam axis 45 tilt of screen observation under φ BGO crystal micro-focused beam I =.8 na measured beam spots hor. 4 ± 4 6 hor hor vert vert vert. Workshop on Scintillating Screen Applications, 5..

15 Simulation of Light Propagation - + Analysis: - ZEMAX calculation of -dim PSF - calculation of -dim beam profile - convolution of PSF and beam profile - horizontal / vertical projection of resulting distribution - determinatiuon of nd moment (standard deviation) +.5 Signal in ROI σ x = 8.4 mu Signal in ROI σ x = mu simulation experiment 4 6 hor. σ y = 6.7 mu different scale! simulation experiment 4 6 hor. σ y = 5.4 mu vert vert. Workshop on Scintillating Screen Applications, 5..

16 Comparison simulation experiment σ x / µ m σ y / µ m sinulation experiment rotation angle / deg rotation angle / deg satisfactory agreement between simulation and measurement simulation reproduces observed trend in beam size measured beam size systematically larger than simulated one effect of extension radius not included in calculation increase in PSF results summarized in IPAC proceedings: G. Kube, C. Behrens, W. Lauth, MOPD88 Workshop on Scintillating Screen Applications, 5..

17 Future Plans continue search for optimum scintillator material direct comparison with OTR diagnostics influence on observation geometry for different materials (and thicknesses) new test MAMI, March COTR generation at scintillators open points contribution of M. Yan influence of luminescent centers on resolution different dopands, different concentration? screen saturation saturation at high intensities (>.4 pc/cm ) observed for YAG:Ce screens (A. Murokh et al., Proc. PAC, ) material properties of interest: band gap scintillation decay time Workshop on Scintillating Screen Applications, 5..

18 Luminescent Types Exciton luminescence: BGO, Ionization/excitation by radiation creates unbound e-h pairs or bound e-h pairs called excitons. Excitons can move rather freely in crystals, caught at impurities, defects, and so on, and the STE (self-trapped excitons) gives luminescence upon radiative recombination. Dopant luminescence: GSO:Ce, Radiative recombination of STE at dopant (activator) ions. Charge-transfer luminescence Belongs to exciton luminescence. Due to charge transfer where initial and final states are different, selection rules for EM transition are loosened, thereby enhancing transition probability. CVL (Core-valence luminescence, Cross luminescence) After excitation of the core-valence electron, an electron in the valence band recombines with the resultant hole radiatively. To avoid Auger process, E VC < E g is necessary. BaF, CsF, LiF,.. M. Kobayashi (KEK): Introduction to Scintillators Workshop on Scintillating Screen Applications, 5..

19 Luminescence luminescence in configurational coordinate diagram M. Kobayashi (KEK): Introduction to Scintillators R = inter-atomic distance between ground state of ligand atom and the excited state of luminescence centre atom Workshop on Scintillating Screen Applications, 5..