AWAKE Electron Spectrometer

AWAKE Electron Spectrometer Simon Jolly 6th November 2013

Spectrometer Specifications • Wakefield accelerated electrons ejected collinear with proton beam: need to separate the 2 and measure energy of electron beam only. • Must be able to resolve energy spread as well as energy: spectrometer must accept a range of energies, probably 0-5 GeV. • Current conceptual layout: • Dipole mounted ~2 m downstream of plasma exit induces dispersion in electron beam. • Scintillator screen 1 m downstream of dipole intercepts electron beam ONLY. • Dispersion gives energy-dependent position spread on screen. • Scintillator imaged by intensified CCD camera viewing upstream face of scintillator screen. Simon Jolly, UCL, AWAKE-UK Meeting

Spectrometer Status • Energy reconstruction with Lotov’s beam distribution show good match (Dan Hall). • Confirmed with CERN that CAD models of experimental area can be shared and modified: • Ans Pardons CERN contact. • CERN uses CATIA, we have Autodesk Inventor: reasonably consistent import/export using native file formats. • Gone rather quiet… • Some discussions with Patric Muggli over vacuum vessel: • Light tight path needs some thought so as not to interfere with beamline maintenance. • Largely dependent on spectrometer layout: camera position. • Scintillator light output: • Scintillator performance crucial for this design. • After hiatus, giving it both barrels: • Lawrence Deacon (postdoc). • James Goodhand (MSci). • GEANT4/BDSIM simulations: start with better-known scintillators… Simon Jolly, UCL, AWAKE-UK Meeting

2 GeV Beam, 1.86 T Field Simon Jolly, UCL, AWAKE-UK Meeting

AWAKE Experimental Layout

Spectrometer Layout Scintillator Screen Protons CERN MBPS dipole Electrons + Protons Camera Plasma cell

Spectrometer: Vacuum Vessel Beampipe Electrons + Protons Vacuum Window Camera Vacuum Vessel Light Tight Path Simon Jolly, UCL, AWAKE-UK Meeting

Scintillator Screen • Default scintillator choice is Lanex: • Manufactured by Kodak. • Used in Medical Physics as X-ray phosphor for imaging. • Gd2O2S:Tb – Gadolinium sensitiser, Terbium dopant activator/wavelength shifter. • Phosphor grains on reflective backing. • Properties don’t seem to be well documented/studied… • Need to simulate light production (photons per MeV conversion efficiency) to ensure we have enough photons emitted in direction of camera. • Is this the correct scintillator for our purposes? • We care about: • Light output. • Radiation hardness. • Area. • We don’t care about: • Speed. Simon Jolly, UCL, AWAKE-UK Meeting Vac. Chamb. (grey)

GEANT4 Simulations (Lawrence) • Have set up simulation using GEANT4/BDSIM • Beginning to use it to test required vacuum level (initially air at STP vs vacuum). • Included scintillation and reflection/refraction processes in screen – currently crystal YAG:Ce quadrupole Coils (yellow) Vacuum Chamber (grey) Yoke (green) Camera plane Scint. screen Poles (blue) Simon Jolly, UCL, AWAKE-UK Meeting Vac. Chamb. (grey)

GEANT4 Simulation Status • Plan to include necessary optical processes in screen – scattering, absorption etc. to obtain realistic optical photon distribution • e.g. for Lanex screen: • Particle size ~ photon wavelength -> Mie Scattering. • Optical surfaces – reflective back surface etc. quadrupole Coils (yellow) Vacuum Chamber (grey) Yoke (green) Camera plane Scint. screen Poles (blue) Simon Jolly, UCL, AWAKE-UK Meeting Vac. Chamb. (grey)

Conclusions • Scintillator selection key to spectrometer performance. • Light output levels from scintillator sets almost entire spectrometer geometry. • Set camera position/distance from solid angle scintillation photon distribution. • Check “standard candle” scintillators – YAG:Ce etc. – in GEANT4 before moving on to granular Lanex. • Need to pin down Lanex properties: previous GEANT4 simulations are scarce… Simon Jolly, UCL, AWAKE-UK Meeting

AWAKE Electron Spectrometer